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-4 272 654 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 invention consists in an electroacoustics transducer including a pair of spaced permanent magnets, a coil having a tunnel therethrough and a flat elongate reed armature having a central portion which extends through the coil tunnel 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 the tunnel is defined solely by the windings of the coil and in that at least one part of the cross-section of the tunnel is dimensioned or shaped to restrict movement of the reed armature in a direction orthogonal to its plane. The part may thus be narrower than the rest of the tunnel in the direction of reed armature movement or the movement restricting part may be engaged by a part of the reed armature before the tunnel wall would otherwise be engaged by the reed armature. Preferably the movement restricting part is not reentrant to avoid problems in winding the coil. For example the movement restricting part consists in cut off corners of an otherwise rectangular cross-sectioned tunnel. The invention may be performed in various ways. Specific embodiments will now be described with reference to the accompanying drawings of which Figures 1 to 9 illustrate embodiments leading to the invention but not falling under the scope of protection of the present claims and which are the subject of other Patent Applications, in which: Figures 1-7 illustrate an arrangement claimed in our co-pending European Patent Application No. 93 923 007.4 and figures 8 and 9 are claimed in co-pending European Patent Application No. 99 102 640.2, but are all retained so as to illustrate the operation of the subsequent embodiments. Figure 1 is a cross sectional stylised view through an electroacoustics 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 to 9 are schematic views generally corresponding to Figure 1 showing alternative arrangements; and Figures 10 to 12 show a number of tunnel cross-sections according to the invention. An electroacoustic transducer 10 is schematically shown in Figures 1 to 3. 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 another approach 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 an insert 26 is pressed into the end of the tunnel 18 or created using settable plastic material. Preferably the restriction so created is confined to the area of the longitudinal axis for the reasons mentioned above. Figures 10 to 12 illustrate embodiments of the invention of the present Application in which the coil 12, which has its tunnel 18 defined purely by its windings, is wound in such a way that at in least one part of its cross-section, the tunnel walls are formed to engage the reed 17 in a way which reduces damaging deflections of the reed. Thus in Figure 10 the coil 12 is wound with projections 27 projecting into the tunnel 18; in Figure 11, which is a longitudinal section of the coil 12, the tunnel has a reduced cross-section at 28 and in Figure 12 the corners of the rectangular tunnel 18 are cut off to provide inclined reed engaging surfaces 29. 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 electroacoustics transducer (10) including a pair of spaced permanent magnets (13, 14), a coil (12) having a tunnel (18) therethrough and a flat elongate reed armature (17) having a central portion which extends through the coil tunnel (18) 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 the tunnel (18) is defined solely by the windings of the coil (12) and in that at least one part (27; 28; 29) of the cross-section of the tunnel (18) is dimensioned or shaped to restrict movement of the reed armature (17) in a direction orthogonal to its plane. A transducer as claimed in Claim 1, characterised in that the movement restricting part (27; 28; 29) of the tunnel (18) is narrower than the rest of the tunnel (18) in the direction of the reed armature movement. A transducer as claimed in Claim 2, characterised in that the movement restricting part (27; 28; 29) is engaged by a part of the reed armature (17) before the tunnel wall would otherwise be engaged by the reed armature (17). A transducer as claimed in Claim 3, characterised in that the movement restricting part (27; 28; 29) is not reentrant. A transducer as claimed in Claim 4, characterised in that the movement restricting part (29) consists in cut off corners of an otherwise rectangular cross-section tunnel (18).

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