Sound Attenuation At High Frequencies
The following figure presents laboratory measured data that indicate how high-frequency sounds are attenuated, in passing through samples of perforated metal having different values for the Transparency Index (TI). The horizontal scale gives the frequency in Hz (cycles per second); the vertical scale gives the attenuation in decibels (abbreviated: dB). It is evident that at frequencies below about 1000 Hz there is little attenuation: the sound passes right through most sheets with no loss whatever .
One third octave band center frequencies in HZ (cps)
Figure 21. Sound attenuation vs frequency for samples of perforated metal having different TI.
But as the frequency increases, there is more and more attenuation. ...meaning that the sound is reflected from the sheet and fails to get through to reach the acoustical treatment that lies behind.
This condition is more severe, the lower the value of TI. For a sheet with TI = 1500, the attenuation of sound at 16,000 Hz is as much as 4.75 dB; for TI = 12,000, the loss is only 1.5 dB at the same frequencies.