GB550877A - Apparatus for testing microphones - Google Patents

Abstract

550,877. Converting. FOUNTAIN, Ltd., G. R., HOULGATE, H. J., and MARKS, P. D. R. July 31, 1941, No. 9737. [Class 38 (ii)] [Also in Group XXXIX] Apparatus for use in testing microphones comprises a sound generator coupled to a tube open at the other end, the open end being of small diameter compared with the wavelength of the sound so that a spherical wavefront is emitted. The apparatus provides a small sound-emitting area close to the microphone the effect being similar to that of the mouth. In the apparatus, shown a loud-speaker unit 1, which may be of the type described in Specification 519,625, [Group XXXIX], is coupled to a tube 2 of small diameter compared with the wave-length of the sound frequency used; for example having an inside diameter of 7/10 inch for a frequency of 400 cycles per second. The length of the tube is adjusted so that resonance occurs with a velocity antinode at the open end. in which circumstance a blast of air is produced simulating that produced by the mouth at the beginning of most words. The microphone 7 to be tested is held adjacent to the end of the tube in an open supporting member 8 adjustably clamped on the tube. The speaker may be driven from an oscillator operating at the desired frequency the input being adjusted until the microphone output, as measured by a voltmeter across an output load, is normal, or the microphone output being measured for constant sound strength. Instead of providing a separate oscillator, the apparatus may be made self-exciting by operating the speaker from an amplifier, to which is connected a microphone opposite an aperture in the tube wall situated at a pressure antinode. Alternatively, a microphone 21 of low acoustic impedance may be coupled to the tube through a high acoustic resistance such as a capillary tube 22, or a small piezo-electric element may be mounted in the tube, or a very small velocity microphone may be located at the mouth. Phase-correcting networks may be employed to bring about the required conditions for oscillation, and the selection of the desired frequency from the several frequencies at which the tube may tend to oscillate may be effected by making the response of the microphone a maximum at that frequency, or by adjusting the phase-frequency characteristic of the microphone so that only at the desired frequency is the phase correct for producing oscillation. Specification 507,183,- [Group XXXIX], also is referred to.