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New Data for Acoutiscal Properties of
Air-Conditioning Silencers

An environmental acoustics research report in which seven laboratories from Canada and the United States participated is the basis for a revision of ASTM E 477, Standard Test Method for Measuring Acoustical and Airflow Performance of Duct Liner Materials and Prefabricated Silencers. The revised standard will provide a better understanding of the acoustical properties of duct-silencers installed in ventilation systems to reduce air-conditioning noise.

Data from the interlaboratory study supports a revised precision statement for Standard E 477 that extends to a 63 Hertz octave band. It will be included in a revision of E 477 by a task group of ASTM Subcommittee E33.08 on Sound Transmission in Committee E33 on Environmental Acoustics that will be reviewed and re-published by the Society.

A well-known test originated in 1973 by ASTM, E 477 is referenced in building codes, and has been adopted by the U.S. Department of Defense and approved by the American National Standards Institute. The precision statement covers both insertion loss and generated noise of silencers.

“It is a summary of a very extensive series of round robins that took five years and gives us a rational basis for the precision and reproducibility of E 477 data,” said the subcommittee chair, Howard Kingsbury, P.E., acoustical consultant, State College, Pa. “Recently, the standard has been extended down in frequency range to include the 63 Hertz octave band. This is the first time anybody has ever looked extensively at such low frequency data obtained in a variety of reverberation rooms.

“The round-robin tests included a 3 ft.-long standard pressure-drop silencer, a 7-ft. long high-insertion loss silencer, and a 5-ft. long packless (no fiber-fill) silencer,” he explained, pointing to the wide of range of silencer-types evaluated.

As stated in the standard, E 477 covers the laboratory testing of duct-liner materials, integral ducts, and in-duct absorptive silencers. Procedures describe measures of acoustical insertion loss, airflow generated noise, and pressure drop as a function of airflow. A provision is included for a simulated semi-reflective plenum to fit around thin-walled duct and silencer test specimens, as the acoustical environments around such thin-walled specimens can affect the measured insertion loss.

For further technical information, contact Howard Kingsbury, P.E., State College, Pa. (phone/fax: 814/238-9688). Committee E33 meets April 9-10 in West Conshohocken, Pa. For meeting or membership details, contact Staff Manager Steve Mawn, ASTM (phone: 610/832-9726). //

Copyright 2001, ASTM