Page 225 - The Master Handbook Of Acoustics

200 CHAPTER NINE

Effect of Carpet Underlay on Absorbance
Hair felt was formerly used almost exclusively as the padding under the
carpet. It is interesting that in the early days of this century hair felt was
used for general acoustical treatment until it was displaced by numer-
ous proprietary materials. Today foam rubber, sponge rubber, felts,
polyurethane, or combinations have replaced hair felt. Foam rubber is
made by whipping a latex water dispersion, adding a gelling agent, and
pouring into molds. The result is always open-celled. Sponge rubber, on
the other hand, formed by chemically generated gas bubbles, can yield
either open or closed cells. Open cells provide the interstices required
for good sound absorption while closed cells do not.
The influence of underlay on carpet absorption is very great. Figure
9-18 shows chamber measurements of absorption coefficients for a sin-
gle Axminster type of carpet with different underlay conditions.
Graphs A and C show the effect of hair felt of 80 and 40 oz/sq yd
weight. Graph B shows an intermediate combination of hair felt and
foam. While these three graphs differ considerably, they all stand in
stark contrast to graph D for the carpet laid directly on bare concrete.
Conclusion: the padding underneath the carpet contributes markedly
to overall carpet absorption.

Carpet Absorption Coefficients
The absorption coefficients plotted in Figs. 9-17 and 9-18 are taken
5
from Harris’s 1957 paper, perhaps the most exhaustive study of carpet
characteristics available. In general, these coefficients are higher than
those in currently published tables. In Fig. 9-19 the coefficients listed
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in a widely used publication and included in the appendix are plot-
ted for comparison with Figs. 9-17 and 9-18. Carpets vary widely,
which can account for some of the great variability with which any
designer of acoustical systems is confronted.

Sound Absorption by People

People absorb sound too. Just how much of this is due to absorption by
flesh and how much by hair and clothing has yet to be reported. The
important thing is that the people making up an audience account for
a significant part of the sound absorption of the room. It also makes an