GB2248997A

GB2248997A - Sound output device

Info

Publication number: GB2248997A
Application number: GB9111775A
Authority: GB
Inventors: Hiro Negishi; Michael David Graham Jewitt
Original assignee: Canon Research Centre Europe Ltd
Current assignee: Canon Technology Europe Ltd
Priority date: 1990-10-17
Filing date: 1991-05-31
Publication date: 1992-04-22
Anticipated expiration: 2011-05-31
Also published as: GB9022553D0; GB2248997B; GB9111775D0; GB2248996A

Abstract

A speaker unit is provided in which a drive unit 18 fits within a housing 10 which is supported cantilever-wise above a sound mirror 14. The axis of the drive unit 18 is offset relative to the axis of the sound mirror 14 to enhance the distribution of sound intensity in a preferred listening direction. A single speaker having concentric first and second diaphragms, when operated at high frequency, directs a narrow beam of sound towards the mirror 14 from which it can be reflected into the room so as to produce a desired sound distribution. The struts 16 have lengths greater than their width and are directed towards the axis of the drive unit 18 to minimise their effect on the sound reaching the listener. <IMAGE>

Description

2 24 3 ?' 7 SOUND OUTPUT DEVICE This invention relates to a sound output

device or speaker unit which may be incorporated into a stereo audio output system, and more particularly into such a system intended to reproduce sound with high fidelity or near high fidelity and to give rise to a relatively wide field of stereophonic sound.

is Our Patent GB-B-2188811 describes a stereo speaker system having a pair of first and second speaker assemblies. Each speaker assembly comprises a speaker having a diaphragm which emits a soundwave in a vertical direction and an acoustic reflector which is disposed above the diaphragm and reflects the soundwave from the diaphragm into a horizontal plane. The surface of the acoustic reflector facing the diaphragm of each speaker assembly is conical with the apex of the cone nearest to the diaphragm. The centre axis of the conical surface of the acoustic reflector is offset from the centre axis of the diaphragm so as to provide a preferential distribution of sound intensity in an intended listening direction and so as to enhance or increase the area over which the stereo image is obtained.

- 2 Further aspects of sound output systems using generally conical sound mirrors are disclosed in EP-A-0320270 and EP-A-0409360.

In one aspect this invention provides a speaker unit having a speaker or drive unit which emits sound in a first direction and an acoustic reflector supported in spaced relationship from the speaker for redirecting sound therefrom into a second direction generally at right angles to the first direction. The reflector has a generally conical surface facing the speaker with the apex of the cone nearest to the speaker. The centre of the speaker is offset from the axis of the conical surface so as to define a direction for sound leaving the speaker, and there is provided a single speaker which has concentric first and second diaphragms for lower and higher frequencies and which is arranged to direct at intermediate and high audible frequencies (e.g. frequencies above, 5 KHz) a relatively narrow beam of sound towards the audio mirror. It has been found that such relatively narrow intermediate and high frequency sound beams when directed onto the audio mirror can be used to produce a distribution of sound which is more reproducible, the sound distribution being controlled by the shape - 'R- of the diffracting or reflective surface of the audio mirror.

In one form of the speaker or drive unit the second diaphragm may fit within but be unattached to the first diaphragm, the first and second diaphragms being independently driven. Thus, the first and second diaphragms could be concentric domes, e.g. as disclosed in our UK Patent Application No. 9026679.2. More preferably, however, the second diaphragm fits within and is attached to the first diaphragm with a single drive vibrating both diaphragms, so that the second diaphragm acts as a socalled "parasitic tweeter", the second diaphragm conveniently being arranged to come into operation at frequencies above 5 KHz. The second diaphragm is preferably relatively narrow and horn-like and assists in the direction of a relatively narrow beam of high frequency sound towards the audio mirror.

