Acoustic Principles

ACOUSTICPRINCIPLEShttp://www.lenardaudio.com

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Acoustic principles"Nothing is more beautiful than silence except

the sounds of nature and music"

Auditoriums well designed give acoustical directivity from stage to audience with a fine balanceof reverberation to enhance performance. When recording, musicians select the rightreverberation for the music. Once recorded extra room reverberation detracts from themusic. The perfect listening room for recorded music is 100% absorbent at all frequencies(free field).

Reverberation is sound reflecting off the floor, walls and ceiling and builds up to apercentage of the direct sound and is different at all frequencies. Some rooms are absorbentat high frequencies, but reverberant at low frequencies and vice versa. As we move closer tothe speakers the direct sound gets louder and clearer. As we move back from the speakersdirect sound diminishes (inverse square law) but the reverberant sound remains constant andlimits intelligibility.

RT60 is time in seconds for reverberation to diminish to - 60dB (1/1,000,000). Withpractice this test can be approximated with a single hand-clap, in a quite room, as in the abovegraph. But with continuous sound (music) reverberation builds up and remains at a constantlevel.Note: Changing the loudness of music does not change the reverb time T60.

RT60 Metric = 0.16/S Imperial = 0.05/S (S surface area. average absorption) ofroom.

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Path-lengths refer to the distance of walls and ceiling, characterising thereverberation. Stage walls and ceiling can have limited controlled acoustic reflection toenhance the performance. Only from the stage. Acoustic path lengths must be as short aspractical. An exaggeration of short acoustic path-lengths is a bathroom. Long acoustic path-lengths are echoes (churches) and cause difficulty for musicians to play in time. Differentreverberation path-lengths suit different music. No single reverberation path-length suits allmusic.

Sound travels at 344 meters / sec (1ft / milli-sec) approx. Walls or ceiling that are 10meters 30ft from the sound source, can reflect sound back to the musicians. The totaldistance being 20 meters 60ft, which is 30 milli-sec delay. Sound reflected back from 10meters and greater cause difficulty for musicians especially when playing modern percussivemusic.

Echo is heard as distinct repeat, 100 milli-seconds (1/10 sec) or greater, from walls andceiling with path-lengths greater than 15 meters (45ft) apart. Echoes cause difficulty formusicians to play in time and destroy intelligibility. Only the egos of deity's suit the excessiveecho and reverberation of churches for pipe organs and choirs to sing their praise.

Auditorium Design. Many auditoriums are visually beautiful but reverberantnightmares. Auditoriums and Concert halls evolved before electronic sound re-enforcementwas available. Mozart hated the excessive echo and reverberation of many large concerthalls, which restricted his music. Mozart often preferred to perform outside. Historicallyreverberation was used to increase sound level to the audience. But the cost of increasingsound level by reverberation is at the loss of intelligibility. However a small amount of shortpath-length reverberation can beautifully enhance a performance. There is no one singlepath-length of reverberation to suit all music. The larger the concert hall, the longer theacoustic path-lengths (echoes) and the slower the music has to be to retain intelligibility.

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The above auditorium is typical of current design. The majority of concert halls of approx2,000 seating have an average reverberation time RT of 1.5 seconds. But auditoriums cannow be anechoic designed . Free of reverberation and echo. Amplification with computermanagement skilfully applied can provide the ideal sound level with correct reverberation,perfectly tailored for any performance imaginable. However the understanding to manageauditorium acoustics requires a specialised field of study combining music, architectural andelectro-acoustics. Formal education in this field has yet to evolve. Greek and Romanarchitecture demonstrated exceptional skill of sound and reverberation management in thedesigning of some amphitheatres.

The original designers of amphitheatres had an understanding of 'critical distance'. As wemove closer to the performance the direct sound gets louder and clearer. As we move awayfrom the performance direct sound diminishes (inverse square law) whereas reverberation canremain constant. Some amphitheatres have the unique capacity to maintain aconstant 'critical distance' providing an even balance of direct and reverberant soundthroughout the theatre. Without understanding 'Critical Distance' all other knowledge onacoustics has no meaning.

