GB956448A - Improvements in or relating to electrodynamic microphones - Google Patents

Abstract

956,448. Microphones. PHILIPS ELECTRICAL INDUSTRIES Ltd. June 21, 1960 [June 24, 1959], No. 21730/60. Heading H4J. In an electrodynamic microphone having a cardioid directional characteristic, a diaphragm is accessible to sound waves at the front and rear, rear access being by way of an acoustic resistance terminated duct which opens directly into the air, and an air chamber having one or more apertures in the microphone housing, all such apertures and a further aperture leading from the air chamber to a further air chamber behind the diaphragm being covered by acoustic resistances, while the ratio between the duct resistance and the total resistance of the housing apertures is such that a desired frequency characteristic and directional characteristic in the low frequency range may be obtained. In the moving coil microphone described, the diaphragm 1 is suspended from the pole plate 7. A duct 2 passes through the centre of the magnet system and terminates in an acoustic resistance 2<SP>1</SP> made for example of gauze. An air chamber 3 is in communication with the air chamber behind the diaphragm via holes 3<SP>1</SP> covered by gauges, and with the outside air via a hole or holes 4 covered by gauzes 4<SP>1</SP>. Various mathematical expressions are derived with the aid of electrical equivalent diagrams, for the conditions governing the directional and frequency characteristics. In general if a lower response is required, e.g. for speech purposes, the resistance terminating the duct is increased and in order to preserve the directional characteristic, the now excessive compliance of air chamber 3 is corrected for, by a decrease in the resistance 4<SP>1</SP> governing the aperture 4. Figs. 5a and 5b show one method of adjusting the response for different purposes with the microphone shown in the " speech position." Switch, 10, deforms U-spring 12 so that the ends of the limbs of the spring, which carry foamed rubber pads 13, are disengaged from gauges 4<SP>1</SP> while a strip of gauze 14 is stretched so as to cover the gauze 2<SP>1</SP> terminating the duct. Further apertures 4 may be provided which are also covered by gauzes, e.g. 4<SP>11</SP>, and which are unaffected by the switch. Various embodiments are described whereby the apertures and ducts may be covered or uncovered by suitable switching means.