Microphone Techniques for Music Sound Reinforcement

5/26/2018 Microphone Techniques for Music Sound Reinforcement

MICROPHONETECHNIQUES FOR MUSIC

REINFORCEMENT

SOUND


3

MICROPHONE

TECHNIQUES FOR MUSIC

SOUND

REINFORCEMENT

INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . .4

MICROPHONE CHARACTERISTICS . . . . . . . . . . . . . .4

MUSICAL INSTRUMENT CHARACTERISTICS . . . . . . .11

ACOUSTIC CHARACTERISTICS . . . . . . . . . . . . . . . .14

MICROPHONE PLACEMENT . . . . . . . . . . . . . . . . . .22

STEREO MICROPHONE TECHNIQUES . . . . . . . . . . .32

MICROPHONE SELECTION GUIDE . . . . . . . . . . . . .34

GLOSSARY . . . . . . . . . . . . . . . . . . . . . . . . . . . . .35


4

Introduction

Microphone techniques (the selection and place-

ment of microphones) have a major influence onthe audio quality of a sound reinforcement sys-tem. For reinforcement of musical instruments,

there are several main objectives of microphonetechniques: to maximize pick-up of suitable

sound from the desired instrument, to minimizepick-up of undesired sound from instruments or

other sound sources, and to provide sufficientgain-before-feedback. Suitable sound from thedesired instrument may mean either the natural

sound of the instrument or some particularsound quality which is appropriate for the appli-

cation. Undesired sound may mean the director ambient sound from other nearby instruments

or just stage and background noise. Sufficientgain-before-feedback means that the desiredinstrument is reinforced at the required level

without ringing or feedback in the sound system.

Obtaining the proper balance of these factorsmay involve a bit of give-and-take with each. In

this guide, Shure application and developmentengineers suggest a variety of microphone tech-

niques for musical instruments to achieve theseobjectives. In order to provide some backgroundfor these techniques it is useful to understand

some of the important characteristics of micro-phones, musical instruments and acoustics.

Microphone Characteristics

The most important characteristics of micro-phones for live sound applications are their oper-ating principle, frequency response and direc-tionality. Secondary characteristics are their

electrical output and actual physical design.

Operating principle - The type of transducerinside the microphone, that is, how the micro-phone picks up sound and converts it into anelectrical signal.

A transducer is a device that changes energyfrom one form into another, in this case, acousticenergy into electrical energy. The operatingprinciple determines some of the basic capabili-

ties of the microphone. The two most common

types are Dynamic and Condenser.

Dynamic microphones employ a diaphragm/voice coil/magnet assembly which forms aminiature sound-driven electrical generator.Sound waves strike a thin plastic membrane(diaphragm) which vibrates in response. Asmall coil of wire (voice coil) is attached to therear of the diaphragm and vibrates with it. Thevoice coil itself is surrounded by a magneticfield created by a small permanent magnet. It isthe motion of the voice coil in this magneticfield which generates the electrical signal corre-sponding to the sound picked up by a dynamicmicrophone.

Dynamic microphones have relatively simpleconstruction and are therefore economical andrugged. They can provide excellent sound quali-ty and good specifications in all areas of micro-phone performance. In particular, they can han-dle extremely high sound levels: it is almostimpossible to overload a dynamic microphone.In addition, dynamic microphones are relativelyunaffected by extremes of temperature or humid-

ity. Dynamics are the type most widely used ingeneral sound reinforcement.

Condenser microphones are based on an electri-cally-charged diaphragm/backplate assemblywhich forms a sound-sensitive capacitor. Here,sound waves vibrate a very thin metal or metal-coated-plastic diaphragm. The diaphragm ismounted just in front of a rigid metal or metal-coated-ceramic backplate. In electrical terms thisassembly or element is known as a capacitor (his-

SO

U

N

D

REIN

FO

RC

EM

EN

T MICROPHONETECHNIQUES FOR MUSIC

SOUND

REINFORCEMENT


torically called a condenser), which has the

ability to store a charge or voltage. When theelement is charged, an electric field is createdbetween the diaphragm and the backplate, pro-portional to the spacing between them. It is thevariation of this spacing, due to the motion ofthe diaphragm relative to the backplate, that pro-duces the electrical signal corresponding to thesound picked up by a condenser microphone.

The construction of a condenser microphonemust include some provision for maintaining theelectrical charge orpolarizing voltage. Anelectretcondenser microphone has a permanent

charge, maintained by a special material deposit-ed on the backplate or on the diaphragm. Non-electret types are charged (polarized) by meansof an external power source. The majority ofcondenser microphones for sound reinforcementare of the electret type.

All condensers contain additional active circuitryto allow the electrical output of the element to beused with typical microphone inputs. Thisrequires that all condenser microphones be pow-ered: either by batteries or byphantom power(a method of supplying power to a microphone

through the microphone cable itself). There aretwo potential limitations of condenser micro-phones due to the additional circuitry: first, theelectronics produce a small amount of noise;second, there is a limit to the maximum signallevel that the electronics can handle. For thisreason, condenser microphone specificationsalways include a noise figure and a maximumsound level. Good designs, however, have verylow noise levels and are also capable of verywide dynamic range.

PHANTOM POWER

Phantom power is a DC voltage (usually 12-48volts) used to power the electronics of a con-denser microphone. For some (non-electret)condensers it may also be used to provide thepolarizing voltage for the element itself. Thisvoltage is supplied through the microphonecable by a mixer equipped with phantom poweror by some type of in-line external source. Thevoltage is equal on Pin 2 and Pin 3 of a typicalbalanced, XLR-type connector. For a 48 voltphantom source, for example, Pin 2 is 48 VDCand Pin 3 is 48 VDC, both with respect to Pin 1which is ground (shield).

Because the voltage is exactly the same on Pin 2and Pin 3, phantom power will have no effect onbalanced dynamic microphones: no current willflow since there is no voltage difference acrossthe output. In fact, phantom power supplieshave current limiting which will prevent damageto a dynamic microphone even if it is shorted ormiswired. In general, balanced dynamic micro-phones can be connected to phantom poweredmixer inputs with no problem.

Fig. 3: phantom power schematic

Condenser microphones are more complex than

dynamics and tend to be somewhat more costly.Also, condensers may be adversely affected byextremes of temperature and humidity which cancause them to become noisy or fail temporarily.However, condensers can readily be made withhigher sensitivity and can provide a smoother, morenatural sound, particularly at high frequencies. Flatfrequency response and extended frequency rangeare much easier to obtain in a condenser. In addi-tion, condenser microphones can be made verysmall without significant loss of performance.

5

MICROPHONE


SOUND

REINFORCEMENT


6

TRANSIENT RESPONSE

Transient response refers to the ability of amicrophone to respond to a rapidly changingsound wave. A good way to understand whydynamic and condenser mics sound different isto understand the differences in theirtransient response.

In order for a microphone to convert soundenergy into electrical energy, the sound wavemust physically move the diaphragm of themicrophone. The amount of time it takes forthis movement to occur depends on the weight(or mass) of the diaphragm. For instance,the diaphragm and voice coil assembly of adynamic microphone may weigh up to 1000times more than the diaphragm of a condensermicrophone. It takes longer for the heavydynamic diaphragm to begin moving than forthe lightweight condenser diaphragm. It alsotakes longer for the dynamic diaphragm tostop moving in comparison to the condenserdiaphragm. Thus, the dynamic transientresponse is not as good as the condensertransient response. This is similar to two

vehicles in traffic: a truck and a sports car.They may have equal power engines but thetruck weighs much more than the car. Astraffic flow changes, the sports car canaccelerate and brake very quickly, while thesemi accelerates and brakes very slowly dueto its greater weight. Both vehicles followthe overall traffic flow but the sports carresponds better to sudden changes.

Pictured here are two studio microphonesresponding to the sound impulse producedby an electric spark: condenser mic on top,

dynamic mic on bottom. It is evident that ittakes almost twice as long for the dynamicmicrophone to respond to the sound. It alsotakes longer for the dynamic to stop movingafter the impulse has passed (notice the rippleon the second half of the graph). Since con-denser microphones generally have bettertransient response then dynamics, they arebetter suited for instruments that have verysharp attack or extended high frequency output

such as cymbals. It is this transient response

difference that causes condenser mics to have amore crisp, detailed sound and dynamic mics tohave a more mellow, rounded sound.

The decision to use a condenser or dynamicmicrophone depends not only on the soundsource and the sound reinforcement system

but on the physical setting as well. From apractical standpoint, if the microphone will beused in a severe environment such as a rockand roll club or for outdoor sound, dynamictypes would be a good choice. In a morecontrolled environment such as a concert hallor theatrical setting, a condenser microphonemight be preferred for many sound sources,especially when the highest sound quality isdesired.

Frequency response - The output level orsensitivity of the microphone over its operating

range from lowest to highest frequency.

Virtually all microphone manufacturers listthe frequency response of their microphonesover a range, for example 50 - 15,000 Hz.This usually corresponds with a graph thatindicates output level relative to frequency.The graph has frequency in Hertz (Hz) on thex-axis and relative response in decibels (dB)on the y-axis.

