Qualification and performance of a reverberation chamber equipped with lightweight diffusers

Abstract

A study was conducted in the reverberation chamber of the Integrated Acoustics Laboratory at Georgia Tech to evaluate the efficacy of lightweight (1.3 kg/m 2 ), fiberglass diffusers in increasing room diffusion while affording easy chamber reconfiguration. To this end, performance and qualification tests were conducted in accordance with the American Society of Testing and Materials (ASTM) standard C423-02a, the American standard for sound absorption measurements in reverberation chambers. Also, comparative absorption tests were conducted per the standard for comparison with other laboratories. Preliminary performance testing per C423-X1 included determining the optimum diffuser surface area and quantifying variation of sound decay rate with loudspeaker position. During performance testing, the sound absorption coefficient of a specimen increased with increasing diffuser area until the coefficient reached a maximum. The configuration that yielded this maximum, identified in Appendix X1 of the standard as optimum, had a diffuser-to-chamber surface area ratio of 19%; the diffusers were stationary for these tests. With respect to loudspeaker position, the decay rate was relatively independent of position. Qualification testing per C423-A3 required demonstration that decay rate was sufficiently independent of measurement location and specimen position. With the optimum stationary diffuser configuration the chamber did not qualify per ASTM C423-A3. In five of eighteen frequency bands, variation of decay rate with respect to measurement location exceeded the qualification limitations; likewise, variation with respect to specimen position exceeded the limits in five bands. However, a combination of rotating and stationary lightweight diffusers produced a sound environment that was diffuse enough to qualify per ASTM C423 in all frequency bands. To compare performance to other laboratories, absorption testing of round robin specimens was replicated in the IAL chamber. It should be noted that all specimens were Amounted according to ASTM Practices E795-00 (to chamber floor); the results of this paper apply to this mounting only. With both diffuser configurations, the measured absorption coefficients and their repeatabilities were within the confidence interval developed by the round robin testing. Interestingly, the absorption coefficients measured with the diffuser configuration that did not qualify the chamber were statistically no different from those from qualified laboratories. These results call into question the assumption that dense diffusers and strict diffusion requirements are necessary for sound absorption measurements in reverberation chambers.