In Part One, we learned how to analyze an acoustical treatment in which perforated metal sheet is mounted over an air- space containing sound absorptive material, in order to make a "Tuned Resonant Absorber" .That is, by the use of a nomogram, we could determine the frequency of resonance where the sound absorption would be especially great; or we could choose the dimensions of the treatment to target a particular frequency range of interest.
Nothing was said there about how much sound absorption would be achieved at the resonance frequency nor about how broad the targeted frequency range would be. We take up these matters here.
Sound Absorption At The Resonance Frequency
In a tuned resonant sound absorber, the sound absorption reaches a maximum value, amax, at the resonance frequency, fR, falling off to lower values at higher and lower frequencies. We can control this maximum value of absorption by the choice of the sound absorptive material with which the airspace is filled. Usually, that material will be a kind of porous blanket or board, made of glass fiber or mineral fiber .
The maximum value of absorption depends only on the flow resistance of that material, and not on any of the physical dimensions of the sound absorptive treatment (such as the depth of airspace, perforation diameter, percent open area, etc.).
