Calculating The Design Parameters
Of The Tuned Absorber
Having determined the desired frequency for maximum absorption, the Nomogram
shown below can be used to calculate the specifications for a tuned absorber to attenuate that frequency. Use of the Nomogram is described in detail below, in the discussion of the Riverbank tests.
The Nomogram works equally well whether you are starting with a desired target frequency range or with a set of constraints relating to available spacer or perforated metal.
The Riverbank Tests
The Riverbank Tests used a test specimen that comprised the basic elements of a Tuned Resonant Absorber, which is illustrated below. The test specimen was two-chambered to provide for comparative experiments. The elements included a sheet of perforated metal backed by a layer of aluminum honeycomb with 1" cells into which fiberglass had been pressed; the thickness of this layer varied in the tests from 1" to 4". This assembly was placed at the top of a box, which was 4" deep from the underside of the perforated sheet to the bottom of the box. Important note: Dr. Schultz points out, "It makes a very important difference whether the airspace behind the (perforated) sheet is continuous or divided into small cells by means of partitions." |
There, the acoustic energy (carried by the back-and-forth motion of the air particles) is converted by the friction into heat and is thereby removed from the acoustical scene."
He cites as an example the power transformer which emits a well-defined sound concentrated around 120Hz. An effective barrier to this sound might require a six-inch layer of sound absorptive blanket. But, the use of perforated metal to make a resonant absorber especially tuned to 120Hz can achieve efficient sound absorption at that frequency with only a thin layer of absorptive material.
"If there is a clearly perceptible pure tone or a prominent frequency, (a squeal, hum or whine, as opposed to a whoosh or roar), this is a good
indication that the disturbing noise is concentrated in a limited frequency range and a tuned resonant sound absorber is called for."
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