In one form the second diaphragm is horn-like and is circular in outline at its inner and outer ends. The first diaphragm may also be circular in outline at its inner and outer ends. In an alternative form, the second diaphragm is horn-like but is elliptical at end and in that case, the first be elliptical at least at its outer first or both diaphragms are elliptical, their major axes are preferably directed generally at right angles to a line joining the centre of the drive unit and the axis of the audio mirror. The reason for the choice of this direction is that in an elliptical speaker the distribution of the sound emitted along the direction of the major axis of the speaker is relatively narrow and the distribution of the sound emitted along the minor axis of the speaker is relatively wide, so that the selected attitude of the speaker gives a wide field of sound.

least at its outer diaphragm may also end. Where the The drive unit or speaker is advantageously contained within a housing which is closed except for an aperture defining a Helmholtz resonator, the aperture being in the form of a tube whose length is adjusted to provide the required resonance frequency, conveniently about 50 to 60 Hz e.g. about 55 Hz to provide an element of built-in bass lift The region of the housing opposite to the diaphragm of the speaker or drive unit is advantageously domed to minimise resonances which can arise from the presence of flat surfaces within the housing.

Both from an aesthetic and from a sound quality standpoint, it is desirable that there should be no obtrusive struts in the intended listening direction. For that purpose the or each strut which extends between the sound mirror and a housing in which the speaker or drive unit is mounted are on the opposite side with respect to the axis of the acoustic mirror from the drive unit. The housing is therefore supported cantilever-wise above the acoustic mirror, or vice versa, and a problem arises as to how to prevent the struts, which have to be relatively large in such a cantilever support arrangement, from interferring with sound quality.

This problem is solved, according to a further preferred feature of the invention, by having struts whose length is greater than their width and which in their longer direction are directed towards or face the axis of the speaker or drive unit. Advantageously, the or each strut has straight sides leading to a curved end which faces towards the axis of the drive unit, and the sides of the struts converge radially in the direction of the drive unit axis. Preferably the opposed sides or other extremities of the or each strut subtend an angle of 5 to 10 degrees e.g. about 7.5 degrees at the axis of the drive unit. Although a single strut is possible, the provision of two struts is preferred, and the provision of more than two struts is less preferred. Where two struts are present their angular spacing measured by the spacing of their median or centre lines is preferably in the range 40 to 60 degrees e.g. 50 degrees. Where the speaker or drive unit has a Helmholtz resonator aperture, that aperture is preferably located between the struts so as not to be noticed when the speaker is in use.

Various forms of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which:

Figure 1 is an exploded view of a speaker; first form of the 1 Figure 2 is a view of the speaker of Figure 1 in vertical section; Figure 3 is a diagrammatic plan view of the speaker showing the relative position of the sound mirror, speaker unit and struts; A 7 Figure 4 is a diagrammatic partly sectioned view of the speaker or drive unit; Figure 5 is a circuit diagram of an equaliser for adjusting an incoming signal having regard to the characteristics of the speaker; and Figures 6 and 7 are respectively a diagrammatic elevation and plan of a second form of the speaker unit.

In the drawings, a loudspeaker intended to be used on a bookshelf or on a stand in a domestic living room or similar environment as part of a stereophonic sound reproduction system has an overall height of about 30Omm and a diameter of about 255mm and is circular when viewed in plan. It comprises a speaker housing 10 supported in cantilever manner above a sound mirror 14 by means of struts 16 that form part of and stand up from the sound mirror 14. The housing 10 contains a drive unit 18 whose sound is directed axially onto the sound mirror 14 from which it is directed towards the listener by reflection at high frequencies and diffraction at low frequencies, the sound at mid-range 8 frequencies being directed towards the listener by a combination of these two mechanisms.

