As a practical matter, Table 4 presents, for a number of currently manufactured Owens-Corning Fiberglas products, the value of S (cgs rayls/in.), the value of R (cgs rayls for a 1/2" layer) and the value of amax at resonance for a tuned resonator filled with such a layer:
Sound absorptive treatment, covered with decorative perforated and drawn metal sheet, provides calm acoustical environment in the elegant dining room of the Scandinavia Hotel in Oslo.
We come now to a complication that we have already encountered (without an explanation) near the end of Part One: namely, it makes a difference where, within the air cavity, the sound absorptive material is placed (See Figures 18 and 19).
We realize that in order for the absorptive layer to work well, turning the sound energy into heat by the friction of the vibrating air particles within the fine pores of the material, there must be freedom for the air particles to move. If anything impedes this motion, then the energy conversion is less efficient and less sound energy is absorbed. And that is just what happens at locations near a hard wall: the wall itself, being rigid, cannot move with the sound wave, and this means that the nearby air particles also cannot move. Thus, any sound absorptive material placed against a hard wall is virtually useless, because there can be no air motion within the material to dissipate the sound energy. Nevertheless, it is common practice to mount sound absorptive layers directly against a wall, because it is very convenient to do so. We must, however, realize that, in such cases, only the outer one-third of the thickness of the layer is effective in absorbing sound. The rest of the material is simply acting as a convenient support!
Therefore, the values of R/pc for the 1/2" layer of material given in Table 4, and the corresponding values of amax, assume that this 1/2" layer is mounted near the perforated metal screen with, say, an inch of empty airspace behind it. so that the entite 1/2" layer is effective.
If the layer were mounted directly against a hard wall, the tabulated values of R/pc would have to be multiplied by 1/3, and the corresponding values of maximum absorption recalculated.
Earlier in this booklet (in Table 1, p.37), we considered the acoustical performance of commonly produced perforated metal products in terms of the Transparency Index, and the corresponding Sound Attenuation and Access Factor at 10,000 Hz. Some were pretty good, some pretty bad.
We now consider four of these same materials in terms of the resonance frequencies that they would produce if mounted in front of a one-inch airspace (the item numbers here are the same as in Table 1):
~ 1 4 5 6
fR (Hz) 5000 3800 3000 3000
So here's an odd situation! Using common perforated sheet, these "resonant absorbers" all resonate at such high frequencies that the resonance phenomenon adds nothing extra to the natural sound absorption of, say, a 1/2" layer of glass fiber with no covering at all! Moreover, no reasonable depth of airspace behind these sheets would decrease the resonance frequency below 1000 Hz; for example, samples #5 and #6 would require a 7-inch airspace to make fR = 1000 Hz.
Tuned resonant sound absorbers evidently require some what out-of-the-way perforation patterns, as we saw in Examples 5, 6 and 7, pages 26-28.
