Transparency with Sound Absorbing Materials

 A Wide Range of Perforations Provide High Transparency 

The first test compares the sound absorption performance of a bare, unprotected 4" blanket of fiberglass with the same material protected by sheets of perforated metal that required the sound to pass through the perforated sheet. The three perforated sheets were of the following specifications:

Chart 1 disclosed that there was virtually no diminishment of the fiberglass blanket's sound absorption performance by the presence of any of the perforated metal sheets; they were equally transparent with only minor and insignificant variations. Each of the perforated-protected tests followed very closely theperformance of the bare blanket at all frequency levels.

CHART 2: ABSORPTION OF MINERAL WOOL OR FIBERGLASS WITH IPA #115

Chart 2 illustrates the results of a second test wherein the IPA #115 perforated sheet, the one with the least Open Area, was used in conjunction with four different sound absorbing materials:

4" 6 pcf Mineral Wool with a NRC of 1.05
4" 3 pcf Mineral Wool with a NRC of 1.10
4" Fiberglass Blanket with a NRC of 1.05
4" Fiberglass Board with a NRC of 1.10

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 the materials.

Place Sound Absorbent Material Against Perforated Metal for Maximum Transparency and Absorbency.

In the tests depicted by Charts 3, 4, and 5, as the diagrams show, the perforated sheet, IPA #115, was mounted over a frame having a rigid back into which fiberglass blankets of varying thicknesses were placed either against the perforated sheet with or without airspace behind it or against the back leaving an airspace between the face of the sound absorbing blanket and the perforated sheet.

In addition to the sound transparency of the #115 material shown in Charts 1 and 2, these tests clearly demonstrated these conclusions:

1. As a general rule, the thicker the absorbent blanket the greater the sound absorbency. But, the thickness of the fiberglass blanket has its most noticeable effect below 500 Hz with the effect increasing towards the lower frequencies.
2. Placement of the absorbent blanket against the perforated metal with an airspace behind does not diminish sound absorbency. On the other hand, the airspace behind does not contribute to sound absorbency.
3. Placement of the sound absorbent blanket away from the perforated metal-leaving an air space between will noticeably reduce sound absorbency. To achieve maximum transparency of the perforated metal requires that the absorbent material be placed against it leaving no airspace.

CHART 3: ABSORPTION OF FIBERGLASS AGAINST IPA #115 IN FRAME

CHART 4: ABSORPTION OF FIBERGLASS WITH AIRSPACE & IPA #115 IN FRAME

CHART 5: ABSORPTION OF 2” FIBERGLASS WITH AIRSPACES & IPA #115 IN FRAME