SO

U

N

D

REIN

FO

RC

EM

EN


SOUND

REINFORCEMENT

Condenser/dynamic scope photo


A microphone whose output is equal at all

frequencies has aflatfrequency response.

Flat response microphones typically have anextended frequency range. They reproduce avariety of sound sources without changing orcoloring the original sound.

A microphone whose response has peaks or dips incertain frequency areas exhibits a shapedresponse.

A shaped response is usually designed to enhance

a sound source in a particular application.

For instance, a microphone may have a peak inthe 2 - 8 kHz range to increase intelligibility forlive vocals. This shape is called apresence peakor rise. A microphone may also be designed to beless sensitive to certain other frequencies. Oneexample is reduced low frequency response (lowend roll-off) to minimize unwanted boominessor stage rumble.

THE DECIBEL

The decibel (dB) is an expression often used inelectrical and acoustic measurements. The deci-bel is a number that represents a ratio of two val-ues of a quantity such as voltage. It is actually alogarithmic ratio whose main purpose is to scalea large measurement range down to a muchsmaller and more useable range. The form ofthe decibel relationship for voltage is:

dB = 20 x log(V1/V2)

where 20 is a constant, V1 is one voltage, V2 isthe other voltage, and log is logarithm base 10.

Examples:

What is the relationship in decibelsbetween 100 volts and 1 volt?

dB = 20 x log(100/1)dB = 20 x log(100)dB = 20 x 2 (the log of 100 is 2)dB = 40

That is, 100 volts is 40dB greaterthan 1 volt.

What is the relationship in decibelsbetween 0.001 volt and 1 volt?

dB = 20 x log(0.001/1)dB = 20 x log(0.001)dB = 20 x (-3) (the log of .001 is -3)dB = -60

That is, 0.001 volt is 60dB less that 1 volt.

Similarly:

if one voltage is equal to the other theyare 0dB different

if one voltage is twice the other they are6dB different

if one voltage is ten times the other theyare 20dB different

7

MICROPHONE


SOUND

REINFORCEMENT

Flat frequency response

Shaped frequency response


8

Since the decibel is a ratio of two values, there

must be an explicit or implicit reference valuefor any measurement given in dB. This is usual-ly indicated by a suffix on the decibel value suchas: dBV (reference to 1 volt which is 0dBV) ordB SPL (reference to 0.0002 microbar which is0dB Sound Pressure Level)

Decibel scalefor dBV or dB SPL

One reason that the decibel is so useful in certainaudio measurements is that this scaling functionclosely approximates the behavior of humanhearing sensitivity. For example, a change of1dB SPL is about the smallest difference in loud-

ness that can be perceived while a 3dB SPLchange is generally noticeable. A 6dB SPLchange is quite noticeable and finally, a 10dBSPL change is perceived as twice as loud.

The choice of flat or shaped response micro-phones again depends on the sound source, thesound system and the environment. Flatresponse microphones are usually desirable toreproduce instruments such as acoustic guitars orpianos, especially with high quality sound sys-tems. They are also common in stereo mikingand distant pickup applications where the micro-

phone is more than a few feet from the soundsource: the absence of response peaks mini-mizes feedback and contributes to a more naturalsound. On the other hand, shaped response micro-phones are preferred for closeup vocal use and forcertain instruments such as drums and guitar ampli-fiers which may benefit from response enhance-ments forpresence orpunch. They are also usefulfor reducing pickup of unwanted sound and noiseoutside the frequency range of an instrument.

Directionality - A microphones sensitivity to

sound relative to the direction or angle fromwhich the sound arrives.

There are a number of different directionalpatterns found in microphone design. Theseare typically plotted in a polar pattern tographically display the directionality of themicrophone. The polar pattern shows thevariation in sensitivity 360 degrees around themicrophone, assuming that the microphone isin the center and that 0 degrees represents thefront of the microphone.

The three basic directional types of micro-phones are omnidirectional, unidirectional,and bidirectional.

The omnidirectional microphone has equaloutput or sensitivity at all angles. Its coverageangle is a full 360 degrees. An omnidirectionalmicrophone will pick up the maximum amountof ambient sound. In live sound situations anomni should be placed very close to the soundsource to pick up a useable balance betweendirect sound and ambient sound. In addition,

an omni cannot be aimedaway from undesiredsources such as PA speakers which may causefeedback.

Omnidirectional

The unidirectional microphone is most sensitiveto sound arriving from one particular directionand is less sensitive at other directions. Themost common type is a cardioid (heart-shaped)response. This has the most sensitivity at0 degrees (on-axis) and is least sensitive at 180

SO

U

N

D

REIN

FO

RC

EM

EN


SOUND

REINFORCEMENT

100=1

101=10

102=100

103=1000

104=10,000

105=100,000

106=1,000,000

0

20

40

60

80

100

120

1. Compare 2. Compress 3. scale (x 20)

b a

b/a


degrees (off-axis). The effective coverage or

pickup angle of a cardioid is about 130 degrees,that is up to about 65 degrees off axis at thefront of the microphone. In addition, the cardioidmic picks up only about one-third as muchambient sound as an omni. Unidirectionalmicrophones isolate the desired on-axis soundfrom both unwanted off-axis sound and fromambient noise.

Cardioid

For example, the use of a cardioid microphone

for a guitar amplifier which is near the drum setis one way to reduce bleed-through of drumsinto the reinforced guitar sound.

Unidirectional microphones have severalvariations on the cardioid pattern. Two of theseare the supercardioid and hypercardioid.

Both patterns offer narrower front pickup anglesthan the cardioid (115 degrees for the supercar-dioid and 105 degrees for the hypercardioid) andalso greater rejection of ambient sound. Whilethe cardioid is least sensitive at the rear (180

degrees off-axis) the least sensitive direction isat 126 degrees off-axis for the supercardioid and110 degrees for the hypercardioid. When placedproperly they can provide more focused pickupand less ambient noise than the cardioid pattern,but they have some pickup directly at the rear,called a rear lobe. The rejection at the rear is-12 dB for the supercardioid and only -6 dB forthe hypercardioid. A good cardioid type has atleast 15-20 dB of rear rejection.Supercardioid

The bidirectional microphone has maximumsensitivity at both 0 degrees (front) and at 180degrees (back). It has the least amount of out-put at 90 degree angles (sides). The coverageor pickup angle is only about 90 degrees atboth the front and the rear. It has the sameamount of ambient pickup as the cardioid.This mic could be used for picking up twoopposing sound sources, such as a vocal duet.Though rarely found in sound reinforcementthey are used in certain stereo techniques,

such as M-S (mid-side).

Microphone Polar Patterns Compared

9

MICROPHONE


SOUND

REINFORCEMENT

Cardioid


10

In sound reinforcement, microphones must oftenbe located in positions where they may pick upunintended instrument or other sounds. Someexamples are: individual drum mics picking upadjacent drums, vocal mics picking up overallstage noise, and vocal mics picking up monitorspeakers. In each case there is a desired soundsource and one or more undesired sound sources.Choosing the appropriate directional pattern canhelp to maximize the desired sound and mini-mize the undesired sound.

Although the direction for maximum pickup isusually obvious (on-axis) the direction for leastpickup varies with microphone type. In particu-lar, the cardioid is least sensitive at the rear (180degrees off-axis) while the supercardioid andhypercardioid types actually have some rearpickup. They are least sensitive at 125 degreesoff-axis and 110 degrees off axis respectively.

For example, when using floor monitors withvocal mics, the monitor should be aimed directlyat the rear axis of a cardioid microphone for

maximum gain-before-feedback. When using asupercardioid, however, the monitor should bepositioned somewhat off to the side (55 degreesoff the rear axis) for best results. Likewise,when using supercardioid or hypercardioid typeson drum kits be aware of the rear pickup of thesemics and angle them accordingly to avoid pick-up of other drums or cymbals.

Monitor speaker placement for

maximum rejection:cardioid and supercardioid

Other directional related microphone characteristics:

Ambient sound rejection - Since unidirectionalmicrophones are less sensitive to off-axis soundthan omnidirectional types they pick up lessoverall ambient or stage sound. Unidirectionalmics should be used to control ambient noisepickup to get a cleaner mix.

Distance factor - Because directional micro-phones pick up less ambient sound than omni-directional types they may be used at some-what greater distances from a sound source andstill achieve the same balance between thedirect sound and background or ambientsound. An omni should be placed closer tothe sound source than a uniabout half thedistanceto pick up the same balance betweendirect sound and ambient sound.

Off-axis coloration - Change in a microphonesfrequency response that usually gets progressive-ly more noticeable as the arrival angle of soundincreases. High frequencies tend to be lost first,often resulting in muddy off-axis sound.

Proximity effect - With unidirectional micro-

phones, bass response increases as the mic ismoved closer (within 2 feet) to the sound source.With close-up unidirectional microphones (lessthan 1 foot), be aware of proximity effect androll off the bass until you obtain a more naturalsound. You can (1) roll off low frequencies onthe mixer, or (2) use a microphone designed tominimize proximity effect, or (3) use a micro-phone with a bass rolloff switch, or (4) use anomnidirectional microphone (which does notexhibit proximity effect).

