On doubly-resonated hot-wire microphones

Abstract

In the selective hot-wire microphone a Helmholtz resonator is used for the detection of sound-waves of definite pitch. The resonator responds to the particular note to which it is tuned, and its response is measured by the change in resistance of an electrically heated platinum-wire grid mounted in the “neck” of the resonator. The magnitude of the response depends in the usual manner on the tuning and damping of the receiving system, and therefore in order to obtain great sensitivity the damping must be small and consequently the resonance must be sharp. For some purposes, however, sharp resonance may be a disadvantage— as, for example, when the source of sound is liable to small variations in pitch, or when allowance must be made for the Doppler effect—while at the same time it may be desired to retain a high degree of sensitivity. Even in cases where very sharp tuning is permissible, the only effective means of increasing the acoustical sensitivity is by reducing the neck of the resonator, and since this tends to expose the hot-wire grid to chance currents of air, accurate observation becomes difficult except under very favourable conditions. To meet such difficulties as these, it has been found convenient to use doubly-resonated microphones in which the hot-wire grid is placed in a short neck between two resonators, only one of which communicates directly with the outside air. In this way it is possible to increase the range of response, and at the same time to obtain greater sensitivity than is practicable with a singly-resonated microphone.