Noise Control Handbook

Environmental Noise Control

ii

TABLE OF CONTENTS

Page

INTRODUCTION. 1

DEFINITIONS.. 1

Sound and Noise 1

Sound Waves... 1

Speed of Sound 2

Wavelength and Frequency... 3

Noise Spectrum 5

Octave Bands... 8

Decibel and A-Weighted Decibel (dBA) Scale 10

Loudness... 12

Sound Pressure Level (SPL) and Sound Power Level (PWL). 14

BASIC CALCULATIONS... 17

Calculating Sound Power from Sound Pressure 17

Calculating the Total PWL for a Single Noise Source... 19

A-Weighting the PWL of a Single Noise Source. 19

Calculating the Total PWL of Numerous Noise Sources.. 20

SOURCE-PATH-RECEIVER. 23

Source Specifics.. 23

Path Specifics.. 25

Receiver Specifics... 34

ACOUSTIC MATERIALS... 38

Sound Absorbing Materials 38

Transmission Loss or Barrier Materials 39

Resonator-Type Materials.. 40

Damping Materials.. 41

Vibration Isolators 41

iii

TABLE OF CONTENTS CONTDPage

ATTENUATION 42

Buffers 42

Natural Barriers 42

Barriers.. 42

Acoustical Enclosures. 43

Acoustical Buildings. 44

Silencers 46

Acoustic Plenums 49

Acoustic Louvers.. 50

Acoustic Lagging.. 51

NOISE CONTROL APPLICATIONS 51

ATCOs Acoustic Assemblies 51

ATCOs Balanced Approach.. 53

Testing and Guarantees. 58

USEFUL SOURCES 61

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FIGURES

PageFigure 1: Behavior of Sound Waves. 2

Figure 2: Wavelength.. 4

Figure 3: Wavelength and Frequency.. 5

Figure 4: Example of a Noise Level Spectrum 5

Figure 5: Discrete Frequency (Tonal) Noise... 6

Figure 6: Broad Band Noise.. 7

Figure 7: Impulsive Noise... 8

Figure 8: Narrow Band, One-Third Octave Band and Octave Band... 9

Figure 9: Comparison Between the Pascal and Decibel Scales.. 10

Figure 10: A, B, C and D Weighting Networks.. 12

Figure 11: Doubling Sound Pressure Adds 3 dB.. 13

Figure 12: Equal Loudness Contours. 14

Figure 13: Comparison of Sound Power (PWL or Lw) and Sound Pressure (SPLor Lp).

18

Figure 14: Structure Borne Noise 23

Figure 15: Near Field and Far Field 26

Figure 16: Sound Intensity... 28

Figure 17: Sound Pressure Decreases 6 dB for Each Doubling of Distance... 29

Figure 18: Sound Propagation from a Line Source.. 30

Figure 19: 3 dB Near Field and 6 dB Far Field Guideline for a Point Source.. 31

Figure 20: What Happens When Sound Waves Encounter an Obstacle. 32

Figure 21: Refraction of Sound 33

Figure 22: Equivalent Continuous Sound Pressure Level (Leq).. 35

Figure 23: Common Noise Level Criteria Used by Regulators.. 36

Figure 24: Transmission Loss (TL) for Two Walls 39

Figure 25: Example of Parallel Baffles... 47

Figure 26: Example of an Absorptive-Reactive Silencer. 49

Figure 27: Example of an Acoustic Plenum.. 50

vFIGURES CONT

Page

Figure 28: Example of an Acoustic Louver 51

Figure 29: Example of a Noise Management Assembly.. 52

Figure 30: Noise Contour Levels at a Power Plant Before Acoustic Treatment.. 54

Figure 31 Noise Contour Levels at a Power Plant After Acoustic Treatment. 55

Figure 32 Example of ATCOs Balanced Approach 57

Figure 33 Sample Acoustical Test. 59

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TABLESPage

Table 1: Relationship Between Sound Power (PWL or Lw) and SoundPressure (SPL or L p)... 16

Table 2: Examples of Sound Power Levels for Select Equipment byOctave Band Frequency.. 19

Table 3: Sampling of Noise from Sources at a Peaking Power Plant. 21

Table 4: Table Method for Adding or Subtracting Decibels.. 22

Table 5: Correction for Background Noise... 25

Table 6: Examples of Community Noise Guidelines.. 36

Table 7: STC Ratings and Their Relationship to Sound ProofingProperties.. 45

1ENVIRONMENTAL NOISE CONTROL

IINNTTRROODDUUCCTTIIOONNThe objective of environmental noise control is to improve the acoustic environment in a

community by reducing noise levels. Noise from industrial operations can affect

neighboring residential areas, ranging from intolerable noise levels to structural

vibrations. Well-planned noise control can eliminate a major component of an industrial

sites impact on its surrounding environment. Noise control is based on what we know

about how sound behaves. For this reason, our look at some of the fundamentals of

environmental noise control begins with basic descriptions of sound and noise, acoustic

materials, and attenuation.

DDEEFFIINNIITTIIOONNSS

SOUND AND NOISE

Noise is usually defined as annoying or unwanted sound. Sound may be defined as

any pressure variation (in air, water or other medium) that the human ear can detect.

A barometer measures pressure changes in air. However, the arrival of a warm or cold

front is too slow and the changes too gradual to be heard and, hence, called sound.

The human ear hears the rapid changes in air pressure that barometers cant

measurechanges that are at least 20 times per second. Pressure changes are

caused by the action of a vibrating objectsuch as a turbine casingon the

surrounding air.

SOUND WAVES

Pressure variations (sound energy) travel through air or other elastic media (such as

water) in the form of sound waves from the sound source to the receptor (microphone,

listeners ears). When a solid object hits the air and does so repeatedlyas a vibrating

2object doesthe air alternately compresses and expands around it and waves of lower

and higher pressure are sent out in all directions from the object. What we sometimes

feel in our ears, and express as sound, is the change from the lower to higher pressure.

Figure 1: Behavior of Sound Waves

Sound vibrations alternately compress and expand air in front of the loudspeaker cone,moving away in the form of a sound wave.

SPEED OF SOUND

The speed at which sound travels varies with the medium. In air, a familiar rule applies.

Do you recall counting three (3) seconds per kilometer (five (5) seconds per mile) every

time you saw lightning to the time you heard thunder? The time lapse corresponds to

the speed of sound in air of 1,238 kilometers (770 miles) per hour. For purposes of

sound measurement, the speed of sound is expressed as 340 meters (372 yards) per

second (at sea level and 15 Celsius).

3WAVELENGTH AND FREQUENCY

The number of pressure changes per second is called the frequency of the sound.

Units of frequency used to be given in cycles per second, but now they are called Hertz

(Hz), to honor H.R. Hertz, the physicist who discovered electromagnetic waves. One

cycle of pressure change is called the period. The period is also called the reciprocal

of the frequency and is given as follows:

Period (T) = 1

Frequency

Knowing the speed and frequency of a sound allows the calculation of its wavelength.

A wavelength is the distance a sound wave travels in the time it takes to complete one

cycle or period.

Wavelength (l) = Speed of Sound ( c )

Frequency (Hz)

4Figure 2: Wavelength

When designing an acoustical solution to industrial noise, it is important to know the

wavelength of the different frequencies. In general, the object in the sound path

must be larger than one wavelength to significantly disturb the sound. At 20 Hz, a

wavelength is about 17 meters (56 feet), so an object must be larger than 17 meters

wide and high to block the sound waves. At 20,000 Hz, the wavelength shortens to 1.7

centimeters (.7 inches). Low frequency noises have long wavelengths and high

frequency noises have short ones. The longer wavelength of a low frequency sound

allows it to slip easily around or over barriers.

5Figure 3: Wavelength and Frequency

NOISE SPECTRUM

Most sound is made up of a number of frequencies just as light is made up of different

colors. A color spectrum results when light passes through a prism. A sound or noise

spectrum is produced when sound is passed through a spectrum analyzer.

Figure 4: Example of a Noise Level Spectrum

6Two types of noise exist: steady noise and non-steady noise. Steady noise with audible

discrete tones is called discrete frequency noise and is the most common noise found

in industry. This type of noise has the characteristic of pure tones over a number of

frequencies. Discrete frequency noise is caused by rotating parts of machines such as

fans, internal combustion engines, transformers and pumps.

Figure 5: Discrete Frequency (Tonal) Noise

The second most common form of industrial noise is called broad band noise. Broad

band noise is steady noise without discrete frequency tones. Sounds are of longer

duration and vary little over time. However, acoustical energy may be heavily

concentrated in one or more areas of the spectrum. Large gas turbines emit peak noise

levels in the lower frequencies. This is called pink noise and is analogous to the pink

and red light at the lower frequencies of the color spectrum. If the noise has

frequencies evenly distributed throughout the audible range, white noise results.