Proximity effect graph

Unidirectional microphones can not only help

SO

U

N

D

REIN

FO

RC

EM

EN


SOUND

REINFORCEMENT

REJECT UNWANTED SOURCES

USING DIRECTIONAL PATTERNS TO


to isolate one voice or instrument from other

singers or instruments, but can also minimizefeedback, allowing higher gain. For thesereasons, unidirectional microphones arepreferred over omnidirectional microphones inalmost all sound reinforcement applications.

The electrical output of a microphone isusually specified by level, impedance and wiringconfiguration. Output level or sensitivity is thelevel of the electrical signal from the micro-phone for a given input sound level. In general,condenser microphones have higher sensitivitythan dynamic types. For weak or distant soundsa high sensitivity microphone is desirable whileloud or close-up sounds can be picked up wellby lower-sensitivity models.

The output impedance of a microphone is rough-ly equal to the electrical resistance of its output:150-600 ohms for low impedance (low-Z) and10,000 ohms or more for high impedance.(high-Z). The practical concern is that low impedancemicrophones can be used with cable lengths of1000 feet or more with no loss of quality whilehigh impedance types exhibit noticeable high

frequency loss with cable lengths greater thanabout 20 feet.

Finally, the wiring configuration of a microphonemay be balanced or unbalanced. A balancedoutput carries the signal on two conductors (plusshield). The signals on each conductor are thesame level but opposite polarity (one signal ispositive when the other is negative). A balancedmicrophone input amplifies only the differencebetween the two signals and rejects any part of thesignal which is the same in each conductor. Anyelectrical noise or hum picked up by a balanced

(two-conductor) cable tends to be identical in thetwo conductors and is therefore rejected by thebalanced input while the equal but oppositepolarity original signals are amplified. On theother hand, an unbalanced microphone outputcarries its signal on a single conductor (plusshield) and an unbalanced microphone inputamplifies any signal on that conductor. Such acombination will be unable to reject any electricalnoise which has been picked up by the cable.Balanced, low-impedance microphones are

therefore recommended for nearly all sound

reinforcement applications.

The physical design of a microphone is itsmechanical and operational design. Types usedin sound reinforcement include: handheld, head-worn, lavaliere, overhead, stand-mounted, instru-

ment-mounted and surface-mounted designs.Most of these are available in a choice of operat-ing principle, frequency response, directionalpattern and electrical output. Often the physicaldesign is the first choice made for an application.Understanding and choosing the other character-istics can assist in producing the maximum qual-ity microphone signal and delivering it to thesound system with the highest fidelity.

Musical Instrument Characteristics

Some background information on characteris-tics of musical instruments may be helpful.Instruments and other sound sources are char-acterized by their frequency output, by theirdirectional output and by their dynamic range.

Frequency output - the span of fundamentaland harmonic frequencies produced by aninstrument, and the balance or relative level ofthose frequencies.Musical instruments have overall frequency

11

MICROPHONE


SOUND

REINFORCEMENT


12

ranges as found in the chart below. The dark

section of each line indicates the range offundamental frequencies and the shadedsection represents the range of the highestharmonics or overtones of the instrument.The fundamental frequency establishes thebasic pitch of a note played by an instrumentwhile the harmonics produce the timbre orcharacteristic tone.

Instrument frequency ranges

It is this timbre that distinguishes the soundof one instrument from another. In this man-ner, we can tell whether a piano or a trumpetjust played that C note. The following graphsshow the levels of the fundamental andharmonics associated with a trumpet and anoboe each playing the same note.

Instrument spectra comparison

The number of harmonics along with the relative

level of the harmonics is noticeably different

between these two instruments and provideseach instrument with its own unique sound.

A microphone which responds evenly to the fullrange of an instrument will reproduce the mostnatural sound from an instrument. A microphonewhich responds unevenly or to less than the fullrange will alter the sound of the instrument,though this effect may be desirable in some cases.

Directional output - the three-dimensional pat-tern of sound waves radiated by an instrument.

A musical instrument radiates a different tonequality (timbre) in every direction, and each partof the instrument produces a different timbre.Most musical instruments are designed to soundbest at a distance, typically two or more feetaway. At this distance, the sounds of the variousparts of the instrument combine into a pleasingcomposite. In addition, many instruments pro-duce this balanced sound only in a particulardirection. A microphone placed at such distanceand direction tends to pick up a natural or well-balanced tone quality.

On the other hand, a microphone placed close tothe instrument tends to emphasize the part of theinstrument that the microphone is near. The result-ing sound may not be representative of the instru-ment as a whole. Thus, the reinforced tonal bal-ance of an instrument is strongly affected by themicrophone position relative to the instrument.

Unfortunately, it is difficult, if not impossible, toplace a microphone at the natural soundingdistance from an instrument in a sound rein-forcement situation without picking up other

(undesired) sounds and/or acoustic feedback.Close microphone placement is usually the onlypractical way to achieve sufficient isolation andgain-before-feedback. But since the soundpicked up close to a source can vary significantlywith small changes in microphone position, it isvery useful to experiment with microphone loca-tion and orientation. In some cases more thanone microphone may be required to get a goodsound from a large instrument such as a piano.

SO

U

N

D

REIN

FO

RC

EM

EN


SOUND

REINFORCEMENT

200 500 50004000300020001000

oboe

trumpet in Bb

frequency


Another instrument with a wide range of charac-teristics is the loudspeaker. Anytime you areplacing microphones to pick up the sound of aguitar or bass cabinet you are confronted withthe acoustic nature of loudspeakers. Each indi-vidual loudspeaker type is directional and dis-plays different frequency characteristics at differ-ent angles and distances. The sound from a loud-speaker tends to be almost omnidirectional atlow frequencies but becomes very directional athigh frequencies. Thus, the sound on-axis at thecenter of a speaker usually has the most bite orhigh-end, while the sound produced off-axis orat the edge of the speaker is more mellow orbassy. A cabinet with multiple loudspeakers hasan even more complex output, especially if it hasdifferent speakers for bass and treble. As withmost acoustic instruments the desired sound onlydevelops at some distance from the speaker.

Sound reinforcement situations typically requirea close-mic approach. A unidirectional dynamicmicrophone is a good first choice here: it canhandle the high level and provide good soundand isolation. Keep in mind the proximity effect

when using a uni close to the speaker: somebass boost will be likely. If the cabinet has onlyone speaker a single microphone should pick upa suitable sound with a little experimentation. Ifthe cabinet has multiple speakers of the sametype it is typically easiest to place the micro-phone to pick up just one speaker. Placing themicrophone between speakers can result instrong phase effects though this may be desirableto achieve a particular tone. However, if thecabinet is stereo or has separate bass and treblespeakers multiple microphones may be required.

Placement of loudspeaker cabinets can also have asignificant effect on their sound. Putting cabinetson carpets can reduce brightness, while raisingthem off the floor can reduce low end. Open-backcabinets can be miked from behind as well as fromthe front. The distance from the cabinet to walls orother objects can also vary the sound. Again,experiment with the microphone(s) and placementuntil you have the sound that you like!

instrument from its softest to its loudest level.

The dynamic range of an instrument determinesthe specifications for sensitivity and maximuminput capability of the intended microphone.Loud instruments such as drums, brass andamplified guitars are handled well by dynamicmicrophones which can withstand high soundlevels and have moderate sensitivity. Softerinstruments such as flutes and harpsichords canbenefit from the higher sensitivity of condensers.Of course, the farther the microphone is placedfrom the instrument the lower the level of soundreaching the microphone.

In the context of a live performance, the relativedynamic range of each instrument determines howmuch sound reinforcement may be required. If allof the instruments are fairly loud, and the venue isof moderate size with good acoustics, no reinforce-ment may be necessary. On the other hand, if theperformance is in a very large hall or outdoors,even amplified instruments may need to be furtherreinforced. Finally, if there is a substantial differ-ence in dynamic range among the instruments,such as an acoustic

guitar in a loud rockband, the micro-phone techniques(and the sound sys-tem) must accom-modate those differ-ences. Often, themaximumvolume of theoverall soundsystem is limitedby the maximumgain-before-feed-

back of the softestinstrument.

An understandingof the frequencyoutput, directional output, and dynamic rangecharacteristics of musical instruments can helpsignificantly in choosing suitable microphones,placing them for best pickup of the desiredsound and minimizing feedback or other unde-sired sounds.

13

MICROPHONE


SOUND

REINFORCEMENT

0 20 40 60 80 100 120

CYMBAL

SNARE DRUM

BASS DRUM

FEMALE VOICE

MALE VOICE

TRUMPET

HAR MONICA

SAX OPHONE

GUI TAR

PIA NO

VIOLIN

Intensity Level in Decibels(at distance of 10 feet)

INSTRUMENT LOUDSPEAKERSDynamic range - the range of volume of an


14

SO

U

N

D

REIN

FO

RC

EM

EN


SOUND

REINFORCEMENT

Acoustic Characteristics

Sound Waves

Sound moves through the air like waves in water.Sound waves consist of pressure variations travel-ing through the air. When the sound wave travels,it compresses air molecules together at one point.This is called the high pressure zone or positivecomponent(+). After the compression, an expan-sion of molecules occurs. This is the low pressurezone or negative component(-). This process con-tinues along the path of the sound wave until itsenergy becomes too weak to hear. The soundwave of a pure tone traveling through air wouldappear as a smooth, regular variation of pressurethat could be drawn as a sine wave.

