GB1599506A

GB1599506A - Dynamic transducer

Info

Publication number: GB1599506A
Application number: GB13050/78A
Authority: GB
Original assignee: Licentia Patent Verwaltungs GmbH
Current assignee: Licentia Patent Verwaltungs GmbH
Priority date: 1977-04-09
Filing date: 1978-04-04
Publication date: 1981-10-07
Also published as: DE2716063A1; SE421099B; IT7822093A0; SE7803728L; IT1095115B; FR2386954B1; DE2716063B2; JPS53146614A; FR2386954A1

Description

(54) IMPROVEMENTS IN OR RELATING TO A DYNAMIC TRANSDUCER (71) We, LICENTIA PATENT VERWALTUNGS GESELLSCHAET MIT BESCHRANKTER HAFTUNG, a company organised under the laws of Germany, of Theoder-Stern-Kai 1,6 Frankfurt/Main 70, Germany, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to dynamic transducers.

Dynamic transducers in the form of loud speakers are known (Instr. A. Control Syst. 48 (1975) 10 pages 41-44) in which a space in a magnetic body called the 'air gap', in which a moving coil of the dynamic arrangement is movable, is filled with a magnetic fluid, for example, magnetic particles colloidally suspended in a carrier liquid. The magnetic fluid is retained in the air gap by the permanent magnetic field field of the magnetic body. Its purpose is to effect an improvement in the acoustic properties, in particular as regards distortion factor and the phase behaviour of the loud speaker system. Also, the manufacture of the loud speaker is simplified since manufacturing discrepancies have less effect. The presence of the filling causes the 'air gap' to assume properties that approximate to the properties of the poles of the magnet.

Obviously, the air gap is reduced in this way to the value zero.

A further important advantage as compared with unfilled air gap is the improved behaviour towards heat generated in the coil. In comparison with air the magnetic fluid has a very much higher thermal conductivity, so that the resistance to the passage of heat from the moving coil winding to the magnet pole body is reduced. The maximum permissible input power is thereby increased for the same maximum temperatures of the moving coil.

It is an object of the invention to provide an improved dynamic transducer.

According to the invention there is provided a dynamic transducer including a magnetic body and a movable coil, the magnetic body defining a cavity within the body with a magnetic pole air gap in which the coil is positioned for movement, the transducer having a quantity of magnetic fluid between the coil and the magnetic body and at least one vent from the cavity by-passing the air gap.

The vent may be a bore through the magnetic body, and may be through a pole piece thereof, and lead to the atmosphere and/or to a closed air volume. The volume may be closed by a diaphragm. The vent, and any closed volume if used, may be dimensioned to provide frequency response adjustment and correction.

Embodiments of the invention will now be described with reference to the accompanying drawings in which: Figure 1 shows a high frequency dome (or calotte) type loud speaker, and Figure 2 shows a cone loud speaker.

It has been found that while the addition of a magnetic fluid as described above produces the advantages mentioned, other disadvantages occur which reduce the expected improvement in efficiency and distortion factor.

The disadvantages result from the magnetic fluid sealing the spaces between the coil and the magnet in the air gap. This seal prevents air movement into and out of the cavity in the magnet body beyond the coil in the air gap with movement of the coil.

As a consequence, the magnetic fluid is moved to and fro in the air gap, in synchronism with the movement of the moving coil, by the action of pneumatic forces. This has an influence on the mechanical behaviour of the magnetic fluid and also on the magnetic properties. The friction of the moving coil in the fluid is increased and the magnetic field, which is established owing to the orientation of the magnetisable particles contained in the fluid, is impaired. Both lead to impairment of the efficiency and the behaviour as regards the distortion factor.

The invention is explained in the following with reference to practical examples illustrated in the drawing.

