WO2002091797A1

WO2002091797A1 - Coil driven diaphragm

Info

Publication number: WO2002091797A1
Application number: PCT/AU2002/000524
Authority: WO
Inventors: Robert Michael Grunberg
Original assignee: Robert Michael Grunberg
Priority date: 2001-05-04
Filing date: 2002-04-30
Publication date: 2002-11-14
Also published as: AUPR475201A0

Abstract

An electro-acoustic transducer (1) has an elongated diaphragm (10) provided with a convex front face located behind a compression plug (7) which closely conforms to, but is spaced from the convex surface of the diaphragm (10). The plug (7) is formed with divergent expanding slits (8) leading into the inlet end (6) of a horn or waveguide formed by flat, parallel horizontal surfaces (4, 5) between which extend divergent vertical surfaces (2, 3). The diaphragm (10) is driven by a moving coil (11) of congruent shape to the perimeter of the front face of the diaphragm (10) and which is located in an air-gap of a magnetic circuit. The diaphragm (10) is at least 1.4 longer than its width, and it has parallel sides apart from its end-portions. The congruent configuration of the moving coil and the diaphragm enables a straight wavefront to be propagated at high frequencies with a relatively low energy loss.

Description

COIL DRIVEN DIAPHRAGM

FIELD OF THE INVENTION

THIS INVENTION relates to a moving coil electro-acoustic transducer and is more specifically concerned with a diaphragm used in such a transducer and which is driven by a coil located in an air gap of a magnetic circuit. The term "transducer" is used in the specification to mean a device for effecting a conversion between electrical and acoustic energy. Thus it may take the form of a loudspeaker assembly, or a microphone assembly, or any other form of assembly which such a conversion is required to occur.

STATE OF THE ART Diaphragms of various profiles and shapes driven by circular moving coils are manufactured relatively cheaply in quantities numbering tens of millions per annum. They provide a practical means of electro-acoustic conversion, and moving coil dynamic loudspeakers are the most efficient class of transducer from the point of view of conversion of input energy to acoustic output energy. A circular driving coil is the most usual shape for effecting the conversion between electrical energy and the acoustic energy radiated from the diaphragm. In the case of a loudspeaker, the coil vibrates the. diaphragm in accordance with the force produced by the electrical input signal applied across the coil which is located in the air-gap of the magnetic circuit. Moving coil diaphragms are of robust enough construction to withstand the vibrational accelerations that give rise to the propagation of acoustic waves and wavefronts. The angular energy distribution is a mathematical function of the diaphragm diameter and the radiated frequency. In practice it is dependent on the shape and size of the diaphragm and any associated baffle or waveguide. Thus, within limits, altering the shape of the radiating surface of a diaphragm can alter the polar energy distribution of the propagating acoustic wave that emanates from it.

Nowadays concerts are often performed in large arenas designed to be used in various configurations for seating an audience numbering many tens of thousands. The audience may be seated in horizontally arranged tiered rows of seats and these rows can be straight, curved, or extend wholly around the central stage where the performers are located. The existence of this type of super arena has led to a demand for high power loudspeaker systems able to project and distribute the sound of the performance occurring on stage and with an acceptable level of fidelity in a way that evenly and uniformly reaches the seated listeners.

In order to meet the requirements for good sound reproduction in large arenas, there has been a trend towards providing vertically stacked arrays of loudspeakers. The arrays are horizontally spaced some distance from one another so as to overlap the 6dB down polar angles from adjacent arrays. A loudspeaker element in a vertically stacked array may have some small vertical orientation with respect to the adjacent loudspeakers and the arrays themselves normally form a vertical curved line. This line array deployment takes account of the sound energy emanating from a section of the line intended to cover a particular section of the seating area. Uniformity of energy distribution throughout the seating area can be pre-computed, at least in theory, as a line array exhibits a 3dB loss per doubling of distance from it. The distance from any particular section of the line to a segment of the audience it is intended to cover, enables the relative level of each loudspeaker in the line to be appropriately set.