Frequency, wavelength and the speed of sound

The frequencyof a sound waveindicates the rateof pressure vari-ations or cycles.One cycle is a

change fromhigh pressure tolow pressureand back to high pressure. The number of cyclesper second is called Hertz, abbreviated Hz.So, a 1,000 Hz tone has 1,000 cycles per second.

The wavelength of a sound is the physical distancefrom the start of one cycle to the start of the nextcycle. Wavelength is related to frequency by thespeed of sound. The speed of sound in air is about1130 feet per second or 344 meters/second. Thespeed of sound is constant no matter what the fre-

quency. The wavelength of a sound wave of anyfrequency can be determined by these relationships:

Loudness

The fluctuation ofair pressure createdby sound is a changeabove and belownormal atmosphericpressure. This iswhat the human earresponds to. Thevarying amount ofpressure of the airmolecules compress-ing and expanding isrelated to the appar-ent loudness at thehuman ear. Thegreater the pressure change, the louder the

sound. Under ideal conditions the human earcan sense a pressure change as small as 0.0002microbars (1 microbar = 1/1,000,000 atmospher-ic pressure). The threshold of pain is about 200microbars, one million times greater! Obviouslythe human ear responds to a wide range ofamplitude of sound. This amplitude range ismore commonly measured in decibels SoundPressure Level (dB SPL), relative to 0.0002microbars (0 dB SPL). 0 dB SPL is the thresholdof hearing Lp and 120 dB SPL is the threshold ofpain. 1dB is about the smallest change in SPLthat can be heard. A 3dB change is generally

noticeable while a 6dB change is very notice-able. A 10dB SPL increase is perceived to betwice as loud!

Sound Propagation

There are four basic ways in which sound canbe altered by its environment as it travels orpropagates: reflection, absorption, diffractionand refraction.

The Wave Equation: c = f lspeed of sound = frequency wavelength

or

wavelength =speed of sound

frequency

for a 500Hz sound wave:

wavelength =1,130 feet per second

500Hzwavelength = 4.4 feet

Approximate wavelengths of common

frequencies:

100 Hz: about 10 feet1000 Hz: about 1 foot10,000 Hz: about 1 inch

Ambient sounds

DISTANCEWAVELENGTH

PRESSURE

+

0_

1 CYCLE

1/2 CYCLE

AMPLITUDE

Schematic of sound wave

140

130

120

110

100

9080

70

60

50

40

30

20

10

0


1. Reflection - Asound wave can be reflected

by a surface or other object if the object is physi-cally as large or larger than the wavelength ofthe sound. Because low frequency sounds havelong wavelengths they can only be reflected bylarge objects. Higher frequencies can be reflect-ed by smaller objects and surfaces as well aslarge. The reflected sound will have a differentfrequency characteristic than the direct sound ifall frequencies are not reflected equally.

Reflection is also the source of echo, reverb, andstanding waves:

Echo occurs when a reflected sound is delayedlong enough (by a distant reflective surface) tobe heard by the listener as a distinct repetition ofthe direct sound.

Reverberation consists of many reflections of asound, maintaining the sound in a reflective spacefor a time even after the direct sound has stopped.

Standing waves in a room occur for certain fre-quencies related to the distance between parallelwalls. The original sound and the reflected sound

will begin to reinforce each other when the dis-tance between two opposite walls is equal to amultiple of half the wavelength of the sound. Thishappens primarily at low frequencies due to theirlonger wavelengths and relatively high energy.

2. Absorption - Some materials absorb soundrather than reflect it. Again, the efficiency ofabsorption is dependent on the wavelength.Thin absorbers like carpet and acoustic ceilingtiles can affect high frequencies only, while thickabsorbers such as drapes, padded furniture andspecially designed bass traps are required to

attenuate low frequencies. Reverberation in aroom can be controlled by adding absorption:the more absorption the less reverberation.Clothed humans absorb mid and high frequen-cies well, so the presence or absence of an audi-ence has a significant effect on the sound in anotherwise reverberant venue.

3. Diffraction - Asound wave will typicallybend around obstacles in its path which aresmaller than its wavelength. Because a low fre-

quency sound wave is much longer than a high

frequency wave, low frequencies will bendaround objects that high frequencies cannot.The effect is that high frequencies tend to have ahigher directivity and are more easily blockedwhile low frequencies are essentially omnidirec-tional. In sound reinforcement, it is difficult toget good directional control at low frequenciesfor both microphones and loudspeakers.

4. Refraction - The bending of a sound waveas it passes through some change in the densityof the environment. This effect is primarilynoticeable outdoors at large distances from loud-speakers due to atmospheric effects such as windor temperature gradients. The sound will appearto bend in a certain direction due to these effects.

Direct vs. Ambient Sound

A very important property of direct sound is that itbecomes weaker as it travels away from the soundsource. The amount of change is controlled bythe inverse-square law which states that the levelchange is inversely proportional to the square ofthe distance change. When the distance from a

sound source doubles, the sound level decreasesby 6dB. This is a noticeable decrease. For exam-ple, if the sound from a guitar amplifier is 100 dBSPL at 1 ft. from the cabinet it will be 94 dB at 2ft., 88 dB at 4 ft., 82 dB at 8 ft., etc. Conversely,when the distance is cut in half the sound levelincreases by 6dB: It will be 106 dB at 6 inchesand 112 dB at 3 inches!

On the other hand, the ambient sound in a roomis at nearly the same level throughout the room.This is because the ambient sound has beenreflected many times within the room until it is

essentially non-directional. Reverberation is anexample of non-directional sound.

For this reason the ambient sound of the roomwill become increasingly apparent as a micro-phone is placed further away from the directsound source. In every room, there is a distance(measured from the sound source) where thedirect sound and the reflected (or reverberant)sound become equal in intensity. In acoustics,this is known as the Critical Distance. If a micro-

15

MICROPHONE


SOUND

REINFORCEMENT


16

phone is placed at the Critical Distance or farther,

the sound quality picked up may be very poor.This sound is often described as echoey, rever-berant, or bottom of the barrel. The reflectedsound overlaps and blurs the direct sound.

Critical distance may be estimated by listeningto a sound source at a very short distance, thenmoving away until the sound level no longerdecreases but seems to be constant. That dis-tance is critical distance.

A unidirectional microphone should be positionedno farther than 50% of the Critical Distance, e.g.if the Critical Distance is 10 feet, a unidirectionalmic may be placed up to 5 feet from the soundsource. Highly reverberant rooms may requirevery close microphone placement. The amount ofdirect sound relative to ambient sound is con-trolled primarily by the distance of the micro-phone to the sound source and to a lesser degreeby the directional pattern of the mic.

Phase relationships and interference effects

The phase of a

single frequency soundwave is always describedrelative to the startingpoint of the waveor 0 degrees. Thepressure change is alsozero at this point. The

peak of the high pressure zone is at 90 degrees,the pressure change falls to zero again at 180degrees, the peak of the low pressure zone is at270 degrees, and the pressure change rises to zeroat 360 degrees for the start of the next cycle.

Two identical sound waves starting at the samepoint in time are called in-phase and will sumtogether creating a single wave with double theamplitude but otherwise identical to the originalwaves. Two identical sound waves with onewaves starting point occurring at the 180 degreepoint of the other wave are said to be out ofphase and the two waves will cancel each othercompletely. When two sound waves of the samesingle frequency but different starting points arecombined the resulting wave is said to have

phase shift or an

apparent startingpoint somewherebetween the origi-nal starting points.This new wavewill have the samefrequency as theoriginal waves butwill haveincreased ordecreased ampli-tude depending onthe degree ofphase difference.Phase shift, in thiscase, indicates thatthe 0 degreepoints of twoidentical wavesare not the same.

Most soundwaves are not a single frequency butare made up of many frequencies. When identicalmultiple-frequency soundwaves combine thereare three possibilities for the resulting wave: a

doubling of amplitude at all frequencies if thewaves are in phase, a complete cancellation at allfrequencies if the waves are 180 degrees out ofphase, or partial cancellation and partial reinforce-ment at various frequencies if the waves haveintermediate phase relationship. The results maybe heard as interference effects.

The first case is the basis for the increased sensi-tivity of boundary or surface-mount micro-phones. When a microphone element is placedvery close to an acoustically reflective surfaceboth the incident and reflected sound waves are in

phase at the microphone. This results in a 6dBincrease (doubling) in sensitivity, compared to thesame microphone infree space. This occurs forreflected frequencies whose wavelength is greaterthan the distance from the microphone to the sur-face: if the distance is less than one-quarter inchthis will be the case for frequencies up to at least18 kHz. However, this 6dB increase will notoccur for frequencies that are not reflected, that is,frequencies that are either absorbed by the surfaceor that diffract around the surface. High frequen-

SO

U

N

D

REIN

FO

RC

EM

EN


SOUND

REINFORCEMENT

Phase relationships

Sound pressure wave

+

+1

0

-1

+1

0

-1a

=

+2

0

-2

+

+1

0

-1

+1

0

-1b

= 0

+

+1

0

-1

+1

0

-1c

=phase shifts

in-phase

1800 outof phase

+2

+1

0

-1

-2

one cycle or one period

90000 1800 2700 3600


cies may be absorbed by surface materials such as

carpeting or other acoustic treatments. Low fre-quencies will diffract around the surface if theirwavelength is much greater than the dimensionsof the surface: the boundary must be at least 5 ft.square to reflect frequencies down to 100 Hz.

