A Case History Illustrating The "Transparency" Approach
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Figure 24. "Hush-house" , designed to confine the noise of jet engine tune-ups.
A typical application where widespread use is made of perforated metal is in the acoustical treatment of large "hush houses" for the run-up and testing of jet engines. In many cases these hush houses are large enough to accommodate an entire airplane for testing.
Since the jet engines on large aircraft are among the noisiest of today's noise sources, it would be intolerable (and a great hazard to hearing) if people had to work in buildings with these engines, unless very effective methods are introduced for controlling and abating the jet noise.
Among the most effective methods is the treatment of the walls and/or ceiling with deep, sound-absorptive material (typically glass fiber blankets or board), covered with perforated metal for protection and ease of maintenance. Example 9:
If we must choose a very economical wall treatment, it might consist of a 1.5-inch layer of glass fiber board, faced with a perforated metal that has been chosen for the best acoustical transparency consistent with high structural integrity and availability . For this purpose one might select a stock perforated sheet of 16 gauge steel (t = 0.0598") with 3/16" holes (d = 0.188") on 5/16" centers (b = 0.313"). These dimensions lead to n = 12 holes/ sq in, p = 32,5% and a = b - d = 0.125". We calculate the Transparency Index to be:
TI = nd2/ta2
= 12 x (0.188)2/0.0598 x (0.125)2
= 445
We can already anticipate from this very low value of TI that we will get some degradation of the performance of the glass fiber board; but the sheet dimensions are in this case determined by structural requirements and availability , so we may not have a better choice.
From the nomogram of Figure 23, above (p. 35, or Appendix D), we find the attenuation at 10,000 Hz to be 4.4 dB and the corresponding Access Factor to be 0.36.
We can interpolate in Figure 22 (p. 33, or Appendix D), which gives the curves of Access Factor vs frequency, in order to estimate the Access Factor at octave band frequencies down to 500 Hz, as follows.
Table 2 gives the sound absorption coefficient at various frequencies for the basic fiber board, as well as the (estimated) Access Factors for the perforated metal, and finally the effective sound absorption coefficients for the composite structure:
Table 2: Effect of perforated metal sheet with a low value of TI on the absorption coefficients for glass fiber board.
Freq 125 250 500 1000 2000 4000 8000
a 0.18 0.40 0.65 0.90 0.95 0.92 0.88
AF 1.0 1.0 1.0 0.98 0.90 0.75 0.49
Comparing a and aeff (see sketch), it is evident that the perforated metal covering is hindering the sound absorption at high frequencies. But this may not be a serious drawback, if there is not much high-frequency energy in the spectrum to be controlled in the first place.
Other acoustical applications that use large quantities of perforated metals as facings for sound absorptive treatments include subway tunnels and stations, and street and highway tunnels. Since all of these treatments are trying to cope with noises having broadband spectrums, the acoustical design approach should be the same in all cases: namely, the Transparency Approach. (See Section IV).
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).
