GB2118398A

GB2118398A - Moving coil electroacoustic transducers

Info

Publication number: GB2118398A
Application number: GB08309762A
Authority: GB
Inventors: Boaz Elieli
Original assignee: Individual
Current assignee: Individual
Priority date: 1982-04-14
Filing date: 1983-04-11
Publication date: 1983-10-26
Also published as: GB2118398B; GB8309762D0

Abstract

A moving coil electroacoustic transducer has a coil 12 located in a magnetic gap 14 and a shorted turn 20, which is mechanically decoupled from the coil, is internally located in the same magnetic gap. The shorted turn drives a first radiating surface 18 and the coil 12 drives a further radiating surface 10. The coil 12 receives its energizing electrical signals via its input terminals 16 while the shorted turn receives its energizing electrical signals exclusively from the coil by means of an electrical transformer action with a high-pass filter characteristics, thus operating as a high frequency transducer without requiring an external electrical crossover network and removing the need for input leads connected to the first radiating surface. <IMAGE>

Description

SPECIFICATION Improvements in moving coil electroacoustic transducers This invention relates to moving coil electroacoustic transducers for use in loudspeakers, earphones and audio frequency sound reproduction devices, and to loudspeakers, earphones and audio frequency sound reproduction devices incorporating such transducers.

Many types of electroacoustic transducers are known, some of which operate by having a voice coil located in a gap. When the voice coil is electrically energised via its input terminals, it is caused to oscillate perpendicular to the magnetic field within the gap and to drive a radiating surface, for example a cone or dome. Sound pressure level (SPL) output in the upper frequency range of such transducers is limited due to the effect of the voice coil inductance and the radiating cone resonant modes, and as a result, the transducer's high frequency output "rolls off" (i.e. falls), above a particular frequency.

In order to extend the transducer's high frequency output level and to improve its acoustical directivity, multiple transducers have been arranged in various ways and confirations to create multi-way transducers and systems. In such an arrangement, an external electrical frequency dividing filter network is necessary, to feed each transducer with signals within its particular bandwidth, and in order to protect the high frequency transducer from thermal and mechanical damage. A common cause of failure in such high frequency transducers in broken voice coil input leads, due to vibration fatique.

No matter how the different transducers have been arranged, the voice coil of each transducer receives its energizing electrical signals through a filter network or direct from the amplifier, via its input terminals.

In accordance with the present invention there is provided an electroacoustic transducer comprising a coil located within a magnetic gap and at least one shorted turn, which is substantially mechanically decoupled from the coil, located in the same magnetic gap, wherein the at least one shorted turn is arranged to drive a radiating surface when an electrical signal is applied to the coil.

The at least one shorted turn is substantially mechanically decoupled from the coil, which means that motion of the coil will not substantially affect the motion of the at least one shorted turn, for instance at frequencies above 3 KH3. Mechanical decoupling cannot give completely independent movement as there will always be some residual transfer of movement passed through the air or other gas, between the coil and the at least one shorted turn.

in operation, when an electrical signal is applied to the coil via its terminals, the at least one shorted turn which is normally located internally to the coil will receive its electrically energizing signal exclusively from the coil by means of an electrical transformer action. Furthermore, this electrical transformer action also provides an electrical high-pass filter to the shorted turn orturns.

It is therefore possible to produce an electroacoustic transducer which will eliminate the necessity for an external frequency dividing filter network, and which will also eliminate the problems associated with broken input leads of the high frequency radiator's voice call.

In a preferred embodiment of the invention, the transducer further comprises a radiating surface driven by the coil.

By providing a further radiating surface driven by the coil, it is possible to configure a transducer which has both a low and/or middle range radiator and a correspondingly higher range radiator and, therefore, to substantially extend and improve the high frequency output of the transducer.

In an embodiment, the at least one shorted turn comprises a continuous metal band.

In an embodiment the at least one shorted turn comprises a continuous shorted winding.

In an embodiment the at least one shorted turn comprises a plurality of independently shorted turns.

In an embodiment, the at least one shorted turn is located internally to the coil.

In an embodiment, the clearance between the at least one shorted turn and the coil is at least 0.01 mm.

In an embodiment, the radiating surface of the at least one shorted turn is internally supported.

By employing an internal supporting suspension, the acoustic output irregularities of the radiating surface driven by the at least one shorted turn are reduced relative to the acoustic output irregularities produced by a conventional radiator having an external suspension.

In an embodiment, the radiating surface driven by the at least one shorted turn comprises a dome, supporting said at least one shorted turn upon its periphery.

It is possible that a conventional electroacoustic transducer may be modified by the placement of such a dome upon the central pole piece of the magnet, such that the periphery of the dome protrudes into the gap between the voice coil and the central pole piece.