The second case occurs when two closely spacedmicrophones are wired out of phase, that is, withreverse polarity. This usually only happens byaccident, due to miswired microphones or cablesbut the effect is also used as the basis for certainnoise-canceling microphones. In this technique,two identical microphones are placed very closeto each other (sometimes within the same hous-ing) and wired with opposite polarity. Soundwaves from distant sources which arrive equallyat the two microphones are effectively canceledwhen the outputs are mixed. However, soundfrom a source which is much closer to one ele-ment than to other will be heard. Such close-talkmicrophones, which must literally have the lips of

the talker touchingthe grille, are usedin high-noiseenvironments

such as aircraftand industrial pag-ing but rarely withmusical instru-ments due to theirlimited frequencyresponse.

Polarity reversal

It is the last case which is most likely in musicalsound reinforcement, and the audible result is adegraded frequency response called comb filter-

ing. The pattern of peaks and dips resembles theteeth of a comb and the depth and location ofthese notches depend on the degree of phase shift.

With microphones this effect can occur in twoways. The first is when two (or more) micspick up the same sound source at different dis-tances. Because it takes longer for the sound toarrive at the more distant microphone there iseffectively a phase difference between the sig-nals from the mics when they are combined

(electrically) in the mixer. The resulting comb

filtering depends on the sound arrival timedifference between the microphones: a largetime difference (long distance) causes combfiltering to begin at low frequencies, while asmall time difference (short distance) movesthe comb filtering to higher frequencies.

The second wayfor this effect tooccur is when asingle micro-phone picks up adirect sound andalso a delayedversion of thesame sound.The delay maybe due to anacoustic reflec-

tion of the original sound or to multiple sourcesof the original sound. A guitar cabinet withmore than one speaker or multiple loudspeakercabinets for a single instrument would beexamples. The delayed sound travels a longerdistance (longer time) to the mic and thus has a

phase difference relative to the direct sound.When these sounds combine (acoustically) at themicrophone, comb filtering results. This timethe effect of the comb filtering depends on thedistance between the microphone and the sourceof the reflection or the distance between themultiple sources.

Reflection comb filtering

17

MICROPHONE


SOUND

REINFORCEMENT

Multi-mic comb filtering


18

The 3-to-1 Rule

When it is necessary to use multiple micro-phones or to use microphones near reflectivesurfaces the resulting interference effectsmay be minimized by using the 3-to-1 rule.For multiple microphones the rule statesthat the distance between microphonesshould be at least three times the distancefrom each microphone to its intended soundsource. The sound picked up by the moredistant microphone is then at least 12dB lessthan the sound picked up by the closer one.This insures that the audible effects of combfiltering are reduced by at least that much.For reflective surfaces, the microphoneshould be at least 11/2 times as far fromthat surface as it is from its intended soundsource. Again, this insures minimumaudibility of interference effects.

3-to-1 rule

Strictly speaking, the 3-to-1 rule is based onthe behavior of omnidirectional microphones.

It can be relaxed slightly if unidirectionalmicrophones are used and they are aimedappropriately, but should still be regarded as abasic rule of thumb for worst case situations.

MICROPHONE PHASE EFFECTS

One effect often heard in sound reinforcementoccurs when two microphones are placed in closeproximity to the same sound source, such as a drumkit or instrument amplifier. Many times this is due tothe phase relationship of the sounds arriving at themicrophones. If two microphones are picking up thesame sound source from different locations, somephase cancellation or summing may be occurring.Phase cancellation happens when two microphonesare receiving the same soundwave but with oppositepressure zones (that is,180 degrees out of phase).This is usually not desired. Amic with a differentpolar pattern may reduce the pickup of unwantedsound and reduce the effect or physical isolation canbe used. With a drum kit, physical isolation of theindividual drums is not possible. In this situation thechoice of microphones may be more dependent onthe off-axis rejection characteristic of the mic.

Another possibility is phase reversal. If there iscancellation occurring, a 180 degree phase flip willcreate phase summing of the same frequencies. Acommon approach to the snare drum is to place onemic on the top head and one on the bottom head.

Because the mics are picking up relatively similarsound sources at different points in the sound wave,you may experience some phase cancellations.Inverting the phase of one mic will sum any frequen-cies being canceled. This may sometimes achieve afatter snare drum sound. This effect will changedependent on mic locations. The phase inversion canbe done with an in-line phase reverse adapter or by aphase invert switch found on many mixers inputs.

Potential Acoustic Gain vs. NeededAcoustic Gain

The basic purpose of a sound reinforcement sys-tem is to deliver sufficient sound level to theaudience so that they can hear and enjoy the per-formance throughout the listening area. As men-tioned earlier, the amount of reinforcement need-ed depends on the loudness of the instruments orperformers themselves and the size and acousticnature of the venue. This Needed Acoustic Gain(NAG) is the amplification factor necessary sothat the furthest listeners can hear as if they wereclose enough to hear the performers directly.

SO

U

N

D

REIN

FO

RC

EM

EN


SOUND

REINFORCEMENT


To calculate NAG: NAG = 20 x log (Df/Dn)

Where: Df= distance from sound source tofurthest listener

Dn = distance from sound source tonearest listener

log = logarithm to base 10

Note: the sound source may be a musical instru-ment, a vocalist or perhaps a loudspeaker

The equation for NAG is based on the inverse-square law, which says that the sound leveldecreases by 6dB each time the distance to thesource doubles. For example, the sound level(without a sound system) at the first row of theaudience (10 feet from the stage) might be a com-fortable 85dB. At the last row of the audience (80feet from the stage) the level will be 18dB less or67dB. In this case the sound system needs to pro-vide 18dB of gain so that the last row can hear atthe same level as the first row. The limitation inreal-world sound systems is not how loud the sys-tem can get with a recorded sound source but

rather how loud it can get with a microphone asits input. The maximum loudness is ultimatelylimited by acoustic feedback.

The amount of gain-before-feedback that a soundreinforcement system can provide may be estimatedmathematically. This Potential Acoustic Gaininvolves the distances between sound system com-ponents, the number of open mics, and other vari-ables. The system will be sufficient if the calculatedPotential Acoustic Gain (PAG) is equal to or greaterthan the Needed Acoustic Gain (NAG). Below isan illustration showing the key distances.

PAG

The simplified PAG equation is:

PAG = 20 (log D1 - log D2 + log D0 - log Ds)-10 log NOM -6

Where: PAG = Potential Acoustic Gain (in dB)

Ds = distance from sound source tomicrophone

D0 = distance from sound source tolistener

D1 = distance from microphone toloudspeaker

D2 = distance from loudspeaker tolistener

NOM = the number of openmicrophones

-6 = a 6 dB feedback stability margin

log = logarithm to base 10

In order to make PAG as large as possible, thatis, to provide the maximum gain-before-feed-back, the following rules should be observed:

1) Place the microphone as close to thesound source as practical.

2) Keep the microphone as far awayfrom the loudspeaker as practical.

3) Place the loudspeaker as close to theaudience as practical.

4) Keep the number of microphones toa minimum.

In particular, the logarithmic relationship meansthat to make a 6dB change in the value of PAGthe corresponding distance must be doubled orhalved. For example, if a microphone is 1 ft.from an instrument, moving it to 2 ft. away willdecrease the gain-before-feedback by 6dB whilemoving it to 4 ft. away will decrease it by 12dB.On the other hand, moving it to 6 in. away

19

MICROPHONE


SOUND

REINFORCEMENT

D0

Ds

D2D1


20

increases gain-before-feedback by 6dB while

moving it to only 3 in. away will increase it by12dB. This is why the single most significantfactor in maximizing gain-before-feedback is toplace the microphone as close as practical to thesound source.

The NOM term in the PAG equation reflects thefact that gain-before-feedback decreases by 3dBevery time the number of open (active) micro-phones doubles. For example, if a system has aPAG of 20dB with a single microphone, addinga second microphone will decrease PAG to 17dBand adding a third and fourth mic will decreasePAG to 14dB. This is why the number of micro-phones should be kept to a minimum and whyunused microphones should be turned off orattenuated. Essentially, the gain-before-feed-back of a sound system can be evaluated strictlyon the relative location of sources, microphones,loudspeakers, and audience, as well as the num-ber of microphones, but without regard to theactual type of component. Though quite simple,the results are very useful as a best case estimate.