Unfortunately, the high frequency radiation from a loudspeaker system employing a vertically stacked line arrays of loudspeakers, with conventional, i.e. circular diaphragms, is not completely phase coherent due to the vertical curvature of the wavefront propagated by each diaphragm. Also, the circular nature of the driving coil and its associated diaphragm, together with the circular shape of other components often used in loudspeakers such as phase plugs and horns or waveguides having circular throats, results in an expanding height of the wavefront which may not be required. Also, the high frequency signals produced by the line tend to be initially propagated as wavelets that, in the far field, and only after a significant energy loss, exhibit a straight vertical wavefront.

OBJECT OF THE INVENTION

Is to provide an improved moving coil diaphragm for an electro-acoustic transducer. THE INVENTION

In accordance with the present invention an elongated, moving coil diaphragm has two parallel sides which are longer in length than the spacing between them and a driving coil of congruent shape to the perimeter of the diaphragm and adapted to be located in a similarly-shaped air-gap of a magnetic circuit, the ratio of the length of each parallel side to the spacing between them being at least 1.4:1.

PREFERRED FEATURES OF THE INVENTION

In practice, the length of each of the parallel sides of the diaphragm is at least four times the spacing between them.

Preferably the opposite end-portions of the diaphragm, or at least one of them, is of part-circular shape and may have the diameter of the circle equal or greater in length than the spacing between the parallel sides. However the opposite end-portions, or each of them may be square cut which enables a stack of aligned such diaphragms to be spaced very close to one another.

UNDERLYING THEORY

Consider first a circular diaphragm having a circular driving coil of a given radius. If the coil is to be mechanically coupled to drive the diaphragm, then the diaphragm obviously requires an area equal to or greater than the circular area of the coil. The coupling of a diaphragm with a smaller radiating area to a coil having the same winding parameters, results in a greater air pressure at the surface of the diaphragm. For example if the area of the diaphragm is halved, then the acceleration of the diaphragm is doubled and an extension of the output frequencies by possibly an extra octave is achievable as compared with the response of a circular diaphragm driving with a circular voice coil having the same parameters. By elongating the voice coil of the diaphragm so that it is no longer circular, the coil is given a major and minor axis. When an elongated coil is mechanically coupled to a radiating diaphragm with the same elongated shape as the coil, the spatial distribution of the energy radiation from the diaphragm will be inherently different from that achieved with a circular diaphragm. Whereas a circular diaphragm exhibits a conical primary lobe of acoustic energy, a elongated diaphragm will exhibit a primary lobe of energy that has a major and a minor axis. If a loudspeaker having an elongated diaphragm with an elongated coil of congruent shape, is used to provide a listening environment in a domestic cinema for example, the asymmetrical distribution of sound energy from the diaphragm can be arranged to give a desired acoustic coverage with less energy being directed from the diaphragm towards the floor and the ceiling. A beneficial consequence of this is that a vertical line array of loudspeakers containing respective diaphragms having their major axes vertical, is able to provide a greater acoustic output with a narrower vertical directivity than is possible with a circular diaphragm. The invention enables a stack of vertically arranged loudspeakers each providing a cylindrical wavefront, to produce a combined wavefront. In front of the vertical line of loudspeakers which is seamlessly cylindrical and with phase coherence up to the higher end of the audible frequency range.

INTRODUCTION TO THE DRAWINGS

The invention will now be described in more detail, by way of example, with reference to the accompanying largely diagrammatic drawings, in which:-

IN THE DRAWINGS

FIGURE 1 is a perspective view of a loudspeaker assembly with portions cut away to show internal details of its construction, the assembly being shown in a simplified form;

FIGURE 2 is a front view of an elongated diaphragm used in the assembly of figure

1 ;

FIGURE 3 is a view corresponding to figure 2 but show front views of several alternative shapes of diaphragm;

FIGURE 4 shows a vertical array of loudspeakers such as would be used in a large arena, the array comprising two vertical lines of conventional loudspeakers with circular diaphragms arranged in oblique planes with each pair of horizontally spaced speakers having a third loudspeaker of the construction shown in figure 1 located between them; FIGURE 5 diagrammatically illustrates horizontal and vertical wavefront patterns obtained from a single line of vertically stacked speakers having circular diaphragms; and,

FIGURE 6 shows a relatively straight vertical wavefront propagated from a vertical line of stacked loudspeakers each having an elongated diaphragm of the shape shown in figure 2.

