Special Considerations: Non-Circular Perforations and "Self-Resistance" Of The Perforated Metal
All of the discussion above has dealt with perforated metal sheet having circular holes, in either straight or staggered patterns. If, instead, the holes are square, we can use the same calculations to a good approximation if we assume an effective hole diameter d' that is equal to (4/7r)Jl2 L = 1.13 L, where L is tl1e length of the side of the square perforation. Use the calculation for the straight pattern.
Another somewhat more complicated difficulty arises when, in an attempt to achieve a high value for the Transparency Index, we end up with very small holes. Then, not only is there a risk that the holes will be clogged upon being repainted, but there may even be unwanted energy loss as the air pumps in and out of the tiny holes. just as if it were lost by friction within a sound absorptive blanket.
This condition would cause no harm if our purpose is to use the perforated metal as a facing for an absorptive blanket: it would only add a bit more to the total sound absorption. But if our goal is to provide a transparent room surface so that sound can pass freely through and back, then we do not want any sound absorbed inadvertently, along the way. We must, therefore, check our sheet dimensions to be sure that the material is sufficiently sound transparent without adding unwanted sound absorption.
But the further discussion of this problem is slightly complicated; it must wait until we have considered the "Tuned Absorber" application, below. (See Section III, C.3).
