Acoustically Speaking, More is Not Always Better

by Mike Nixon, CSI, EA Acoustical Engineering, Inc.

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There are many aspects of acoustics and noise control that are not well understood by the average person. This lack of understanding can lead to unnecessary higher costs, and sometimes, less than desired performance.

For example, consider a one inch thick fabric covered acoustical wall panel. In most situations where these panels are employed to enhance speech intelligibility in a room, the one inch thick panel gives the best bang for the buck. If the designer or specifier will look at the performance data at each of the frequencies, rather than relying on the single NRC number, it will become quite evident that the amount of sound being absorbed at the mid frequency voice range is in excess of 0.90 or 90%.

Increasing the thickness beyond one to two inches is likely to increase the absorption value to something in excess of 1.00 or 100%. In a case where the panels are deployed in a continuous array, it is almost impossible, under normal circumstances, to get more than 100% absorption, so for acoustical analysis it is best to consider the total amount of absorption achievable as 100%. Thus, the difference between a one inch thick panel and a two inches thick panel at the mid?voice frequency range is in the order of 10%, but you may be paying as much as 30-40% more for the thicker panel. This does not make economic sense.

The value of a two inches thick acoustical panel, compared to a one inch thick panel, is primarily in low frequency performance, so if low frequency is a concern, by all means use the thicker panel. In most applications, improving voice communication is the reason for acoustical wall panels, so be sure to look at the mid?frequency performance in particular.

Yet another cost saving application lies in the edge effect and the acoustical phenomenon of diffraction. With today's manufacturing techniques, some fiberglass acoustical core materials are so resilient and abuse resistant that there is no longer any need to harden or reinforce the panel edges, which adds significantly to the cost. With soft resilient abuse?resistant edges, the acoustical absorbing surface of a 24 inches by 48 inches panel is now increased to 26 inches by 50 inches, about 12.8%.

The ASTM acoustical test laboratory requires an acoustical sample eight feet square (often made up of eight panels, 24 inches by 48 inches each) but if the 64 square feet of test sample panels are separated from one another the absorptive edges add to the total absorption. At the same time the test sample is still listed as only 64 square feet. Thus, through an anomaly in the test procedure, the absorption data is reported to be higher than appears to be possible.

Yet another acoustical anomaly lies in the fact that the same eight panels used in the test can be separated by as much as two feet, resulting in an increase in acoustical performance of as much as a 30%. The increase is due to diffraction. It is less important to understand the nature and technicalities of this phenomenon than it is to accept it as fact. Diffraction improves acoustical performance far more than appears to be possible.

In summary, it is possible to get equal or better acoustical performance with fewer panels at a much lower cost per square foot. Making the right selection is a matter of perspective; more is not necessarily better, acoustically speaking.

Mike is president of EA Acoustical Engineering, Inc., and serves as a CSI representative on ANSI committees that deal with acoustics. His e-mail address is mtnixon@aol.com

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2001 Mike Nixon

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