Erling Frederiksen J(slashed oh)rgen I. Christensen
Dept. of Microphone Devel., Bruel & Kj(ae ligature)r, 2850 N(ae ligature)rum, Denmark
IEC and ANSI standards that specify the performance of instruments for the measurement of sound intensity are expected to be released in the near future. The drafted standards prescribe the minimum performance of sound intensity probes in standing wave fields. Generally, the measurement of sound intensity has the greatest interest in connection with reactive sound fields. Unfortunately, the more reactive the sound field, the higher are the requirements to the instrument for the same measurement accuracy. As well-defined reactive fields can be produced inside tubes, standing wave tubes have been selected for testing of intensity probes. The standing wave performance of intensity probes, equipped with pressure-sensing microphones, has been calculated and measured. Results obtained by the two methods will be shown. A description will be given of the calculation model and of the specially designed standing wave tube that has a frequency-independent standing wave ratio of 24 dB ((plus or minus)0.5 dB) between 40 and 500 Hz. This performance has been obtained by terminating the test tube with many long and narrow plastic tubes.