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Critical distance"Know thy Critical Distance" is the 1st commandment of acoustics. Critical Distance is thedistance from the sound source where the direct and reverberant sound energies becomeequal. The more reverberant a room is, thecloser the Critical Distance is to the soundsource. The more absorbent a room is, the further the Critical Distance is from the soundsource. (Critical Distance is different at all frequencies).

For good acoustic design the Critical Distance should be as far as possible from the soundsource and the resultant reverberation minimal and even at all frequencies. Direct sound fromthe speaker system diminishes in level as a function of the distance (inverse square law)whereas reverberation constantly spreads throughout the room. Because there is newincoming sound from the speakers reverberation keeps building up until the new incomingsound equals the sound absorbed (steady-state).

When the reverberant sound becomes 12dB or greater than the direct sound all intelligibility islost. The simplest way to find 'Critical Distance' is to play compressed pop music throughthe sound system. Begin with one speaker (left or right). Walk back and forth around theroom, and you will be surprised how easy it is to identify the critical distance. Repeat the

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exercise with the other speaker, then both speakers. Its surprising how accurate our ears arewhen compared with acoustic measurement microphones.

The more reverberant the room is the closer the Critical Distance. The more absorbent the room is the further the Critical Distance.

Near field or Direct field is inside the Critical Distance. Far field or Reverberant field is outside the Critical Distance.

Critical Distance: Dc = 0.14/√QR (Q = directivity factor 1 of sound source. R = roomconstant)

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Sound absorptionAcoustical absorption of furnishing and curtain fabrics against walls readily absorb highfrequencies but have limited absorption at low frequencies. The further curtain fabrics areplaced away from walls, the better the absorption is to include lower frequencies. The amountof sound energy absorbed depends on type of material, weight and pleating width. Rock wool(fibreglass) has the highest absorption capacity, converting molecular air movement to heat (atmolecular level). Fibreglass consists of minute razor sharp fibres that are irritant and need tobe contained within fabric.

Brick, stone, concrete, reflect all sound. Timber, gyprock, steel, reflect most high frequenciesand a % low frequency is absorbed by the wall. The remaining low frequency energy that isnot reflected or absorbed passes through the wall. Nothing can be done about sound thatpasses through a wall. Bass frequencies are the most difficult to absorb.

The 1/4 wave-length rule. Acoustical absorbent material must be placed away from wallsand ceiling at a distance of 1/4 wavelength of the lowest frequency to be absorbed. This willinclude all higher frequencies if the absorbent material is soft furnishing or fibreglass. Pleasenote that the ceiling should also be included. Understandably this will slightly reduce thephysical size of the room. Acoustically the room will sound and feel LARGER. Also anacoustic absorbent environment is relaxing and calming.

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Bass trap refers to distance the absorbent material is from a wall to include absorbing bassfrequencies. Lowest frequency absorbed is governed by the material being at a distance of1/4 wavelength from a wall. Recording studios can have fabric up to 6ft / 2meters fromwalls. At 1/4 wavelength the molecular air movement is maximum, and is converted to heatby the absorbent material. The remaining sound that gets through the absorbent material isreflected back from the wall and again absorbed by the absorbent material.

Standing Waves are bass frequencies reflected back from walls and ceiling. The reflectedbass interferes with the new incoming bass frequencies, causing cancellations at differentpoints throughout the room. Each bass note will behave differently and the cancelled pointswill be in different positions. Moving speakers or listening position does not solve theproblem. The only solution is to insure that the room is 100% absorbent at all bassfrequencies. Standing waves also refer to how a string behaves on a musicalinstrument. There are excellent descriptions of standing waves on other web sites whichinclude animation. Right mouse click to open in new window and allow time to downloadanimation. While waiting to download continue reading.

