Patent GB-625662-A (GB)
Title: Improvements in and relating to earphones
IPC: H04R1/10
Filed: 1945-12-12
Abstract: 625,662. Instruments for deaf persons ;  earpiece attachments. STEVENS, A. H. (Brush Development Co.). Dec. 12, 1945, No. 33602. [Class 40 (iv)] An insert earphone comprises a piezo-electric crystal transducer mounted on an annular frame to which is secured the peripheral edge portion of a diaphragm, an inner portion of which is connected to the transducer, the frame, transducer and diaphragm which together constitute an operative sub-assembly being mounted in a housing which provides a substantially air-tight enclosure for the transducer except for one opening which when the earphone is being worn is directed into the wearer's ear. In one embodiment, the annular frame 14, Fig. 1, which is preferably made of compliant plastic material, is provided with three notches 15 which support three corners respectively of the crystal unit 16. The fourth corner is connected by the drive pin 17 to the apex of the diaphragm 12, Fig. 5, which is peripherally mounted on a shoulder 9, Fig. 1, on the opposite side of the frame, to which it is secured by adhesive. The shoulder 9 and the notches 15 of the frame accurately position the diaphragm and crystal unit respectively and ensure that the drive pin 17 of the crystal unit is in exact alignment with the apex of the conical portion of the diaphragm. The diaphragm is protected from subsequent damage by a guard plate 25, Figs. 1, 5, secured to its edge, thus allowing the sub-assembly to be handled without the necessity of extreme care. The crystal unit 16 comprises two slabs 20, Fig. 1, each having a lead 40 which is wrapped around the terminal bar 41 and is held by a clip 31 connected to the corresponding external wire 29 or 30, Fig. 5. The earphone case is made in two halves, the " outside " portion 26, Fig. 2, and the " inside portion 27, Fig. 3, which fits into the wearer's ear. The outside portion 26 is assembled with a circular guard plate 28 (to protect the crystal unit), and the external lead-wires 29, 30 which enter through a hole 32 in the case pass through a further hole 33 in a raised lug 34 on the guard plate 28 and terminate in the clips 31. As the portion 26 is egg-shaped in plan any external pull on the wires 29, 30 causes the guard plate 28 to wedge into the narrow end of the case and prevent further movement. The portion 26 is pushed on to the sub-assembly with its edge fitting the step 51, Fig. 1, on the frame 14. The inside portion 27 also pushes on to the sub-assembly and engages the diaphragm guard plate.25, the outside edge of the diaphragm 12, and a further step 47, Fig. 1, on the mounting frame 14. Adhesive may be applied between the frame and the case portions to hold them together and to prevent moisture from entering the earphone. The inside portion 27 comprises a body part 48 of substantially the same plan as portion 26, a horn-shaped lug 49 which lies in the groove at the top of the wearer's external ear, and a snout 50 which projects into the ear canal. All three portions are formed from yieldable plastic material so that they adjust their shape to fit differently shaped ears. The walls of the snout 50 decrease in thickness towards the apex at which an opening 57 is provided. The earphone is retained in position by the horn 49 and the smaller end of the portion 26 engaging with the external parts of the ear. The diaphragm 12 is shaped in plan similarly to the body part 48 of the inside portion 27, i.e. the ends 65 and 66, Fig. 5, are arcs of circles (the latter having the smaller radius) and are joined by straight or slightly curved lines. This shape provides a diaphragm of maximum area for the earphone, but if the diaphragm is flat, it introduces inefficiency in operation. Therefore the diaphragm is made substantially conical in crosssection and is driven from off-centre, Fig. 5. In a second embodiment (Figs. 7 to 10, not shown), the diaphragm is circular in plane with an off-centre conical portion. A roughly hemispherical housing replaces the " outside " case portion 26, a flat metal cap fits over the diaphragm side of the frame and is provided with a sound outlet within a raised connector button. An earpiece moulded to fit the individual wearer's ear is snapped on to this button to complete the earphone. The Specification discusses the conditions for optimum sound output. It also discloses a method of constructing the sub-assembly of the embodiment of Fig. 1. The diaphragm 12 is shaped by pressing a sheet of metal between a pair of forming dies. The female die is then removed and the unwanted material is sheared off by means of a cutting die which co-operates with the male forming die and is retained in position to act as a jig. A mounting frame 14 is placed on top of the diaphragm where the cutting die locates it accurately in position. The crystal unit is next placed on the notches 15, and the drive pin 17 is passed through a hole in the apex of the diaphragm 12 into a positioning hole provided in the male forming  die. Adhesive is applied as required to secure the various components together. Finally the cutting die is removed, the sub-assembly is lifted off the male forming die, and the guard plate 25 is fitted. The method of construction is illustrated in Figs. 11 to 13 (not shown). Specification 318,934, [Class 40 (iv)], is referred to.