7Figure 6: Broad Band Noise

The noise levels shown in Fig. 6 were emitted by the engine exhaust of a Solar Mars Centaur

40S.

Other industrial noises are non-steady and consist of fluctuating noise (noise that

doesnt remain at any constant level over a given period of time), intermittent noise

(noise that returns to the ambient or background level), and, more commonly,

impulsive noise (sounds of short duration with high peak pressures). Peak pressures

rise at least 40 dB in 0.5 seconds.1

1 Henning E. Von Gierke and Charles W. Nixon, Damage Risk Criteria for Hearing and Human Body Vibration,in Noise and Vibration Control Engineering: Principles and Applications. Leo L. Beranke and Istavn L. Vr, eds.New York.: John Wiley & Sons, Inc., p. 595.

8Figure 7: Impulsive Noise

OCTAVE BANDS

Frequencies are divided into octaves, just like octaves on a piano. An octave band is

defined as a range of frequencies extending from one frequency to exactly double that

frequency. For example, the 1000 Hz octave band is centered at 1000 Hz and extends

from 707 Hz to 1414 Hz. Nine octave bands are most often used when measuring

sound.

Most Commonly Used Octave Bands in Industrial Noise Studies

31.5 Hz 63 Hz 125 Hz 250 Hz 500 Hz 1000 Hz 2000 Hz 4000 Hz 8000 Hz

When analyzing noise at an industrial site, a noise spectrum is studied. However, it is

not practical to examine the acoustic energy generated at every frequency at the same

time this would create an enormous amount of data. Instead, the frequency range is

apportioned into a set of broader ranges, each containing lesser amounts of detail.

Examples of the three most common types of frequency analyses are narrow band,

one-third octave band and the octave band.

9Figure 8: Narrow Band, One-Third Octave Band and Octave Band

For most industrial noise analysis, the octave band provides a sufficient level of detail.

Occasionally, a finer breakdown than an octave band is required, particularly when the

noise emitted is tonal. Tonal or discrete frequency sounds are characterized by spikes

of high energy at specific frequencies in an otherwise continuous noise spectrum. To

pinpoint these energy spikes, a more detailed noise analysis (using one-third octave

band) is undertaken. For even greater accuracy, a narrow band analysis over specified

narrow frequency ranges can be performed.

The frequency of a sound produces its distinctive tone. The rumble of the lowest notes

of the largest pipe organ has a low frequency, while a flute produces a high frequency

tone. Machines like gas turbines generate both low and high frequency sounds. Some

sources dont cause various frequencies of sound. Instead, they generate a single

frequency or pure tone.

10

DECIBEL (DB) AND A-WEIGHTED DECIBEL (DBA) SCALE

The size or amplitude of pressure changes is measured in decibels or dB. The

weakest sound the human ear can hear has an amplitude of around 20 millionths of a

Pascal (20mPa) the scale used to measure barometric pressure. A pressure change

of 20mPa is equivalent to 5 billion times less than normal atmospheric pressure.

Because the range of sound pressures in a typical room is so huge, using the Pascal

scale to measure noise would be close to impossible. The decibel scale was devised to

make calculations of noise levels manageable.

The decibel (dB) is a unit of logarithmic measure, which uses 2 x 10 5 Pa as the

starting point of zero (0) dB. Zero dB or 2 x 10 5 Pa is the lowest pressure a young

adult can detect of a pure tone at 1000 Hz. Most continuous noise sources emit sound

pressure levels between 0 to 150 dB. A level of 150 dB is equivalent to a jet aircraft at

take off. Noise levels over 150 dB can occur. For example, a blowdown vent opening

can produce sounds of 170 dB, while the space shuttle is recorded at 180 dB.

Figure 9: Comparison Between the Pascal and Decibel Scales

Sound Pressure (Pascals) Sound Pressure Level (Decibels)

Jet Engine (25 m)

Rock Concert

Heavy Truck

ConversationalSpeech

Unsilenced Turbine Inlet (3 m)Unsilenced Turbine Exhaust (3 m)

Inside Turbine EnclosureCooling Tower (3 m)Transformers (3 m)

HRSGInside Powerhouse Building

Lube Oil Cooler (3 m)

Inside Control Room

Equipment ExamplesExamples

11

The decibel scale is a closer approximation to the sounds heard by the human ear than

the Pascal scale, because the human ear is able to react to exponential changes in

sound pressure. However, the decibel scale still doesnt replicate what the human ear

actually hears. This is because the human ear is more sensitive to sound at

frequencies between 1000 and 5000 Hz and less sensitive to higher and lower

frequency sounds. To quantify the sensitivity of humans to sound the A-weighted

decibel or dBA scale (also written dB(A)) was created. A correction factor was devised

to change unweighted decibels (dB), also known as the linear scale, to A-weighted

decibels (dBA).

For purposes of noise control, both the dB and dBA scale can be used interchangeably.

Sometimes it is necessary to convert from the dB to dBA scale and vice versa. For

example, a manufacturer might provide the noise level of a machine in dB, whereas the

community noise requirement is stated for dBA. In this case, initial calculations of the

noise level might be made in dB, then converted to dBA.

There exist three additional weighting networks B, C, and D which are either

used in special circumstances or are obsolete. When low frequency noise is of

concern, C weightings are used because they attenuate low frequencies much less than

the other weightings. D weightings are used when very high frequencies, like those

emitted from jet engines, need to be attenuated. The B weightings, emphasizing middle

frequencies, are no longer in use.

Example: A 100 dB sound in the 31.5 Hz band has a correction factor of 39.4. Subtract 39.4 from 100 dB(i.e., 100 dB 39.4 = 60.6 dBA). The answer60.6 dBAis how loud the 100 dB sound isperceived by the human ear in the 31.5 Hz band. By contrast, the same 100 dB sound is perceived bythe human ear exactly as 100 dBA when frequencies are in the 1000 Hz band (i.e., 100 dB 0 = 100dBA).

12

Figure 10: A, B, C and D Weighting Networks

Frequency Curve A dB Curve B dB Curve C dB Curve D dB

10 -70.4 -38.2 -14.3 -26.512.5 -63.4 -33.2 -11.2 -24.516 -56.7 -28.5 -8.5 -22.520 -50.5 -24.2 -6.2 -20.525 -44.7 -20.4 -4.4 -18.5

31.5 -39.4 -17.1 -3 -16.540 -34.6 -14.2 -2 -14.550 -30.2 -11.6 -1.3 -12.563 -26.2 -9.3 -0.8 -1180 -22.5 -7.4 -0.5 -9

100 -19.1 -5.6 -0.3 -7.5125 -16.1 -4.2 -0.2 -6160 -13.4 -3 -0.1 -4.5200 -10.9 -2 0 -3250 -8.6 -1.3 0 -2315 -6.6 -0.8 0 -1400 -4.8 -0.5 0 -0.5500 -3.2 -0.3 0 0630 -1.9 -0.1 0 0800 -0.8 0 0 01000 0 0 0 01250 0.6 0 0 21600 1 0 -0.1 5.52000 1.2 -0.1 -0.2 82500 1.3 -0.2 -0.3 103150 1.2 -0.4 -0.5 114000 1 -0.7 -0.8 115000 0.5 -1.2 -1.3 116300 -0.1 -1.9 -2 108000 -1.1 -2.9 -3 8.5

10000 -2.5 -4.3 -4.4 612500 -4.3 -6.1 -6.2 316000 -6.6 -8.4 -8.5 -420000 -9.3 -11.1 -11.2 -7.5

LOUDNESS

Sound is defined as any pressure variation heard by the human ear. This translates

into a range of frequencies from 20 Hz to 20,000 Hz for a healthy human ear. In terms

of sound pressure, the human ears range starts at the threshold of hearing (0 dB) and

ends at the threshold of pain (around 140 dB).

13

The human ear is less sensitive to sound pressure variations in the low frequencies

compared to the higher frequencies. A 50 Hz tone must be 15 dB higher than a 1000

Hz tone at a level of 70 dB to be perceived as the same loudness by the listener. As a

rule of thumb, a doubling in the loudness of the sound occurs with every increase of

10 dB in sound pressure. Similarly, for each 10 dB decrease in sound pressure, the

loudness is cut in half.

The 10 dB loudness rule is not the same as a common guideline used when decibels

are added (or subtracted) together. In the latter guideline, a doubling in sound pressure

results in a 3 dB increase in the noise level (not a 10 dB increase as with loudness).

