Page 266 - Master Handbook of Acoustics

Carpet and its underlay can provide significant absorption at mid and high frequencies. Suppose that
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carpet is placed in a recording studio with a floor area of 1,000 ft . Further, suppose that
reverberation time is specified to be about 0.5 second, which requires 1,060 sabins of absorption in

this room. At higher audio frequencies, a heavy carpet and pad with an absorption coefficient of
around 0.6 gives 600 sabins of absorption at 4 kHz. This is about 57% of the required absorption for
the entire room before the absorption of walls and ceiling are even considered. The acoustical design
is quite limited before it is started.
There is another, more serious problem. This high absorbance of carpet is only at higher audio
frequencies. Carpet having an absorption coefficient of 0.60 at 4 kHz may offer only 0.05 at 125 Hz.

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In other words, the 1,000 ft of carpet introduces 600 sabins at 4 kHz but only 50 sabins at 125 Hz.
This is a major problem encountered in many acoustical treatments. The unbalanced absorption of
carpet can be compensated for in other ways, principally with resonant-type, low-frequency
absorbers.

To compound the problem of unbalanced absorption of carpet, dependable absorption coefficients
are hard to come by. An assortment of types of carpet and variables in underlay add to the uncertainty.
Experience and judgment must be used when deciding which absorption coefficients should be
employed, particularly in wall-to-wall carpeting.

Effect of Carpet Type on Absorbance

There are considerable variations in sound absorption between types of carpet. Figure 12-19 shows

the difference between a heavy Wilton carpet and a velvet carpet with and without a latex backing.
The latex backing seems to increase absorption materially above 500 Hz and to decrease it a modest
amount below 500 Hz.