Tests 3, 4, and 5:

These three employed #115 test samples mounted over a frame having a rigid back into which glass fiber blankets of varying thicknesses were placed. In some tests the sound-absorbing blanket was placed against the perforated sheet with or without airspace behind it. In others the blanket was placed against the back leaving an airspace between the face of the blanket and the perforated sheet.

The tests clearly demonstrated:

      As a general rule, the thicker the absorbing blanket, the greater the sound absorbency. But, the thickness of the absorbing blanket showed its greatest effect below 500 Hz with the effect increasing towards the lower frequencies.

      Placement of the absorbent blanket against the perforated metal with an airspace behind it does not diminish sound absorbency. On the other hand, the airspace behind does not contribute to sound absorbency.

     Placement of the sound absorbent blanket away from the perforated metal-leaving an airspace between-noticeably reduced sound absorbency. To achieve maximum transparency of the perforated metal sheet and the greatest sound absorbing efficiency requires that the absorbent material be placed against the perforated sheet.

Test 6:

Demonstrated that when a polyethylene film was placed as a protective cover between the absorbent blanket and the perforated sheet, there was a substantial loss in absorbency at frequencies above 500 Hz and the loss increased as frequencies went up. Below 500 Hz and the loss increased as frequencies went up. Below 500 Hz, the absorbency loss was negligible. Loss also increased with the thickness of the polyethylene film.

(Refer to explanations of Tuned Resonant Absorber in the preceding section.)

Riverbank's test device comprised the basic elements of a Tuned Resonant Absorber with the notable exception that the perforated metal sheet was backed by a layer of aluminum honeycomb with 1" cells.

For the tests, glass fiber was pressed into the cells to varying thicknesses 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.

Dr. Schultz explained the need for this design: "When the airspace is continuous, the behavior of the absorber changes greatly at different angles of incidence of the sound. As the sound direction changes from perpendicular to the surface of the absorber (angles of incidence = 0) to the grazing incidence of 90, the resonance frequency changes drastically, rising away from the intended frequency to as much as three octaves higher."

"By contrast, with the partitioned backstructure, not only does the resonance frequency remain the same as the angle of incidence increases, but the bandwidth of high sound absorption actually becomes broader."

The Nomogram chart following this write-up illustrates a test which used an aluminum sheet .080" thick perforated with 1/8" (.125") holes on 2 1/4" straight row centers, providing an unusually small percentage of open area, .2437%. The target frequency was a low 125 Hz. Clearly the Tuned Resonant Absorber performed as expected with a Sound-Absorbing Coefficient of 1.0, very close to 100% efficiency.

To determine the resonance frequency of the TRA used in the test discussed above, Dr. Schultz's Nomogram for doing so is shown following this write-up. The elements of the TRA are as follows:

t= thickness of the sheet = .080"

e= the effective throat length of the holes in the sheet,
(e= t+.8d) = .080 + .125 x .8) = .18"

h= distance from the perforated sheet to the back of TRA = 4"

P= Percentage of open area = .2437%

Using a ruler, connect the point .18 on the "e" scale with the point .2437 on the P scale. 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 for this TRA has been determined to be 125 Hz.

You can use this nomogram to solve for any missing component of a TRA you are designing. Clean copies of this nomogram are found in the Appendix of Dr. Schultz's book, ACOUSTICAL USES FOR PERFORATED METALS, available from the I.P.A. or Hendrick Manufacturing Co.. It can be reproduced on any copying machine.