Introduction to Acoustics

Table 11.1 Absorption coefficients and NRC values for common materials
Building Acoustics 11.1 Room Acoustics 391
Material 125 Hz 250 Hz 500 Hz 1000 Hz 2000 Hz 4000 Hz NRC
Painted drywall 0.10 0.08 0.05 0.03 0.03 0.03 0.05
Plaster 0.02 0.03 0.04 0.05 0.04 0.03 0.05
Smooth concrete 0.10 0.05 0.06 0.07 0.09 0.08 0.05
Coarse concrete 0.36 0.44 0.31 0.29 0.39 0.25 0.35
Smooth brick 0.03 0.03 0.03 0.04 0.05 0.07 0.05
Glass 0.05 0.03 0.02 0.02 0.03 0.02 0.05
Plywood 0.58 0.22 0.07 0.04 0.03 0.07 0.10
Metal blinds 0.06 0.05 0.07 0.15 0.13 0.17 0.10
Thick panel 0.25 0.47 0.71 0.79 0.81 0.78 0.70
Light drapery 0.03 0.04 0.11 0.17 0.24 0.35 0.15
Heavy drapery 0.14 0.35 0.55 0.72 0.70 0.65 0.60
Helmholtz resonator 0.20 0.95 0.85 0.49 0.53 0.50 0.70
Ceramic tile 0.01 0.01 0.01 0.01 0.02 0.02 0.00
Linoleum 0.02 0.03 0.03 0.03 0.03 0.02 0.05
Carpet 0.05 0.05 0.10 0.20 0.30 0.40 0.15
Carpet on concrete 0.05 0.10 0.15 0.30 0.50 0.55 0.25
Carpet on rubber 0.05 0.15 0.13 0.40 0.50 0.60 0.30
Upholstered seats 0.19 0.37 0.56 0.67 0.61 0.59 0.55
Occupied seats 0.39 0.57 0.80 0.94 0.92 0.87 0.80
Water surface 0.01 0.01 0.01 0.01 0.02 0.03 0.00
Soil 0.15 0.25 0.40 0.55 0.60 0.60 0.45
Grass 0.11 0.26 0.60 0.69 0.92 0.99 0.60
Cellulose spray (1 ′′ ) 0.08 0.29 0.75 0.98 0.93 0.76 0.75
to be reflective while those with NRC values greater than
0.40 are considered to be absorptive. When significant
sound energy must be absorbed, as may be the case for
eliminating echoes or standing waves, materials having
higher absorption coefficients are usually recommended.
A few cautionary notes apply. NRC values are convenient
to use for rating the absorption characteristics of
a material. However, they should only be used when the
sound sources of interest are within the 200–2000 Hz
range. For sources outside of this range, and especially
below this range, materials effective for the specific frequency
of interest must be used. Also note that some
manufacturers specify NRC and absorption coefficient
values that are greater than 1.0. Methods used to measure
absorption coefficients can artificially raise their values
above 1.0; yet such values inaccurately imply that more
energy is absorbed by a material than is incident upon
it, which is a physical impossibility. Therefore, any published
absorption coefficient or NRC values greater than
1.0 should be not be considered as greater than 1.0.
Another absorption metric gaining increasing application
is the sound absorption average (SAA). The SAA
is a single number rating that is the average, rounded off
to the nearest 0.01, of the sound absorption coefficients
of a material for the twelve one-third octave bands from
200 through 2500 Hz. Although the SAA replaces the
NRC rating, as directed by the ASTM C 423 since the
year 2000, most product literature still uses NRC values.
The method by which these materials are mounted
affects their sound absorption effectiveness, especially
for low frequencies (below 500 Hz). Figure 11.7 shows
Absorption coefficient α
1.0
0.8
0.6
0.4
Typical absorptive material
against reflective surface
0.2
Same material with airspace
behind
0
125 250 500 1k 2k 4k
Octave band center frequency (Hz)
Fig. 11.7 The general effect of an air space between absorptive
material and its mounting surface
Part C 11.1