30079_24.pdf

CHAPTER 24

NOISE MEASUREMENT

AND CONTROL

George M. Diehl, RE.

Consulting Engineer

Machinery Acoustics

Phillipsburg, New Jersey

24.1 SOUND CHARACTERISTICS

711

24.14 MACHINES IN

SEMIREVERBERANT

LOCATIONS

716

24.2 FREQUENCY AND

WAVELENGTH

712

24.15 TWO-SURFACE METHOD

717

24.3 VELOCITYOFSOUND

712

24.16 MACHINERYNOISE

CONTROL

719

24.4 SOUND POWER AND SOUND

PRESSURE

712

24.17 SOUNDABSORPTION

719

24.5 DECIBELS AND LEVELS

712

24.18 NOISE REDUCTION DUE TO

INCREASED ABSORPTION

IN ROOM

24.6 COMBINING DECIBELS

712

720

24.7 SOUND PRODUCED BY

SEVERAL MACHINES OF THE

SAME TYPE

24.19 SOUNDISOLATION

720

713

24.20 SINGLEPANEL

721

24.8 AVERAGINGDECIBELS

715

24.21 COMPOSITE PANEL

721

24.9 SOUND-LEVEL METER

715

24.22 ACOUSTICENCLOSURES

722

24.10 SOUND ANALYZERS

715

24.23 DOUBLEWALLS

723

24.11 CORRECTION FOR

BACKGROUND NOISE

715

24.24 VIBRATION ISOLATION

723

24.12 MEASUREMENTOF

MACHINE NOISE

24.25 VIBRATIONDAMPING

725

716

24.26 MUFFLERS

725

24.13 SMALL MACHINES IN A

FREE FIELD

716

24.27 SOUND CONTROL

RECOMMENDATIONS

727

24.1 SOUND CHARACTERISTICS

Sound is a compressional wave. The particles of the medium carrying the wave vibrate longitudinally,

or back and forth, in the direction of travel of the wave, producing alternating regions of compression

and rarefaction. In the compressed zones the particles move forward in the direction of travel, whereas

in the rarefied zones they move opposite to the direction of travel. Sound waves differ from light

Mechanical Engineers' Handbook, 2nd ed., Edited by Myer Kutz.

waves in that light consists of transverse waves, or waves that vibrate in a plane normal to the

direction of propagation.

24.2 FREQUENCY AND WAVELENGTH

Wavelength, the distance from one compressed zone to the next, is the distance the wave travels

during one cycle. Frequency is the number of complete waves transmitted per second. Wavelength

and frequency are related by the equation

v = /A

where v = velocity of sound, in meters per second

/ = frequency, in cycles per second or hertz

A = wavelength, in meters

24.3 VELOCITYOFSOUND

The velocity of sound in air depends on the temperature, and is equal to

v = 20.05 V273.2 4- C° m/sec

where C° is the temperature in degrees Celsius.

The velocity in the air may also be expressed as

v = 49.03 V459.7 + F° ft/sec

where F° is the temperature in degrees Fahrenheit.

The velocity of sound in various materials is shown in Tables 24.1, 24.2, and 24.3.

24.4 SOUND POWER AND SOUND PRESSURE

Sound power is measured in watts. It is independent of distance from the source, and independent

of the environment. Sound intensity, or watts per unit area, is dependent on distance. Total radiated

sound power may be considered to pass through a spherical surface surrounding the source. Since

the radius of the sphere increases with distance, the intensity, or watts per unit area, must also decrease

with distance from the source.

Microphones, sound-measuring instruments, and the ear of a listener respond to changing pres-

sures in a sound wave. Sound power, which cannot be measured directly, is proportional to the mean-

square sound pressure, /? 2 , and can be determined from it.

24.5 DECIBELSANDLEVELS

In acoustics, sound is expressed in decibels instead of watts. By definition, a decibel is 10 times the

logarithm, to the base 10, of a ratio of two powers, or powerlike quantities. The reference power is

1 pW, or 10- 12 W. Therefore,

(24>1 )

L W= 1010 S^

where L w = sound power level in dB

W = sound power in watts

log = logarithm to base 10

Sound pressure level is 10 times the logarithm of the pressure ratio squared, or 20 times the

logarithm of the pressure ratio. The reference sound pressure is 20 ^Pa, or 20 x 10~ 6 Pa. Therefore,

L ^ 201 °s 2OTIcF

< 24 - 2 >

where L p = sound pressure level in dB

p = root-mean-square sound pressure in Pa

log = logarithm to base 10

24.6 COMBININGDECIBELS

It is often necessary to combine sound levels from several sources. For example, it may be desired

to estimate the combined effect of adding another machine in an area where other equipment is

operating. The procedure for doing this is to combine the sounds on an energy basis, as follows:

