Isodynamic electromagnetic acoustic transducer with stressed diaphragm

Abstract

 An electromagnetic acoustic transducer having a substantially "flat" diaphragm bears a spiral or meandering current conductor. The diaphragm is positioned adjacent one or more fixed permanent magnets (12,18) which create a magnetic field orthogonal to the current path of the conductor such that the vector force is perpendicular to the plane of the diaphragm. The diaphragm (13) is stressed to form a bowed shape permitting damping elements to be provided without deleteriously affecting the resonance of the transducer.

Description

[0001] The present invention relates to electromagnetic acoustic transducers, and particularly to such transducers of the type suitable for use in headphones or in loudspeakers.

[0002] Electromagnetic acoustic transducers of the general type to which the invention relates have a substantially flat diaphragm having fixed to at least one face thereof an elongate electric current conductor in a spiral or meandering configuration, at least one fixed permanent magnet adjacent to but spaced from the diaphragm on at least one side thereof, the magnet presenting a plurality of magnetic poles arranged so as to produce a magnetic field substantially orthogonal to the majority of the current paths of the conductor such that the interaction between the magnetic field produced thereby and a current flowing through the conductor portions produces a force thereon, and therefore on the diaphragm, substantially normal to the general plane of the diaphragm. Instead of a single magnet with a multiple pole magnetisation, a plurality of magnets, suitably supported may be employed.

[0003] This construction is particularly suitable for the manufacture of transducers for headphones or for loudspeakers in the form of the so-called "flat" loudspeakers. As used in this specification the term "flat" loudspeaker will be understood to refer to the external shape of the transducer, the thickness of which is considerably less than the height or width. Typically, but not exclusively, the thickness of a flat loudspeaker is less than 20% of the next smallest dimension, which is usually the width.

[0004] In transducer constructions of the type defined above, if the diaphragm has a spiral conductor configuration the magnetic poles defined by the magnet or magnets are substantially concentric circular poles, whereas for a meandering conductor (in which usually the majority of the length of the conductor is formed as straight parallel conductor portions) the magnet or magnets are formed to provide a plurality of straight parallel poles. Although constructions employing a magnet or magnet system on any one side of the diaphragm can be employed, greater efficiency is obtained by using a magnet or magnet system on both sides of the diaphragm, and it is clearly an advantage for the magnet or magnet system to be positioned-as closely as possible to the diaphragm in order to obtain the strongest magnetic field for interaction with the current flowing in the conductor on the diaphragm; particularly for use in headphones it is desirable, in order to minimise the weight of the transducer and the amount of material used in the magnets, to use the smallest magnets possible for this purpose. However, the smaller the magnets the weaker the magnetic field and the less efficient the transducer, and therefore, the nearer the diaphragm must be positioned to the magnets.

[0005] The proximity of the magnets to the diaphragm, however, is limited by the maximum excursions which the diaphragm will make during operation since it is essential that the diaphragm should never actually contact the magnets during use because this would give rise to undesirable noises such as clicking or buzzing which would spoil the performance of the transducer. However, the maximum excursions which the diaphragm makes, when producing full volume sound, that is when the current flowing through the conductor is a maximum, occur relatively infrequently in use and thus, for the majority of the period of use, the magnets are spaced further from the diaphragm than they need be.

[0006] In order to overcome this disadvantage the transducers of the general type discussed above have been provided with resilient buffer elements of soft and yieldable material such as foam rubber or batted cotton fibres which allow the diaphragm to vibrate normally, but which prevent physical contact between the diaphragm and the magnets. This procedure has not been entirely successful, however, since the interposition of the resilient buffer elements has modified the resonance frequency of the diaphragm, particularly where unidirectional sound radiation is required. The present invention seeks to provide a transducer of the general type discussed above in which the resonance frequency can be suitably controlled, but in which vibration of the diaphragm can never result in physical contact between the diaphragm and the magnets. According to the present invention there is provided an electromagnetic acoustic transducer incorporating a diaphragm having fixed to at least one face thereof an elongate electric current conductor in a spiral or meandering configuration, one or more fixed.permanent magnets adjacent to but spaced from the diaphragm on each side thereof, the magnet or magnets presenting a plurality of elongate poles lying in substantially parallel planes and being positioned so as to produce a magnetic field orthogonal, or at least substantially orthogonal, to the current path of the said elongate electric current conductor, such that the interaction between the magnetic field and a current flowing through the said elongate electric current conductor produces a force on the diaphragm substantially normal to the said parallel planes defined by the magnetic poles, in which the diaphragm is stressed to a bowed shape between the said magnets.

