Once a sound absorptive material is chosen to match the noise control task at hand, we must select the proper kind of perforated metal to serve as a protective covering. We must decide which perforation pattern, AMONG THOSE PATTERNS THAT ARE READILY AND CURRENTLY AVAILABLE, provides the greatest transparency .
Most people assume that the greater the percent open area of the sheet, the more easily sound can go through it. In a general way, this assumption is correct. but not always. For example, we could make a sheet with 10% open area in two ways: either by making a single large hole at the center or by very fine perforations overall.
Figure 14. Two samples of perforated metal with the same percentage of open area.
In the first case, instead of a transparent facing material, we would have a small completely open area at the center of the sheet (10% of the total area); but the rest of the sheet would be completely opaque to sound, reflecting ALL of it.
In the second case, the entire sheet is almost completely transparent to sound, because the tiny solid areas between the holes are too small to intercept the sound waves.
For high transparency, the most important consideration is to have many small perforations, closely spaced. It is better to minimize the bar size (the size of the solid portions between the perforations) and (to a lesser extent) to minimize the sheet thickness, rather than to concentrate on percent open area.
In order to help the designer choose a suitably trans- parent sheet for such applications, we have introduced a parameter called the Transparency Index (TI) given by the following formula:
TI = nd2/ta2 = 0.04 P/1rta2
where:
n = number of perforations per sq in;
d = perforation diameter (in);
t = sheet thickness (in);
a = shortest distance between holes (in);
a = b - d, where b = on-center hole spacing (in);
p = percent (not fractional) open area of sheet.