The 3dB rule applies only when identical noise sources are added (or subtracted). For

example, adding together two identical noise sources of 85 dB results in a total noise

level of 88 dB (85 dB + 85 dB = 88 dB).

Figure 11: Doubling Sound Pressure Adds 3dB

14

The human ears ability to hear logarithmic changes in sound pressure explains why

loudness increases 10 dB but the noise level from identical sources increases by only

3dB. In practice, loudness plays a small role in noise control because it is subjective

and varies from person to person. What is interpreted as loud noise by one individual

may not be loud or noise to another. Of note is that human beings do not hear sounds

in the very low frequencies. However, you may recall feeling rather than hearing

sound. Vibrations from very low frequency sounds can rattle dishes and shake home

foundations even though they cant be heard.

Figure 12: Equal Loudness Contours

Equal loudness curves show the relative lack of sensitivity of the human ear to lowfrequencies.

SOUND PRESSURE LEVEL (SPL) AND SOUND POWER LEVEL (PWL)

Sound pressure is the change in pressure of the air above and below the average

atmospheric pressure. When dealing with sound, the changes an acoustical engineer

records can be hugefrom as small as a millionth of a Pascal (also recorded in

15

pounds per square inch, abbreviated as psi) to larger pressure changes like

explosions inside reciprocating engines and gas turbines.

To measure such wide pressure changes (or amplitude), sound pressure is converted

into decibels, and referred to as the Sound Pressure Level (SPL or Lp ). The scale

starts at zero decibels and the international standard of pressure change of 2 x 10 5 Pa.

The equation used to calculate the Sound Pressure Level is:

SPL or Lp = 10 log10 (p2 / p20) [dB]Or, in a simpler form as:

SPL or Lp = 20 log10 p + 94 [dB]Where:

SPL or Lp = Sound Pressure Levelp = root-mean-square (rms) sound pressure (Pascals or Pa)p0 = international reference pressure of 2.0 x 10 5 Pa

Most manufacturers will make available the Sound Pressure Levels of their machines.

These machines, such as gas turbines, emit energy in the form of power, heat and

sound. The power is expressed in horsepower, the unit used to describe your cars

performance. The acoustic energy radiating from a machine is termed sound power.

Sound power is defined as the average rate at which sound energy is radiated from a

sound source. It is measured in watts (W). The Sound Power Level, abbreviated as

PWL or Lw, is sound energy after it is converted into decibels. Like sound pressure, a

reference sound power has been established. This reference is 10 12 x watt (W).

The equation used to calculate the Sound Power Level is:

PWL or Lw = 10 log10 (W / W0) [dB]

Or, in a simpler form as:

PWL or Lw = 10 log10 (W) + 120 [dB]Where:

PWL or Lw = Sound Power LevelW = acoustic energy of the source given in watts (W)W0 = international reference sound power of 10 12 Watt (W)

16

The PWL or Lw is constant for a given source and is independent of the acoustic

environment. It cannot be measured directly, but must be calculated from the Sound

Pressure Level. This is because PWL can be thought of as similar to the watt rating of

a light bulb. SPL, on the other hand, is like the amount of light produced at a given

distance from the bulb in a specific environment. Sound pressure is relatively easy to

measurethe pressure variations felt by the human eardrum are the same pressure

variations detected by a microphone used to record the sound.

Table 1: Relationship between Sound Power (PWL or L w) and SoundPressure (SPL or Lp)

Pressure and Pressure Level:

Source Pascal (Pa) Decibels (dB)

Average hearing threshold 2 x 10 5 0

Whisper 2 x 10 3 40

Conversation 4 x 10 -2 65

Train Station 2 x 10 0 100

Jet aircraft at takeoff 6 x 10 1 130

Power and Power Level:

Source Watts (W) Decibels (dB)

Conversational voice 10 5 70

Piano 10 2 100

Orchestra 10 0 120

Jet aircraft at takeoff 10 2 140

Space shuttle 10 6 180

Example:1.0 watt of acoustic energy is the equivalent of 120 dB:

PWL or Lw = 10 log (1 watt / 10 12 watts)= 10 log (1012 )= 10 (12)= 120 dB

17

Note: Unless otherwiseindicated, all acousticcalculations involvingdistance use metric units.

BBAASSIICC CCAALLCCUULLAATTIIOONNSS

CALCULATING SOUND POWER FROM SOUND PRESSURE

The Sound Power Level (PWL or Lw) of noisy

equipment is what we use to determine the amount of

attenuation needed to meet the noise level

requirement. As mentioned, the PWL cannot be

measured; it must be calculated. To calculate the PWL, we first measure the Sound

Pressure Levelusually at one meter from the machine. Also needed to calculate the

PWL is the size (or dimension) of the noise source. Manufacturers will often make

available the SPL and equipment dimensions upon request.

An equation that gives an approximate calculation of the PWL from the SPL of a noise

source is:2

PWL or Lw @ SPL + 10 log (A ) [dB]

Where:

SPL = Sound Pressure Level of the sound source at a specified distance

Area = height x width x length in square meters (m2)

As mentioned, the Sound Pressure Level is relatively easy to measure; a microphone

picks up the same pressure changes as the human ear. However, the sound pressure

2 The precise equation is:

PWL = SPL + 10 log [P02 * A/W0 r C]Where:

SPL = Sound Pressure Level of the sound source at a specified distanceP02 = reference pressure of 20 x 10 5

A = area of sound source in square meters (m2 )C = speed of sound which is 340.3 meters per secondr = density of medium; 1.225 kilograms per cubic meter in air

Since Po2 = (20 x 10 5 )2 Pa 2

W0rC = 1 x 10 12 x 1.225 kg/m3 x 340.3 m2

And Po2 W0 rC= 0.96 and 10 log (0.96) = -0.18;Hence the formula, PWL or Lw @ SPL + 10 log (A ) represents an approximation of the Sound Power Level.

18

measurement doesnt represent the acoustical energy (sound power) of a machine. To

use an analogy from another kind of energy electrical energy heating the head of

a pin and a stovetop element to exactly the same temperature takes different levels of

energy. The amount of electricity used to heat the pin is much less than the energy

emitted by the element. This same analogy can be applied to sound. A radio and

orchestra might produce the same Sound Pressure Level (e.g., 85 dB) at a certain

distance, but the orchestra emits substantially higher amounts of acoustical energy with

a correspondingly greater impact on the environment.

Figure 13: Comparison of Sound Power (PWL or Lw) and Sound Pressure (SPLor Lp)

The PWL also needs to be calculated in each octave band. Recall the noise peaks that

occur at discrete frequencies for most industrial equipment. The peak noise level is

often the level that is attenuated, particularly when it is causing discomfort to residents

in the neighborhood.

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Table 2: Examples of Sound Power Levels for Select Equipment by OctaveBand Frequency *

Sound Power Level (PWL or Lw) in dB (relative to 10 12 Watts)Octave Band Frequency (Hz)

Equipment Item 31.5 63 125 250 500 1000 2000 4000 8000LM6000 Enclosure 124.5 120.5 117.5 113.5 106.5 100.5 84.5 87.5 77.5HRSG Body 122.0 114.0 106.0 103.0 99.0 97.0 98.0 96.0 89.0Inlet Filter 116.0 120.0 112.0 108.0 107.0 113.0 107.0 102.0 92.0* PWLs for select equipment at 110 MW power station in Iroquois Falls, Ontario.

CALCULATING THE TOTAL PWL FOR A SINGLE NOISE SOURCE

After a machines PWL is calculated for each octave band frequency, the next step is to

enter the calculated PWLs into the following formula to obtain the Total PWL:

n

Total Sound Power Level (PWL) = L w, Total = 10 * log10 [ S 10 Lw, i /10 ]

i = 1

Where:Lw, I = Sound Power Level or PWL for each octave band frequency = sum of number of PWLs

The total PWL should always be higher than the highest PWL recorded by octave

banda quick way to check whether your calculation is on track.

A-WEIGHTING THE PWL OF A SINGLE NOISE SOURCE

Sometimes it is necessary to A-weight the Sound Power Level if a communitys noise

by-law is stated in dBA. To obtain the total A-weighted PWL for single noise source, a

Example:Calculating the total PWL for a LM6000 enclosure at Iroquois Falls, Ontario:

PWL or LwTotal = 10 * log10 (10 124.5/10 + 10120.5/10 + 10117.5/10 + 10113.5/10 + 10106.5/10 + 10100.5/10 + 1084.5/10 + 1087.5/10 + 1077.5/1)

PWL or Lw Total = 10 * log 10 (4.783 x 1012)PWL or Lw Total = 126.8 dB

20

correction factor, given in Figure 10, is added to the unweighted PWL (known as the

linear PWL) at each octave band frequency. Then, the A-weighted PWLs for each

octave band are inserted into the formula for calculating the Total Sound Power Level to

obtain the PWL expressed in dBA.