Understanding principles of basic acoustics canhelp to create an awareness of potential influ-ences on reinforced sound and to provide someinsight into controlling them. When effects ofthis sort are encountered and are undesirable, itmay be possible to adjust the sound source, use amicrophone with a different directional charac-teristic, reposition the microphone or use fewermicrophones, or possibly use acoustic treatmentto improve the situation. Keep in mind that inmost cases, acoustic problems can best be solvedacoustically, not strictly by electronic devices.

General Rules

Microphone technique is largely a matter of per-sonal tastewhatever method sounds right forthe particular instrument, musician, and songis right. There is no one ideal microphone to useon any particular instrument. There is also noone ideal way to place a microphone. Chooseand place the microphone to get the sound youwant. We recommend experimenting with avariety of microphones and positions until you

create your desired sound. However, the desired

sound can often be achieved more quickly andconsistently by understanding basic microphonecharacteristics, sound-radiation properties ofmusical instruments, and acoustic fundamentalsas presented above.

Here are some suggestions to follow when mik-ing musical instruments for sound reinforcement.

Try to get the sound source (instrument, voice,or amplifier) to sound good acoustically(live) before miking it.

Use a microphone with a frequency responsethat is limited to the frequency range of theinstrument, if possible, or filter out frequenciesbelow the lowest fundamental frequency of theinstrument.

To determine a good starting microphone posi-tion, try closing one ear with your finger.Listen to the sound source with the other earand move around until you find a spot thatsounds good. Put the microphone there.However, this may not be practical (or healthy)

for extremely close placement near loudsources.

The closer a microphone is to a sound source,the louder the sound source is compared toreverberation and ambient noise. Also, thePotential Acoustic Gain is increasedthat is,the system can produce more level before feed-back occurs. Each time the distance betweenthe microphone and sound source is halved, thesound pressure level at the microphone (andhence the system) will increase by 6 dB.(Inverse Square Law)

Place the microphone only as close as neces-sary. Too close a placement can color thesound sources tone quality (timbre), by pick-ing up only one part of the instrument. Beaware of Proximity Effect with unidirectionalmicrophones and use bass rolloff if necessary.

Use as few microphones as are necessary to geta good sound. To do that, you can often pickup two or more sound sources with one micro-

SO

U

N

D

REIN

FO

RC

EM

EN


SOUND

REINFORCEMENT


phone. Remember: every time the number of

microphones doubles, the Potential AcousticGain of the sound system decreases by 3 dB.This means that the volume level of the systemmust be turned down for every extra mic addedin order to prevent feedback. In addition, theamount of noise picked up increases as doesthe likelihood of interference effects such ascomb-filtering.

When multiple microphones are used, the dis-tance between microphones should be at leastthree times the distance from each microphoneto its intended sound source. This will helpeliminate phase cancellation. For example, iftwo microphones are each placed one footfrom their sound sources, the distance betweenthe microphones should be at least three feet.(3 to 1 Rule)

To reduce feedback and pickup of unwantedsounds:

1) place microphone as close as practicalto desired sound source

2) place microphone as far as practicalfrom unwanted sound sources such asloudspeakers and other instruments

3) aim unidirectional microphone towarddesired sound source (on-axis)

4) aim unidirectional microphone awayfrom undesired sound source (180degrees off-axis for cardioid, 126degrees off-axis for supercardioid)

5) use minimum number of microphones

To reduce handling noise and stand thumps:

1) use an accessory shock mount (suchas the Shure A55M)

2) use an omnidirectional microphone

3) use a unidirectional microphone witha specially designed internal shockmount

To reduce pop (explosive breath sounds

occurring with the letters p, b, and t):

1) mic either closer or farther than 3inches from the mouth (because the3-inch distance is worst)

2) place the microphone out of the pathof pop travel (to the side, above, orbelow the mouth)

3) use an omnidirectional microphone

4) use a microphone with a pop filter.This pop filter can be a ball-type grilleor an external foam windscreen

If the sound from your loudspeakers is distort-ed even though you did not exceed a normalmixer level, the microphone signal may beoverloading your mixers input. To correct thissituation, use an in-line attenuator (such as theShure A15AS), or use the input attenuator onyour mixer to reduce the signal level from themicrophone.

Seasoned sound engineers have developedfavorite microphone techniques through years ofexperience. If you lack this experience, the sug-gestions listed on the following pages shouldhelp you find a good starting point. These sug-gestions are not the only possibilities; othermicrophones and positions may work as well orbetter for your intended application.RememberExperiment and Listen!

21

MICROPHONE


SOUND

REINFORCEMENT


22

MICROPHONE


SOUND

REINFORCEMENT

Lead vocal:

Handheld or on stand, microphonewindscreen touching lips or just afew inches away

Backup vocals:

One microphone per singer.Handheld near chin or stand-mounted.Touching lips or a few inches away

Choral groups:

1 to 3 feet above and 2 to 4 feet infront of the first row of the choir,aimed toward the middle row(s) ofthe choir, approximately 1 micro-phone per 15-20 people

Miniature microphone clippedoutside of sound hole

Miniature microphone clipped

inside sound hole

Acoustic guitar:

8 inches from sound hole

3 inches from sound hole

4 to 8 inches from bridge

6 inches above the side, over thebridge, and even with the frontsoundboard

miniature microphone clippedoutside of sound hole

miniature microphone clippedinside sound hole

Bassy, robust(unless an omniis used)

Bassy, robust(unless an omni isused)

Full range,good blend,semi-distant

Natural,well-balanced

Bassy, less

string noise

Bassy

Very bassy, boomy,muddy, full

Woody, warm,

mellow. Midbasy,lacks detail

Natural, well-balanced,slightly bright

Natural, well-balanced

Bassy, lessstring noise

Minimizes feedback and leakage.Roll off bass if desired for morenatural sound.

Minimizes feedback and leakage. Allowsengineer control of voice balances. Rolloff bass if necessary for more naturalsound when using cardioids.

Use flat-response unidirectional micro-phones, Use minimum number ofmicrophones needed to avoid overlap-ping pickup areas.

Good isolation. Allows freedom ofmovement.

Reduces feedback.

Good starting placement when leakageor feedback is a problem. Roll off bassfor a more natural sound (more for auni than an omni).

Very good isolation. Bass rolloffneeded for a natural sound.

Reduces pick and string noise.

Less pickup of ambience and leakagethan 3 feet from sound hole.

Good isolation. Allows freedom ofmovement.

Reduces feedback.

Microphone Placement Tonal Balance Comments

VO

CALS

STRING

S


23

MICROPHONE


SOUND

REINFORCEMENT

Banjo:

3 inches from center of head

3 inches from edge of head

Miniature microphone clipped totailpiece aiming at bridge

Violin (fiddle):

A few inches from side

Cello:

1 foot from bridge

Miniature microphone attached to

strings between bridge and tailpiece

6 inches to 1 foot out front, justabove bridge

A few inches from f-hole

Wrap microphone in foam padding(except for grille) and put behindbridge or between tailpiece and body

Harp:

Aiming toward player at part ofsoundboard, about 2 feet away

Tape miniature microphone tosoundboard

Bassy, thumpy

Bright

Natural

Natural

Well-defined

Bright

Well-defined

Full

Full, tight

Natural

Somewhatconstricted

Rejects feedback and leakage.Roll off bass for natural sound.

Rejects feedback and leakage.

Rejects feedback and leakage. Allowsfreedom of movement.

Well-balanced sound.

Well-balanced sound, but little isolation.

Minimizes feedback and leakage.

Allows freedom of movement.

Natural sound.

Roll off bass if sound is too boomy.


See Stereo Microphone Techniquessection for other possibilities.



General string instruments (mandolin, dobro and dulcimer):

Acoustic bass (upright bass, string bass, bass violin):


24

MICROPHONE


SOUND

REINFORCEMENT

Grand piano:

12 inches above middle strings, 8inches horizontally from hammerswith lid off or at full stick

8 inches above treble strings, asabove

Aiming into sound holes

6 inches over middle strings,8 inches from hammers, with lid onshort stick

Next to the underside of raised lid,

centered on lid

Underneath the piano, aiming up atthe soundboard

Surface-mount microphone mountedon underside of lid over lower treblestrings, horizontally close to ham-mers for brighter sound, further fromhammers for more mellow sound

Two surface-mount microphonespositioned on the closed lid, under the

edge at its keyboard edge, approxi-mately 2/3 of the distance from mid-dle A to each end of the keyboard

Surface-mount microphone placedvertically on the inside of the frame,or rim, of the piano, at or near theapex of the pianos curved wall


Natural, well-balanced,slightly bright

Thin, dull, hard,constricted

Muddy, boomy,dull, lacks attack

Bassy, full

Bassy, dull, full

Bright, well-balanced

Bright, well-balanced, strong

attack

Full, natural

Less pickup of ambience andleakage. Move microphone(s) far-ther from hammers to reduce attackand mechanical noises. Good coinci-dent-stereo placement. See StereoMicrophone Techniques section.

Place one microphone over bassstrings and one over treble strings forstereo. Phase cancellations may occurif the recording is heard in mono.

Very good isolation. Sometimessounds good for rock music. Boostmid-bass and treble for more naturalsound.

Improves isolation. Bass rolloff andsome treble boost required for morenatural sound.

Unobtrusive placement.

Unobtrusive placement.

