Matching the Sound Absorption II. Matching the Sound Absorption to the Frequencies Where the Noise Problem Lies
Figure 2. In refurbishing the Rotunda at the Rotunda at the University of Virginia, there was a conflict between the architect's wish to preserve the original appearance of Thomas Jefferson's handsome plaster dome and the need for acoustical treatment to quiet the room. The original plaster was replaced with curved, finely perforated sheet metal behind which sound-absorbing blankets were hidden, with a resulting appearance indistinguishable from that of plaster.
Aft View of Fan-exhaust Duct / Front View of Inlet Duct
Figure 3. Inlet and exhaust ducts of jet engine, lined with sound absorptive treatment that is faced with perforated metal.
Before beginning to design noise control measures using perforated materials, you must decide what kind of noise problem you have.
As suggested under applications 1 and 2, above, perforated metals can be used in two completely different ways in acoustical applications.
In the first application, we want the sheet to be as transparent as possible to sound of all frequencies. This would be the choice if we want to absorb noise that contains energy in a broad range of frequencies, or if we want the sound of an orchestra in a concert hall to pass freely through a false, decorative, perforated surface in order to reach specially designed acoustical treatment behind the sheet.
If on the other hand, we wish to absorb sound in a relatively narrow band of frequencies, we use the perforated sheet as an integral part of a tuned Resonant Sound Absorber. A common application for this kind of treatment is in the inlet of a jet engine.
The design procedures for these two applications are quite different. They are described in Sections III and IV.
Figure 2. In refurbishing the Rotunda at the Rotunda at the University of Virginia, there was a conflict between the architect's wish to preserve the original appearance of Thomas Jefferson's handsome plaster dome and the need for acoustical treatment to quiet the room. The original plaster was replaced with curved, finely perforated sheet metal behind which sound-absorbing blankets were hidden, with a resulting appearance indistinguishable from that of plaster.