GB1379372A - Electroacoustic transducer having an inverted diaphragm - Google Patents

Abstract

1379372 Receivers NORTHERN ELECTRIC CO Ltd 10 Jan 1973 [10 Jan 1972] 1422/73 Heading H4J In a telephone receiver, a diaphragm having a balanced armature electro-mechanical drive is mounted so as to divide a casing into two cavities a coupling cavity and a damping cavity, and the free volume the coupling of cavity in which the electro-mechanical drive is positioned is made at least three times the volume of the damping cavity ring. As shown an aluminium casing 1 having sound orifices 2 has ring mounted therein a conical diaphragm 12 so as to form two cavities, the upper of which is totally enclosed and includes as part of its boundary a closure plate 62 of acoustical resistance material e.g. sintered stainless steel, and a backplate 30 of thermoplastics material, this upper cavity providing a damping effect at the natural resonance frequency of the armature diaphragm assembly. The lower cavity contains a balanced armature drive and also has a thin acoustically transparent plastics membrane 60 mounted across the face portion 5 having the sound orifices. The armature shown in more detail in Fig. 3, comprises an integral structure of e.g. permalloy, and includes a central portion 21 connected by torsion bars 22 to arms 24 on which are mounted U-shaped non-magnetic shims 25. e.g. of brass, to provide a clearance between the magnet pieces 72, 74 and the armature. A linkage bar 100 of a copper beryllium alloy is spot-welded to the armature. Permanent magnet 75 is of Alnico V and polepieces 72, 74 are of permalloy. A hole 14 in the diaphragm 12 provides at least a 6 db/octave roll off below about 425 Hz which is approximately reverse to the transmission characteristics of a telephone line. With the above construction the upper (damping) cavity can be made to be about <SP>1</SP>/ 3 of the free volume of the lower (coupling) cavity and this is said to cause a poor coupling to a users' ear to affect the frequency response to a lesser extent than with prior art receivers. Canadian Specification 496,187 is referred to.