A Microphone Calibration Facility

The free-field frequency response of a microphone is conveniently determined in a large anechoic chamber. However, anechoic chambers that are effective below 100 cps are large in size and often prohibitive in cost. This paper discusses an economical microphone calibration facility which consists of (a) a small anechoic chamber approximately 10-ft cube having a useful free-field frequency range from 400 cps to 100 kc, and (b) a long hard wall duct with an 8-ft terminating wedge. This duct has a useful frequency range from 20 to 700 cps and permits achieving a plane progressive wave in this range. The performance characteristics and the cost of this facility are described in this paper.