Table 24.1 Velocity of Sound in Solids

Longitudinal Bar Velocity

Plate (Bulk) Velocity

Material

cm/sec

fps

cm/sec

fps

2.1 x 10 4

6.4 x 10 5

1.72 x 10 4

5.24 X 10 5

Aluminum

1.12 x 10 4

—

3.40 X 10 5

7.15 X 10 3

Antimony

2.18 x 10 5

5.87 X 10 3

1.79 X 10 5

1.39 X 10 4

Bismuth

4.25 X 10 5

1.12 X 10 4

3.42 X 10 5

9.12 X 10 3

Brass

2.78 X 10 5

7.87 X 10 3

2.40 X 10 5

1.72 x 10 4

Cadmium

5.24 x 10 5

1.41 x 10 4

4.30 x 10 5

1.51 x 10 4

Constantan

4.60 x 10 5

1.17 x 10 4

3.58 x 10 5

1.56 X 10 4

Copper

4.76 x 10 5

1.17 X 10 4

3.58 X 10 5

1.06 x 10 4

German silver

3.24 x 10 5

6.66 x 10 3

2.03 X 10 5

Gold

1.57 X 10 4

—

4.79 X 10 5

1.92 X 10 4

Iridium

5.85 X 10 5

1.70 X 10 4

5.17 X 10 5

7.87 X 10 3

Iron

2.40 X 10 5

4.10 X 10 3

1.25 X 10 5

Lead

1.61 X 10 4

—

4.90 X 10 5

1.53 x 10 4

Magnesium

4.66 x 10 5

1.26 x 10 4

3.83 X 10 5

1.84 x 10 4

Manganese

5.60 x 10 5

1.56 x 10 4

4.76 X 10 5

1.30 X 10 4

Nickel

3.96 X 10 5

9.19 X 10 3

2.80 X 10 5

1.18 x 10 4

Platinum

3.60 x 10 5

8.66 x 10 3

2.64 x 10 5

2.00 x 10 4

Silver

6.10 x 10 5

1.66 X 10 4

5.05 X 10 5

Steel

1.10 XlO 4

—

3.35 x 10 5

1.09 x 10 4

Tantalum

3.32 x 10 5

8.96 X 10 3

2.73 X 10 5

1.79 X 10 4

Tin

5.46 X 10 5

1.41 X 10 4

4.31 X 10 5

1.37 x 10 4

4.17 x 10 5

Tungsten

1.25 x 10 4

3.81 X 10 5

Zinc

1.64 XlO 3

—

5.00 x 10 4

Cork

1.88 X 10 4

5.72 x 10 5

Crystals

Quartz X cut

1.78 x 10 4

5.44 X 10 5

1.57 x 10 4

4.78 x 10 5

1.48 x 10 4

4.51 X 10 5

Rock salt X cut

1.23 X 10 4

3.76 x 10 5

Glass

Heavy

1.15 x 10 4

3.49 X 10 5

1.57 x 10 4

4.80 x 10 5

flint

1.49 x 10 4

4.55 X 10 5

1.73 X 10 4

5.26 x 10 5

Extra heavy

flint

1.55 x 10 4

4.71 X 10 5

1.86 x 10 4

5.66 x 10 5

Heaviest crown

1.74 x 10 4

5.30 x 10 5

1.81 X 10 4

5.57 X 10 5

Crown

1.76 X 10 4

5.37 X 10 5

Quartz

1.30 XlO 4

—

3.95 X 10 5

Granite

9.88 X 10 3

—

3.01 X 10 5

Ivory

1.25 x 10 4

—

3.81 X 10 5

Marble

1.48 X 10 4

—

4.51 X 10 5

Slate

Wood

Elm

3.31 X 10 3

—

1.01 X 10 5

1.35 X IQ 4

—

4.10 X IQ 5

Oak

L p = 10 log [10° 1L l + 10° 1L 2 + • • • + 10° 1L «]

(24.3)

where L p = total sound pressure level in dB

L 1 = sound pressure level of source No. 1

L n - sound pressure level of source No. n

log = logarithm to base 10

24.7 SOUND PRODUCED BY SEVERAL MACHINES OF THE SAME TYPE

The total sound produced by a number of machines of the same type can be determined by adding

10 log n to the sound produced by one machine alone. That is,

Table 24.2 Velocity of Sound in Liquids

Temperature

Velocity

0 F

cm/sec

fps

0 C

Material

3.97 x 10 3

1.21 x 10 5

Alcohol, ethyl

12.5

54.5

3.84 x 10 3

1.17 X 10 5

4.33 X 10 3

1.32 X 10 5

Benzene

3.81 X 10 3

1.16 x 10 5

Carbon bisulfide

3.28 x 10 3

.00 X 10 5

Chloroform

3.31 X 10 3

.01 X 10 5

Ether, ethyl

6.30 X 10 3

.92 X 10 5

Glycerine

4.76 X 10 3

.45 X 10 5

Mercury

3.35 X 10 3

.02 X 10 5

Pentane

4.36 X 10 3

.33 x 10 5

Petroleum

4.49 X 10 3

.37 X 10 5

Turpentine

3.5

38.3

4.20 x 10 3

1.28 x 10 5

80.6

4.69 x 10 3

1.43 x 10 5

Water, fresh

62.6

4.95 X 10 3

1.51 X 10 5

Water, sea

62.6

L p (n) = L p + 10 log /i

where L p (n) — sound pressure level of n machines

L p = sound pressure level of one machine

n = number of machines of the same type

In practice, the increase in sound pressure level measured at any location seldom exceeds 6 dB,

no matter how many machines are operating. This is because of the necessary spacing between

machines, and the fact that sound pressure level decreases with distance.

