AU777224B2

AU777224B2 - Electro-acoustic converter

Info

Publication number: AU777224B2
Application number: AU15640/02A
Authority: AU
Inventors: Barry Arnstein
Original assignee: Individual
Current assignee: Individual
Priority date: 2001-02-16
Filing date: 2002-02-15
Publication date: 2004-10-07
Anticipated expiration: 2022-02-15
Also published as: AU1564002A

Description

P/00/01i1 Regulation 3.2

AUSTRALIA

Patents Act 1990

ORIGINAL

COMPLETE SPECIFICATION STANDARD PATENT Invention Title: Electro-acoustic converter The following statement is a full description of this invention, including the best method of performing it known to me: Freel~iIls Carter Smith Beadle Melboume\003997381 Printed 15 February 2002 (14:43) page 2 Freehills Carter Smith Beadle Melboume\003997381 Printed 15 February 2002 (14:43) page 2 ELECTRO-ACOUSTIC CONVERTER The present invention relates generally to loud speakers and like devices for converting variations of electrical energy into corresponding variations of acoustic energy, such as, sound. The present invention is suitable for use both in specialised loud speakers forming part of hi-fidelity sound reproduction systems, and as a loud speaker for general applications in televisions, radio and other consumer markets, and it will be convenient to describe the invention in relation to such an exemplary, non limiting applications.

The basic operation of loud speakers and like electro-acoustic converters is .well known. In such devices, an electrically driven vibration inducing member, such as a moving coil or piezo-electric crystal, is attached to one end of a ooooo diaphragm. Variations in the electrical signal applied to the vibration inducing member are converted into mechanical vibrations which are amplified by the diaphragm and result in the production of sound waves in the air.

In dynamic or moving-coil loud speakers the vibration inducing member includes a moving coil, attached to the diaphragm, which oscillates in an annular cavity of either a specially shaped permanent magnet or an electro magnet. The efficiency of such a loud speaker in converting electrical energy into sound energy ooooo S"is governed by three main factors, namely, the product of the active links of the moving coil and the magnetic field strength, the effective area of the diaphragm, and the total moving mass of the diaphragm, moving coil assembly and the acoustic air load. An increase in the first two parameters, either singly or together will result in an increase in the efficiency of the loud speaker. However, an increase in the mass will result in a reduction in the loud speaker efficiency.

All loud speakers are carefully designed in an attempt to optimise these parameters, based on the specific market the loud speaker is intended for and the Freehills Carter Smith Beadle Melboume\003997208 Printed 15 February 2002 (12:13) -3manufacturing cost of the loud speaker. However, difficulties arise in the optimisation of these parameter. For example, whilst an increase in the area of the diaphragm can often provide the most effective loud speaker gain improvement, high frequency beaming and roll off is found to occur once the area of conventional dome type diaphragms is increased beyond a certain point.

An object of the present invention is to ameliorate or overcome one or more disadvantages of known electro acoustic converters.

The present invention provides an electro-acoustic converter, comprising: a barrel shaped diaphragm formed about a longitudinal axis and comprising first and second annular opposed ends; a first moving coil drive system for inducing a vibrational movement in the first annular end in the direction of the longitudinal axis; and a second moving coil drive system for inducing a vibrational movement in the second annular end of the diaphragm in the direction of the longitudinal axis, wherein the first and second moving coil systems act simultaneously to the first and second annular ends to either compress or stretch the diaphragm.

20 An electro-acoustic converter having these features enables a greater area of diaphragm to be used in the reproduction of sound waves than do existing electroacoustic converters, without the high frequency limitations of such standard "converters.

S o o oo 25 A series of electro-acoustic converters according to the present invention, each adapted for the optimal reproduction of a preselected frequency range, may be mounted in a single loud speaker arrangement. For example, the electro-acoustic converters may be arranged in a stack, or in end to end relation. Such an arrangement has been found to optimise the omni-directional sound dispersal of such a loud speaker arrangement.

Melboume\004525004 Printed 6 August 2004 (12:21) The following description refers in more detail to the various features of the electro-acoustic converter of the present invention. To facilitate an understanding of the invention, reference is made in the description to the accompanying drawings where the electro-acoustic converter is illustrated in a preferred embodiment. It is to be understood that the electro-acoustic converter of the present invention is not limited to the preferred embodiment illustrated in the drawings.