DESCRIPTION OF PREFERRED EMBODIMENT

Figure 1 shows a loudspeaker 1 having a pair of divergent vertical side wails 2 and 3 extending between a pair of horizontal walls 4 and 5 of sector shape. The walls 2, 3, 4

•, Q and 5 provide a waveguide or horn having an input throat 6 containing a compression plug 7 penetrated by divergent slits 8 which extend horizontally and inwardly to a part- cylindrical concave surface 9 forming the entry end of the plug 7. The surface 9 of the plug 7 extends in spaced parallel relationship with a convex frontal surface of a vertically elongated diaphragm 10 of arched cross-section. The diaphragm 10 has the extremities

15 of the arch terminating on a bobbin (not shown) on which is wound a diaphragm driving coil 11 shown in dotted outline and of congruent shape to the front face of the diaphragm as shown in figure 2. The driving coil 11 is located in a correspondingly shaped air-gap of a magnetic circuit (not illustrated) which generates a uniformly distributed magnetic flux along the length of the air-gap. The technology of positioning a

20 driving coil of a diaphragm in an air-gap in a magnetic circuit is well known to one skilled in the art and will not therefore be described in more detail.

As is apparent from figure 2, the diaphragm is of elongated shape and has two straight parallel sides 12 and 13 and two semi-circular ends 14 and 15 which are continuous with the sides 12 and 13. The expression "elongated" is used throughout this patent 25 specification to mean that the ratio of the length of each of the parallel sides of the diaphragm to the spacing between them, exceeds 1.4. In practice, an elongated diaphragm having a spacing between its parallel walls of 25 mm. and shaped as shown in figure 2, has the same sort of frequency response and horizontal polar distribution as a domed tweeter which beams at 20 KHz. If the spacing between the parallel sides of the diaphragm is reduced to 12.5 mm., the equivalent domed tweeter would beam at 40 KHz. The actual length of the elongated diaphragm determines its radiating area and this, in turn, determines the lower frequency limit and the maximum acoustic power output from the diaphragm.

The elongated diaphragm of figure 2 does not have to have semi-circular ends. Its frontal aspect can have a large number of different shapes some of which are shown diagrammatically in figure 3. Both end-portions of the diaphragm may be part-circular. On the other hand it may have one end-portion circular only. The diameter of the circle can be the same or greater than the space between the parallel sides of the diaphragm.

Figure 4 shows a vertical array 20 of stacked loudspeakers of an overall shape likely to be used in a large arena when several of such stacks would be spaced horizontally from one another in order to provide the required acoustic coverage to an audience seated on tiered seating around the arena.

The vertical array of speakers 20 are arranged in two parallel vertical lines 21 , 22 each formed from conventional speakers having circular diaphragms 24 and driving coils. These are arranged in horizontal pairs, as shown, and the circular diaphragms 24 of each pair are arranged to lie in respective planes which are oblique to one another. Towards the lower end of the loudspeaker stack, the loudspeakers follow a slight curve as shown.

Arranged between each pair of circular diaphragms 24 is a vertically elongated diaphragm 26 of a third loudspeaker of the shape shown in figures 1 and 2. The purpose of each of the diaphragms 26 is to provide a non-dispersive vertical wavefront for the higher frequency acoustic signals received by listeners spaced some distance in front of the array, to improve the sound fidelity of the array at a distance from the loudspeakers.

Figure 5 illustrates the at 40 and 41 the shapes of the acoustic vertical wavefronts obtained at 0.1 metres and 10 metres from a vertical row of loudspeakers 30 having circular diaphragms 31 as diagrammatically illustrated. A polar diagram produced by the array in the horizontal plane is shown at 42 in figure 5. It will be noticed that the wavefront 40 in the vertical plane suffers a loss in energy of the higher frequencies at positions corresponding to the spaces between the diaphragm. This energy loss occurs close to the array. This is indicated by the undulations in the wavefront 40, so that at 10 metres from the array, denoted by the wavefront 42, the energy level at the upper end of the frequency range is substantially less compared with the frequencies at the lower portion of the loudspeaker's frequency range. The energy loss is due to phase differences of the radiated wavelets from the spaced apart circular diaphragms.