Excellent isolation. Experimentwith lid height and microphoneplacement on piano lid for desiredsounds.

Excellent isolation. Moving lowmic away from keyboard six inches

provides truer reproduction of thebass strings while reducing dampernoise. By splaying these two micsoutward slightly, the overlap in themiddle registers can be minimized.

Excellent isolation. Minimizeshammer and damper noise. Best ifused in conjunction with two sur-face-mount microphones mountedto closed lid, as above.


STR

IN

GS


25

MICROPHONE


SOUND

REINFORCEMENT

Upright piano:

Just over open top, above treblestrings

Just over open top, above bassstrings

Inside top near the bass andtreble stings

8 inches from bass side ofsoundboard

8 inches from treble side ofsoundboard

1 foot from center of soundboard onhard floor or one-foot-square plateon carpeted floor, aiming at piano.

Soundboard should face into room

Aiming at hammers from front, sever-al inches away (remove front panel)

1 to 2 feet from bell. A couple ofinstruments can play into one

microphone

Miniature microphone mountedon bell

Natural (butlacks deep bass),picks up ham-mer attack

Slightly full ortubby, picks uphammer attack

Natural, picks uphammer attack

Full, slightlytubby, nohammer attack

Thin, constricted,no hammer attack

Natural, goodpresence

Bright, picks uphammer attack

On-axis to bellsounds bright; to

one side soundsnatural or mellow

Bright

Good placement when only onemicrophone is used.

Mike bass and treble strings forstereo.

Minimizes feedback and leakage.Use two microphones for stereo.

Use this placement with thefollowing placement for stereo.

Use this placement with thepreceding placement for stereo.

Minimize pickup of floor vibrationsby mounting microphone in low-profile shock-mounted microphone

stand.

Mike bass and treble strings forstereo.

Close miking sounds tight andminimizes feedback and leakage.

More distant placement gives fuller,more dramatic sound.

Maximum isolation.


Brass (trumpet, cornet, trombone, tuba):

The sound from these instruments is very directional. Placing the mic off axis with the bellof the instrument will result in less pickup of high frequencies.


26

MICROPHONE


SOUND

REINFORCEMENT

French horn:

Microphone aiming toward bell

Afew inches from and aiming into bell

A few inches from sound holes

A few inches above bell and aimingat sound holes

Miniature microphone mounted on bell

Afew inches from area betweenmouthpiece and first set of finger holes

A few inches behind players head,aiming at finger holes

Woodwinds (Oboe, bassoon, etc):

About 1 foot from sound holes

A few inches from bell

Natural

Bright

Warm, full

Natural

Bright, punchy

Natural, breathy

Natural

Natural

Bright

Watch out for extreme fluctuationson VU meter.


Picks up fingering noise.

Good recording technique.

Maximum isolation, up-front sound.

Pop filter or windscreen may berequired on microphone.

Reduces breath noise.

Provides well-balanced sound.



Flute:

The sound energy from a flute is projected both by the embouchure and by the first openfingerhole. For good pickup, place the mic as close as possible to the instrument. However,if the mic is too close to the mouth, breath noise will be apparent. Use a windscreen on themic to overcome this difficulty.

Saxophone:

With the saxophone, the sound is fairly well distributed between the finger holes and the bell.Miking close to the finger holes will result in key noise. The soprano sax must be consideredseparately because its bell does not curve upward. This means that, unlike all other saxo-phones, placing a microphone toward the middle of the instrument will not pick-up the soundfrom the key holes and the bell simultaneously. The saxophone has sound characteristics sim-ilar to the human voice. Thus, a shaped response microphone designed for voice works well.

W

IN

DINS

TRUM

ENT

S


27

MICROPHONE


SOUND

REINFORCEMENT

Harmonica:

Very close to instrument

Accordion:

Miniature microphone mountedinternally

4 inches from grille cloth at centerof speaker cone

1 inch from grille cloth at center ofspeaker cone

Off-center with respect to speakercone

3 feet from center of speaker cone

Miniature microphone draped overamp in front of speaker

Microphone placed behind openback cabinet

Bass guitar amplifier/speaker:

Mike speaker as described inElectric Guitar Amplifier section

Mike speaker as described inElectric Guitar Amplifier section

Full, bright

Emphasizedmidrange


Bassy

Dull or mellow

Thin, reducedbass

Emphasizedmidrange

Depends onposition

Depends onplacement

Depends onbrand of piano

Minimizes feedback and leakage.Microphone may be cupped in hands.

Minimizes feedback and leakage.Allows freedom of movement.

Small microphone desk stand may beused if loudspeaker is close to floor.


Microphone closer to edge ofspeaker cone results in duller sound.

Reduces amplifier hiss noise.

Picks up more room ambience andleakage.

Easy setup, minimizes leakage.

Can be combined with mic in frontof cabinet, but be careful of phasecancellation.

Improve clarity by cuttingfrequencies around 250 Hz andboosting around 1,500 Hz.

Roll off bass for clarity, roll offhighs to reduce hiss.


Electric guitar amplifier/speaker:

The electric guitar has sound characteristics similar to the human voice. Thus, a shapedresponse microphone designed for voice works well.

Electric keyboard amplifier/speakers:


28

MICROPHONE


SOUND

REINFORCEMENT

Leslie organ speaker:

Aim one microphone into toplouvers 3 inches to 1 foot away

Mike top louvers and bottom bassspeaker 3 inches to 1 foot away

Mike top louvers with two micro-phones, one close to each side. Panto left and right. Mike bottom bassspeaker 3 inches to 1 foot away andpan its signal to center

Natural, lacksdeep bass



Good one-mike pickup.

Excellent overall sound.

Stereo effect.


Drum kit:

In most sound reinforcement systems, the drum set is miked with each drum having its ownmic. Using microphones with tight polar patterns on toms helps to isolate the sound from eachdrum. It is possible to share one mic with two toms, but then, a microphone with a widerpolar pattern should be used. The snare requires a mic that can handle very high SPL, so adynamic mic is usually chosen. To avoid picking up the hi-hat in the snare mic, aim the nullof the snare mic towards the hi-hat. The brilliance and high frequencies of cymbals are pickedup best by a flat response condenser mic.

DRU

M

KIT

Front View Top View

1. Overhead-Cymbals:

One microphone over center of drumset, about 1 foot above drummershead (Position A); or use two spacedor crossed microphones for stereo(Positions A or B). See StereoMicrophone Techniques section

Natural; soundslike drummerhears set

Picks up ambience and leakage. Forcymbal pickup only, roll off low fre-quencies. Boost at 10,000 Hz foradded sizzle. To reduce excessivecymbal ringing, apply masking tapein radial strips from bell to rim.


29

MICROPHONE


SOUND

REINFORCEMENT

2. Snare drum:

Just above top head at edge ofdrum, aiming at top head. Comingin from front of set on boom(Position C); or miniature micro-phone mounted directly on drum

Remove front head if necessary.Mount microphone on boom arminside drum a few inches from beaterhead, about 1/3 of way in from edgeof head (Position D); or place sur-face-mount microphone inside drum,on damping material, with micro-phone element facing beater head

4. Tom-toms:

One microphone between every twotom-toms, close to top heads (PositionE); or one microphone just aboveeach tom-tom rim, aiming at tophead (Position F); or one microphoneinside each tom-tom with bottomhead removed; or miniature micro-phone mounted directly on drum

5. Hi-hat:

Aim microphone down towards thecymbals, a few inches over edgeaway from drummer (Position G).Or angle snare drum microphoneslightly toward hi-hat to pick upboth snare and hi-hat

Full, smooth

Full, goodimpact

Full, goodimpact

Natural, bright

Tape gauze pad or handkerchief ontop head to tighten sound. Boost at5,000 Hz for attack, if necessary.

Put pillow or blanket on bottom ofdrum against beater head to tightenbeat. Use wooden beater, or loosenhead, or boost around 2,500 Hz formore impact and punch.

Inside drum gives best isolation.Boost at 5,000 Hz for attack, ifnecessary.

Place microphone or adjust cymbalheight so that puff of air from closinghi-hat cymbals misses mike. Roll offbass to reduce low-frequency leak-age. To reduce hi-hate leakage intosnare-drum microphone, use smallcymbals vertically spaced 1/2 apart.


3. Bass drum (kick drum):

Placing a pad of paper towels where the beater hits the drum will lessen boominess. If youget rattling or buzzing problems with the drum, put masking tape across the drum head todamp out these nuisances. Placing the mic off center will pick up more overtones.


30

MICROPHONE


SOUND

REINFORCEMENT

6. Snare, hi-hat and high tom:

Place single microphone a few inch-es from snare drum edge, next tohigh tom, just above top head of tom.Microphone comes in from front ofthe set on a boom (Position H)

7. Cymbals, floor tom and high tom:

Using single microphone, place itsgrille just above floor tom, aimingup toward cymbals and one of hightomes (Position I)

Natural

Natural

In combination with Placements 3and 7, provides good pickup withminimum number of microphones.Tight sound with little leakage.

In combination with Placements 3and 6, provides good pickup withminimum number of microphones.Tight sound with little leakage.


