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:
