Metal Foams: A Design Guide

Sound absorption and vibration suppression 173
Table 12.2 Sound-absorption coefficient at indicated frequency
Material 500 Hz 1000 Hz 2000 Hz 4000 Hz
Glazed tiles 0.01 0.01 0.02 0.02
Concrete with roughened surface 0.02 0.03 0.04 0.04
Timber floor on timber joists 0.15 0.10 0.10 0.08
Cork tiles on solid backing 0.20 0.55 0.60 0.55
Draped curtains over solid backing 0.40 0.50 0.60 0.50
Thick carpet on felt underlay
Expanded polystyrene, 25 mm
(1 in.) thick, spaced 50 mm
0.30 0.60 0.75 0.80
(2 in.) from solid backing
Acoustic spray plaster, 12 mm
0.55 0.20 0.10 0.15
( 1
2
in.) thick, on solid backing 0.50 0.80 0.85 0.60
Metal tiles with 25% perforations, with
porous absorbent material laid on top 0.80 0.80 0.90 0.80
Glass wool, 50 mm, on rigid backing 0.50 0.90 0.98 0.99
12.2 Sound absorption in metal foams
Absorption is measured using a plane-wave impedance tube. When a plane
sound wave impinges normally on an acoustic absorber, some energy is
absorbed and some is reflected. If the pressure pi in the incident wave is
described by
pi D A cos ⊲2 ft⊳ ⊲12.2⊳
and that in the reflected wave (pr) by
� �
pr D B cos 2 f t
��
2x
c
⊲12.3⊳
then the total sound pressure in the tube (which can be measured with a
microphone) is given by the sum of the two. Here f is the frequency (Hz), t
is time (s), x is the distance from the sample surface (m), c is the velocity of
sound (m/s) and A and B are amplitudes.
The absorption coefficient ˛ is defined as
˛ D 1
� �2 B
A
⊲12.4⊳