GB545790A - Improvements in microphones - Google Patents

Abstract

545,790. Sound-directing appliances. MARCONI'S WIRELESS TELEGRAPH CO., Ltd., Jan. 31, 1941, No. 1326. Convention date, Jan. 31, 1940. [Class 13] [Also in Group XXXIX] Relates to directional microphones of the kind in which the sound is picked up at a number of points, for example by a number of open-ended tubes communicating with a common diaphragm as in Specification 479,328. In order to make the directional characteristic less dependent on the frequency of the sound received, the pick-up tubes or other devices are divided into groups, each group being effective over a different frequency range, and one or more groups of the tubes &c. is associated with an acoustic filter. In Fig. 1, the two groups of tubes 9, 11, responsive to low and high frequencies respectively, are coupled through lowpass and high-pass acoustic filters, 13, 23 with the diaphragm 3 of an electro-dynamic microphone. The low-pass filter 13 comprises a cylindrical tube with an apertured partition 15, and the high-pass filter 23 comprises a cylinder with two openings 25, 27 in one side thereof. In a modified arrangement Fig. 2 (not shown), the tubes 9 are arranged in a circle around the tubes 11. When more than two groups of tubes are used, the intermediate groups terminate in band pass acoustic filters. In a further modification shown in Figs. 4 and 5 (Fig. 4 being a developed view) the tubes of the several groups are symmetrically arranged in relation to a circular diaphragm. The three shorted tubes 31a, 31b, 31c constituting one group are arranged as shown in Fig. 5, as are also the groups 33a, 33b, 33c; 35a, 35b, 35c ; and 37a, 37b, 37c. Further groups 39, 41, 43 are arranged as shown. The tubes of each group terminate in similar acoustic filters 36, 42, 48 &c. With the usual pressure-actuated microphone, the output will decrease with rising frequency; this may be avoided by using a condenser or crystal microphone feeding a resistance equal to the microphone impedance in the highest frequency, or by using a stiffness-controlled dynamic microphone.