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.