CALCULATING THE TOTAL PWL OF NUMEROUS NOISE SOURCES

In most industrial facilities, sound is emitted from many sources. Table 3 gives a

sampling of some of the major noise sources associated with a single gas turbine at a

peaking power plant, which are often driven by two or more gas turbines.

Example: Calculating A-weighted PWLs using the table method. Taking the linear PWL at each frequency for acombustion exhaust, apply the correction factor from Table 3 to obtain the A-weighted result.

31.5Hz 63Hz 125Hz 250Hz 500Hz 1000Hz 2000Hz 4000Hz 8000HzTake Unweighted PWLs LM 6000

Enclosure124.5 120.5 117.5 113.5 106.5 100.5 84.5 87.5 77.5

Add A-Weighted Correction Factor -39.4 -26.2 -16.1 -8.6 -3.2 0 1.2 1.0 -1.1

Obtain A-Weighted PWL Result 85.1 94.3 101.4 104.9 103.3 100.5 85.7 88.5 76.4

21

Table 3: Sampling of Noise from Sources at a Peaking Power PlantSound Power Levels at Center Octave Bands dB (relative to 10 12 Watts)

Source Description 31.5Hz 63Hz 125Hz 250Hz 500Hz 1000Hz 2000Hz 4000Hz 8000Hz TotaldBInlet Gas Turbine 100.8 99.9 93.0 95.2 93.5 87.9 86.7 87.0 90.0 105.0

Turbine Vent Fan 108.2 108.2 110.1 104.1 101.0 92.1 96.8 95.3 91.1 114.5

Load Compartment Vent Fan 103.1 103.1 100.3 96.5 90.2 85.6 85.10 79.5 78.0 107.6

Load Compartment Discharge 89.0 102.0 93.0 96.0 95.0 102.0 103.0 110.0 98.0 112.3

Lube Oil Demister Vent 92.0 96.0 96.0 98.0 99.0 91.0 83.3 72.0 87.0 104.1

Accessory Module 103.0 106.0 99.3 97.1 95.9 95.4 97.7 91.7 88.3 109.5

Inlet Plenum 86.4 89.0 86.1 88.0 86.9 87.7 96.9 87.4 76.8 99.5

Turbine Compartment 108.1 109.9 104.5 102.8 100.4 98.4 103.5 98.7 93.8 114.1

Exhaust Diffuser 114.5 112.0 110.0 103.3 102.4 99.8 98.1 96.9 93.8 117.8

Load Compartment 103.1 104.9 104.8 100.3 94.9 92.7 96.6 92.7 85.8 110.2

Generator 101.9 101.8 101.4 98.0 100.3 98.8 98.0 93.0 84.0 108.9

Expansion Joint 100.8 108.8 105.8 98.8 87.8 84.8 77.8 60.8 43.8 111.3

Transition Duct 101.4 109.4 108.4 103.4 91.4 93.4 81.4 51.4 36.4 112.9

Exhaust Stack Casing 92.3 85.3 63.3 53.3 38.3 46.3 45.3 41.3 30.3 93.1

Exhaust Stack Opening 131.0 142.0 146.0 145.0 137.0 139.0 132.0 115.0 98.0 150.1

Fin Fan Cooler 57.0 96.0 88.0 93.0 92.0 90.0 89.0 88.0 69.0 100.3

Total dB 131.2 142.0 146.0 145.0 137.0 139.0 132.0 116.4 103.4 150.1

The same formula for adding (or subtracting) PWLs for a single noise source is used for

adding (or subtracting) multiple-source PWLs. The difference is that all source PWLs

are typically added (subtracted) up over a single octave band (down a column), then a

grand total is calculated for all noise sources over the nine octave bands. However,

you can add (subtract) over the individual noise sources first (across a row) and arrive

at the same grand total.

Example:Calculating the total PWL for all the noise sources in Table 3 at the 31.5 Hz octave band is:

PWL or Lw, Total = 10 * log10 (10 100.8/10 + 10 108.2/10 + 10 103.1/10 + 1089.0/10 + 1092.0/10

+ 1086.4/10 + 10108.1/10 + 10114.5/10 + 10103.1/10 + 10101.9/10 +10100.8/10 + 10101.4/10 + 1092.3/10 + 10131.0/10 + 1057.0/10)

PWL or Lw, Total = 10 * log 10 (1.28 x 1013 )PWL or Lw, Total = 131.2 dB

22

A popular method for adding (or subtracting) PWLs is the table method. For example,

first find the difference between the two loudest sources in PWLs. Next, go to Table 5

and add the specified number of dB that correspond to the difference. The sum should

then be combined with the highest remaining level and so on, until all levels are

combined.

Table 4: Table Method for Adding or Subtracting Decibels

Difference between levels dBNumber of dB to be added to the

higher level0 3.01 2.52 2.13 1.84 1.55 1.26 1.07 0.88 0.69 0.5

10 0.412 0.314 0.216 0.1

Example:Using the table method to determine the PWL of three of the power plant noise sources in the 31.5octave band in the example in Table 4: turbine vent noise level of 108.2 dB, a generator noise level of101.9 dB and lube oil demister vent noise level of 92.0 dB. Start by subtracting the noise level of theturbine vent noise level from the generator (108.2 dB 101.9 dB = 6.3 dB). Looking at Table 5, a 6.3dB difference means 1.0 should be added to the highest noise level.

108.2 dB 103.4 dB = 6.1 dB; 6.1 dB converts to 1.0 dB108. 2 dB + 1.0 dB = 109.2 dB for turbine vent and generator noise

Add the lube oil demister vent noise to the subtotal. The difference between 109.2 dB and 92.0 dB is17.2 dB. Looking at 17.2 dB in Table 5, 0.1 is added to the subtotal.

109.2 dB + 0.1dB = 109.3 dB for total noise.

23

SSOOUURRCCEE--PPAATTHH--RREECCEEIIVVEERR

All noise propagation can be broken into three parts:

The source

The path

The receiver

The source radiates sound based on its sound power (PWL). The path is how the

sound travels through the air. The receiver is what the sound impinges upon (person,

microphone, etc.).

SOURCE SPECIFICS

In industry, the most common noise sources are described as a point source, like a

gas turbine, or a line source, like a pipeline. In the free field, sound propagates

outward from point sources in uniform, concentric circles and from line sources as a

cylindrical wave, much like a weather front. Free field conditions exist when no

obstacles block the sound path. Noise from a source can either be air borne or

structure borne. Noise that travels through the air and through building walls and

openings is called air borne noise. Structure borne noise is a term used to describe

mechanical vibrations carried from machinery through to a buildings structure.

Figure 14: Structure Borne Noise

24

Whether a point or line source, occupational health standards in most countries limit

employees exposure to the noise. For example, the Occupational Safety and Health

Administration (OSHA) sets 85 dBA over an eight hour period as the maximum

admissible noise exposure limit in the workplace. The OSHA standard is representative

of a source noise limit. With this standard in mind, plant equipment is typically

ordered to emit sounds of no more than 85 dBA at one meter (3 feet).

Normally 10 to 12 measurements of the sound pressure around the periphery of a

machine at one meter (3 feet) are taken to obtain the source noise level. However, the

number of measurements vary by machine shape and size. National and international

standard institutes, such as ASTM (American Society for Testing and Materials), ANSI

(American National Standards Institute), CSA (Canadian Standards Association) and

ISO (International Standards Organization) publish guidelines on how to construct a grid

over equipment and gather point measurements at different frequencies.

Microphones are located at the points and, a sound level meter set to A-weighting,

measures sound levels at mid-band frequencies of 63, 125, 250, 500, 1000, 4000, and

8000 Hz. The measurements are averaged for each frequency and corrected for the

machines measuring surface area to find the Sound Power Level. The floor is

assumed to reflect the sound energy and so it is not included in the measuring surface.

Often in industry, background or ambient noise exists along with the source noise.

Industrial parks, for example, can emit high ambient noise levels from the many

industries on site. To get an accurate reading of noise from a specific source, the noise

level of the source must be at least 10 dB higher than the ambient noise level.

The following steps are recommended to obtain measurements of noise for a source

under conditions of background noise:

1. Measure the total noise level with all equipment running.

2. Shut down all equipment and measure the background noise level alone.

3. Determine the difference between the two measurements.

25

If the total noise level is 10 dB greater than the ambient noise level, then

background noise wont interfere with a true measurement of the total noise level. If the

background noise level is 3 dB or less, then an accurate measure is not possible. If the

background noise is between 3 dB and 10 dB, a correction is necessary. To make

corrections the following table method can be used.