room standing waveswww.kettering.edu/~drussell/demos.htmlwww.isvr.soton.ac.uk/SPCG/Tutorial/Tutorial/Tutorial_files/Web-basics.htmstringed instrumentswww.id.mind.net/~zona/mstm/physics/waves/standingWaves/standingWaves1/StandingWaves1.html

Panel Absorbers consist of large sheets of plywood formed into complex architecturalshapes. The panels can break up standing waves, deflect high frequencies and resonate toabsorb bass energy. The formulas governing their behaviour are complex and the outcome isunpredictable and unknown until constructed. Almost without exception they require timeconsuming trial and error modifications to get them to work as predicted. There are only a fewacoustical architects that have mastered them. The below formula gives an approximationonly.

fres. = √60/md (fres = frequency of max absorption) (m = panel mass Kg/m2 (d = depth of air spacein meters)www.primacoustic.com/indexstudio.htm

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Anechoic chamber is 100% absorbant at all frequencies. No sound can enter or escapefrom the room and is 100% silent. The closest we can experience this is in an open field,forest or desert on a perfectly still night. Simply described as free field. No sound is reflectedor returned. Everyone should experience being in an anechoic chamber or spend time asilent free field to attain a reference. Surprising how different and revealing a sound systemactually sounds and therapeutically humbling a reality change can be.

Recording studio control rooms often havewalls and ceiling slope outward and upward, away from the speakers and screen. Absolutelyno sound should reflect from the rear wall. For amplified performance including cinema's, allwalls and ceiling, yes ceiling, should be as close to 100% absorbent as possible at allfrequencies (free field).

Echo and excessive reverberation destroys intelligibility and enjoyment for theaudience. Absolutely no echo must be allowed to be reflected from the back wall to thestage. The further away from the stage performance the more acoustically absorbent theroom should become.

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For live acoustic performance the stage walls and ceiling can have a small % of controlledacoustic reflection to enhance the performance. Only from the stage. Acoustic path lengthsmust be as short as practical. An exaggeration of short acoustic path lengths is abathroom. Long acoustic path lengths are echoes (churches) and cause difficulty formusicians to play in time.

Sound system placement. Facing speakers directly forward adds excessive reflection fromwalls, and further reduces intelligibility. Many roadie sound engineers incorrectly mix in mono,in front of one speaker stack facing forward.

The speaker system should be turned inward to improve directivity, and minimise wallreflection. The angle that speakers could be turned inward can only be approximated byacademic calculation. The most suited angle has to be found by trial and error. Whereverpossible mixing should be from the centre, in stereo, where sound from left and right speakersintersects and at a distance no further back than where direct sound from the speakers is equalto the reflected reverberant energy of the room (CriticalDistance). www.genelec.com/support/flushmount.php

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The above picture is to bring attention to the importance of acoustical absorption ofceilings. Many cinema complexes provide acoustical absorption on walls, but forget aboutceilings. Below is the address of a company that supplies and consults on acousticalabsorption, with many excellent pictures of applications as above. www.acousticalsurfaces.com

Architectural Acoustics

The information on this site is not meant to substitute academic text books. The aim is toprioritize the order of information to enable good acoustic design, often omitted in academictext.An excellent referred text is 'Sound System Engineering' by Don and Carolyn Davis.

A Weighting sound measurement is non-linear and scaled in reference to our subjectivehearing at low level. Our hearing is very sensitive at low level at the higher frequencies 500Hzto 4KHz, and less sensitive at bass frequencies. A weighting is used for noise measurementof office, work-place, and external traffic environment. A weighting is notappropriate for musicand entertainment venues.

C Weighting sound measurement is flat and therefore the correct method of measurementfor music and entertainment venues. At higher power (music) our hearing tends to be even atall frequencies especially bass.

Note Building noise specifications are referenced to A Weighting sound measurement, andoften limited to frequencies within voice range (250, 500, 1000 and 2000 Hz). Many architectshave failed to fully understand the difference between A and C weighting specifications whendesigning entertainment venues. Bass energy is the most difficult to control, and the leastunderstood, and therefore the largest problem in litigation issues of noise pollution.

