GB969000A - Improvements in or relating to electro-acoustical apparatus - Google Patents

Abstract

969,000. Microphones. ELECTRO-VOICE Inc. Oct. 14, 1960 [Oct. 22, 1959], No. 35326/60. Heading H4J. A microphone having a unidirectional characteristic, comprises a transmission line coupled to one side of a diaphragm of the microphone, the line having a plurality of sound paths of different lengths, such that at the lowest frequency of the response range the largest path is less than half a wavelength longer than the shortest path, and an acoustic impedance coupled to the other side of the diaphragm so that in the lower part of the frequency range of the microphone, its characteristic is cardiod. A moving coil 74 (Fig. 3) type transducer, the magnet 54 of which is provided with a bore 56 aligned with the transmission line 10, is mounted in a housing 12 at one end of the transmission line 10. The latter consists of a pipe 18, which has a number of cloth covered holes 16 along its length, one end of the pipe 18 being closed by a plug 20 (Fig. 2) which has a hole 22 connecting with the outer air. A sleeve 24 (Fig. 1) encloses pipe 18 and is provided with a number of slots 26 covered with cloth 28. The acoustic impedance " behind " the diaphragm 72 comprises two air chambers defined by cover 86, disc 82 and plate 76 (Fig. 3). Cover 86 has a slot 92 (Fig. 1) open to the outer air and positioned in the vicinity of a mounting bracket 14 for the microphone. Further cloth covered holes in disc 82 and plate 76 connect the first air chamber to the space behind the diaphragm 72. The acoustic impedance is adjusted so that the microphone acts as a pressure gradient type having a cardiod characteristic below a frequency in the range 200-800 c.p.s. For frequencies above this frequency, the impedance is very great and the microphone functions as a pressure-type line microphone having a highly directive characteristic.