In Figure 1 there is illustrated an embodiment of the invention applied to a transducer exemplified as an electrodynamic high frequency loud speaker of the type with a calotte or dome diaphragm 1, and a moving coil 2 driving the diaphragm. The magnetic body is composed of a magnetic material plate base 6 with a pole core 11, an annular permanent magnet 5 and an upper pole plate 4. The pole plate 4 surrounds the pole core 11 and forms an air gap 3 in which the moving coil 2 is movably arranged. The free spaces in the air gap 3 between the moving coil 2 and the pole plate 4 on one side and between the moving coil 2 and the pole core 11 on the other side are each filled with a magnetic fluid 30. A closed volume in cavity 12 thereby results beneath the moving coil 2. The pole core 11 has a central bore 8 which leads from the air gap beneath the diaphragm to a closed volume resonance chamber 70 located beneath the base plate 6 and formed by a housing 7. This arrangement serves for correction of the loud speaker frequency response.

The cavity 12 beneath the moving coil 2 is connected by a vent such as bore 10 to the central bore 8 in the pole core 11, so that pressure equalisation can take place from the air contained in cavity 12 to the considerably larger air volume in chamber 70 and bore 8.

Instead of the bore 10 there may be provided in base plate 6 a vent of a bore 9 which is indicated by broken lines in Figure 1.

In Figure 2 are illustrated further practical details of an embodiment of the invention as applied to the example of a loud speaker of the cone 13 type. This loud speaker, similarly to the loud speaker of Figure 1, has a diaphragm 13, a moving coil 19 in an air gap 31, a pole core 24 and a magnetic fluid filling 20.

The large cone diaphragm 13, the outer fastening of which is not illustrated for reasons of space, is centered at its lower zone by a centering diaphragm 14 so that the moving coil 19 in the liquid-filled air gap 31 does not come into contact with the pole core 24 or the pole plate 16. For pressure equalisation there is provided a vent or a bore 21 leading from the space air volume in cavity 22, beneath the moving coil 19, to the space beneath the centering diaphragm 14. It is also possible to provide, instead of the bore 21 or additionally thereto, a vent of a bore 23, drawn in broken lines in Figure 2, which leads out to the atmosphere.

The openings or vents shown in Figures 1 and 2 as leading from the space beneath the moving coil to a further volume, which in the invention serve for pressure equalisation, may at the same time be used as a means of frequency correction, by suitably dimensioning the bores and/or the connected volumes.

Openings dimensioned in this way are known in themselves in transducers with an air gap without a magnetic fluid filling (e.g. from the Telefunken 'th 300' earphones).

The openings may be provided with an air permeable foam material which prevents particles from entering the volumes. While the transducers described have a diaphragm arranged to couple to the air to generate sound clearly such diaphragms are not essential for other transducers, e.g. vibrator drivers.

When applied to low frequency loud speakers the invention is useful in improving the quality of the loud speaker system, in particular noise induced by the movement of the magnetic fluid on the moving coil is suppressed.

It is not only the quality of the reproduction that is improved, but also the aforementioned disadvantages are mitigated by ensuring the retention of the magnetic fluid in the air gap with movement of the coil for all demands that are made on the transducer, while gaining the advantages of an air gap filled with magnetic liquid. Although the above-described embodiments relate to loudspeakers, the invention may be applied to other dynamic transducers such as ear-phones.

WHAT WE CLAIM IS: 1. A dynamic transducer including a magnetic body and a movable coil, the magnetic body defining a cavity with the body with a magnetic pole air gap in which the coil is positioned for movement, the transducer having a quantity of magnetic fluid between the coil and the magnetic body and at least one vent from the cavity by-passing the air-gap.

2. A transducer according to Claim 1 in which the vent is a bore through the magnetic body from the cavity to another, larger, closed volume.

3. A transducer according to Claim I or Claim 2 in which the vent is a bore through the magnetic body to the surrounding atmosphere.

4. A transducer according to any of Claims 1 to 3 in which the vent is a bore through a central pole piece of the magnetic body.

5. A transducer according to Claim 1 or Claim 2, or Claim 4 when dependent thereon, in which the vent extends to a said volume closed by a diaphragm.

