Stability of Western Electric Microphones

Abstract

The absolute sensitivities of several Western Electric 640 AA condenser microphones have been measured using reciprocity techniques. All readings are made in terms of the settings of a precision potentiometer. By taking six different pairs of readings instead of the minimum number of three which the reciprocity technique requires, each calibration was checked for self-consistency and any erroneous reading detected immediately. Changes in sensitivity may be measured to ±0.03 db; all measurements have been made at 400 cps. The temperature coefficients of sensitivity varied from one microphone to another; from as little as −0.0025 db/°C to +0.022 db/°C. When measurements were first commenced the microphones showed considerable scatter, as much as ±0.2 db, but after a large number of measurements they settled down to much more steady values. In all cases it was found that the microphone sensitivities were a function of the ambient pressure, thus indicating that the stiffness of the air cavity behind the diaphragm contributes to the total stiffness. In most cases this contribution was of the order of 10%.