(1) Stopping sound The only way to stop all sound from entering or escaping a room is toconstruct double brick walls, double sealed ceilings, double sealed doors etc. This isapproached from the theory of 2 rooms, one within the other with an air gap in between. Thisis justified by recording, radio and TV studios, but is not economical practical for most homesand venues. The closer to achieving this the better with double-glazed windows, solid timberdoors, sealing air gaps, multiple baffled air conditioning etc.

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Material attenuation approx 125 Hz 500 Hz 4K Hz

Brick 100mm 4in -30dB -40dB -60dB

Concrete 200mm. 8in -38dB -50dB -60dB

Chipboard 20mm. 1 3/4in -20dB -22dB -33dB

Plasterboard 12mm. 1in -15dB -24dB -35dB

Plasterboard x 2 -25dB -33dB -40dB

Window 6mm. 1/4in -22dB -28dB -36dB

Double glazed 12mm. 1/2in -30dB -37dB -52dB

Door plywood -12dB -22dB -24dB

Door solid 50mm 2in -22dB -26dB -35dB

Sheet steel 1.6mm. 16swg -12dB -27dB -43dB

The above table shows approx attenuation -dB of reducing sound getting through a buildingmaterial. Increasing the thickness of a building material x 2 increases attenuation by approx -6dB. Building materials are specified with Sound Transmission Class (STC) and NoiseReduction Coefficient (NRC). Education of STC and NRC is available on many buildingmaterial suppliers web sites, including building construction details.STC and NRC only refer to isolation in speech frequencies (250, 500, 1000 and 2000 Hz) andprovide no information of a materials ability to reduce low frequency noise, eg. bass in musicetc.www.stcratings.com

(2) Absorbing sound within a room is essential. But internal absorption has only limitedability to reduce sound that passes through walls. Absorbing sound that has been createdinside the room limits reverberation therefore reducing overall sound energy. Absorbing the

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majority of sound before it strikes the first wall, reduces sound reflected to other walls. Again,this can only indirectly help reduce some sound getting through walls.

Material absorption approx 125 Hz 500 Hz 4K Hz

Brick or concrete 0.01 0.02 0.02

Plasterboard wall 0.3 0.06 0.04

Plywood wall 0.2 0.1 0.1

Curtains heavy pleated 0.15 0.5 0.6

Curtains flat 0.05 0.2 0.35

Fibreglass board 25mm. 1in 0.06 0.6 0.98

Fibreglass board 100mm. 4in 0.9 0.99 0.99

Carpet 0.01 0.1 0.4

The above table shows approx absorption of a material as a ratio. It can be seen that aplywood wall absorbs bass but reflects hi frequencies. Plywood and many other low weightbuilding materials can act as low frequency resonant absorbers as described above in Panelabsorbers. Increasing the thickness of a building material x 2 increases attenuation by approx-6dB.

Absorption coef. α = sound absorbed by a material as ratio 0 to 1. Frequency dependant.Absorption coef. = average absorption of a room as ratio 0 to 1 Frequency dependant.(fully reflective is 0 = 0% absorption) (0.5 = 50% absorption) (1 = 100% absorption)

Air attenuation / 100 meters (300ft) is approx -3dB/octave from approx 1K Hz dependant onhumidity and temp.www.acoustics.com

(3) Understanding dB for sound absorption.

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3dB = x 2 power change we only hear as a bit less or bit more as loud.10dB = x 10 power change we only hear as double or half as loud.

α 0.5 absorbs 50% sound energy, and 50% reflected.50% = -3dB, only heard as a bit less as loud to the ear.

α 0.9 absorbs 90% sound energy, and 10% reflected.90% absorption = -10dB approx only heard as 1/2 as loud to the ear.

