ELECTROMAGNETIC SOUNDING DEVICE

Abstract: An electromagnetic sounding device applicable to a paging receiver or like radio receiver has a miniature and thin configuration and is stably operable even when subjected to a substantial magnitude of impact.

Description: BACKGROUND OF THE INVENTION The present invention relates to a miniature electromagnetic sounding device applicable to, for example, a portable radio receiver for a radio paging system. A paging receiver which is a typical example of portable radio apparatuses known in the art receives a signal of a predetermined format from a central station. If the signal received is identical with a code which is assigned to the paging receiver, the receiver produces sound to alert the user to the reception of a call. Since this kind of receiver is usually carried in the user's breast pocket or the like, its overall configuration should preferably be as miniature as a cigarette lighter or the like. Therefore, a sounding device installed in such a portable receiver has to be designed extremely small size. Other prerequisites with a sounding device for this kind of application are that it produces an acoustic output above a certain reference level which overcomes possible noisy environment, and that it sufficiently withstands shocks and impacts which will be applied when it is let fall while being carried. However, prior art paging receivers such as one disclosed in Japanese Utility Model Publication (Kokai) No. 57-125100 have substantial overall thickness due to their inherent structures. Should they be reduced in thickness, they would fail to achieve the sufficient acoustic output. In the specification of United States Patent No. 4,413,253, there is described a sounder in which a diaphragm closes one end of a tuned chamber. The other end of the chamber is closed by a plate through which a hollow tube extends at right angles to the plane of the diaphragm. By varying the length and diameter of the tube the resonant frequency of the chamber may be altered. It is an object of the present invention to provide a thin and miniature electromagnetic sounding device. It is another object of the present invention to provide a generally improved electromagnetic sounding device. According to the present invention, an electromagnetic sounding device is as claimed in claim 1. BRIEF DESCRIPTION OF THE DRAWINGS The above and other objects, features and advantages of the present invention will become more apparent from the following detailed description taken with the accompanying drawings in which: Fig. 1 is a sectional side elevation showing a prior art electromagnetic sounding device; Fig. 2A is a plot representative of a sound pressure characteristic particular to the prior art sounding device; Fig. 2B is a view similar to Fig. 2A, showing a sound pressure characteristic achievable with the present invention; Fig. 3 is a sectional side elevation of an electromagnetic sounding device embodying the present invention; Fig. 4 is a plan view of the sounding device of Fig. 3; Fig. 5 is a fragmentary sectional side elevation of the sounding device as shown in Fig. 3; and Fig. 6 is an exploded perspective view of the sounding device as shown in Fig. 3. DESCRIPTION OF THE PREFERRED EMBODIMENT To better understand the present invention, a brief reference will be made to a prior art electromagnetic sounding device which is disclosed in Japanese Utility Model Publication (Kokai) No. 57-125100. As shown in Fig. 1, the prior art sounding device 10 includes a magnetic force generating section which is made up of a base 12 having a center pole 14, and an annular coil 16 and a permanent magnet 18 which are concentrically arranged around the center pole 14. The sounding device 10 also includes a vibrating section defined above the magnetic force generating section and made up of a movable iron piece type vibrator 20 and a support frame 22 which supports the vibrator 20. The vibrator 20 is caused to vibrate by a static magnetic force which is exerted by the permanent magnet 18. Specifically, the vibrator 20 is constituted by a flexible diaphragm 20a and a magnetic disk 20b fixed to the diaphragm 20a by spot welding. A film 24 made of synthetic resin is disposed above the vibrator 20 and supported by the rim of a frame 22. A first presser member 26 is located above the film 24 and also supported by the rim of the frame 22, the member 26 being notched to have windows 26a. Disposed above the first presser member 26 is a second presser member 28 which is provided with openings 28a. All the structural elements mentioned so far are accommodated in a casing 30 which is formed with a window 30a. A terminal plate 32 constitutes the bottom of the casing 30. Leads for feeding signal voltage to the coil 16 are connected to the terminal plate 32. When signal voltage is applied to the coil 16 via the terminal plate 32, the vibrator 20, i.e., diaphragm 20a is caused into vibration to produce a sound pressure corresponding to the signal voltage. The sound pressure is radiated to the outside as an acoustic output by way of the openings 28a of the second presser member 28 and the window 30a of the casing 30. Fig. 2A shows a sound pressure characteristic particular to the prior art sounding device 10 as