4.11 SOUND TRANSMISSION CLASS

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In the previous sections, we considered some relatively simple panel constructions

and presented techniques for estimation of the transmission loss

134 Chapter 4

Copyright © 2003 Marcel Dekker, Inc.

curve. In practice, partitions separating two spaces are often much more

complicated in construction than those previously discussed. In these cases,

the estimation of the transmission loss curve by analytical or numerical

techniques may not be practical. For this reason, the transmission loss

curve must be measured experimentally. The standard test technique is

described by the American Society of Testing Materials standard (ASTM,

1983).

It is often convenient to have available a single-figure rating that can

be used to compare the performance of partitions in reducing noise transmission.

Prior to 1970, the arithmetic average of the transmission loss at

nine test frequencies was used to rate partitions (Faulkner, 1976). This

technique has the weakness inherent in all ‘‘averaging’’ systems, i.e., two

specimens may have the same average transmission loss but quite different

frequency curves. The use of average transmission loss values does not

present an overall picture of the ability of the material to reduce noise

transmission over the entire range of frequencies of interest to the acoustic

designer.

The sound transmission class (STC) rating was developed to provide a

single-number rating of partitions, and yet provide additional information

about the frequency spectrum of the transmission loss (ASTM, 1984). The

STC rating generally correlates the impressions of the sound insulation

characteristics of walls for transmission of such sounds as speech, radio,

television, and other broadband noise sources in buildings. The STC rating

is defined as the value of the transmission loss at 500 Hz which approximates

a standard TL curve, measured in sixteen 1/3 octave band intervals from

125 Hz through 4000 Hz. The standard curve has three portions: (a) from

125 Hz to 400 Hz, in which the curve increases 3 dB for each 1/3 octave

increase; (b) from 400 Hz to 1250 Hz, in which the curve increases 1 dB

for each 1/3 octave increase; and (c) from 1250 Hz to 4000 Hz, in which

the curve is constant at a value 4 dB higher than the value at 500 Hz.

The STC rating is determined by comparison of the experimental

measurements of the transmission loss with the standard TL curve, subject

to two conditions:

1. No single value of the experimental TL may be more than 8 dB

below (less than) the standard curve.

2. The sum of the deviations below the standard curve cannot

exceed 32 dB. The STC rating is generally specified to 1 dB significant

figures.

A graphical technique may be used to determine the STC rating;

however, the following procedure is much more adaptable to computer

utilization.

Transmission of Sound 135

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Step 1. Measure the TL value for the sixteen 1/3 octave bands having

center frequencies from 125 Hz through 4000 Hz.

Step 2. Calculate the corresponding difference _ between the STC-50

standard curve S50 and the TL at each 1/3 octave band center frequency,

where:

_ ј S50 _ TL (4-193)

The standard STC-50 curve is given in Table 4-4.

Step 3. Calculate the first estimate for the STC rating (STC1) from the

following expression:

STC1 ј 50 _ р_Юmax ю 8 ј 58 _ р_Юmax (4-194)

The quantity р_Юmax is the largest (algebraically) value of the differences _.

The calculation in Step 3 meets the first requirement of the STC rating; i.e.,

none of the experimental points will lie more than 8 dB below the standard

curve.

Step 4. Determine the deficiencies at each experimental point, where

the deficiency Def is defined by the following:

Def ј S50 _ р50 _ STC1Ю _ TL ј _ _ р50 _ STC1Ю (4-195)

The deficiency is the difference between the standard STC curve for STC1

and the experimental data points for the transmission loss. A positive value

of the deficiency corresponds to a point below the standard curve.

Step 5. Add all of the positive values of the deficiencies, which are the

values that lie below the first estimate (STC1) curve.

Step 6. (A) if the sum of the positive values of the deficiencies is 32 dB

or less, the STC is equal to the first estimate value, STC1:

If _рюDefЮ _ 32 dB; then STC ј STC1

136 Chapter 4

TABLE 4-4 Values for the Standard STC-50 Curve.

1/3 Octave band

center frequency, Hz S50, dB

1/3 Octave band

center frequency, Hz S50, dB

125 34 800 52

160 37 1,000 53

200 40 1,250 54

250 43 1,600 54

315 46 2,000 54

400 49 2,500 54

500 50 3,150 54

630 51 4,000 54

Copyright © 2003 Marcel Dekker, Inc.

