Economic Design and Construction of Anechoic Chambers
Authors: Duda John , Oppenheim Arthur 
Published: September 2005

For the past 16 years, the design of anechoic chambers has been the subject of a good deal of acoustic research. Prior research has yielded a design employing a large number of light-density wedges (2.S lb/ft3) which had to be supported a fixed distance from the chamber wall. The design considerations which formed the nucleus of a research program by the Industrial Acoustics Company, Inc., were the following: (1) The wedge had to be made of a rigid material. (2) The over-all length of any anechoic wedge must be maintained as short as physically possible. (3) The wall area covered by a single wedge should be made as large as possible. The first series of investigations made was to determine whether or not the conventional air space could be eliminated. Two figures showing test results of two particular experiments indicate that, if the wedge sizes were maintained constant, there is no advantage in having the air space behind them. It was also observed that the low-frequency absorption could be matched to the high-frequency absorption by altering the wedge-shaped portion, or tapered length, of the anechoic wedge as well as the base depth. In order to maintain as small as physically possible the over-all length of our anechoic-chamber wedges, experiments were performed to show that the standard 4-in.-thick IAC Noise-Lock panel could be used as part of the wedge's length without affecting the acoustic property, in any manner, of either clement. A graph is presented showing the final design curve of the IAC wedge for use in obtaining the length of the anechoic wedge required for any given cutoff frequency, as well as actual test results of wedges constructed to check the IAC design curve. The test method used in the laboratory was to analyze a standing wave pattern produced by a sound source emitting a pure tone. The schematic drawings given show a typical wedge arrangement in the test duct. The method of test used to evaluate a completed installation is slightly different. It is accomplished by generating a pure tone at one point in the room, moving the microphone away from the sound source until the entire room dimension is traversed, and then by comparing the results with those predicted by the inverse-square law. The correlation between IAC laboratory tests and field installations shows excellent agreement, within ±1-db measurement accuracy. The elimination of the air space behind the wedge allows a more efficient use of wedge material. This advantage, plus the use of a more rigid glass-fiber material than previously used, enables a more pleasing appearance to be maintained in the anechoic chamber, as well as a less elaborate construction for movable portions of an IAC anechoic chamber (such as the wedge door or window-wedge plugs). All these features are important to the economic design and over-all performance of an anechoic chamber. The incorporation of a larger base area and a shorter wedge length, coupled with the use of prefabricated structures in which the additional design criteria of quiet intake and exhaust of ventilation air can be provided, as well as the support of the entire structure on suitable vibration eliminators, is strongly recommended. In order to show that a more economical structure can now be manufactured than heretofore possible, a comparison is made of prefabricated and concrete rooms utilizing a new or old wedge design.