Metal Foams: A Design Guide

174 Metal Foams: A Design Guide
It is the fraction of the incident energy of the sound wave which is absorbed by
the material. The upper figure shows the value of ˛ for a good absorber, glass
wool: at frequencies above 1000 Hz the absorption coefficient is essentially
1, meaning that the sound is almost completely absorbed. The central figure
shows absorption in a sample of Alporas foam in the as-received (virgin) state:
Absorption
Absorption
Absorption
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
1
0.8
0.6
0.4
0.2
0
800
b
a
400
1200
600
800
1600
1000
Frequency
Alporas compressed 10%
2000
Frequency
1200
Glass Wool
1400
2400
1800
Alporas as received
Figure 12.1 Sound absorption, measured in a plane-wave impedance tube,
for glass fiber, Alporas foam in the as-received condition, and Alporas foam
after 10% compression to rupture the cell faces
2800