| Nomogram 1 and Charts 7 and 8 illustrate a test which used an aluminum sheet .080" thick perforated with 1/8" (.125) holes on 2 1/4" straight row centers providing an open area of .2437%. The Nomogram elements for this test, therefore were:
t = The thickness of the sheet = .080"
e = The effective throat-length of the holes in the perforated sheet,
(t + .89d) = .080 + (.125X.8) = .18”
h = The distance from the perforated sheet to the backing = 4"
p = The percentage of Open Area,
(O.A.) = .2437%
To determine the target frequency which this Tuned Resonant Absorber will
attenuate using the Nomograph, first calculate the "e" dimension, which is.18”.
Using a ruler, connect the point .18” on the "e" scale with the point .2437% on the p scale, (it will be necessary to estimate the position of this point on the Nomograph). Now place your ruler on the point where this line crosses the M line and draw a line to the 4" position on the "h" scale. Where this line crosses the "f' scale, you'll find the target frequency that should be most highly attenuated by this Tuned Resonant Absorber. |
The target frequency in this test was determined to be 125Hz.
Charts 7 and 8 reporting on the results of two tests conducted by Riverbank Labs, the first with a 1" thick absorbing layer and second with a 4" thick absorbing layer, demonstrate clearly the effectiveness of the Tuned Resonant Absorber principle and the accuracy of Dr. Schultz's methods and Nomograph for determining the components of an efficient system.
Chart 7 illustrates the test results for the TRA using a 1" thick absorbing layer. The target frequency is clearly 125Hz; a "Sound Absorption Coefficient" of 1.0 is very close to 100% efficiency.
Chart 8 illustrating the test using a 4" sound absorbing layer, shows a slight shift to 100 Hz as the frequency most efficiently attenuated though 125Hz is also efficiently removed, as well. It also illustrates an overall increase in sound absorbed. Both of these results can be attributed to the thicker sound absorbing layer.
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