Patent GB-382668-A (GB)
Title: Improvements in electrodynamic devices for use in receiving or transmitting sound
IPC: H04R1/22
Filed: 1931-07-31
Abstract: 382,668. Sound receiving or transmitting devices. ELECTRICAL RESEARCH PRODUCTS, Inc., 195, Broadway, New York, U.S.A.-(Assignees of Thuras, A. L. ; 243, Waverly Place, New York, U.S.A.) July 31, 1931, No. 21905. Convention date, Jan. 31. Addition to 333,578, [Class 40 (iv), Telephones &c.]. [Class 40 (iv).] In an electrodynamic transmitter or receiver of the kind described in the parent Specification in which the diaphragm has an air damping device arranged behind it to control the movements of the diaphragm over substantially the whole range of audible sound frequencies, a second acoustic impedance element is associated with the diaphragm to control its vibrations within a limited portion of the said range of frequencies. The impedance devices both con-' sist of air chambers with restricted orifices, the mass, elasticity, and resistance to flow of the air being suitably proportioned. The invention is shown in Figs. 2 and 3 as applied to an electrodynamic transmitter. A magnet shell 10 has an integral central pole-11 to which is attached an extension 12. A dome-shaped polepiece 14 is secured to the extension 12, and -an annular polepiece 15 is fixed to the shell 10 by pins 16 held in soft metal inserts, an annular air-gap 15<1> being formed between the parts 14, 15. The polepiece 14 holds in position on the part 12 a flanged ring 13 of phosphor-bronze or the like. Within the gap 15<1> is a coil 25 carried by a diaphragm 23, which has a central dished rigid portion and is clamped at its flat peripheral portion between a ring 27 and the polepiece 15. Paper washers 26 space the diaphragm from the polepieces so as to provide an air space between the diaphragm and polepieces. An annular damping plate 18 of brass or aluminium &c. is attached to the polepiece 15 by screws 19 passing through three equally spaced lugs 20. The plate 18 has an undercut surface with a ring-shaped ridge 181, which, together with the wall 13<1> of the ring 13, and with the inner part of the plate 18 located behind the magnetic air gap 15<1>, forms an annular air chamber. The chamber is sealed at the back by a paper &c. washer 21, and communicates through the narrow annular passage 182 with the air in the shell 10. The acoustic impedance device constituted by the air chamber and passage provided by the plate 18 is so proportioned that the motion of the diaphragm is controlled over substantially the whole frequency range. For certain devices, such as receivers and carbon microphones, this impedance should vary inversely as the frequency, but in a device such as the transmitter described, the impedance should be constant over substantially the whole of the range. In accordance with the invention, in order to compensate for some distortion which occurs in a certain portion of the high-frequency range, an auxiliary damping device is provided by forming a second air chamber 38 between the polepieces 14 and the pole extension 12. This chamber communicates through a number of apertures 39 with the space between the diaphragm 23 and the member 14, the apertures being proportioned to give the necessary resistance to air flow. The mass reactance of the diaphragm which causes distortion in the higher portion of the frequency range is thereby neutralized. Fig. 6 shows a modification of the auxiliary impedance device, wherein a chamber in the central polepiece 40 is bridged by a member 41 which carries a dome-shaped closure member 42. A restricted annular passage is formed between the rim of the part 42 and the edge 45 of the chamber. This construction may be slightly modified by making the head of the closure member 42 larger than the aperture in the polepiece, and inserting behind it a spacer shaped to provide restricted slots. A further modification is described and shown in Fig. 8, in which the auxiliary damping element is provided by a hollow plug 65 screwed into a recess 64 in the central polepiece 57. The plug has a number of holes 67 in the bottom, which are closed by a removable plate 68, and a dome-shaped closure member 69 is secured at the outer end of the chamber, with interposition of a spacing member 47, formed to provide restricted apertures. Measurement of the values of the stiffness and mass of the air within the chamber in the plug is made, and the proper adjustments are effected, before the damping element is inserted in the magnet structure. In this construction, the main damping plate 55 is attached to the annular polepiece 53, a spacing washer 55a being interposed, so that restricted slots 56 are formed. The damping plate lies against a sealing ring 57 of soft rubber.