Perforated Metal Sheet With Properties Chosen to Target a Limited Range of Frequencies for Optimum Sound Absorption
In the transparency application discussed above, the function of the perforated metal was to act as a protective covering for something else: it must get out of the way and let some other material do its acoustic job.
Now we consider an application where the perforated metal takes an active part in determining the acoustical properties of the treatment. In many noise control applications, the problem is to reduce noise that occurs only in a limited range of frequencies.
For example, an enclosure around a power transformer must be especially effective at a frequency of 120 Hz (which is the most prominent noise component of the 60-cycle line frequency).
Or, the absorptive lining for the compressor inlet or the exhaust in a jet engine should be most efficient in absorbing sound at the blade passage frequency of the rotor, about 2000 Hz.
One of the great advantages of perforated metal is that it can be used as an element in a "tuned resonant sound absorber" to provide remarkably high sound absorption in the targeted frequency range without requiring a large amount of spacer absorptive material. Naturally, it sacrifices high absorption efficiency at frequencies outside this range. In this application, the perforated metal is used in combination with a trapped layer of air, in order to modify the acoustical performance of the absorptive material. This is done by setting up an acoustical resonance condition, which concentrates the sound absorption into a particular frequency range of special interest. It works as follows:
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).
