GB409368A - Improvements in or relating to acoustic devices of the electrodynamic type - Google Patents

Abstract

409,368. Transmitters. ELECTRICAL RESEARCH PRODUCTS, Inc., 195, Broadway, New York, U.S.A.-(Assignees of Thuras, A. L. ; 243, Waverly Place, New York, U.S.A.) Oct. 31, 1932, No. 30582. Convention date, April 12. Addition to 333,578, [Class 40 (iv)]. [Class 40 (iv).] Relates to an electrodynamic acoustic device, more particularly a transmitter, of the kind described in the parent Specification in which a diaphragm has a coil attached thereto suspended in an annular magnetic gap and a chamber having restricted openings is provided under the diaphragm, the chamber and openings being so dimensioned as to exert a damping effect on the movement of the diaphragm over a predetermined frequency range and is characterised in that the openings are overdimensioned and their proper degree of restriction is achieved by closing them with porous material, such as silk fabric. The transmitter shown comprises a magnetic casing 11 having a centre pole 12 and carrying an annular pole plate 14, all these parts being preferably made of hardened cobalt steel containing approximately 35 per cent cobalt. The pole 12 carries a block 16 of magnetic iron which is capped by a dome portion 17 of cobalt iron containing approximately 50 per cent cobalt. An annular insert 15 of this alloy is carried by the plate 14. A diaphragm 20 carrying the coil 27 comprises a central dome portion 21, an annular concentrically corrugated portion 22 and a rim portion 23 which is clamped between washers 24 to the insert 15 by the shallow mouthpiece 25. A plate member 37 of non-magnetic material, Fig. 6, has a collar portion 38 which embraces the block 16 whilst the slots 42 in the intermediate portion 41 thereof are covered by a fabric annulus 43 and arranged immediately beneath the air gap 18. The rim portion 40 of member 37 is secured to the insert 15, sealing means 44 being interposed to ensure that air flowing from or to the space under the diaphragm passes only through the slots 42 and fabric 43. In order to increase the response of the device at low frequencies, the space 46 at the rear of the diaphragm is connected with the front of the diaphragm by a tube 47 threaded at one end to engage the mouthpiece 25. The domed portion 17 of the central pole is also formed with a chamber 30 communicating through openings 31 with the space between the domed portion of the diaphragm and the member 17, the openings being covered with fabric 33. According to a modification of this arrangement (Fig. 4, not shown), the openings are formed in a separate plate held in a recess in the member 17 by the screw 29. The chamber 30, openings 31 and fabric 33 control the vibratory characteristics of the diaphragm over a band of frequencies between approximately 2000 and 5000 cycles per second whilst the chamber 32, air-gap 18 and fabric annulus 43 control the diaphragm over the entire frequency range. Specifications 382,668 and 388,069 are referred to.