Page 255 - Master Handbook of Acoustics

FIGURE 12-9 The average absorption characteristics of eight acoustical tile brands of 3/4-in

thickness. The vertical lines show the spread of the data.

Effect of Thickness of Absorbent

It is logical to expect greater sound absorption from thicker porous materials, but this logic holds
primarily for lower frequencies. Absorption is greatest when the porous material is placed at a

distance of a quarter wavelength (λ/4) from a hard reflective surface (or odd multiples of this
dimension); this is the point where particle velocity is greatest; in practice, this can be difficult to do.
Figure 12-10 shows the effect of varying absorbent thickness where the absorbent is mounted directly
on a solid surface (mounting type A). There is little difference above 500 Hz in increasing the
absorbent thickness from 2 to 4 in, but there is considerable improvement below 500 Hz as thickness
is increased. There is also a proportionally greater gain in overall absorption in a 1-in increase of

thickness in going from 1 to 2 in than going from 2 to 3 in or 3 to 4 in. A 4-in thickness of glass-fiber
3
material of 3-lb/ft density has essentially perfect absorption over the 125-Hz to 4-kHz region.