Table 5: Correction for Background Noise

dB difference between sound pressurelevel and background sound pressure level

alonedB to subtract from sound pressure level

Less than < 6 1.06 1.07 1.08 0.59 0.5

10 0.5Greater than > 10 0.0

Source: ANSI, S12.34 - 1988

PATH SPECIFICS

Under free field conditions, point sources produce noise that spreads uniformly as a

sphere, much like water ripples on a pond. By contrast, sound flows from line sources

as a cylindrical wave. The sound field within close proximity to a noise source is called

the near field. A person is considered to be standing in the near field if he or she is

within one size of the noisy object in distance away. Size is measured according to

the largest dimension of the object. So, if the object is a building and the largest

dimension is the buildings height, then the near field would start at the point away from

the building that is equivalent to its height.

26

Figure 15: Near Field and Far Field

Standing 3 meters (10 feet) away from this 15 meter (50 feet) high power plant would put aperson in the near field. Standing at a distance more than 15 meters away places her in thefar field.

In the free field, the SPL increases the closer you move toward the noise source and

decreases the further you move away. More precisely, the SPL increases or

decreases as the inverse square of distance. The formula used to calculate the SPL

at a known distance away from a noise source in the free field is:

Lp(R2) = Lp(R1) 20 log 10 ( R2 ) [dB] R1

Where:Lp (R1) = Sound Pressure Level at the initial locationLp (R2) = Sound Pressure Level at the new locationR1 = distance from the noise source to the initial locationR2 = distance from the noise source to the new location

27

A popular method is to decrease the SPL by 6 dB for every doubling of distance away

from the source. If you are located one meter away from a point source, then move one

meter further away, the SPL drops by 6 dB. If you move to 4 meters away, it drops by

12 dB, at 8 meters by 18 dB, and so on. This method is derived from the inverse

square law of sound intensity.

Sound intensity is defined as the sound power per unit area. To understand the

concept of sound intensity, think of sound radiating outward from a point source. Under

free field conditions, this sound is of uniform intensity (power per unit) in all directions.

The sound power passing through a small area (d) near the sound source is the same

sound power passing through areas further away (2d, 3d, and 4d), but each successive

area gets larger while the sound intensity decreases with distance.

Example:The sound level specification you are given is 75 dB for the compressor package at 50 meters away.You have a residence 800 meters away from the facility. The SPL at the residence would be 51 dB,calculated as follows:

SPL or Lp (R2) = Lp (R1) 20 log 10 ( R2, decibels) R1

SPL or Lp (800 meters) = Lp (50 meters) 20 log (800/50)SPL or Lp (800 meters) = 51 dB

Example:Using the 6 dB rule, you also get 51 dB at 800 meters, the equivalent of using the formula:

Distance (m) Sound Level (dB)50100200400800

1600

756963575145

28

Figure 16: Sound Intensity

The same sound energy is distributed over successively larger areas as distance from thesound source is increased.

The uniform, concentric circles are actually spheres. As the area of a sphere is 4pr2 ,

the area of a small segment on the surface of the sphere varies in relation to the square

of the radius. Doubling the distance from d to 2d reduces the intensity to , tripling the

distance reduces the intensity to 1/9, and quadrupling the distance reduces the intensity

to 1/16. Intensity of sound is inversely proportional to the square of the distance

in a free field. 3

The inverse square law for intensity becomes the inverse distance law for sound

pressure. That is, sound pressure varies inversely as the first power of distance.

When sound pressure is plotted against distance units, this means that sound pressure

is reduced 6 dB for each doubling of the distance. This is called the 6 dB rule.

3 F. Alton Everest. The Master Handbook of Acoustics. 3rd Ed. New York.: Tab Books, 1994, page 68.

29

Figure 17: Sound Pressure Decreases 6 dB for Each Doubling of Distance

The inverse square law holds true only for discrete distance points and under free field

conditions. If sound values between distance points (e.g., 425.5 meters) are required,

the calculation rather than the table method is used.

For a line source, the sound spread equates to a 3 dB loss per doubling of distance.

The formula for calculating noise levels at different distances from a line source is:

Lp (R2) = L p(R1) 10 log 10 ( R2 ) [dB] R1

Where:Lp (R1) = Sound Pressure Level at the initial locationLp (R2) = Sound Pressure Level at the new locationR1 = distance from the noise source to the initial locationR2 = distance from the noise source to the new location

30

Figure 18: Sound Propagation from a Line Source

In the near field, noise from a point source diverges from the 6 dB guideline. Because

point sources are typically housed in buildings, the building behaves as a plane source,

rather than a point source. Sound is radiating outward from a flat surface. With plane

sources like buildings, there is minimal noise reduction until the radial distance (r = b/p,

where b is the width of the building) is reached. The radial distance is roughly one-third

a buildings width. At this point and as far as the far field, the Sound Pressure Level

Example:The sound level specification you are given is 55 dB for a paper recycling bailer at 200 meters away. You havea residence 800 meters away from the facility. The SPL at the residence would be 51 dB, calculated as follows:

L (R2) = L(R1) 10 log 10 ( R2, decibels) R1

L (800 meters) = L (200 meters) 10 log10 (800/200)L (800 meters) = 55 dB 10 log10 (800/200)L (800 meters) = 51 dB

31

diverges at the same rate as a line source (-3 dB per doubling of distance), then

changes to 6 dB in the far field.

Figure 19: 3dB Near Field and 6 dB Far Field Guideline for a Point Source

The near field-far field guideline applies only in the free field. In practice, sound waves

regularly collide with obstacles. Think of the static on your car radio as you drive into a

tunnel. When a sound wave encounters an obstacle, five phenomena can occur:

absorption, reflection, transmission, diffraction and refraction.

Some of these conditions can occur at the same time. Part of a sound waves energy is

absorbed and part is reflected when it strikes a surface. This fact is important when

considering how to attenuate noise. For example, the more porous a surface, the more

sound is absorbed rather than reflected.

32

When an object is a certain thicknesslike a wallpart of the sound waves energy is

transmitted through it. In general, more sound energy will pass through a thin wall

than a thick one. If sound-absorbing material is also added inside of the wall, then the

amount of noise that gets through to the other side will be less than if the wall were left

untreated. The amount of noise lost when sound waves pass through a wall or barrier

is called Transmission Loss (TL). This is the difference between the noise level

measured on the source side of a noise barrier, and the level measured on the receiver

side.

Figure 20: What Happens When Sound Waves Encounter an Obstacle

Diffraction is a change in the direction of travel of sound when the sound encounters

an obstacle. Objects capable of diffracting (bending) sound must be large compared

to the wavelength of the sound. For low frequency noise, with its long wavelength, a

barrier must be acoustically large (larger than the wavelength of the sound) to change

the sound path.

33

Refraction changes the direction of travel of the sound by differences in the speed of

propagation. Wind and temperature changes are most common causes of refraction.

Sound travels faster in warmer air than in cooler air causing the tops of the wavefronts

to go faster than the bottom parts. Under normal conditions, air temperatures decrease

as altitude increases. This causes sound waves to refract upwards which decreases

audibility along the ground. Sometimes, the temperature is higher above the ground

than near the grounda condition called a temperature inversioncausing sound

waves to bend back toward the ground and increase audibility. Temperature inversions

are especially common at dawn, dusk, and in cold winter conditions.

Also, because winds aloft are usually faster than at ground level, the upper part of a

sound wave travels faster than the lower part when travelling with the wind. The

sound wave travels slower when traveling against the wind. Refraction of the noise

toward the ground occurs in the first instance and refraction away from the ground in the

latter case.

Figure 21: Refraction of Sound

34

RECEIVER SPECIFICS

Most municipalities set a dB or, more frequently, a dBA limit at the nearest sensitive

receiver (NSR), usually defined as the property line of an industrial, commercial or

residential building or its outside wall. A property line noise limit is typically used to

control noise from stationary sources like power plants and compressor stations. A

time limit, during which noise is either prohibited or required to stay below a certain

dBA level, is frequently combined with the property line limit. When using time limits, an

allowable day-time noise level is specified which is higher than a night-time noise

level.

Some localities define permissible noise levels for areas. In the case of area limits,

noise is restricted to a dBA level at the boundary of the nearest sensitive area (NSA).

Industrial zones allow higher noise levels than residential areas that have higher noise

levels than noise sensitive ones like hospitals or nursing homes. What becomes

interesting from a noise control perspective is when industrial areas abut noise sensitive

zones.

