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Transparency Approach

We begin with a closer look at the "Transparency Approach" in the use of perforated metals. We learned in Part One how to define a Transparency Index as an indicator of how easily sound can pass through a particular sample of perforated metal at high frequencies (see page 14). We now look at what this means in practice.

Subway station in Vienna; note the perforated metal facing for the sound absorptive ceiling treatment.

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.

Sometimes the acoustical treatment that lies behind the sheet is a hard, sound-diffusing surface, intended to break up the sound waves and reflect them back to the room, as in a concert hall, Then this attenuation must be counted twice: once on the way in and once on the way back.

Access To The Sound Treatment

On the other hand, if the acoustical treatment is in- tended to absorb the incident sound, then we must determine how much the perforated metal degrades the intrinsic absorptive properties of the material installed behind it, by preventing the sound from getting access to the absorptive material.

For this purpose, we introduce the Access Factor (AF), illustrated in the following figure for the same samples of perforated metal that we saw above, in Figure 21.

Figure 22. Curves showing the Access Factor vs frequency for the same samples of perforated metal as in Fig. 21.