Hypothetical example (not calculating for distance of walls) the sound would have to bereflected 6 times through an acoustical absorbent material of α 0.9 for it to be reduced to -60dBRT60.Simply put, to reduce the amount of echo and reverberation by 1/2 to our hearing the amountof acoustical absorption required may be x 10 greater than one would have assumed.

(4) Reverberation path-lengths First is the direct sound striking a wall.

A % is reflected, a % is absorbed, a % gets through the wall.The reflected sound then strikes another wall.A % is reflected, a % is absorbed, a % gets through the wall.The reflected sound then strikes another wall.A % is reflected, a % is absorbed, a % gets through the wall. so on and so on.

Sound may have to be reflected many times through the absorbent material on walls to bereduced to RT60 -60dB 1/1,000,000 of its original energy. A larger room 2 x surface areawith same absorbent material will have 1/2 the RT60. But a larger room has longer pathlengths. If the path-lengths are 20 meters (60ft) or greater the reflections will be heard asdistinct repeats (echoes). Reverberation is bad but echoes are infinitely worse. The largerthe room the greater the absorbent coef of the material on the walls and ceiling will need be, toinsure zero echoes.

Calculations for designing rooms with the appropriate acoustical absorption must include thesubjective loudness of how we hear sound. Our ears expand when it is quite to hear detailand contract when loud. Many architects make errors by not including calculations for thesubjective hearing experience of loudness variation and loss of intelligibility and annoyancecaused by echo and reverberation. The result is that most entertainment venues, workenvironments and homes have less acoustical absorption than required, or at worst, noacoustical absorption at all.

Repeat. A room that is larger requires more absorbent material with a higher absorbent coef.

(5) Room Constant R is a modified ratio number representing direct to reverberantsound. The R number is academic and has no significance on its own but is used for makingfurther calculations. An example is Critical distance and Articulation index. Roomconstant calculation is not always needed because a simple listening test can achieve mostresults required just as accurately. However, understanding the principles behind RoomConstant and Room Loss is important.

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Room constant calculation assumes a position from the sound source from where the inversesquare law applies. In most cases this is at 1 meter. 3ft.

(a) In theory a 100% reverberant room the critical distance would be close to the soundsource. The ratio between direct and reverberant sound would be close to 1:1.Room constant R = small number approx 1.

(b) In theory a 100% absorbent room, the critical distance would be at the walls. Thereverberation would be close to 0. The ratio between direct and reverberant sound would bevery large.Room constant R = large number, similar to surface area of room S.

Room Constant: R = S /1 - . small number = reverberant. Large number =absorbent.(S = surface area of room) ( = average absorption coef) Frequency dependant.

The drawings above and below are simplified to give a basic understanding of the principlesdescribed. No matter what calculations of room acoustics are being looked at, always keep

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the knowing of 'Critical Distance' as priority. The outcome objective is for the CriticalDistance to be as far as possible from the sound source at all frequencies. Maintain this asthe primary objective and you will never become lost.

The more reverberant the room is the closer the Critical Distance.The more absorbent the room is the further the Critical Distance.T60 is measurement of time reverberation diminishes to one millionth (-60dB).

(a) Assume curtain material in cinema has absorbent coef 0.9 (90%) at hi-frequencies.Reverberation time short T60 = 1/10 sec (100 milli-sec). Hi-frequencies sound clear.Critical distance is further away.

(b) Assume same curtain material in cinema has absorbent coef 0.1 (10%) at bassfrequencies.Reverberation time long T60 = 1.5 sec Bass sounds muddled.Critical distance is close to sound system.

(6) Regulation and Litigation Many entertainment venues are in suburbs where noiseregulations are strict. Complying with regulations by driving at the speed limit, may beacceptable on the road. But saving $ in building construction by doing the least possible tocomply with noise regulations is risky. Heavy metal, rap and techno is offensive to themajority of the conservative population, regardless of how far below the regularity noise levelthe music is heard at especially bass.

Many venues complying with noise regulations have still been closed down, sometimesresulting in successful litigation against architects by venue owners. Architects have a legalresponsibility to correctly advise venue owners of the regulatory and social non-acceptance ofnoise pollution.