In the case of speakers 43 having aligned vertical diaphragms 44 as shown in figure 6, these diaphragms are effective at the upper range of acoustic frequencies to transmit a vertical wavefront 45 which has a much lower energy loss with distance from the diaphragms than occurs with a spaced apart array circular diaphragms. The wavefront 45 is relatively non-dispersive in the vertical and thus the high frequency energy received by a listener in front of the array is much greater in the upper range of audio frequencies.

OPERATION OF PREFERRED EMBODIMENT

If now the vertical line of aligned diaphragms shown in Figure 6 is placed between two lines of circular diaphragms respectively, with each vertical diaphragm between two horizontally spaced circular diaphragms which are arranged in respective oblique planes as shown in figure 4, the overall wavefront at a distance from the array of figure 4 is much more coherent at the higher frequency range than is the case without the elongated diaphragms 24, and thus a reproduction pattern is obtained which displays a higher fidelity across the range of acoustic frequencies than is obtainable with a vertical line of spaced apart circular diaphragms of the same radiating area.

It should be noted that the use of the elongated diaphragm with a congruent driving coil is not limited to use with a diaphragm array as shown in figure 4. There are numerous applications where a relatively low energy loss at acoustically high frequencies is useful, in conjunction with a wavefront which is non-dispersive, or relatively so, in one plane only, and the diaphragm of the invention is able to provide this. MODIFICATIONS OF THE PREFERRED EMBODIMENT

The diaphragm shown in figure 1 does not necessarily require a compression plug 7 in front of it. It may, for example, be located directly in the throat of the waveguide. Also the use of a waveguide in conjunction with the diaphragm is not essential, and it may be used as a direct radiator.

Although the diaphragm described is of arched cross-section, it may have other cross- sectional shapes and could, in some cases, be flat.

Finally although the diaphragm has been specifically described in relation to a sound- generating loudspeaker, it may also have useful applications in microphones or other forms of electro-acoustic transducers, because of the shape of its polar distribution.

Claims

CLAIMS:

1 . An elongated moving-coil diaphragm having two parallel sides which are longer in length than the spacing between them, and a driving coil of congruent shape to the perimeter of the diaphragm adapted to be located in a similarly-shaped air-gap of a magnetic circuit, the ratio of the length of each parallel side to the spacing between them being at least 1 .4:1 .

2. A diaphragm as set forth in claim 1 , in which the length of each of the parallel sides of the diaphragm is at least four times the spacing between them.

3. A diaphragm as set forth in claim 1 or claim 2, and of arched cross-section.

4. A diaphragm as set forth in any one of the previous claims, in which the parallel sides of the diaphragm terminate, at least one pair of their ends in a part-circular end- portion of the diaphragm.

5. A diaphragm as set forth in claim 4, in which the part-circular end-portion of the diaphragm has a diameter greater than the spacing between the parallel sides.

6. A diaphragm as set forth in any one of the preceding claims, incorporated into an inlet end of an acoustic waveguide which flares outwardly in one plane only towards its outlet end, the direction of elongation of the diaphragm being perpendicular to said one plane.

7. A diaphragm as set forth in claim 6, in which a compression plug having outwardly divergent slits is located between the throat of the waveguide and an elongated convex concave surface of the diaphragm, the contour of the surface of the plug facing the diaphragm being spaced from it and conforming to the convex surface of the diaphragm, the spacing between the plug and the diaphragm being such that they are acoustically matched to one another.

8. A diaphragm as set forth in any one of the preceding claims, located between two circular diaphragms and having the plane containing its longitudinal axis perpendicular to the plane containing the line joining the centres of the circular diaphragms.

9. A diaphragm as set forth in claim 8 in which the two circular diaphragms respectively lie in oblique planes.

1 0. A diaphragm as set forth in either one of claims 8 and 9, forming part of an array of diaphragms comprising a line of aligned elongated diaphragms closely spaced to one another and each flanked at its opposite sides by two circular diaphragms.

1 1 . A diaphragm as set forth in claim 1 , or any one of claims 1 to 3, or any one claims 6 to 10 when dependent thereon, in which the diaphragm, when viewed from the front, is rectangular in shape.