[0007] By stressing the diaphragm to a bowed shape between the magnets it is possible to control the tension and therefore the resonant frequency of the diaphragm permitting resilient damping elements to be provided on each side thereof without deleteriously affecting the resonance of the transducer.

[0008] Preferably the diaphragm is clamped at its periphery and stressed to a bowed shape by contact with the resilient buffer and support member against at least the central portion on the concave face thereof. This is made from a specially selected grade of polyurethane foam. Alternatively, however, it could be made of soft down or mineral wool. The resilient buffer and support member may cover an area less than, equal to or more than the area of the radiating window of the diaphragm (but less than the total diaphragm area) and this element acts not merely to prevent the diaphragm from hitting the magnets, but also to tension the diaphragm and to control its resonance frequency.

[0009] In a preferred embodiment of the invention the periphery of the diaphragm is reinforced by a peripheral diaphragm support laminated thereto on at least one face thereof. Prior to lamination the diaphragm can be tensioned in one or all directions, and this pretensioning also assists in controlling the resonance frequency of the diaphragm.

[0010] Rather than being provided on one face only, there may be a peripheral diaphragm support laminated to each face of the diaphragm at the periphery thereof.

[0011] In a preferred embodiment of the invention the magnet or magnets on each side of the diaphragm is or are supported on a backing plate and the backing plates are spaced a predetermined distance apart by a rigid peripheral support frame, the periphery of the diaphragm being clamped between resilient peripheral diaphragm support members compressed between the backing plates, the combined thickness of which resilient peripheral diaphragm support members is greater than the thickness of the rigid peripheral support frame. The diaphragm is thus clamped by the resilient peripheral diaphragm support members at the perimeter and accurately located in position thereby. These resilient peripheral diaphragm support members are preferably made of a foam material, such as a closed cell foam.

[0012] In order to obtain variations in the compression of the resilient peripheral diaphragm support members, and in the spacing.between the magnets and the diaphragm, there is preferably further provided at least one spacer ring between the rigid peripheral support frame and one of the said backing plates. By interchanging this spacer ring for others of different thickness the effective thickness of the support frame can be changed.

[0013] The backing plates, in a simple embodiment, may both have openings therein through which acoustic radiation generated-by vibrations of the diaphragm can be transmitted.

[0014] Control on the performance of the transducer can also be achieved by varying the size of the magnet or magnets on at least one side of the diaphragm; these magnets may therefore cover an area less than the free unsupported area of the diaphragm itself.

[0015] In a preferred embodiment of the invention there is provided a further resilient buffer element between the diaphragm and the magnets on the convex side of the diaphragm, this buffer element being in contact with at least the central region of the diaphragm although it may be in contact with the diaphragm over substantially the whole of the free unsupported area thereof.

[0016] Various different shapes may be employed in order to obtain further variations in the performance of the transducer. For example, although a transducer having a circular diaphragm will be described herein by way of example, it will be understood that the diaphragm may be rectangular, elliptical or, indeed, any other regular or irregular shape, and may be tensioned differently in one or more directions, or may be tensioned equally in all directions.

[0017] Likewise, the magnet structure may be one employing a single magnet plate having a plurality of holes therein and magnetised to present a plurality of magnetic poles extending substantially parallel to the conductors on the diaphragm (that is, substantially parallel spaced poles in the case of a meandering conductor pattern, and substantially concentric circular poles in the case of a spiral conductor pattern), or a plurality of magnets each presenting a single elongate polar region may be provided as a magnet system or magnet assembly.

[0018] One embodiment of the present invention will now be more particularly described, by way of example, with reference to the accompanying drawings, in which:

Figure 1 is a schematic axial section of a first embodiment of the invention; and

Figures 2A-2C are three frequency response curves illustrating the effect of variations in the dimensions and physical properties of the component parts of the embodiment of Figure 1.