The dBA limit in noise guidelines is sometimes qualified with the symbol Leq. Leq is

defined as the equivalent continuous sound pressure level, and represents an

average of the noise history at a given site or location. The Leq is used when it is

important to consider variations in Sound Pressure Levels over time. It is usually

appraised hourly and then averaged over 24 hours, using the following formula:

n

Leq = 10 log (1/T S ti 10 Li/10)

i = 1

Where:T = total time (usually 24 hours)ti = usually an hourly time interval (with S ti = T)Li = Sound Pressure Level at time ti, measured in dBA (and converted to dB, if

required)

35

People are more sensitive to noise at night than they are during the day. Background

levels drop during the night-time when people are at home asleep. The day-night level,

Ldn, is an energy average of the 24 hour Leq for a day, with a 10 dBA penalty added to

the sound level for the hours between 10 p.m. and 7 a.m. The CNEL (Community

Noise Exposure Level) is the same as the Ldn but with a 5 dBA penalty added to the 10

dBA penalty from 10 p.m. to 7 a.m.

Figure 22: Equivalent Continuous Sound Pressure Level (Leq)

Other communities base their noise requirements on the existing background sound

level, L90 or L95 (the noise level present 90% or 95% of the time) with noise levels

allowed to reach a certain level over the ambient level (e.g., 5 dBA). Other communities

specify that the sound level must not exceed a certain limit 75% of the time (L75), 50 %

of the time (L50), or 10% of the time (L10). Still other communities specify noise limits for

each octave band.

36

Figure 23: Common Noise Level Criteria Used by Regulators

Table 6: Examples of Community Noise Guidelines

Municipality Sound Level Location

Miami, Florida Ambient + 10 dBA or 75 dBA Industrial property line

Toronto, Ontario 83 dBA L90 15 meters from equipment

World Health Organization

(WHO)

55 dBA Leq

Daytime

At residence

Puerto Rico 75 dBA L10 Industrial property line

Denver, Colorado 80 dBA Industrial property line

Salinas, California 60 CNEL

80 Ldn

Industrial property line

New York City, New York 70 dBA 25 feet from equipment

37

To measure the effect of noise from an industrial site on the NSR, an ambient noise

survey is conducted. Of interest is the total Sound Pressure Level generated at the

NSR by the many sound sources on the industrial site. The level of ground absorption,

site topography, placement of buildings, and atmospheric conditions influence the

sound pressure levels at the NSR. Sound pressure measurements at the receiving

property are typically taken every hour over a 24 hour period under calm and dry

weather conditions. Microphones are placed at a height of 1.5 meters (5 feet) above

the ground or surface and away from any natural or artificial structure.

For most noise, an octave band analysis suffices. When audible discrete frequency

tones exist, a narrower band analysis is usually performed (either one-twelfth or one-

third octave band). If noise is fluctuating, the maximum and minimum values during the

time the noise is on are recorded. For intermittent noise, the average noise level is

recorded during the on time. The maximum or peak noise level in addition to the

average noise level is captured when impulsive noise is the problem.

Ambient measurements are especially important when siting a plant or station. How the

facility is situated has a strong bearing on how much noise it will contribute at the NSA

or NSR. By configuring the plant design so that noise is channeled away rather than

toward the NSR or NSA, significant cost savings for attenuation can be realized.

Measuring the ambient noise level at a fully operational plant is sometimes necessary.

The need arises when documentation is required to determine the source and level of

noise affecting an NSR. Taking noise measurements at built-up sites may be

complicated. Sound pressure patterns are often disturbed by buildings and other

structures as well as landscaping. Since it is important to take measurements under

free field conditions, sound pressure may have to be measured in locations away from

structures, then extrapolated out to the NSR or back to the noise source.

Directional noise from existing facilities is also common. Sound from building

openings, such as exhaust stacks and ventilation and combustion outlets, emit more

38

noise in the front of the openings than to the sides. Frequency and the area of the

opening influence the directivity effect. The higher the frequency and larger the

opening, the greater the sounds impact. Sound pressure measurements at more than

20 locations may be needed to determine the directivity effect.

AAccoouussttiicc MMaatteerriiaallss

Acoustical materials are divided into the following basic types:

1. Sound absorbing materials

2. Transmission loss or barrier materials

3. Resonator-type materials

4. Damping materials

5. Vibration isolators

SOUND ABSORBING MATERIALS

Sound absorbing materials are porous materials such as rock wool, mineral wool,

glass fiber, and foam. The effectiveness of acoustical material to absorb sound depends

on its thickness, amount of airspace, and density. For every inch of thickness of a

porous material (e.g., rock wool) sound loss is about 1 dB at 100 Hz to 4 dB at 3000 Hz.

The amount of sound absorbed at the surface of a material is described by an

absorption coefficient ( a ). The absorption coefficient relates to sound reflection,

where a high a equals low reflected energy and a low a equals high reflected energy.

Marble slate has an absorption coefficient of 0.01 (almost no absorption and high

reflection). Some specially constructed sound rooms score as high as 1.0 (total

absorption and no reflected energy).

The absorption coefficient of a material typically increases with frequency. At low

frequencies, porous materials absorb less sound, so that materials must be thicker to be

effective. The overall performance of a sound-absorbing material is often described by

39

the Noise Reduction Coefficient (NRC). The NRC is the arithmetic average of the

absorption coefficient at 250, 500, 1000, and 2000 Hz.

Sound absorption differs from sound insulation. Sound absorption relates to sound

reflection, whereas sound insulation relates to the amount of acoustic energy able to

pass through material. The sound absorption provided by a 10 centimeter-thick (4-inch

thick) fiberglass acoustical blanket is high, but its insulation quality is low. Sound is able

to travel through the material to the other side. By contrast, a lead wall absorbs almost

no sound but it is a very good insulator.

TRANSMISSION LOSS OR BARRIER MATERIALS

Lead is an example of a transmission loss or barrier material. Barrier materials are

dense and rigid and are defined in terms of their Transmission Loss (TL).

Transmission Loss is defined as the logarithmic ratio of the sound power on one side of

a barrier (wall or partition) to the sound power transmitted to the other side. The higher

the TL, the better a barrier material is at limiting or attenuating the amount of sound

travelling through it. For example, a wall or barrier having a TL of 45 dB reduces a 120

dB interior noise level to 75 dB. A wall with a TL of 60 dB reduces the same amount of

noise to 60 dB.

Figure 24: Transmission Loss (TL) for Two Walls

40

TL is calculated using the following equation:

TL (dB) = 10 log 1/t = 10 log Wi/Wt

Where:t = sound transmission coefficient; ratio of the PWL incident on one side to PWL on

the other sideWi = incident sound power (PWL on source side)Wt = transmitted sound power (PWL on the receiver side)

As a general rule, the heavier and thicker the wall the greater the attenuation of the

sound or higher the TL. This is because it is difficult for sound waves in air to move

or excite a dense, heavy wall. Sound transmission through walls, floors or ceilings

varies with sound frequency, and the weight and stiffness of the construction. This

gives rise to the effect known as the mass law in acoustics which states that for each

doubling of the surface weight of the wall, there will be about 5 or 6 dB less transmitted

sound. The mass law also states that for each doubling of the frequency (Hz) there will

be about 5 or 6 dB less transmitted sound. Doubling of the frequency has about the

same effect as doubling the surface weight.

RESONATOR-TYPE MATERIALS

Perforated metal wall liners or tiles are examples of resonator materials. The holes in

the liner or tile act as resonate types of sound absorbers. A common resonator is the

opening of a pop bottle or jug; blowing across the opening produces a tone at its natural

frequency of resonance. When the diameter of the hole or length of cavity behind it is

changedas when a larger pop bottle is used or you fill the bottle with waterthe

frequency of resonance also changes.

When a metal perforated liner is applied, sound impinging on the holes is absorbed into

the cavities, but a portion is reradiated back toward the sound source in the form of a

hemisphere. Because the sound energy is bounced back toward the source in semi-

41

circular waves, sound is actually diffused and noise levels are reduced. The holes of

liners can be sized and aligned in such a way that sound is absorbed and diffused at

specific frequencies.

DAMPING MATERIALS

Damping materials are used to reduce structure borne noise. Structure-borne noise

is a term used to describe mechanical vibrations carried from machinery through to a

buildings structure. For example, an engine bolted onto a metal skid thats bolted to the

floor transmits huge amounts of acoustical energy through to the structure. Vibrations

from rattling machinery travel easily through solid structures like wood, steel, concrete

or masonry. With wood, concrete and bricks, vibrations are attenuated 2 dB in 30

meters (100 feet), while steel requires 20 times the distance for the same attenuation.

