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.