Patent GB-485171-A (GB)
Title: Improvements in and relating to microphones for speech transmission
IPC: H04R1/38
Filed: 1936-11-12
Abstract: 485,171. Microphones; transmitters. SHEVLIN, J. T. (Siemens & Halske Akt.- Ges.) Nov. 12, 1936, No. 30919. [Class 40 (iv)] A sound-receiving instrument for the transmission of speech without frequency distortion from places having a large volume of extraneous or background noise comprises means where - by the mouth of a speaker is disposed so near to the microphone that the sound waves from the mouth act substantially as spherical waves on that side of the microphone diaphragm which faces towards the mouth and a pressure-gradient microphone which has a frequency response curve of such a steepness for plane wavesi.e. waves originating from a sound source relatively distant from the microphonethat a frequency response curve substantially horizontal within the speech-frequency range is produced for the spherical waves emitted by the mouth of the speaker-whereby the ratio of speech to noise is increased. The desired form of response to spherical and planefronted waves may be obtained by the choice of the type of microphone together with the nature of the diaphragm mounting and /or restraint. The required e.m.f.-frequency response curves e, k for plane and spherical sound waves are indicated in Fig. 3, I, A and II, C, which relate respectively to a displacement microphone, e.g. of the piezo-electric, condenser, carbon granule type-with a diaphragm tuned to a comparatively high frequency, and to a velocity microphone-e.g. an electromagnetic or electrodynamic instrumentwith its diaphragm movement restrained chiefly by friction. In cases where the response to spherical waves is not substantially level with respect to frequency in the speechfrequency range, electrical networks or acoustic devices are employed in conjunction with the pressure-gradient microphone. The necessary modification of the response curve for spherical waves in all cases gives rise to a response curve for flat waves which is steep in the speechfrequency range. A hand-set constructed to ensure that in normal use the mouth of the speaker is adjacent the diaphragm of the microphone is shown in Fig. 5 in which the diaphragm mounted in the casing M is open on both sides to sound vibrations and the microphone is laterally offset from the handle H so that the lips of the user will be adjacent an opening M<1> leading to one side of the diaphragm. In another construction, Fig. 6, the pressure-gradient microphone 2 is positioned in a casing provided with an opening 1 for speech and apertures 3 .. 3 on each side of the microphone which prevent resonances and allow the air vibrations corresponding to noise &c. to act equally on each side of the microphone. The casing also contains a packing 4 of acoustic damping material. The pressure-gradient microphone may be formed by the combination of two pressureoperated microphones spaced a small distance apart and electrically connected in opposition, as indicated in Fig. 4, in which the microphones 1, 2 are of the piezo-electric type. Fig. 7 shows an electromagnetic microphone operated as a pressure-gradient instrument and having its diaphragm frictionally restrained by the air cushion between the diaphragm and perforated plates b, d provided with small apertures c which are relatively widely spaced. The plate d may be omitted and the instrument may be of the moving-coil or moving-conductor type. In a modification of Fig. 7, Fig. 10, a plate b located between the diaphragm a and an electrically-inoperative diaphragm e has perforations c extending through it and elongated recesses f closed at one end. The microphone shown in Fig. 10 is constructed to give a unidirectional response.