Damping materials create mechanical resistance to the structure-borne sound by

converting sound energy into heat through friction. An example of a damping material is

the spray-on coating compound placed under automobiles.

VIBRATION ISOLATORS

Vibration isolation is also used to reduce the transmission of noise through a

structure. Vibration isolators lower the vibration at its source. They are elastic

elements, such as coiled springs, and rubber, felt, cork or glass fiber materials, which

are as different as possible from the structure or mechanism. Vibration isolators can be

made from elastomers (compressed or shear, ribbed Neoprene); other compressed

material (cork); fibrous mats (felt and glass fiber); and metal springs. Vibration isolators

are often used in conjunction with damping materials. For example, steel springs are

undamped and placing them on elastomer pads, improves their level of vibration

isolation.

42

AATTTTEENNUUAATTIIOONN

Once the noise sources are identified and measured, the next step is to attenuate the

noise. Attenuation is defined as the difference in dB or dBA between two points in and

along the path of sound propagation. The aim of attenuation is to reduce or divert the

amount of sound energy reaching the receiver. The key to attenuation is to apply noise

control materials and measures that are both effective and economical. Noise controls

range from the simple to complex.

BUFFERS

One of the simplest attenuation methods is to place enough distance between the noise

source and the NSR so that noise is not a concern. Establishing a buffer zone is

possible when land is readily available. However, it usually takes a large amount of

land to stop noise from affecting the surrounding environment. Recalling the 6 dB rule,

it could take as much as 1,800 meters (approximately 5,900 feet) to reach 75 dB at the

NSR when the source noise is a high as 140 dB.

NATURAL BARRIERS

Shrubs, trees and berms are often used as natural noise blockers. For trees to be

effective barriers, they must be in a continuous stand, 50 feet tall, 100 feet deep, have

dense foliage down to the ground, and be evergreen. When only a line of deciduous

trees is planted, noise easily travels through the stand, particularly during the winter

when trees lose their foliage. Berms are more effective in stopping high frequency

noise. Low frequency noise, with its long wavelength, can easily slip over berms.

BARRIERS

Barriers are free-standing walls or structures intended to block source noise. The

barrier functions by absorbing a large amount of the sound energy and/or deflecting it

away from the source. Barriers reduce sound levels, but work best at reducing high

frequency noise. Barriers are most effective when they are at least three times larger

43

than the wavelength of the major noise contributor.4 For best results, barriers should

have a high transmission loss and be highly absorptive. Barriers made from a

combination of sound-absorbing and transmission loss materials give highest acoustic

performance. Concrete walls are often used as barriers. As a dense material, concrete

is a better sound insulator than sound absorber, so barriers made from concrete reflect

sound rather than absorb it.

When a barrier is wrapped around a noise source, it acts as a partial enclosure.

Partial enclosures come in a variety of configurations: two-sided, three-sided with a

roof, four-sided without a roof, and so on. Barriers and partial enclosures can be

effective and economical noise reducers, lowering noise levels by up to 12 or 15 dB.

ACOUSTICAL ENCLOSURES

If more than 12 to 15 dB of noise reduction are required, a total enclosure may be

needed to contain the noise. Typically, acoustic enclosures are modular boxes with

relatively high transmission loss and absorptive internal surfaces placed over noise

sources. The Insertion Loss (IL) is a measurement of enclosure performance, defined

as the reduction of sound pressure level at some position that occurs after the

enclosure is installed.

Insertion loss of an acoustic enclosure can be estimated as:

IL = TL + 10 log a

Where:TL = Transmission Lossa = absorption coefficient

By virtue of their design, enclosures can create heat build-up. Heat build-up is handled

by adding a ventilation blower, with silencers for intake and exhaust air. Fans and

4 Paul Jensen, Charles R. Jokel, and Laymon N. Miller, Industrial Noise Control Manual. Reprint. Cambridge,Massachusetts: Bolt Beranek and Newman, 1984: p.56.

44

internal ducting also are needed to supply cool air and remove hot air. The minimum

flow rate of cooling air, Q (in cubic meters or feet per minute), depends on W, the watts

of heat generated, and DT, the temperature rise permitted. At sea level, Q = 1.76 W/

DT.

Most enclosures need openings to provide gas, water and/or steam, electricity and

lighting. Access to the machine through doors or removable panels is also required for

maintenance and servicing. The enclosure must be air tight to reduce the amount of

interior noise radiating through ventilation openings, engine intake and exhaust ducts,

cracks under doors and at panel joints, pipe penetrations and other openings. Even a

slight opening (such as which occurs along an ill-fitting panel joint) can cause a huge

reduction in attenuation (as high as 30 dB).5

ACOUSTICAL BUILDINGS

Sometimes, acoustical equipment enclosures are not enough to reduce noise to

required levels. Standard enclosures provided by manufacturers are designed to meet

an 85 dBA limit (at one meter), but higher attenuation is sometimes needed.

Customized, highly acoustical enclosures or acoustical treatment of the building in

addition to the enclosure provide alternatives.

An acoustical building is similar to an enclosure, but on a larger scale. The building

walls and roof are termed the acoustic envelope. In the design of the envelope, mass

law applies so that thick, dense walls provide better attenuation. However, few walls or

barriers behave exactly according to the mass law; they have elasticity so that

vibrations can occur. Because of this, the envelope is usually comprised of all the

materials used to attenuate sound: acoustical materials, barrier materials, damping

materials, and vibration isolators.

5 Lewis H. Bell. Industrial Noise Control: Fundamentals and Applications. New York and Basel: Marcel Dekker,Inc., 1982.

45

The acoustic performance of a wall structure of a building is often described by an STC

(Standard Transmission Class) rating. The American Society for Testing Materials

(ASTM) has introduced the Standard Transmission Classification (STC) to allow for the

comparison of various types of acoustical walls and roofs according to their

Transmission Loss properties. The STC rating is derived from the TL value of a wall

measured at different octave band frequencies. The TL values are plotted on semi-log

paper against a reference contour produced by the ASTM, producing the STC value.

The higher the STC rating, the better a wall or roof insulates against noise. For

example, a wall of STC 50 dB has greater attenuation capability than a wall of STC 40

dB. Without the STC, comparisons are difficult because actual measurements of

Transmission Loss deviate widely even in controlled acoustic laboratories, where

resonance and other elements affect a sounds behavior.

Table 7: STC Ratings and Their Relationship to Sound Proofing Properties

STC RatingSoundproofing

Properties Speech Comparisons

25-30 Poor Normal speech understood easily anddistinctly through a wall

30-35 Fair Loud speech understood; normal speechaudible but understood with difficulty

35-40 Good Loud speech audible but not understood;normal speech inaudible

40-50 Very Good Loud speech and average radio and TV;only faintly audible

50+ Excellent Very loud noises and hi-fi faint orinaudible

The STC standard applies to frequencies from 125 to 4000 Hz. For this reason, the

standard does not sufficiently consider the importance of low frequency attenuation,

with the result that walls appearing to have adequate STC ratings often fall below what

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is required. The ASTM also cautions that its system is not intended for use with

external wall structures or barriers.

Openings can also have a significant effect on the TL of a building wall or roof. As an

example, a heavy metal plate with holes over 13% of its surface will transmit 97% of the

sound impinging on it. The PWL or Lw of a sound that will pass through an opening is

approximately determined using the equation:

Lw = Lp + 10 log A

Where:Lp = Sound Pressure Level measured at or near the openingA = the cross-sectional area of the opening in square meters

To reduce the amount of interior noise radiating through apertures, the building must be

made airtight and silencers installed where air is ventilated.

SILENCERS

Silencers or mufflers are widely used to control noise from building openings. There is

no technical distinction between a silencer or muffler, and the terms are used

interchangeably.

Silencer performance is described using the same terms that are applied to acoustic

enclosures or buildings.

1. Insertion Loss (IL) is the difference in sound pressure at the same point before andafter a silencer has been installed. Dynamic insertion loss (DIL) is the reduction inthe sound level under actual operating conditions.

2. Transmission Loss (TL) is the ratio of the sound power impinging upon the silencer(at the source side or silencer entrance) to the sound power transmitted by thesilencer (at the receiving side or the silencer exit).

3. Noise reduction (NR) is defined as the difference between the Sound PressureLevel (SPL) measured at the source side of a muffler and the Sound Pressure Level(SPL) measured at the receiving side.

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Silencers are of two basic types: 1) absorptive or 2) reactive.