[0019] Referring first to Figure 1 the embodiment shown is a flat electromagnetic acoustic transducer incorporating a diaphragm 13 on which is formed, by known means, a meandering conductive track on each opposite face. The meandering conductive track has a plurality of parallel spaced portions which are aligned with the gaps between adjacent poles of a magnet system comprising an array of elongate front magnets 12 on one side of the diaphragm and an array of elongate rear magnets 18. As is known for transducers of this general type the magnets 12 are magnetised to present elongate magnetic poles and are orientated so that adjacent poles are of opposite polarity. Likewise, the rear magnets 18 are so arranged that they provide elongate strip poles of alternate polarity, like poles facing each other across the diaphragm so that the magnetic field in the region of the diaphragm extends generally parallel to the plane of the diaphragm and orthogonally with respect to the length of the parallel portions of the conductor strip attached thereto. Preferably the diaphragm 13 has corresponding elongate conductor strips on each face thereof and is made from a non-magnetic plastics material. Although the 'magnets 12 and 18 are shown as a series of individual magnets, it will be appreciated that each set may be constituted by a single magnet one on each side of the diaphragm, and magnetised to present a plurality of poles in corresponding positions.

[0020] Likewise, although parallel elongate magnetic poles are shown in the drawing, it will be appreciated that the invention is equally applicable to constructions in which the magnetic poles are concentric circles and the conductor strip on the diaphragm is a spiral or a plurality of spirals.

[0021] The periphery of the diaphragm 13, which may be a .rectangular, circular, elliptical or other form, is laminated on each side to a peripheral diaphragm support member 10, the diaphragm being pretensioned prior to lamination. The magnets 12 are supported on a backing plate 11 and the magnets 18 are supported on a backing plate 17, the two backing plates being held parallel to one another and spaced a predetermined distance apart by a main frame 20 which surrounds the periphery of the diaphragm 13. A spacer ring 16 is provided to adjust the spacing of the backing plates 11 and 17 (by substitution of spacer rings of differing thicknesses) and the whole assembly is secured together by means of bolts passing through the two backing plates 11, 17 and the main support frame 20. The diaphragm 13 is located accurately in position by means of two resilient peripheral diaphragm support members 15a, 15b which are compressed between the backing plates 11, 17.

[0022] Between the magnets 18 on the rear face of the diaphragm 13, and the diaphragm itself is located a rear polyurethane foam buffer and support member 19 which is shaped to stress the diaphragm to a curved shape, concave towards the rear magnets 18. Between the front magnets 12 and the diaphragm 13 is located a front resilient buffer element 14 which extends over an area which is substantially smaller than the free unsupported area of the diaphragm, that is the area bounded on the outside by the inner periphery of the annular peripheral diaphragm support to which the diaphragm is laminated. The thickness of the front resilient buffer 14 is substantially less than that of the buffer and support member 19 on the rear face of the diaphragm, this buffer serving purely to prevent contact between the diaphragm 13 and the magnets 12.

[0023] In this embodiment each of the front and rear backing plates 11, 17 respectively have apertures therein permitting acoustic radiation generated by vibrations of the diaphragm 13 to pass in both directions. In-order to exercise further control over the resonance of the diaphragm a closed acoustic cavity may be provided on the rear side of the diaphragm. In Figure 2 are shown frequency response curves illustrating variations in the components of the embodiment of Figure 1. In Figure 2a the ideal frequency response is shown in broken outline whilst the solid line represents the form of the frequency response if the foam material of the resilient buffer support 19 is too thick or too stiff. The same effect occurs if the spacer ring 16 is too thin which would result in the gap between the magnets being too small. As can be seen the frequency response falls off too rapidly at the low frequency end of the spectrum and has a peak at the lower end of what would otherwise be the level frequency response region.

[0024] Figure 2b illustrates the frequency response obtained if the foam material of the resilient buffer and support member 19 is too thin or too soft. The same effects can be achieved if the spacer ring 16 is too thick which would make the gap between the magnets too large. As can be seen the frequency response extends further than the ideal response, and instead of being level has a pronounced peak and valley at the lower end.

[0025] Figure 2c shows the effect of making the rear magnets of insufficient depth, assuming the foam materials of the resilient buffer and support member 19 are correct and the spacer.ring 16 is of appropriate thickness. As can be seen the frequency response falls off steadily from a peak rather than being level over the required frequency range.

