River Bank Test
These tests were sponsored by the IPA to validate data developed by Theodore J. Schultz, Ph.D. and presented in his book ACOUSTICAL USES FOR PERFORATED METALS published by the Industrial Perforators Association. The perforated patterns tested are shown below.
The test's objectives were:
- Determine which perforated metal specifications would provide a high degree of sound transparency.
- Demonstrate the theories regarding Tuned Resonant Absorbers set forth by Dr. Schultz.
Wide Range of Perforations Provide High Transparency
Test 1:
Compared the sound absorption performance of a bare, unprotected 4" blanket of glass fiber with the same material protected by perforated metal sheets of the specifications shown above.
Results showed that there was no diminishment of the glass fiber blanket's absorption performance by the presence of any of the perforated metal sheets. Each of the perforated-protected tests followed very closely the performance of the bare blanket at all frequency levels.
Test 2:
Focused on the use of IPA pattern #115, the pattern with the least Open Area (23%) in conjunction with 4 different sound-absorbing materials. Again, the results demonstrated a high degree of transparency for the #115 material. The differences between the sound absorbing performances of the various materials were small at their greatest divergences and the presence of the perforated metal had no effect on their performances.
Place Sound Absorbent Material Against the Perforated Metal for Maximum Transparency and Absorbency
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.
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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.
Dr. Schultz's Calculations Relating to Tuned Resonant Absorbers were Clearly Demonstrated.
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.