One microphone: Use Placement 1. Placement 6 may work if the drummer limits playing toone side of the drum set.

Two microphones: Placements 1 and 3; or 3 and 6.

Three microphones: Placements 1, 2, and 3; or 3, 6, and 7.

Four microphones: Placements 1, 2, 3, and 4.

Five microphones: Placements 1, 2, 3, 4, and 5.

More microphones: Increase number of tom-tom microphones as needed. Use a small micro-phone mixer (such as the Shure M268) to submix multiple drum microphones into one channel.

Timbales, congas, bongos:

One microphone aiming downbetween pair of drums, just abovetop heads

Tambourine:

One microphone placed 6 to 12inches from instrument

Natural

Natural

Provides full sound with goodattack.

Experiment with distance andangles if sound is too bright.

DRU

M

KIT


31

MICROPHONE


SOUND

REINFORCEMENT

Steel Drums:

Tenor, Second Pan, GuitarOne microphone placed 4 inchesabove each pan

Microphone placed underneath pan

Cello, BassOne microphone placed 4 - 6inches above each pan

Xylophone, marimba, vibraphone:

Two microphones aiming downtoward instrument, about 1 1/2 feetabove it, spaced 2 feet apart, or angled135 apart with grilles touching

Glockenspiel:

One microphone placed 4 - 6 inches

above bars

Bright, withplenty of attack

Natural

Natural

Bright, with lots

of attack.

Allow clearance for movement of pan.

Decent if used for tenor or second pans.Too boomy with lower voiced pans.

Can double up pans to a singlemicrophone.

Pan two microphones to left and rightfor stereo. See Stereo MicrophoneTechniques section.

For less attack, use rubber mallets

instead of metal mallets. Plasticmallets will give a medium attack.


Stage area miking Tonal Balance Comments

Downstage:Surface-mount microphones alongfront of stage aimed upstage, onemicrophone center stage; use stageleft and stage right mics as needed,approximately 1 per 10-15 feet

Upstage:Microphones suspended 8 -10 feetabove stage aimed upstage, onemicrophone center stage; use stageleft and stage right mics as needed,approximately 1 per 10-15 feet

Spot pickup:Use wireless microphones onprincipal actors; mics concealed inset; shotgun microphones fromabove or below

Voice range,semi-distant

Voice range,semi-distant

Voice range,on mic

Use flat response, unidirectionalmicrophones. Use minimum num-ber of microphones needed to avoidoverlapping pickup area. Use shockmount if needed.

Use flat response, unidirectionalmicrophones. Use minimumnumber of microphones needed toavoid overlapping pickup area.

Multiple wireless systems mustutilize different frequencies. Uselavaliere or handheld microphonesas appropriate.


32

STEREO

M

IC

RO

PHO

N

ETEC

H

N

IQ

U

ES MICROPHONE


SOUND

REINFORCEMENT

Stereo Microphone Techniques

These methods are recommended for pickup oforchestras, bands, choirs, pipe organs, quartets,soloists. They also may work for jazz ensem-bles, and are often used on overhead drums andclose-miked piano.

Use two microphones mounted on a single standwith a stereo microphone stand adapter (such asthe Shure A27M). Or mount 2 or 3 microphoneson separate stands. Set the microphones in thedesired stereo pickup arrangement (see below).

For sound reinforcement, stereo mic techniquesare only warranted for a stereo sound system andeven then, they are generally only effective forlarge individual instruments, such as piano ormiramba, or small instrument groups, such asdrum kit, string section or vocal chorus.Relatively close placement is necessary toachieve useable gain-before-feedback.

Coincident Techniques

Microphone diaphragmsclose together and alignedvertically; microphonesangled apart. Example:1350 angling (X-Y).

MS (Mid-Side)

A front-facing cardioid car-tridge and a side-facing bidi-rectional cartridge are mount-ed in a single housing. Theiroutputs are combined in amatrix circuit to yield discreteleft and right outputs.

Near-CoincidentTechniques

Microphones angled andspaced apart 6 to 10 inchesbetween grilles. Examples:1100 angled, 7-inch spacing.

Comments

Tends to provide a narrowstereo spread (the reproducedensemble does not alwaysspread all the way between thepair of playback loud-speak-ers). Good imaging. Mono-compatible.

Comments

Provides good stereo spread,excellent stereo imaging andlocalization. Some types allowadjustable stereo control.Mono-compatible.

Comments

Tends to provide accurateimage localization.

Musical Ensemble

(Top View)

Musical Ensemble

(Top View)

Musical Ensemble

(Top View)


33

MICROPHONE


SOUND

REINFORCEMENT

Spaced Techniques

Two microphones spacedseveral feet apart horizontal-ly, both aiming straight aheadtoward ensemble. Example:Microphones 3 to 10 feetapart.

Three microphones spacedseveral feet apart horizontal-ly, aiming straight aheadtoward ensemble. Centermicrophone signal is splitequally to both channels.Example: Microphones 5feet apart.

Comments

Tends to provide exaggeratedseparation unless microphonespacing is 3 feet. However,spacing the microphones 10feet apart improves overallcoverage. Produces vagueimaging for off-center soundsources. Provides a warmsense of ambience.

Improved localization com-pared to two spaced micro-phones.

Musical Ensemble

(Top View)

Musical Ensemble

(Top View)


34

BETA 58A

SM58

BETA 57A

SM57

BG3.1

BG2.1BG1.1

BETA 87

SM87

BG5.1

WH10XLR

SM10A

SM12A

512

SM81

SM7

BETA 87

SM87

BG5.1

SM81

SM94

BG4.1

V O C A L S

PERFORMANCEVOCAL (dynamic)

PERFORMANCEVOCAL (condenser)

HEADWORNVOCAL

STUDIOVOCAL

ENSEMBLEVOCAL

BETA 56

BETA 57A

SM57

BG3.1

BG2.1

BG6.1

BETA 52

SM7

BETA 57A

BETA 56

SM57

BETA 52

SM91A

BETA 57A

SM57

BETA 57A

BETA 56

SM57

BG3.1

SM98A1

BETA 57A

BETA 56

SM57

BG6.1

I N S T R U M E N T S

GUITARAMPLIFIER

BASSAMPLIFIER

KICKDRUM

SNAREDRUM

TOMSRACK & FLOOR

SM81

SM94

BG4.1

SM98A

BETA 56

BETA 57A

SM57

SM81

SM94

BG4.1

SM81

BETA 57A

SM57

SM81

SM91

BG4.1

OVERHEADCYMBALS HIGH HAT2

CONGA MALLETINSTRUMENTS2

MARIMBA& OTHERPERCUSSION2

PIANO2

SM81

SM94BG4.1

SM114

SM98A4

BETA 52

SM81SM94

BG4.1

SM98A3

BETA 56BETA 57A

SM57

SM81

SM98ABG4.1

SM98A3

SM7BETA 56

BETA 57A

SM57

STRINGS ACOUSTICBASS

BRASSINSTRUMENTS

WOODWINDS SAXAPHONE

SM81

SM94

BG4.1

BETA 57A

SM57

SM114

520D Green Bullet

SM57

SM58

BETA 57A

BETA 56

SM57

BG3.1

SM91A

SM81

SM94

BG4.1

SM81 (pair)

SM94 (pair)

BG4.1 (pair)

VP88

ACOUSTICGUITAR

HARMONICA LESLIESPEAKER

ORCHESTRA2 LIVE CONCERTRECORDING OR

STEREOPICKUP/AMBIENCE2

SM81

SM94

BG4.1

SM58S

565

BG3.1

BG2.1

BG1.1 (Hi or Lo Z)

SAMPLING KARAOKEThis guide is an aid in selecting microphones for various applications. Microphone sound quality andappearance are subject to specific, acoustic environments, application technique and personal taste.

1 With A98MK drum mount kit.2 For single point stereo miking, use VP88 MS Stereo Microphone.3 Bell-mounted with A98KCS clamp.4 With RK279 mounting kit for instrument applications.

SHU

RE

M

IC

RO

PH

O

N

E

S

ELEC

TIO

N

G

U

ID

E


3-to-1 Rule-When using multiple microphones,the distance between microphones should be atleast 3 times the distance from each microphoneto its intended sound source.

Absorption-The dissipation of sound energy bylosses due to sound absorbent materials.

Active Circuitry-Electrical circuitry whichrequires power to operate, such as transistors andvacuum tubes.

Ambience-Room acoustics or naturalreverberation.

Amplitude-The strength or level of soundpressure or voltage.

Audio Chain-The series of interconnected audioequipment used for recording or PA.

Backplate-The solid conductive disk that formsthe fixed half of a condenser element.

Balanced-A circuit that carries information bymeans of two equal but opposite polarity signals,on two conductors.

Bidirectional Microphone-A microphone thatpicks up equally from two opposite directions.The angle of best rejection is 90 deg. from thefront (or rear) of the microphone, that is, directlyat the sides.

Boundary/Surface Microphone-Amicrophonedesigned to be mounted on an acoustically reflec-tive surface.

Cardioid Microphone-A unidirectional micro-phone with moderately wide front pickup (131deg.). Angle of best rejection is 180 deg. from thefront of the microphone, that is, directly at