To validate the data developed by Theodore J. Schultz, Ph.D., the Industrial Perforators Association contracted the Riverbank Acoustical Laboratories to test his findings. The following is a summary report on the tests conducted in demonstrating the two acoustical capabilities of perforated metals - allowing a maximum amount of sound to pass through it in order for it to be absorbed by material behind it and their capability in a tuned resonant sound absorber to alternate specific, narrow frequency ranges. The objectives of the tests were:
B. Demonstrate the theories regarding Tuned Resonant Absorbers set forth by Dr. Schultz.
Transparency with Sound Absorbing Materials
The first test compared the sound absorption performance of 4" thick fiberglass with and without different perforated metal patterns covering it. The three perforated metal patterns used for the test and the results of the tests are shown below.
As shown in Chart 1, the tests proved that there was virtually no diminishment of the fiberglass's sound absorption performance by the presence of any of the perforated metal patterns. Each of the perforated-protected tests followed very closely the performance of the bare fiberglass at all frequency levels.
The second test compared four different sound-absorbing materials when covered with IPA pattern #115, the one with the least open area. The four different sound absorbing materials and the results of the test are shown in Chart 2.
Note: NRC stands for Noise Reduction Coefficient, a standard measure for sound absorption which is reflected in the Y Axis Scale. A material with a NRC of 1.10 is approximately 5% more efficient as a sound absorber than a material with an NRC of 1.05.
The test results demonstrate, again, a high degree of transparency for the IPA # 115 material. Additionally, we can see a rather significantly better sound absorption by Fiberglass Board in the lower frequencies and noticeably weaker performance of the 6 pcf Mineral Wool material below 1000 Hz. But, the differences are small and clearly the presence of the perforated metal had no effect on the sound absorbing performance of any of these materials.
Three more tests were performed using IPA pattern #115. In these tests, the perforated metal was mounted on a frame and fiberglass blankets of varying thicknesses were placed at different distances behind it. In addition to the sound transparency of pattern #115 proved in the test results clearly demonstrated in Charts 1 and 2, these tests provided three conclusions. They are:
1. As a general rule, the thicker the absorbing blanket the greater the sound absorbency. But, the thickness of the fiberglass blanket showed its greatest effect below 500 Hz with the effect increasing toward the lower frequencies.
2. Placement of the fiberglass blanket against the perforated metal with an airspace behind it does not diminish sound absorbency. On the other hand, the air space behind does not contribute to sound absorbency.
3. Placement of the fiberglass blanket away from the perforated metal with an airspace between noticeably reduced sound absorbency. To achieve maximum transparency of the perforated metal and the greatest sound absorbing efficiency requires that the absorbent material be placed against the perforated sheet - leaving no airspace.
The next test was conducted to determine sound absorbency loss when a sheet of polyethylene film was placed as a protective cover between the fiberglass blanket and the perforated metal. The results of the test are depicted in Chart 6.
Chart 6 shows that while absorbency loss below 500 Hz was negligible, above it there was a substantial loss and the degree of which became greater as the frequency increased. The chart also shows that loss directly correlated with the thickness of the poly film.
Frequency Range Attenuation
Dr. Schultz's Calculations Relating to Tuned Resonant Absorbers are clearly demonstrated. Dr. Schultz writes in his book, "One of the great advantages of perforated metal is that it can be used as an element in a "tuned resonant absorber" to provide remarkably high sound absorption in the targeted frequency range without requiring a large amount of space or absorptive material... the perforated metal is used in combination with a trapped layer of air, in order to modify the acoustical performance of the absorptive material. This is done by setting up an acoustical, resonance condition, which concentrates the sound absorption into a particular frequency range of special interest."
"All resonant devices have a preferred frequency (of oscillation)...In a resonant absorber, the oscillation involves the motion of air particles in and out of the holes in the metal sheet in response to an incident sound wave. The preferred frequency of this oscillation is determined by the mass of the air in the perforations and the springiness of the trapped air layer." "At that resonance frequency, the air moves violently in and out of the holes, which pumps air particles back and forth vigorously within the adjacent sound absorptive layer.
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."
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."
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.
In his book, Dr. Schultz explains in detail how a narrowing of the frequency range around the targeted frequency will result from having a shallower air space between the perforated sheet and the solid backing, (scale h on the nomogram). With a perforated sheet that provided a higher value on the "e" scale and the same Open Area on the "P" scale in combination with a 2" air space, (h scale), the same target frequency would have been maintained, but a more focused range of frequencies would be attenuated. (See dotted lines on Nomogram 2).