Perforated Metal Sheet With High Transparency, for Use in Broad-band Sound Absorptive Treatments
In this application, perforated metal sheet is used as a sound-transparent protective covering or sound Absorptive als that actually do the work of absorbing the sound.
In this case, because the perforated metal is chosen to be completely transparent to sound, it does not alter the intrinsic performance of the absorptive material in any way. The following figure shows typical sound absorption efficiency for glass fiber materials at different frequencies.
Figure 12. Sound absorption coefficients vs frequency, for glass fiber materials of different thickness.
A layer only one inch thick is quite effective at high frequencies but very poor at low frequencies. It would be a suitable match for quieting noises having frequency spectrums like that of the electric drill that are rich in high frequencies. (The match does not have to be perfect; it is sufficient to follow general trends).
On the other hand, a six inch layer is extremely efficient at all frequencies (about 99% of the incident noise energy is absorbed). The problem is that it takes up a lot of space and is expensive.
