Handbook for Sound Engineers

Acoustical Treatment for Indoor Areas 101

5.2.1.4 Absorption Ratings

There are three single number ratings associated with

absorption, all of which are calculated using the Sabine

absorption coefficients. The first and most common is the

Noise Reduction Coefficient (NRC). The NRC is the

arithmetic average of the 250, 500, 1000, and 2000 Hz

octave-band Sabine absorption coefficients, rounded to

the nearest 0.05.^5 The NRC was originally intended to be

a single number rating that gave some indication of the

performance of a material in the frequency bands most

critical to speech.

To partly address some of the limitations of the NRC,

the Sound Absorption Average (SAA) was developed.^5

Similar to NRC, the SAA is an arithmetic average, but

instead of being limited to four octave bands, the Sabine

absorption coefficients of the twelve octave bands

from 200 through 2500 Hz are averaged and rounded to

the nearest 0.01. Table 5-1 provides an example calcula-

tion of both NRC and SAA for a set of absorption coeffi-

cients.

Finally, ISO 11654 provides a single number rating
for materials tested in accordance with ISO 354 called
the weighted sound absorption coefficient (Dw).^12 A
curve matching process is involved to derive the Dw of a
material. Additionally, shape indicators can be included
in parentheses following the Dw value to indicate areas
where absorption has significantly exceeded the refer-
ence curve. Table 5-1 shows the Dw for the set of absorp-
tion coefficients, with the LM indicating that there may
be excess low and mid-frequency absorption offered that
is not otherwise apparent from the Dw value. This is

useful in that it indicates the actual octave-band or

octave band absorption coefficients are probably

worth looking into in greater detail.

None of the metrics described above gives an accu-

rate representation of the absorptive behavior (or lack

thereof) of a material. NRC averages four bands in the

speech frequency range. The problem, of course, is that

many different combinations of four numbers can result

in the same average, as shown in Table 5-2. The same

can be said for SAA. Nonetheless, NRC and SAA can be

compared to give a little bit more information than each

rating gives on an individual basis. If NRC and SAA are

very close, the material probably does not have any

extreme deviations in absorption across the speech range

of frequencies. If SAA is drastically different from NRC,

it may be indicative of some large variations at certain

octave bands. These are, of course, only single

number ratings; none of them takes into account the

performance of the material below the 200 Hz octave

band. They can, at most, provide a cursory indication of

the relative performance of a material. A full evaluation

of the performance of a material should always involve

looking at the octave or octave band data in as much

detail as possible.

Table 5-1. Sample Sabine Absorption Coefficient

(DSAB) Spectrum with Corresponding Single Number

Ratings, NRC, SAA, and Dw.

(^1) / 3 Octave Band
Center Frequency
DSAB^1 / 3 Octave Band
Center Frequency
DSAB
100 Hz 0.54 1250 Hz 0.39
125 Hz 1.38 1600 Hz 0.31
160 Hz 1.18 2000 Hz 0.30
200 Hz 0.88 2500 Hz 0.23
250 Hz 0.80 3150 Hz 0.22
315 Hz 0.69 4000 Hz 0.22
400 Hz 0.73 5000 Hz 0.20
500 Hz 0.56
630 Hz 0.56 NRC = 0.55
800 Hz 0.51 SAA = 0.53
1000 Hz 0.47 Dw = 0.30 (LM)
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Table 5-2. Two Different Sample Sabine Absorption
Coefficient (DSAB) Spectra with Equal NRC and SAA.
DSAB
(^1) / 3 Octave Band
Center Frequency
Material 1 Material 2
100 Hz 0.54 0.01
125 Hz 1.38 0.01
160 Hz 1.18 0.09
200 Hz 0.88 0.18
250 Hz 0.80 0.33
315 Hz 0.69 0.39
400 Hz 0.73 0.42
500 Hz 0.56 0.57
630 Hz 0.56 0.58
800 Hz 0.51 0.67
1000 Hz 0.47 0.73
1250 Hz 0.39 0.69
1600 Hz 0.31 0.60
2000 Hz 0.30 0.58
2500 Hz 0.23 0.65
3150 Hz 0.22 0.67
4000 Hz 0.22 0.80
5000 Hz 0.20 0.77
NRC = 0.55 0.55
SAA = 0.53 0.53
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