Loudspeaker Diaphragm

Abstract

A diaphragm assembly for an electro-acoustic transducer has a central soft dome portion with a semi-rigid surround such that both the dome and surround can undergo substantial excursions. An electro-acoustic transducer incorporating the diaphragm has a totally enclosed chamber to the rear of the voice coil. The diaphragm assembly is particularly suited to the frequency range of 500 c/s to 5 Kc/s at powers of up to 50 watts.

Description

BACKGROUND TO THE INVENTION

1. Field of the Invention

This invention relates to diaphragm assemblies for electro-acoustic transducers, particularly loudspeakers. The invention is also concerned with electro-acoustic transducers incorporating such diaphragm assemblies, and more particularly with a mid-range soft dome pressure loudspeaker.

B 2. Description of the Prior Art

Diaphragm assemblies of many different forms have been devised for use in loudspeakers of the moving-coil type. In one known loudspeaker, in which the diaphragm is of conical or frusto-conical shape with the moving coil mounted at the apex of the cone, the diaphragm cone is formed from a layer of woven fabric impregnated with a hardened synthetic resin and with a layer of compacted fibrous material bonded thereto. This results in a diaphragm having a substantial degree of rigidity and stiffness and is designed to give a good response at high frequencies.

In another known form of diaphragm assembly, in which the diaphragm is of the dome type having its concave face towards the coil, the diaphragm comprises a soft fabric dome which is first impregnated to render it self-sustaining and yieldable but leaving the fabric interstices open, and which is then coated with a soft film closing the fabric interstices, rendering the diaphragm impervious to air and defining a radiating area. The coil is then fixed to a short axial flange at the periphery of the composite diaphragm. This diaphragm assembly is designed for use in the range 2,000 to 18,000 cycles per second.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a diaphragm assembly which is able to handle high power signals without excessive distortion and which is particularly applicable to the middle range of frequencies from about 500 c/s to 5 Kc/s.

It is a further object of the invention to provide an electro-acoustic transducer which has the ability to handle signals of up to 50 watts with low resonance and low distortion.

In accordance with the present invention there is provided a diaphragm assembly for an electro-acoustic transducer, the assembly comprising a soft dome portion of resilient, self-sustaining character, and a semi-rigid surround secured to the periphery of the dome portion to extend substantially radially of the central axis of the dome portion, the combination being such that when said surround is firmly clamped at its periphery both the dome portion and the surround are able to undergo substantial excursions in response to signals applied to a coil secured to said combination.

Preferably, the surround is corrugated over a part of its area and is formed from a fabric base impregnated with a hardenable synthetic resin.

An electro-acoustic transducer in accordance with the invention comprises a diaphragm assembly in accordance with the invention and includes magnetic circuit means generating a flux effective on said coil, and housing means defining a totally enclosed chamber to the rear of the coil.

In a preferred embodiment, the magnetic circuit means comprises an annular pole piece having a radially outwardly prominent portion defining a surface facing the coil, the pole piece being provided with a non-magnetic perforated dome at its forward end projecting into the dome portion of the diaphragm assembly and substantially parallel thereto.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be more fully understood from the following description of a preferred embodiment of loudspeaker which is given by way of example and with reference to the accompanying drawings, in which:

FIG. 1 is a longitudinal cross-sectional view through the loudspeaker; and,

FIGS. 2A, 2B and 2C in combination comprise an exploded view of the loudspeaker illustrating in greater detail the various components and their inter-relationship.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The loudspeaker illustrated in the drawings is a mid-range soft dome pressure loudspeaker designed primarily to operate between the range of 500 c/s and 5 Kc/s, but which will operate .+-. 3db up to 10 Kc/s. The power handling capacity is 50 watts to the DIN 45-500 specification. The harmonic distortion is extremely low and can be better than 2 percent throughout the working range.

Referring now to the drawings, it will be seen that the loudspeaker basically comprises a front mounting assembly, a diaphragm assembly, a magnet assembly and a back pressure system. At the front of the loudspeaker is an annular mounting plate 10 provided with appropriate holes to receive brass screws 12 and dowels 13. Behind the mounting plate 10 and arranged to fit into a recess 11 in the rear of the plate is an annular strip 14 of a synthetic material such as Tesamol. A protective grill 16 of domed configuration is adapted to fit into the central aperture in the mounting plate 10 with its peripheral flange 17 seating against a radially inwardly projecting flange provided on the mounting plate 10. Behind the grill 16 are a pair of sealing and electrically insulating washers 18, and between these washers 18 pass electrically conductive beryllium strips 20 which form part of the diaphragm assembly and which, as will be described hereinafter, provide the electrical connection between an external current source and the coil of the loudspeaker. In FIG. 2A the strips 20 are not shown positioned between the washers 18 simply for ease of illustration.

The diaphragm assembly which is shown in exploded view in FIG. 2A comprises a stainless steel diaphragm housing 22 of annular shape and having a central aperture therethrough; a pair of rubber clamping washers 24 which are seated when assembled against a shoulder 25 of the housing 22; a soft dome 26; a diaphragm surround 28; the beryllium strips 20; and a coil 30 mounted on a former 31.

The dome 26 is outwardly convex relative to the coil 30 and may be made in the following way. A soft woven fabric material, such as cambric, is first coated with a layer of a synthetic thermosetting resin, such as an epoxy resin, and is heated and pressed in a die so that the material has the required domed shape and although deformable will retain its shape. The dome is then treated with a plastics material which is unaffected by climatic changes, for example heat and dampness, in order to produce a finished dome which is of the soft type, being resilient, impermeable to air, and sufficiently rigid to return to its original shape if deformed slightly.

