Acoustical Environment for Calibration of Microphones

Acoustical environment consisting of a duct and an anechoic box to simulate free-field conditions for measurement of the performance characteristics of microphones is described. The duct simulates free-field conditions over the frequency region of 50–800 cps., and the anechoic box over the frequency region of 500–10,000 cps.Measurements of sound pressure were made to evaluate the free-field conditions in the duct and in the anechoic box. The responses of two Western Electric 640-AA microphones were compared in the duct and anechoic box by the substitution method. The results show that the response curves agree within ±0·5 db. of the data supplied by the manufacturers. The experimental arrangement for obtaining a continuous frequency response curve is described.