Figure 16. Section through a tuned resonant sound absorber.
All resonant devices have a preferred frequency of operation. For example, a ball suspended on a rubber band oscillates at only one frequency, when disturbed: that frequency is determined only by the mass of the ball and the springiness of the rubber band. In a resonant sound absorber, the oscillation involves the motion of air particles, in and out of the holes in the metal sheet, in response to an incident sound wave. The preferred frequency of this oscillation is determined by the mass of the air in the per forations and the springiness of the trapped air layer .
At that resonance frequency, the air moves violently in and out of the holes, which pumps the air particles back and forth vigorously within the adjacent sound absorptive layer. There, the acoustic energy (carried by the back-and-forth motion of the air particles) is converted by friction into heat and is thereby removed from the acoustical scene.
The practical advantage of the tuned resonant sound absorber is this: we have seen (page 11) that it requires a six-inch layer of sound absorptive blanket if we wish to attenuate sound effectively at low frequencies. Yet, as we have noted above, the treatment of a power transformer requires maximum absorption around 120 Hz. The one-inch layer of glass fiber (shown in the earlier figure on page 11) is only about 5% efficient at that frequency. But the use of perforated metal to make a resonant sound absorber especially tuned to 120 Hz can achieve efficient sound absorption at that frequency without requiring so much space and with only a thin layer of absorptive material.
The first clue, to help us decide whether the resonant absorber will be the best approach, is found by listening to the noise. If there is a clearly perceptible pure tone or a prominent frequency (a squeal, hum or whine, as opposed to a whoosh or roar, like a waterfall), this is a good indication that the disturbing noise is concentrated in a limited frequency range, and a tuned resonant sound absorber is called for.
The problem now is to pinpoint that frequency fR where the maximum sound absorption is desired.
Here one can sometimes rely on the manufacturer's information about the noisy device in question.
Alternatively, one would make a frequency analysis of the noise, using a Sound Level Meter with a set of frequency filters, as described above (page 6).
Figure 16. Section through a tuned resonant sound absorber.