actually measured when the input signal supplied was a rectangular wave of 1.4 ρ₋ρ and the distance from a microphone was 10 centimeters. In Fig. 2A, the ordinate and the abscissa indicate respectively the acoustic output P₀ (dB) and the resonance frequency f₀ (kHz) of the diaphragm 20a. As shown in Fig. 1, the prior art sounding device 10 use the two presser members 26 and 28 in addition to the magnetic force generating section and vibrating section. To accommodate such extra members, the casing 30 has to be provided with a substantial thickness, or height, H although its width L is not questionable. This obstructs the miniaturization of the device 10. In this connection, a magnetic sounding device actually marketed with the above-described structure has a height H of 6 millimeters and a width L of 10 millimeters. Referring to Figs. 3 to 5, an electromagnetic sounding device in accordance with the present invention is shown and generally designated by the reference numeral 40. As shown, the device 40 includes a base 42 which serves as a base pole. A cylindrical center pole 44 is studded on a central part of the base 42. An annular coil 46 is fit over the center pole 44 while a permanent ring magnet 48 is located to surround the annular coil 46. The upper ends of the center pole 44 and ring magnet 48 are substantially coplanar with each other. The base pole 42, center pole 44, coil 46, and ring magnet 48 constitute a magnetic force generating section. A support frame 50 is composed of synthetic resin or like material and disposed at the circumferential edge portion of the base pole 42. The upper end of the support frame 50 is provided with an annular knife edge configuration as indicated by 50a. A thin flexible diaphragm 52 is supported on the knife edge 50a of the support frame 50. A magnetic disk 54 is coaxially positioned on the upper surface of the diaphragm 52 and fixed at its central portion 56 to the latter by spot welding, whereby the diaphragm 52 is provided with permeability. The diaphragm 52 and magnetic disk 54 in combination constitute a vibrating section. The edge portion of the diaphragm 52 rests on and is magnetically retained on the knife edge 50a of the support frame 50 by the acctraction of the magnetic force generating section, while being regulated in deformation by that force. Defined between the underside of the diaphragm 52 and the coplanar upper ends of the center pole 44 and ring magnet 48 is a narrow air gap magnetic field 58. A DC magnetic flux ⊘m created by the ring magnet 48 extends throughout the base pole 42 and center pole 44, as represented by a dash-and-dot line in Fig. 3. In this condition, the edge portion of the diaphragm 52 is retained on and pressed against the knife edge 50a. A casing 60 made of synthetic resin or the like is engaged with and covers the support frame 50 while defining a narrow air gap 62 between its inner surface and the upper surface of the diaphragm 52. As shown in Figs. 3 and 5, a plurality of (usually four or so) short lugs 64 and a plurality of shoulders 66 each being contiguous with a respective one of the lugs 64 are provided on the inner surface of the casing 60 and arranged at equally spaced locations along the circumference of the the casing 60. Specifically, each lug 64 extends downward slightly beyond the general plane of the diaphragm 52 as viewed in Fig. 3. An air gap 66a is defined between the lugs 64 and the periphery of the diaphragm 52 while an air gap 66b is defined between the lugs 64 and the circumferential wall of the casing 60, the air gaps 66a and 66b serving to minimize obstruction to the propagation of sound. On the other hand, each shoulder 66 extends radially inwardly beyond the edge of the diaphragm 52 and is dimensioned short enough to define an adequate air gap 66c in cooperation with the diaphragm 52. The shoulders 66 and the diaphragm 52 usually remain spaced from each other. A rectangular elongate window 68 is formed through the circumferential wall of the casing 60 so that an acoustic output P₀ may be propagated therethrough to the outside. A piece of cloth 70 is attached to the casing 60 to cover the window 68, preventing dust and other impurities from entering the casing 60. A terminal plate 74 is located on the bottom of the base 42 and frame 50. Fig. 6 is an exploded perspective view of the sounding device as mentioned above. In operation, signal voltage e is applied to the annular coil 46 to cause current i to flow, as shown in Fig. 3. Then, the DC magnetic flux Øm from the ring magnet 48 (indicated by a dash-and-dot line in Fig. 3) and the magnetic flux Øs which is developed by the voltage e (indicated by a dashed line in Fig. 3) are superposed to generate a vibromotive force in the diaphragm 52. As a result, the diaphragm 52 vibrates to develop a sound pressure P₁ in the air gap 62. This sound pressure P₁ is propagated radially into a space 72 which is defined around the diaphragm 52. Further, the sound pressure P₁ is guided along the periphery of the diaphragm 52 to the window 68 of the casing 60, as indicated by arrows in Fig. 4. Consequently, an acoustic output P₀ is intensely radiated to the outside via the window 68. As shown and described, the device 40 has the window 68 which is formed through