You may wonder, then, why it would not ALWAYS be best to choose the most transparent possible material.
We have illustrated by the preceding examples that, generally speaking, perforated sheets with small holes close together give the greatest transparency; but for practical reasons, very tiny holes should be avoided because they may get clogged with dust or filled with paint when the sheet is repainted.
Perhaps more important, very finely perforated sheets tend to be fragile and are much more expensive to manufacture.
Therefore, it is sometimes best to choose the gage of the metal first, based on cost, availability or other reasons; then choose the possible hole size, and jockey the other parameters to achieve the desired transparency.
IMPORTANT NOTICE TO USERS AND SPECIFIERS OF PERFORATED METALS
At this point, large-scale users of perforated metals are probably reaching for their hand-held calculators to find out the values of TI for their most popular products. DON'T BOTHER! Instead, look at Table 1 on page 37, where the calculations are already done for some commonly produced perforated sheets.
NEXT, DON'T PANIC! You will immediately notice that many popular products have values of TI that are nowhere near the 10,000 recommended above for perfect acoustic transparency.
No matter. Most acoustical problems are concerned with frequencies in the mid-range of 1000- 4000 Hz. And a glance ahead at Figure 21 (Page 32) will assure you that practically any common perforated metal is nearly totally transparent to sound at those frequencies and below.
So why the emphasis on performance at 10,000 Hz in the Transparency Index? The answer is that a useful distinction in the transparency of perforated metals is possible only at very high frequencies. For example, if we decided to rate our Transparency Index at 1000 Hz, say, this would be no good at all, because all the samples would get the same (nearly perfect) rating.
We concentrate here on the 10,000 Hz frequency so that people who are interested in making distinctions in transparency can do so meaningfully.
In all cases, it is important to match the perforated product to the specific needs of the problem at hand.