[0026] The embodiment of the invention described herein is secured together by bolts passing through the frame member 20 and the backing plates 11 and 17. Alternatively, the backing plates may simply be screwed into the frame member 20, or the frame 20 may have pins or other projections which can be swaged with heat to hold the plates in position once located. Likewise, locating pins (not shown) on the frame 20 may project into corresponding openings in the peripheral diaphragm support 10 to ensure that the conductor tracks on the diaphragm are accurately lined up with the magnets 12, 18 carried by the backing plates 11, 17.

Claims

1. An electromagnetic acoustic transducer incorporating a diaphragm having fixed to at least one face thereof an elongate electric current conductor in a spiral or meandering configuration, one or more fixed permanent magnets adjacent to but spaced from the diaphragm on each side thereof, the magnet or magnets presenting a plurality of elongate magnetic poles lying in parallel planes and being positioned so as to produce a magnetic field orthogonal or substantially orthogonal to the current path of the said elongate electric current conductor such that the interaction between the magnetic field and a current flowing through the said elongate electric current conductor produces a force on the diaphragm substantially normal to the said parallel planes defined by the magnetic poles, in which the diaphragm is stressed to a bowed shape between the said magnets.

2. An electromagnetic acoustic transducer as claimed in Claim 1, in which the diaphragm is clamped at its periphery and stressed to a bowed shape by contact with a resilient buffer and support member against at least the central portion on the concave face thereof.

3. An electromagnetic acoustic transducer as claimed in Claim 1 or Claim 2, in which the periphery of the diaphragm is reinforced by a peripheral diaphragm support laminated thereto on at least one face thereof.

4. An electromagnetic acoustic transducer as claimed in Claim 3, in which there is a peripheral diaphragm support laminated to each face of the diaphragm at the periphery thereof.

5. An electromagnetic acoustic transducer as claimed in any preceding claim, in which the magnet or magnets on each side of the diaphragm is or are supported on a backing plate and the backing plates are spaced a predetermined distance apart by a rigid peripheral support frame, the periphery of the diaphragm being clamped between resilient peripheral diaphragm support members compressed between the backing plates, the combined thickness of which resilient peripheral diaphragm support members is greater than the thickness of the rigid peripheral support frame.

6. An electromagnetic acoustic transducer as claimed in Claim 5., in which the resilient peripheral diaphragm support members are made of a foam material.

7. An electromagnetic acoustic transducer as claimed in Claim 6, in which the resilient peripheral diaphragm support members are made of a closed cell foam material.

8. An electromagnetic acoustic transducer as claimed in any of Claims 2 to 7, in which the central resilient buffer and support member in contact with at least the central region of the diaphragm is made of a polyurethane foam material.

9. An electromagnetic acoustic transducer as claimed in any of Claims 5 to 8, in which there is further provided at least one spacer ring between the rigid peripheral support frame and one of the said backing plates.

10. An electromagnetic acoustic transducer as claimed in any of Claims 3 to 9, in which the diaphragm is pretensioned in at least one direction before lamination to the reinforcing peripheral diaphragm support.

11. An electromagnetic acoustic transducer as claimed in Claim 10, in which the diaphragm is pretensioned equally in all directions prior to lamination to the reinforcing peripheral diaphragm support.

12. An electromagnetic acoustic transducer as claimed in any of Claims 5 to 11, in which there are openings in both backing plates permitting radiation from both faces of .the diaphragm.

13. An electromagnetic acoustic transducer as claimed in any preceding claim, in which the magnet or magnets on at least one. side of the diaphragm cover an area less than the free unsupported area of the diaphragm itself.

14. An electromagnetic acoustic transducer as claimed in any preceding claim, in which there is provided a further resilient buffer element between the diaphragm and the magnets on the convex side of the diaphragm in contact with at least the central region of the diaphragm.

15. An electromagnetic acoustic transducer as claimed in Claim 14, in which the said further resilient buffer element is in contact with the diaphragm over substantially the whole of the free unsupported area thereof.

16. An electromagnetic acoustic transducer as claimed in any preceding claim, in which the diaphragm is substantially circular.

17. An electromagnetic acoustic transducer as claimed in any of Claims 1 to 15, in which the diaphragm is substantially elliptical.

18. An electromagnetic acoustic transducer as claimed in Claim 17, in which the tension in the diaphragm is different in different directions.

Drawing