Absorptive silencers contain acoustic materials and rely on the absorptive properties

of these materials to limit noise. They are used to treat noise where large volumes of air

or gas need to be moved at relatively low static pressure, such as on the intake

(suction) and exhaust (discharge) of centrifugal compressors, forced draft fans, gas

turbines, steam or process vents and similar equipment.

The simplest form of an absorptive silencer is a parallel baffle. Parallel baffles look like

a line of furnace filters, each covered by a perforated liner. The filter part is a fibrous

material (usually glass or mineral wool). The acoustical performance of baffles

increases with the thickness of the absorbing materials, the narrowness of the spacing

and longer the length. Baffles are placed parallel to the air or gas flow and are

particularly useful in applications where pressure losses need to be kept at a minimum.

Baffles are typically inserted into ducts, stacks, etc. which accommodate inlet or

discharge flows.

Figure 25: Example of Parallel Baffles

Baffles

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A parallel baffle can be made in tubular form to allow for interfacing with circular inlets

and exhausts. The tube, called an absorptive silencer, consists of straight runs of

acoustically-lined baffles inserted behind perforated metal sheets and wrapped around

in heavy gauge steel. When a silencer is placed at an inlet opening, a thicker baffle is

able to give high attenuation, particularly in the lower frequencies. For exhaust

openings, a thick baffle can actually decrease attenuation. What is done when noise

and flow move in the same directionas is the case with discharge systemsis to

narrow the space between baffles rather than increase their thickness.

Reactive silencers dont contain absorptive materials but work on the principle of

reflection and dissipation of sound waves. The reactive (reflective) silencer contains

one or more chambers and perforated tubes inside a casing, but no absorption

materials. A portion of the sound energy entering the silencer is reflected from the

chamber casing back to the sound source. Another portion is dissipated through the

perforations in the tubes. For higher acoustic performance, multiple chambers and

perforated tubes of different sizes are used. The reactive silencer is used primarily for

low frequency control from blowers and compressors.

Higher performance silencers combine both absorptive and reactive principles in their

construction. Custom-made silencer designs with multiple chambers in addition to

acoustically-lined baffles are often required to meet operational requirements. Lagging

of the silencer is also sometimes needed to improve acoustic performance.

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Figure 26: Example of an Absorptive-Reactive Silencer

Source: Jim R. Cummins and Bill Golden. Silencer Application Handbook. Stoughton, WI:Universal Silencer, 1993, p. 49.

ACOUSTIC PLENUMS

A type of chamber that operates like a reactive silencer is called a plenum. When used

for noise control, plenums are lined with porous materials. Plenums are also used to

slow down high velocity air. As a chamber, acoustic plenums can be found just about

anywhere in industry. For example, acoustic plenums are especially designed for the

inlet and exhaust ends of gas turbines. When required, an entire building can be

designed and acoustically-lined to work as an acoustic plenum.

Multiple Chambers

Absorption Material

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Figure 27: Example of an Acoustic Plenum

ACOUSTIC LOUVERS

Louvers are designed to eliminate the line-of-sight from the source to the outside. They

can also be acoustically treated to limit noise from air flowing in and out of a building.

Louvers are overlapping slats designed to admit air into a building and exclude rain.

The slats are typically lined with porous materials. Like baffles, the spacing and length

of the slats and thickness of porous material determines acoustical performance.

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Figure 28: Example of an Acoustic Louver

ACOUSTIC LAGGING

Lagging or wrapping of acoustical material is another method of noise control. Lagging

is often placed around pipes but acoustical wrapping can be applied to noisy equipment

or even silencers. Lagging typically consists of sound absorbing material (fibrous glass,

mineral wool, or polyurethane foam) with an outer layer of dense vinyl or sheet metal.

NNOOIISSEE CCOONNTTRROOLL AAPPPPLLIICCAATTIIOONNSS

ATCO ACOUSTIC ASSEMBLIES

ATCO has developed a line of Noise Management assemblies from sound-absorbing,

barrier, and resonator-type materials and that include vibration isolation and damping.

The assemblies are either whole-wall systems or acoustic panels. Whole walls are

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erected in layers at the site, starting from the inside. The Noise Management panels

are factory-manufactured and assembled in situ.

Figure 29: Example of a Noise Management Assembly

Each assembly starts with a perforated metal liner. The liner can serve two purposes. It

protects the sound absorbing materials and may act as a resonate type of sound

absorber. Liners can be selected based on the dominant noise frequency. Since

industrial noise is generally broad band with a heavy low frequency component, a liner

that resonates at the lower frequencies may be used.

In colder climates, and where building codes require it, a fire-resistant vapor barrier is

installed next to the liner to control condensation. Next, a layer of acoustic material is

applied. Multiple acoustic layers are used if the wall must achieve very high acoustic

performance. To achieve such performance, a barrier material or septum layer (or

layers) is placed between the acoustic materials. The septum layer is dense and has

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high Transmission Loss. The outermost layer of the wall structure is a protective, leak

proof facing (e.g., metal cladding, brick, etc.).

ATCOs acoustic assemblies are applied over structural steel frames rather than affixed

to concrete block walls because the assemblies can be made highly sound absorptive.

A concrete block wall is massive but it is very reflective and even when absorptive

materials are applied to the surface, sound waves passing through the materials are

reflected off the concrete blockssome, back into the room. In addition, because

ATCOs assemblies have both high absorption and transmission loss, they can be

significantly lighter than concrete to achieve the same attenuation level. When using

steel framework, damping and vibration isolators are used to reduce flanking (also

called structure borne noise).

ATCOS BALANCED APPROACH

Acoustically treating the enclosure or building envelope represents one aspect of noise

control. A balanced approach is needed to provide both effective and economical

noise reduction. In a balanced approach, all noise sources are identified, which can be

over 200 in a facility like a power plant. The Sound Power Levels of each source is

entered into an acoustic model. The model generates noise level contours from the

industrial site out to the NSR before acoustic treatment. Many contour maps use

purple and red to display high noise levels, and shades of green to represent lower

noise levels.

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Figure 30: Noise Contour Levels at a Power Plant Before Acoustic Treatment

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Figure 31: Noise Contour Levels at a Power Plant After Acoustic Treatment

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A benefit to using computer modeling is that various acoustic treatments can be applied

to a site on paper. This allows a view of the acoustic alternatives before any

commitment is made to the type (and cost) of treatment. The various acoustic

treatment scenarios include one or all of the noise control elements: acoustic envelope,

silencers, plenums, lagging, and so on. In a balanced design, the aim is to select an

acoustical approach that meets the noise requirement at an affordable price. For

example, making the walls and roof of higher attenuation, the acoustical target for the

exhaust silencer could be relaxed often a cheaper alternative.

Figure 32 depicts ATCOs balanced approach. Walls with higher STC values are used

to the north and west of the power plant, closest to the affected residences. Less

acoustic (and less expensive) walls are used to the south and east, furthest away from

the community. Silencers are placed at building openings to limit noise. Plus, the DIL

performance of the silencers is balanced with the TL performance of the buildings walls

to achieve the most cost-effective acoustic treatment.

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Figure 32: Example of ATCOs Balanced Approach

Northwest View of the Acoustical Treatment of a 110 MW Power Plant

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Southeast View of the Acoustical Treatment of a 110 MW Power Plant

TESTING AND GUARANTEES

ATCOs assemblies are tested at certified acoustical laboratories. Tests involve the

determination of the NRC (Noise Reduction Coefficient) and STC (Standard

Transmission Class). Sound pressure measurements are made at all frequencies.

Measurements within the range of 100 to 5000 Hz are conducted in an acoustical

laboratory. To test acoustic performance below the 100 Hz octave band (31.5 to 100

Hz), tests must be conducted in the field. The reason why tests below 100 Hz are not

made is due to the small size of most acoustical laboratories, which do not permit

accurate recording of long low frequency wavelengths.

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Figure 33: Sample Acoustical Test

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Because ATCO tests the Noise Management assemblies in the laboratory as well as

in the field, the company can guarantee their acoustic performance. ATCO also

guarantees that the noise target will be met using its balanced approach to the noise

problem.

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Useful Sources

Bell, Lewis.H. (1973). Fundamentals of Industrial Noise Control. Trumbull, CT: HarmonyPublications.

Bell, Lewis H. (1982). Industrial Noise Control: Fundamentals and Applications. NewYork and Basel: Marcel Dekker, Inc.

Everest, F. Alton. (1994). The Master Handbook of Acoustics. 3rd ed. New York.: TabBooks.

Jensen, Paul; Jokel, Charles R.; Miller, Laymon N. Industrial Noise Control Manual.Rev. ed. Cambridge, MA: Bolt Berank and Newman, Inc., 1984.