A Case History Illustrating The "Transparency" Approach
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.