Table 24.3 Velocity of Sound in Gases

Temperature

Velocity

0 F

cm/sec

fps

0 C

Material

1.09 X 10 3

3.31 X 10 4

Air

1.13 x 10 3

3.43 X 10 4

1.48 X 10 3

4.15 X 10 4

Ammonia gas

8.50 x 10 2

2.59 x 10 4

Carbon dioxide

1.09 X 10 3

3.33 X 10 4

Carbon monoxide

6.76 X 10 2

2.06 X 10 4

Chlorine

1.01 x 10 3

3.08 X 10 4

Ethane

1.04 X 10 3

3.17 X 10 4

Ethylene

4.20 X 10 3

1.28 X 10 5

Hydrogen

9.71 X 10 2

2.96 X 10 4

Hydrogen chloride

9.48 x 10 2

2.89 x 10 4

Hydrogen sulfide

1.41 X 10 3

4.30 X 10 4

Methane

1.06 X 10 3

3.24 X 10 4

Nitric oxide

1.10 X 10 3

3.34 X 10 4

Nitrogen

1.15 X 10 3

3.51 X 10 4

8.53 X 10 2

2.60 X 10 4

Nitrous oxide

1.04 X 10 3

3.16 X 10 4

Oxygen

1.08 X 10 3

3.28 X 10 4

6.99 x 10 2

2.13 x 10 4

Sulfur dioxide

3.3IxIO 2

1.01 X 10 4

Water vapor

3.45 X IQ 2

1.05 X IQ 4

100

212

24.8 AVERAGINGDECIBELS

There are many occasions when the average of a number of decibel readings must be calculated.

One example is when sound power level is to be determined from a number of sound pressure level

readings. In such cases the average may be calculated as follows:

Ll = 10 log I - [10 01L l + 10° 1L 2 + • • • + 10°- 1L «] 1

(24.4)

where L p = average sound pressure level in dB

L 1 = sound pressure level at location No. 1

L n = sound pressure level at location No. n

n = number of locations

log = logarithm to base 10

The calculation may be simplified if the difference between maximum and minimum sound pres-

sure levels is small. In such cases arithmetic averaging may be used instead of logarithmic averaging,

as follows:

If the difference between the maximum and minimum of the measured sound pressure levels is

5 dB or less, average the levels arithmetically.

If the difference between maximum and minimum sound pressure levels is between 5 and 10 dB,

average the levels arithmetically and add 1 dB.

The results will usually be correct within 1 dB when compared to the average calculated by Eq.

(24.4).

24.9 SOUND-LEVEL METER

The basic instrument in all sound measurements is the sound-level meter. It consists of a microphone,

a calibrated attenuator, an indicating meter, and weighting networks. The meter reading is in terms

of root-mean-square sound pressure level.

The A-weighting network is the one most often used. Its response characteristics approximate the

response of the human ear, which is not as sensitive to low-frequency sounds as it is to high-frequency

sounds. A-weighted measurements can be used for estimating annoyance caused by noise and for

estimating the risk of noise-induced hearing damage. Sound levels read with the A-network are

referred to as dBA.

24.10 SOUNDANALYZERS

The octave-band analyzer is the most common analyzer for industrial noise measurements. It separates

complex sounds into frequency bands one octave in width, and measures the level in each of the

bands.

An octave is the interval between two sounds having a frequency ratio of two. That is, the upper

cutoff frequency is twice the lower cutoff frequency. The particular octaves read by the analyzer are

identified by the center frequency of the octave. The center frequency of each octave is its geometric

mean, or the square root of the product of the lower and upper cutoff frequencies. That is,

/O = VJTF;

where / 0 = the center frequency, in Hz

/! = the lower cutoff frequency, in Hz

/ 2 = the upper cutoff frequency, in Hz

/! and / 2 can be determined from the center frequency. Since / 2 = 2/, it can be shown that f 1 =

/ 0 /V2 and / 2 = V2 / 0 .

Third-octave band analyzers divide the sound into frequency bands one-third octave in width. The

upper cutoff frequency is equal to 2 1/3 , or 1.26, times the lower cutoff frequency.

When unknown frequency components must be identified for noise control purposes, narrow-band

analyzers must be used. They are available with various bandwidths.

24.11 CORRECTION FOR BACKGROUND NOISE

The effect of ambient or background noise should be considered when measuring machine noise.

Ambient noise should preferably be at least 10 dB below the machine noise. When the difference is

less than 10 dB, adjustments should be made to the measured levels as shown in Table 24.4.

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