This calculation meets the second criterion for the STC rating; i.e., the sum

of the deficiencies (deviations from the STC curve) must not be greater than

32dB.

(B) On the other hand, if the sum of the positive values of the deficiencies

is greater than 32dB, the first estimate for the STC must be adjusted

to meet the STC criterion.

If _рюDefЮ > 32dB

then calculate the adjustment:

Adj ј

_рюDefЮ_32

NPD

The quantity NPD is the number of positive values of the deficiencies. The

adjustment Adj is rounded up to the next whole integer. The sound transmission

class is found by applying the adjustment to the initial estimate for

the STC:

STC ј STC1 _Adj

The application of this technique is illustrated in the following

example.

Example 4-11. The measured values of the transmission loss for a partition

are given in Table 4-5. Determine the STC rating for the partition.

The calculations are summarized in Table 4-5, where the second column

contains the experimental values for the TL, the third column contains

the standard curve for STC-50, and the fourth column contains the values

for the differences _, calculated from Eq. (4-193). For example, for the

125 Hz 1/3 octave band,

_ ј S50 _ TL ј 34 _ 22 ј 12 dB

The largest value of the differences is 31 dB, which occurs for the 500 Hz 1/3

octave band.

The first estimate for the STC rating is found from Eq. (4-194):

STC1 ј 58 _ р_Юmax ј 58 _ 31 ј 27

The values in the fifth column (Def) are calculated from Eq. (4-195). For

example, for the 125 Hz 1/3 octave band,

Def ј _ _ р50 _ STC1Ю ј _ _ р50 _ 27Ю ј _ _ 23

Def ј 12 _ 23 ј _11

In this example, there are NPD ј 10 positive values of the deficiencies.

The sum of these 10 positive values of Def is:

Transmission of Sound 137

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_рюDefЮ ј 5 ю 5 ю 8 ю 6 ю 5 ю 7 ю 2 ю 3 ю 3 ю 1 ј 45 > 32

An adjustment is required:

Adj ј

45 _ 32

10 ј 1:3

The actual value of the adjustment is found by rounding up to the nearest

integer, or

Adj ј 2dB

The STC rating for the partition is, as follows:

STC ј STC1 _ Adj ј 27 _ 2 ј 25 dB

The sound transmission class rating may be written as STC-25.

If we want to generate the STC-25 standard curve, we may displace the

STC-50 standard curve such that the value at the 500 Hz point is equal to

25 dB. In general,

SSTC ј S50 _ STC (4-196)

For the STC-25 standard curve, STC ј S50 _ 25. These values are shown in

the last column in Table 4-5.

138 Chapter 4

TABLE 4-5 Solution for Example 4-11.

1/3 Octave band

center frequency, Hz

TL (expt.),

dB

S50,

dB

_,

dB

Def,

dB

S25,

dB

125 22 34 12 _11 9

160 21 37 16 _7 12

200 21 40 19 _4 15

250 22 43 21 _2 18

315 18 46 28 ю5 21

400 21 49 28 ю5 24

500 19 50 31 ј _max

ю8 25

630 22 51 29 ю6 26

800 24 52 28 ю5 27

1,000 23 53 30 ю7 28

1,250 29 54 25 ю2 29

1,600 32 54 22 _1 30

2,000 31 54 23 0 30

2,500 28 54 26 ю3 30

3,150 28 54 26 ю3 30

4,000 30 54 24 ю1 30

Copyright © 2003 Marcel Dekker, Inc.

The sound transmission class rating may be used as a design criterion

for partitions within or between dwellings, between areas in an office building,

or within schools, theaters, etc. Detailed criteria are given by the U.S.

Dept. of Housing and Urban Development (HUD) (Berendt et al., 1967).

The HUD recommendations are classified according to the environment in

which the dwelling is located:

. Grade I: surburban and outer urban residential areas. These are

considered ‘‘quiet’’ areas, as far as background noise is concerned.

The A-weighted sound levels during nighttime would be in the

range of 35dBA to 40dBA or lower.

. Grade II: residential urban and surburban areas. These are considered

‘‘average’’ areas, as far as the background noise level is

concerned. The nighttime levels are generally around 40 dBA to

45dBA. This is probably the most commonly used category for

design.

. Grade III: urban areas. These are considered ‘‘noisy’’ areas, and

this category is considered as the minimumrecommended category.

The sound levels during nighttime are generally 55dBA or higher.

Selected values of the STC criteria are given in Table 4-6.

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