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.