In the drawings: Figure 1 is a cross sectional view of one embodiment of an electro-acoustic converter according to the present invention; Figure 2 is a schematic diagram illustrating the movement of the diaphragm of the electro-acoustic converter of Figure 1; Figure 3 is a schematic diagram illustrating the area of the diaphragm of the electro-acoustic converter of Figure 1; Figure 4 is a cross sectional diagram showing one embodiment of a loud speaker arrangement including three electro-acoustic converters of the type shown in Figure 1 and, Melboume\004525004 Printed 6 August 2004 (12:21) Figure 5 is a schematic diagram illustrating the effective area of the diaphragm of a conventional dome style speaker.

Referring now to Figure 1, there is shown generally an electro-acoustic converter 1 including a diaphragm 2 having a barrel shaped body formed about a longitudinal axis 3 between a first annular end 4 and a second opposed annular end As can be seen from this Figure, the barrel shaped diaphragm 2 is curved so that the diameter of the diaphragm at the two opposed annular ends 4, 5 is less than the diameter of the diaphragm at a point intermediate these two ends. Typically, the 10 diaphragm may be formed from a textile or like material, but other materials known to persons skilled in the sound reproduction field may also be used.

In this embodiment, a first moving coil 6 is attached around the first annular ooooo end 4 of the diaphragm 2. The moving coil 6 is located within the electro-acoustic converter 1 in an annular cavity 7 within a permanent upper magnet assembly 8.

Electrical oscillations in the current flowing through the moving coil 6 in the presence of the magnet field generated by the permanent magnet 8 cause the moving coil 6, and the first annular end 4 of the diaphragm 2 to oscillate in the annular cavity 7.

The permanent magnet 8 is mounted to a circular upper mounting plate 9 and ooee• S"covered by an upper casing 10. An upper acoustic absorbent ring 11 is mounted between the upper casing 10 and the permanent magnet 8.

In other embodiments of the invention, the permanent magnet may be replaced by an electro magnet. Alternatively, the moving coil 6 and permanent magnet 8 may be replaced by a crystal drive system, in which the first end 4 of the diaphragm is attached to a piezo-electric or like crystal which undergoes periodic variations in thickness (oscillations) in the presence of an alternating voltage. In such an arrangement, these oscillations are transmitted to the diaphragm 2 both the Freehills Carter Smith Beadle Melboume\003997208 Printed 15 February 2002 (12:13) -6attachment of the first end 4 to the crystal. It is to be appreciated that these arrangements represent only three possible embodiments of electrically driven vibration generators which induce a vibrational movement in the first annular end 4 of the diaphragm 2 in the direction of the longitudinal axis 3, and that other arrangements may be envisaged by a skilled person in the field of loud speaker design.

Similarly, the electro-acoustic converter 1 may further include a second moving coil 12 attached around the second annular end 5 of the diaphragm 2. The moving coil 12 may be located within an annular cavity 13 of a permanent magnet 14. Once again, electrical oscillations in the current flowing through the moving coil 12 in the presence of the magnetic field generated by the permanent magnet 14 cause the moving coil, and the second end 5 of the diaphragm 2 to which it is attached, to vibrate in the direction of the longitudinal axis 3. The permanent magnet 14 is mounted to a circular lower mounting plate 15 and housed within a lower casing 16. A lower acoustic absorbent ring 17 may be provided between the permanent magnet 14 and the lower casing 16.

o• In other embodiments, the permanent magnet 14 may be replaced by a electro magnet. Alternatively, the moving coil 12 and the permanent magnet 14 may be replaced by a crystal drive system as described previously.

Spacer rods, such as those referenced 18 and 19 may be provided to maintain a fixed relation between the upper and lower mounting plates, and the permanent magnets 8 and 14 mounted thereto. Spacer rod screws, such as those referenced and 21, may be provided to affix the spacer rods 18, 19 to the mounting plates 9, Similarly, casing bolts, such as those referenced 22 and 23 may be provided to affix the upper and lower casings 10 and 16 to the upper and lower mounting plates 9 and Freehills Carter Smith Beadle Melboume\003997208 Printed 15 February 2002 (12:13) -7- At least part of the interior of the diaphragm 2 may be filled with sound damping material 24. The central pole of one or both of the permanent magnets 8, 14 may be hollow, and the damping material 24, may project into that hollow.

