Motional Impedance Analysis Applied to a Dynamic Microphone

Abstract

The receiver analysis developed by R. D. Fay gives a method for calculating the complete performance of an electromagnetic speaker from a plot of electrical impedance measured when the speaker is loaded by a closed tube of variable length. This of course includes the performance of the speaker as a microphone, although the statement is not explicitly made. The present article applies the above analysis to a dynamic microphone and compares the sensitivity thus obtained with an independent calibration at lower audiofrequencies. This method yields a primary calibration, in that neither calibrated source nor comparison microphone is required. The high mechanical resistance of a dynamic microphone and its low coupling constant, as compared with an electromagnetic speaker, result in a much smaller variation in electrical impedance, with changing air load. The variation is of the order of 5 percent of the minimum impedance, as compared with perhaps 80 percent change in speaker impedance. The cross-sectional area of the terminating tube is found to be of considerable importance. The effect of this choice of tube area on over-all accuracy is discussed.