the circumferential wall of the casing 60. The device 40 therefore can be provided with a miniature and thin configuration. For example, assuming that the width l of the device 40 is substantially the same as the width L of the prior art device 10, i.e., 10 centimeters, the thickness or height h of the device 40 is as small as 2 millimeters which is one-third of the thickness H of 6 millimeters of the device 10. In addition, since the sound pressure P₁ developed in the air gap 62 is readily propagated to the window 68 by way of the circumferential space 72 as indicated by arrows in Fig. 4, the propagation loss of sound pressure P₁ in the path defined within the casing 60 is small enough to allow the acoustic output P₀ to be intensely radiated to the outside. Furthermore, the diaphragm 52 is laid on and supported by the knife edge 50a along its peripheral portion due to the magnetic force of the ring magnet 48 and, therefore, it is entirely free from restraint. This extremely increases Q of the vibration system to thereby further improve conversion sensitivity. Fig. 2B shows a curve representative of a sound pressure characteristic of the device 40 which was measured under the same conditions as the prior art device 10. As shown, the device 10 achieves an acoustic output which is comparable with that of the prior art device 10, despite the miniature and thin configuration. Specifically, the acoustic output P₀ attainable with the device 10 is higher than 80 dB or, rather, close to 90 dB. When an impact whose magnitude is greater than 5G is applied to the device 40 such as when it is let fall, lateral displacement and vertical displacement of the diaphragm are effectively eliminated by the lugs 64 and the shoulders 66, respectively. Hence, the device 40 is operable with stability overcoming such a magnitude of impact. It is to be noted that the lugs 66 are omissible if the air gap 62 is relatively narrow. In summary, it will be seen that the present invention provides an electromagnetic sounding device which is miniature, thin, and operable efficiently and stably even when a substantial magnitude of impact is applied thereto from the outside. Various modifications will become possible for those skilled in the art after receiving the teachings of the present disclosure without departing from the scope of the present invention as defined in the appended claims.

Claims: An electromagnetic sounding device (40) including electro-magnetic force generating means (46, 48) for generating a magnetic force, diaphragm vibration means (52, 54) arranged to vibrate in response to variations in the magnetic force created by the generating means (46, 48), a support (50) for supporting both the magnetic force generating (46, 48) and the vibration means (52, 54) and a casing (60) enclosing the support (50), the magnetic force generating means (46, 48) and the vibration means (52, 54), characterised in that there are provided a space (72) between an edge of the vibration means (52, 54) and a circumferential wall of the casing (60) and a window (68) in the circumferential wall of the housing and in that the propagation path of sound pressure P1, which is developed by the diaphragm vibration means (52, 54), is in a direction parallel to the plane of the diaphragm vibration means (52, 54) to the space (72) from which the sound pressure P1 is radiated via the window (68). A sounding device as claimed in claim 1, characterised is that the magnetic force generating means (46, 48) includes a base (42) constituting a base magnetic pole (42), a centre magnetic pole (44) provided in a central part of the base (42), a coil (46) surrounding the centre pole (44), and a permanent magnet (48) surrounding the coil (46). A sounding device as claimed in claim 2, characterised in that the support (50) includes a member located at a circumferential edge portion of the base (42) and provided with a knife edge portion (50a) at the upper end of the member. A sounding device as claimed in claim 3, characterised in that the vibration means (52, 54) includes a diaphragm (52) laid on and magnetically retained on the knife edge portion (50a) of the support frame member and a magnetic plate (54) which is rigidly mounted in a central part of the diaphragm (52). A sounding device as claimed in claim 4, characterised in that the casing (60) includes a plurality of lugs (64) which extend individually from an inner surface of the casing (60) and are arranged at equally spaced locations along the circumference of the casing (60). A sounding device as claimed in claim 5, characterised in that the casing (60) further includes a plurality of shoulders (66) which are positioned radially inwardly of the lugs (64). A sounding device as claimed in claim 6 characterised in that the lugs (64) each extend below the general plane of the diaphragm (52) in such a way that an air gap is defined between the lugs (64) and the periphery of the diaphragm (52) and an air gap (66b) is defined between the lugs (64) and the circumferential wall of the casing (60), the air gaps (66a) and (66b) minimising any obstruction to the propagation of sound.

Inventors: KAIDO, SABURO
Applicants: NEC CORPORATION
Application year: 1988
Main classification: G10K9
IPC codes: H04R13, G10K9, B06B1