(7) Calculations and Testing Procedures Calculations for designing a studio orentertainment venue must always contain the knowing of Critical distance at all frequencies, aspriority. The absolute rule is that form (visual) must follow function (sound).

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The Sydney Opera House is without doubt one of the the worlds worst example of anacoustical environment that had been only conceived from a visual design. This resulted fromthe initial conditions locked in by visual design, 'form following function' in the wrongorder. As a visual tourist attraction it is sucessful but as an entertainment venue it isgovernment subsitised.

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Acoustical termsHearing damage is specified from 84 dB SPL+ for 4hrs or more from continuous industrialmachine noise. This specification may vary. Time is halved for each 3dB increase(87dB/2hrs) (90dB/1hr) etc. Some people (not all) with hearing damage caused by loud noise,suffer hearing sensitivity loss in the 1K to 3K Hz range only, regardless of what frequenciescaused the damage. Normally our hearing is maximally sensitive in the 1K to 3K Hzrange. Its this range that enables us to interpret intelligibility in speech. Whencommunicating with someone who suffers loss of sensitivity in this range, it is necessary tospeak to them slowly, not loudly.

Reverberant noise of city streets, work places and recreational venues is often in excess ofwhat is safe to experience. The reverberant noise from people talking loudly in restaurantsand bars with hard ceilings can exceed 90 dB SPL. Excessive room reverberation can holdnoise at a constant level similar to machine noise. It is simply cheaper to make buildings withhard reverberant surfaces. High-powered sound systems are often blamed but not always theproblem. The transient peaks of music can be held at a continuous level by reverberationeasily adding another 20dB to 30dB more sound energy. Litigation for hearing damage willhopefully bring about social change to force venues and public spaces to be designed moreacoustically absorbent.

Without quiet environments in which music can be enjoyed, our way of life can find noorder. It is relatively simple to create acoustically absorbent environments. It only takes thewill to do so.

The 1st commandment 'know thy Critical Distance'

Acoustic terms and calculations

Absorption coef α = noise absorbed by a material, frequency dependant. Specified from 0 to 1(fully reflective is 0 = 0% absorption) (0.5 = 50% absorption) (1 = 100% absorption)

Absorption coef = average absorption of room.

Acoustical Masking is any sound of 6dB+ that masks others of similar frequency.

Anechoic: is 100% acoustically absorbent room.

Critical Distance is distance from source where direct and reverberant sound is equal.

Critical Distance Dc = 0.14/√QR (Q = directivity factor 1 of sound source. R = room constant)

Directivity factor Q1 = sound dispersing spherically. Q2 = sound dispersing hemi-spherically. etc

Directivity Index is directivity factor expressed in dB. eg. hemispherical dispersion Q2 = +3dB

Echo: is sound reflected back from 10 meters or more, heard as distinct repeat.

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Inverse square law (-6dB/2D) As direct sound doubles in distance, energy diminishes to 1/4

Mean Free Path 4V/S Average distance of reflections. (V volume. S surface) of room.

Mean Free Time Average time of reflections, calculated from mean free path.

Path length is the distance of walls and ceiling from which the sound is reflected back.

Reverberation is sound reflected back from less than 10 meters, not heard as distinct repeat.

Room Constant R = S /1 - small number = reverberant. Large number = absorbent.

RT60 is time reverberation diminishes to - 60dB (1/1,000,000) measured at each 1/3 octave.

RT60 Metric = 0.16/S Imperial = 0.05/S (S surface area. average absorption) of room.

Standing Waves are bass wavelengths cancelled or increased, reflected from walls or ceiling.

Sabin absorption = to 1 square ft of open window.

Sabin Metric absorption = to 1 square meter of open window.

Sabin of person is approx 0.5 Sabin.

Sound transmission class specification of noise reduction through building material

Wavelength (Greek letter symbol Lambda) = velocity of sound / Frequency

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