The diaphragm surround 28 has a peripheral portion which is clamped between the washers 24, a corrugated portion radially inwardly of the peripheral portion, and an inclined portion angled to correspond with the rim of the dome 26 to which it is secured, for example by means of glue or cement. Although secured to the dome 26 on assembly, the diaphragm surround 28 is made separately from a linen material impregnated with a hardenable synthetic resin material, such as an incompletely cured resin, for example a phenol-formaldehyde resin. The corrugated portion is formed during a heating and pressing operation, which also hardens the resin and produces a semi-rigid surround which is impermeable to air. The surround 28 although semi-rigid must be able to follow the movements of the soft dome 26 and thus has the ability to make a large excursion from its rest position. This is particularly important in providing a low resonance without distortion, especially when the loudspeaker has to handle input signals of high power, for example of the order of 50 watts.

The voice coil 30 is mounted as shown in FIG. 2A on a former 31 which is preferably a cylindrical epoxy resin coated paper tube. On assembly, the former 31 is secured, for example by glue or cement, to the diaphragm surround 28 and dome 26 to make a unitary assembly, with the beryllium strips 20 in contact with the coil 30. The unitary assembly is held in place by the periphery of the diaphragm surround 28 being clamped between the two rubber washers 24.

Behind the diaphragm assembly is provided a termination washer 32 which is also of annular shape and which includes electrical connection tags 33 adjacent to its periphery. The beryllium strips 20 leading to the coil 30 are soldered to the tags 33. To the rear of the termination washer 32 and positioned on the axis of the loudspeaker is the magnet assembly which is shown in detail in FIG. 2B. The magnet assembly comprises an annular pole piece 34 which has a radially prominent portion 39 adjacent to its forward end. The forward end of the pole piece 34 has also an axially prominent portion 35 on which is seated a perforated dome 36 which in the assembled state of the loudspeaker projects into the fabric dome 26 in substantially parallel relationship thereto. The dome 36 may be made for example of perforated brass sheet material. The aperture through the pole piece 34 is filled by a foam plug 37 which is normally of greater diameter than the aperture so that when assembled it is compressed and projects forwardly of the front face of the pole piece, as shown in FIG. 1. The plug 37 fulfils a damping function. Around the pole piece 34 is fitted an annular foam strip 38 which, when assembled, is compressed by an annular magnet 40 surrounding the pole piece 34. The magnet 40 is preferably of the ceramic type with a strength of for example 15,000 gauss. The magnet assembly is held together by being secured between front and rear plates 42 and 44 respectively, for example by means of brass screws 46. The coil 30 is positioned in the air gap defined between the pole piece 39 and the facing surface of the front plate 42.

As shown in FIG. 2C the back pressure system comprises a back pressure cover 48 substantially filled by a foam plug 50. The back cover 48 fits around the rear plate 44 of the magnet assembly and is secured thereto to provide a totally enclosed back pressure system behind the coil 30. This means that the loudspeaker is not affected by the size of the cabinet in which it is mounted.

Also shown on the back pressure cover 48 are external tags 52 and electrical leads between these tags and the tags 33 on the termination washer 32.

The diaphragm assembly of the present invention is particularly advantageous for handling large power capacity signals by its use of a semi-rigid diaphragm surround which can also undergo a large excursion to produce a low resonance without distorting.

Although the foregoing description has been given with particular reference to loudspeakers, it is to be understood that the invention is of application to electro-acoustic transducers generally and is limited only by the claims appended hereinafter.

Claims

I claim:

1. An electro-acoustic transducer having a diaphragm assembly comprising a central dome portion of resilient, yieldable, self-sustaining character, a surround supporting said dome portion which is of a material different from that of the dome portion and which has a greater rigidity than the dome portion, the surround having a circumferential inner marginal portion inclined to the central axis of the dome portion, the inclined portion having the dome portion rigidly secured to one face thereof so as to rigidly secure the surround to the periphery of the dome portion, the surround extending substantially radially outwardly of the central axis of the dome portion, the surround being circumferentially corrugated over a part of its area and being arranged to follow the translational movements of the dome portion, a voice coil, and a former carrying the voice coil, the former being secured to the other face of the inner marginal portion so that the voice coil is spaced axially from the dome portion on the concave side thereof.

2. An electro-acoustic transducer as claimed in claim 1, including magnetic circuit means generating a flux effective on said coil, and housing means defining a totally enclosed chamber to the rear of the coil.

3. An electro-acoustic transducer as claimed in claim 2, in which said magnetic circuit means comprises an annular pole piece having a radially outwardly prominent portion defining a surface facing the coil, the pole piece being provided with a non-magnetic perforated dome at its forward end projecting into the dome portion of the diaphragm assembly and substantially parallel thereto.

4. An electro-acoustic transducer as claimed in claim 3, in which the annular pole piece has its central aperture filled with acoustic damping means.

5. An electro-acoustic transducer as claimed in claim 2, in which said housing means is substantially filled with acoustic damping means.

6. An electro-acoustic transducer as claimed in claim 4, in which said acoustic damping means is a compressible foam material.

7. An electro-acoustic transducer as claimed in claim 5, in which said acoustic damping means is a compressible foam material.

8. A transducer as claimed in claim 1, which includes clamp means holding the surround, the surround being formed from a fabric base impregnated with a hardenable synthetic resin and having a non-corrugated flat peripheral portion gripped by said clamp means.

9. A transducer as claimed in claim 1, in which the dome portion is formed from a fabric base impregnated with a thermosetting resin and coated with a plastics material which is resistant to climatic changes.