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.
Why Perforated Metals Are Often The Best Choice
You probably already know that perforated metal sheet is often used as a facing for acoustical treatments, but if more people also realized that for many applications perforated metal is the best available facing material, there would be many more such applications.
Figure 13. Although it is not conspicuous, the ceiling of this classroom is made of perforated metal with glass fiber blanket in the space behind.
A great disadvantage of other commonly used sound absorptive treatments is that they cannot be cleaned or repainted without seriously degrading their sound absorptive properties. Perforated metals are unique as components of acoustically absorptive treatments because they can be cleaned or refinished without harming the absorptive properties for which they were designed, subject only to the proper choice of perforation size and spacing, described later.
Other important advantages of perforated metals in such applications are:
- inherent structural strength, compared with woven or felted facing materials; they can stand alone, if necessary;
- ability to be formed into complex curved shapes for architectural (visual) purposes;
- resistance to abuse and damage
- Finally, the chief architectural advantage of perforated metal is that it is basically uninteresting. It can be made to look like something else: for example, plain plaster.
Unfortunately, its neutral appearance creates difficulties for us when we try to illustrate this advantage in this booklet; photographs don't show up what is really going on!
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.
The formula is valid for either straight or staggered perforations. An approximation for the value of a, when you do not know the value of b, is:
a = d[(const./P1/2) -1]
The value of the constant is 9.5 for staggered and 8.9 for straight perforations.
We can predict from the value of TI the amount by which sound waves at the very high frequency of 10 kHz are attenuated in passing through the sheet, according to the curve in Figure IS, and from this we can develop a curve for the attenuation at lower frequencies. (See Part Two, Section II).