The action of the two moving coils 6 and 12 in alternately compressing and stretching the flexible diaphragm 2 of an appropriate barrel shaped profile causes the vibrational movement of the diaphragm 2 in the direction of the longitudinal axis 3 to be transformed into a corresponding vibrational movement in the central portion of the diaphragm 2 in a direction perpendicular to the longitudinal axis 3, 10 and the radiation of sound waves from the diaphragm 2 in that perpendicular direction. Figure 3 shows a section of the surface of the diaphragm 2. In this embodiment, the diaphragm 2 has a 5.67 centimetre radius curvature, and the separation between the first annular end 4 and the second annular end 5 is *oooo S"centimetres. Intermediate the two annular ends 4, 5, the diameter of the diaphragm 2 is 5.6 centimetres. Simultaneous compressional stretching of the diaphragm 2 at the first and second annular ends 4, 5 by one millimetre causes a corresponding two millimetre movement of the diaphragm 2 at this intermediate position.

As can be seen by this Figure, during the vibrational movement of the diaphragm 2, nodes are established at points A and B, respectively proximate the first annular end 4 and the second annular end 5. That portion of the diaphragm 2 ooe i between the nodes A and B effectively contributes to the sound production of the diaphragm 2. In a diaphragm 2 having the above described dimensions, around seventy per cent of the diaphragm is contributing to the sound output. This effective diaphragm area is shown by the shaded area referenced 30 in Figure 3.

For the purposed of comparison, a mid range loud speaker of a conventional dome style, including a two inch dome of an actual diameter of 54 millimetres and a linear voice coil excursion of two millimetres will be considered. The effective area of such a two inch dome is illustrated in Figure 5. Such a dome is typically intended for a frequency range of 800 Hz to 4 kHz the corresponding mid range loud speaker Freehills Carter Smith Beadle Melboume\003997208 Printed 15 February 2002 (12:13) -8shown in Figure 1, by contrast, has an effective diaphragm area some four times that of the dome driver. If the same voice coil excursion as for the dome loud speaker, namely 2 millimetres, were to be used, the average displacement of the barrel shaped diaphragm will also be close to two millimetres. This is four time the volume displacement of the dome style loud speaker. Since it is possible to achieve sound pressure levels (peak) of the order of 120 decibels with a two inch dome, it may no longer be necessary to achieve higher sound pressure levels in practice. For example, if the design goal is around 120 decibels, then by reducing the excursion of the voice coils to 0.5 millimetres, the efficiency of the magnet assembly can be raised to achieve the required sound pressure level.

oooo The electro-acoustic converter shown in Figure 1 can be implemented as a series of electro-acoustic converters mounted in a single loud speaker arrangement.

Such an arrangement 40 is illustrated in Figure 4. The loud speaker arrangement comprises a base electro-acoustic converter 41, a mid range electro-acoustic converter 42 and a trebly electro-acoustic converter 43. Each of the electro-acoustic converters 41 to 43 are mounted in the loud speaker arrangement 40 in an end to end or stacked relation by the mounting together of adjacent casings. The dimensions and parameters of each of the electro-acoustic converters 41 to 43 are selected so as to optimise the sound reproduction characteristics respectively in the base, mid range and treble frequency range. Stacking of the electro-acoustic converter 41 to 43, as shown in Figure 4, assists in providing an omni directional sound dispersal. For uses where a more directions sound field coverage is required, such as public address and cinema applications, horn structures or reflector canopies (not shown) may be easily designed and adapted for use with the loud speaker arrangement 40 by a skilled person in the field of loud speaker design.

Those skilled in the art will appreciate that there may be many variations and modifications of the configuration described herein which are within the scope of the present invention.

Freehills Carter Smith Beadle Melboume\003997208 Printed 15 February 2002 (12:13)

Claims

1. An electro-acoustic converter, including an electro-acoustic converter, comprising: a barrel shaped diaphragm formed about a longitudinal axis and comprising first and second annular opposed ends; a first moving coil drive system for inducing a vibrational movement in the first annular end in the direction of the longitudinal axis; and a second moving coil drive system for inducing a vibrational movement in the second annular end of the diaphragm in the direction of the longitudinal axis, wherein the first and second moving coil systems act simultaneously to the first and second annular ends to either compress or stretch the diaphragm.

2. An electro-acoustic converter according to claim 1, where the first moving coil drive system comprises a first vibration coil attached around the first annular end of the diaphragm.

3. An electro-acoustic converter according to claim 1, where the second moving coil drive system comprises a vibration coil attached around the second annular end the diaphragm.

4. A loud speaker arrangement comprising a plurality of electro-acoustic converters according to claim 1, where the electro-acoustic converters are mounted in the loud speaker arrangement in an end to end relation. 25

5. An electro-acoustic converter substantially as hereinbefore described with ."reference to the accompanying drawings. DATED:

6 August 2004 FREEHILLS CARTER SMITH BEADLE Patent Attorneys for the Applicant: BARRY ARNSTEIN Melboume\004525004 Printed 6 August 2004 (12:25)