6. A transducer according to any one of the preceding claims in which the vent is formed to have a shape to affect the frequency response of the transducer.

7. A transducer according to Claim 2 in which the closed volume is formed to affect the frequency response of the transducer.

8. A transducer according to any one of the preceding claims in which the vent includes a body of air-permeable material to control air and particle movement in the vent.

9. A transducer according to any one of the preceding claims arranged as a loud speaker.

10. A transducer according to any one of Claims 1 to 8 arranged as an ear phone.

11. A transducer substantially as herein described with reference to Figure 1 or Figure 2 of the accompanying drawings.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. composed of a magnetic material plate base 6 with a pole core 11, an annular permanent magnet 5 and an upper pole plate 4. The pole plate 4 surrounds the pole core 11 and forms an air gap 3 in which the moving coil 2 is movably arranged. The free spaces in the air gap 3 between the moving coil 2 and the pole plate 4 on one side and between the moving coil 2 and the pole core 11 on the other side are each filled with a magnetic fluid 30. A closed volume in cavity 12 thereby results beneath the moving coil 2. The pole core 11 has a central bore 8 which leads from the air gap beneath the diaphragm to a closed volume resonance chamber 70 located beneath the base plate 6 and formed by a housing 7. This arrangement serves for correction of the loud speaker frequency response. The cavity 12 beneath the moving coil 2 is connected by a vent such as bore 10 to the central bore 8 in the pole core 11, so that pressure equalisation can take place from the air contained in cavity 12 to the considerably larger air volume in chamber 70 and bore 8. Instead of the bore 10 there may be provided in base plate 6 a vent of a bore 9 which is indicated by broken lines in Figure 1. In Figure 2 are illustrated further practical details of an embodiment of the invention as applied to the example of a loud speaker of the cone 13 type. This loud speaker, similarly to the loud speaker of Figure 1, has a diaphragm 13, a moving coil 19 in an air gap 31, a pole core 24 and a magnetic fluid filling 20. The large cone diaphragm 13, the outer fastening of which is not illustrated for reasons of space, is centered at its lower zone by a centering diaphragm 14 so that the moving coil 19 in the liquid-filled air gap 31 does not come into contact with the pole core 24 or the pole plate 16. For pressure equalisation there is provided a vent or a bore 21 leading from the space air volume in cavity 22, beneath the moving coil 19, to the space beneath the centering diaphragm 14. It is also possible to provide, instead of the bore 21 or additionally thereto, a vent of a bore 23, drawn in broken lines in Figure 2, which leads out to the atmosphere. The openings or vents shown in Figures 1 and 2 as leading from the space beneath the moving coil to a further volume, which in the invention serve for pressure equalisation, may at the same time be used as a means of frequency correction, by suitably dimensioning the bores and/or the connected volumes. Openings dimensioned in this way are known in themselves in transducers with an air gap without a magnetic fluid filling (e.g. from the Telefunken 'th 300' earphones). The openings may be provided with an air permeable foam material which prevents particles from entering the volumes. While the transducers described have a diaphragm arranged to couple to the air to generate sound clearly such diaphragms are not essential for other transducers, e.g. vibrator drivers. When applied to low frequency loud speakers the invention is useful in improving the quality of the loud speaker system, in particular noise induced by the movement of the magnetic fluid on the moving coil is suppressed. It is not only the quality of the reproduction that is improved, but also the aforementioned disadvantages are mitigated by ensuring the retention of the magnetic fluid in the air gap with movement of the coil for all demands that are made on the transducer, while gaining the advantages of an air gap filled with magnetic liquid. Although the above-described embodiments relate to loudspeakers, the invention may be applied to other dynamic transducers such as ear-phones. WHAT WE CLAIM IS:

1. A dynamic transducer including a magnetic body and a movable coil, the magnetic body defining a cavity with the body with a magnetic pole air gap in which the coil is positioned for movement, the transducer having a quantity of magnetic fluid between the coil and the magnetic body and at least one vent from the cavity by-passing the air-gap.