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A large reverberation room of approximately 310 m3 (11,000 ft3) used in the air conditioning, heating and refrigeration industry, was in need of improvements to meet the updated requirements of the American Heating and Refrigeration Institute (AHRI) Standard 220. In addition, it was desired to extend the measurement qualification of the room down to the 63 Hz octave band. The initial qualification test results showed that the room did not qualify for the extended low frequency range and also had some irregularities in the 100 Hz third octave band. This paper reports the results of a three-part investigation to correct reverberation room response irregularities in the 100 Hz third octave band, to establish performance that qualifies relative to the most recent standard, and to determine and integrate the means by which its qualification could be extended down to the frequency bands of 50, 63, and 80Hz.

Anechoic/hemi-anechoic chamber qualification testing is typically conducted by measuring the free-field response as a microphone is traversed away from a central noise source toward the chamber perimeter. Based on ISO standard 3745, the microphone can be moved either incrementally or continuously. If using broad band random noise, the measurement must last a sufficient period of time to ensure an accurate averaged level is recorded. For low frequencies this can require extended periods of time. Attempting to establish a microphone traverse rate slow enough to allow sufficient time averaging becomes tedious. Over distances of 3 meters, this can require a draw that takes 20 minutes or longer. This paper discusses techniques explored to produce results that are quicker and more accurate than by typically applied methods.