Electroacoustic transducer

Abstract: This invention relates to electroacoustic transducers which incorporate a reed armature 17. Formations (25, 26, 27, 28 and 29) are provided for mounting the deflection of the reed 17.

Description: This invention relates to electroacoustic transducers of the type which incorporate a reed armature. An electric transducer of this general type is described in our British Patent GB-A-2095510, and typically includes a pair of spaced permanent magnets, a coil having a tunnel therethrough and a reed armature having a central portion which extends through the coil and a tip portion which lies at least partially between the magnets. The arrangement is such that when the moving part of the reed shifts in one direction or another away from a centralised position between the two poles, the magnetic flux is caused to flow in one direction or the other along the reed and hence through the coil. The reed is attached to a diaphragm and in this way the vibrations of the diaphragm caused by received sound are converted into corresponding currents in the coil or vice versa. It is very easy to damage the reed by over deflection, if the transducer experiences a shock e.g. from being dropped, in addition the tip portion may strike the magnet with considerable force. It will be understood that a similar configuration can be used for a receiver or loudspeaker. In one transducer, described in US-A-4272654 according to which the preamble of claim 1 has been delimited, the coil tunnel has a restricted central portion which limits the degree of deflection available to the reed and hence reduces the possible damage. However, because coils in such transducers are almost always formerless, this solution is only available when the coil is set in epoxy having been wound on a flexible removable plastic former. This method of construction is only appropriate for certain types of transducers and has a number of limitations. The present invention consists in an electroacoustic transducer including a pair of spaced permanent magnets, a coil having a tunnel therethrough and a reed armature having a central portion which extends through the coil and a tip portion which lies at least partially between the magnets, the reed armature being mounted for deflection towards or away from the respective magnets, characterised in that a restriction having the form of an aperture is defined in a spacer located between the coil and the magnets for limiting the available deflection of the reed armature, the dimension of the aperture, in the direction of deflection of the reed armature, being smaller than the corresponding dimension of the tunnel. It is preferred that the restriction is constructed only to engage an area close to or adjacent the longitudinal axis of the reed armature. The invention may be performed in various ways and specific embodiments will now be described with reference to the accompanying drawings of which Figures 1 to 7 illustrate embodiments leading to the invention and which are the subject of another Patent Application, in which: Figure 1 is a cross sectional stylised view through an electroacoustic transducer with the reed in its central position; Figure 2 shows the reed in its extreme upward position; Figure 3 shows the reed in its extreme downward position; Figure 4 is a view from above of the reed of the transducers of Figures 1 to 3; Figure 5 is a cross sectional view of the reed of Figure 4 along the line IV - IV; Figures 6 and 7 are schematic views generally corresponding to Figure 1 showing other illustrative embodiments; and Figures 8 and 9 are schematic views generally corresponding to Figure 1 showing arrangements in accordance with the invention; An electroacoustic transducer 10 is schematically shown in Figures 1 to 3. The Figures illustrate an arrangement claimed in our co-pending European Patent Application No. 93923007.4, but are retained so as to illustrate the operation of the subsequent embodiments. The transducer 10 comprises a coil 12, magnets 13,14, pole pieces 15,16 and reed armature 17. As can be seen in Figure 1 the coil 12 defines a central tunnel 18 and the magnets 13,14 are spaced apart. The reed armature 17 extends along the tunnel 18 and between the magnets 13,14. A central portion 19 of the reed 17 lies within the tunnel 18 and, adjacent one end of that central portion 19 are formed opposed formations 20,21. As can be best seen in Figures 4 and 5 the formations 20,21 are generally semi-spherical and are pressed out of the plane of the reed. As can be seen in Figures 2 and 3 the formations 20,21 respectively engage the coil 12 when the reed is deflected upwardly or downwardly beyond its normal working range. Preferably the formations are sized to prevent the tip 22 of the reed 17 striking the magnets 13,14, but considerable protection can also be provided if they are dimensioned so that they strike the coil 12, before the tip 22 strikes either the magnet 13 or magnet 14. It has been discovered that a transducer constructed in this manner can be dropped from, typically, twice the height, without incurring damage to the reed, as compared with an identical transducer without the formations. Put another way this means it can, typically, receive up to 4 times the shock without damage. It will be appreciated that the formations could have other conventional shapes, but they are confined to the general area of the longitudinal axis of the reed 17 so that their existence introduces as little restriction as possible on the rotational or lateral position of the reed 17 within the tunnel 18. In a construction where this was not a factor, the formations could be at the sides of the reed 17 or extend right across it. As has been explained above this construction has particular advantages; for example no extra mass is added to the reed 17. However many of these advantages can be obtained with other arrangements and these are briefly described in connection with Figures 6 to 9. In Figure 6 the formations 20,21 are formed by blobs of adhesive or other settable material. In Figure 7 the formations 20,21 are formed by a sleeve 23 slid onto the reed 17. Figures 8 and 9 show the approach according to the present invention in which a restriction is introduced either between the magnets 13,14 and the coil 12 or between the magnets 13, 14 and the coil 12 and at the magnet end of the tunnel 18, so that the restriction engages the reed 17 to limit the deflection in a similar manner to that achieved by the formations mentioned above. Thus in Figure 8 a spacer 24 has a restricted opening 25 whilst in Figure 9 a restriction in the form of an insert 26 is located at the end of the tunnel 18 by means of a portion pressed into the end of the tunnel 18 or is created using settable plastic material. Preferably the restriction so created is confined to the area of the longitudinal axis for the reasons mentioned above. It will be understood that the restriction principles recited in this description are applicable to many other reed mountings and configurations provided they fall within the scope of protection of the claims.

Claims: An electroacoustic transducer (10) including a pair of spaced permanent magnets (13, 14), a coil (12) having a tunnel (18) therethrough and a reed armature (17) having a central portion which extends through the coil (12) and a tip portion (22) which lies at least partially between the magnets (13, 14), the reed armature (17) being mounted for deflection towards or away from the respective magnets (13, 14), characterised in that a restriction having the form of an aperture (25) is defined in a spacer (24; 26) located between the coil (12) and the magnets (13, 14) for limiting the available deflection of the reed armature (17), the dimension of the aperture (25), in the direction of deflection of the reed armature (17), being smaller than the corresponding dimension of the tunnel (18). A transducer as claimed in Claim 1, characterised in that the reed armature (17) is elongate and the restriction is constructed to engage only an area on or adjacent the longitudinal axis of the reed armature (17).

Inventors: HARRINGTON STEVEN JOHN DR, SALVAGE RICHARD JAMES, HARRINGTON, STEVEN JOHN DR., SALVAGE, RICHARD JAMES
Applicants: KNOWLES ELECTRONICS CO, KNOWLES ELECTRONICS CO.
Application year: 1993
Main classification: H04R11
IPC codes: H04R11, H04R25