The invention will be further described by way of example, with reference to the accompanying drawings, wherein: Figure 1 is a schematic section through a conventional transducer; Figure 2 illustrates the SPL output characteristics of a conventional transducer; Figure 3 is a schematic section through a transducer according to one embodiment of the present invention; Figure 4 is a representation of the SPL output characteristics of a transducer according to one embodiment of the present invention; and Figure 5 is a representation of a typical voltage response of the least one shorted turn relative to a voltage input to the outer coil in an embodiment of the present invention.

Figure 1 shows a conventional moving coil electroacoustic transducer having an annular front piate 7, a pole piece 8, an annular permanent magnetic element 6 and a radiating cone 10 driven by a voice coil 12 located in a magnetic gap 14. The coil is energized via its input terminals 16 and moves along the axis of the arrow 9 which is coaxial with the longitudinal axis of the pole piece 8.

Turning to Figure 2, it will be noted that the output of the transducer shown in Figure 1 rolls off, i.e. falls, above the "roll off" frequency (frollofi).

Figure 3 shows a section through a moving coil electroacoustic transducer substantially similar to that of Figure 1, which embodies the present invention. The transducer comprises an annular front plate 7, a pole piece 8, an annular permanent magnetic element 6 and a radiating cone 10 driven by a voice coil 12, located in a magnetic gap 14. The voice coil 12 is energized via its input terminals 16 and the transducer is suitable for the reproduction of low or/and middle range audio frequency signals. An additional inner radiating dome 18 having a shorted turn 20 is internally supported on the pole piece 8 by suspension means 22. The voice coil 12 and the shorted turn 20 are concentric and substantially mechanically decoupled from each other, with both operating in the same magnetic gap 14.

An acoustically transparent dust dome 24 is provided on the radiator 10.

Only the voice coil 12 receives its energizing electrical signals directly via its input terminals 16, while the shorted turn 20 of the radiating dome 18 receives its energizing electrical signals exclusively from the coil 12 by means of an electrical transformer action. This electrical transformer action between the voice coil 12 and the shorted turn 20, also provides a high pass frequency filter to the shorted ttirn 20.

Figure 4 is a representation of typical output characteristics of a transducer in accordance with the present invention. The dotted line indicates the overall output level of the transducer, while the output level of the radiator driven by the voice coil and the output level of the inner radiating dome drivnen by the shorted turn are indicated by the solid lines (A) and (B), respectively.

The output level, A, of the radiator driven by the voice coil is similar two the output level of an identical transducer which does not include an inner radiating dome, and it will "roll off" at frequency frofloffas with a conventional transducer. The output level, B, of the inner radiating dome, which is horn-loaded by the cone radiator, will substantially extend and enhance the high frequency output of the transducer.

The acoustic crossover frequency, shown in Figure 4 as "f crossover", can be controlled by the suitable selection of the electrical parameters of the voice coil and the shorted turn or turns, together with the mechanical parameters of the inner radiating dome, the shorted turn or turns, the suspension means, and the cone profile.

Figure 5 is a representation of a typical voltage response of the shorted turn relative to a voltage input to the voice coil i.e. transformer voltage response.

The electroacoustic transducer in accordance with this invention, will perform as a multi-way transducear with substantially extended and improved high frequency output, and with improved high frequency power handling capabilities. In addition, it will eliminate both the necessity for an external frequency dividing filter network, and problems associated with broken input leads of the high frequency radiator's voice coil.

Various modifications may be made within the scope of the present invention. For example, the coil may be fixed relative to the magnet. The shorted turn or turns may drive a conventional conical radiator rather than a dome.

Claims

1. An electroacoustic transducer comprising a coil located within a magnetic gap and at least one shorted turn which is substantially mechanically decoupled from the coil, located in the same magnetic gap, wherein the at least one shorted turn is arranged to drive a radiating surface when an electrical signal is applied to the coil.

2. A transducer as claimed in claim 1, further comprising a radiating surface driven by the coil.

3. Atransduceras claimed in claim 1 or 2 wherein the at least one shorted turn comprises a continuous metal band.

4. A transducer as claimed in claim 1 or 2, wherein the at least one shorted turn comprises a continuous shorted winding.

5. A transducer as claimed in claim 1 or 2, wherein the at least one shorted turn comprises a plurality of independently shorted turns.

6. A transducer as claimed in any one of claims 1 to 5, wherein the at least one shorted turn is located internally to the coil.

7. A transducer as claimed in claim 6, wherein the clearance between the shorted turn and the coil is at least 0.01 mm.

8. A transducer as claimed in any one of claims 1 to 7, wherein the radiating surface driven by the at least one shorted turn comprises a dome, supporting said at least one shorted turn upon its periphery.

9. An electroacoustic transducer substantially as hereinbefore described with reference to the accompanying Figure 3.