The housing 10 comprises a domed top cover 20 and an apertured lower plate 22 which are sealed together by means of an O-ring seal 24. The dome 20 is a moulding in ABS or other plastics material, and the plate 22 is of a zinc alloy. The function of the O-ring seal 24 is to accommodate differences in the expansion characteristics between the two materials and also to provide an air seal for an internal cavity 26 defined beneath the dome 20. Apart from enhancing the appearance of the speaker, the use of the semi-circular domed top cover 20 minimises the presence of flat surfaces within the housing 10 which can give rise to undesired sound reflections. The plate 22 is apertured in a listener-facing direction at 28 to receive the drive unit 18 which is attached by screws or other suitable means and it is formed with a second aperture 30 at a radially opposite position to the aperture 28. A port tube 32 fits into the aperture 30 and extends into the cavity 26. The dome 20 and the lower plate 22 are held together by means of a strap 34. A space occupied by a body of absorbent material 36 within the cavity 26 behind the drive unit 18 leads to a free space 38 adjacent to the port-defining tube 32. The length of the tube 32 is adjusted in relation to the characteristics of the drive unit 18 and the housing 10 to provide the correct acoustic loading and to provide an acoustic filter having a resonance of 55 Hz. The effect of the resonant cavity which acts as a Helmholtz resonator is to increase the efficiency of the loudspeaker at low frequencies. Low frequency sound from the rear face of the drive unit 18 exits omni-directionally from the port tube 32 and adds to the sound coming from the front face of the drive unit 18. The position of the port tube 32 is not critical but in the present case is advantageously located between the pair of struts 18 directly opposite to the aperture 28 with respect to the centre of the plate 22.

The drive unit 18 is shown in more detail in Figure 4 and has a motor unit 44 and a double diaphragm arrangement in which there is a main diaphragm 46 which is effective at low and intermediate frequencies and a horn-like parasitic tweeter 48 which fits within the diaphragm 46 on which it is mounted. The main diaphragm 46 and parasitic tweeter 48 are concentric and are conventional in construction. The parasitic - 10 tweeter 48 becomes effective at about 5 KHz. The outer diameter of the main diaphragm 46 is 90 mm and the outer diameter of the parasitic tweeter 48 is 48 nim.

The sound mirror 14 provides a stand for the speaker and is of diameter about 250mm and of overall height about 10Omm It is hollow and defines an internal space which is closed off by means of a base tray 50 which accommodates a terminal fitting 52 for receiving the signal to be reproduced and also an equaliser network 54. The apex 56 of the sound mirror 14 when viewed in plan is located approximately on the edge of the drive unit 18 as shown so that the sound is preferentially directed onto one side of the sound mirror.

The sound mirror is a solid of revolution of a curvilinear profile with a straight region adjacent to the apex 56 and with a concave lower region of nominal radius of curvature 390-41Omm to diffuse the sound to provide, at least at high frequencies, a beam of sound which spreads out at an angle of +25 to -5 degre.es in a direction parallel to the axis of the sound mirror 14 and in a plane normal to the axis of the sound mirror 14 has an angular distribution of approximately frequencies frequency, the sound degrees. The resulting sound intensity is sufficiently steady within the range of audible throughout this radial and axial or horizontal and vertical distribution to give satisfactory listening and a useful stereo effect. Frequencies throughout the audible range are present in the distributed sound. The above-mentioned advantageous distribution of sound arises from the combination of a single drive unit that directs sound, at least at high frequencies, in a relatively narrow beam towards the sound mirror together with the offset relationship between the drive unit and the sound mirror which enables the sound to be distributed in a preferential listening direction.

The drive unit or loudspeaker 18 directs low mid-range and high frequency sound towards mirror 14. The sounds.of lowest frequency, omni-directionally, is to act i.e. less than 200 Hz, is emitted and the only effect of the sound mirror 14 somewhat as a horn. At frequencies of 200 Hz to 1 KHz the mirror 14 modifies the sound, it is believed by scattering. The resulting adjustment in sound pressure level enables the sound pattern produced by the speaker unit to be adjusted to reduce colouration and to achieve a preferred polar directivity. At a transition range of frequencies of from 1 to 5 KHz which is believed to be important from the standpoint of perception of the stereo image, the direction of sound from the mirror 14 becomes increasingly directional as the frequency rises. At frequencies above 5 KHz, the sound from the drive unit 18 is reflected from the sound mirror 14 in a manner that can be generally predicted by geometrical acoustics.

The curvature of the mirror 14 can be convex in both horizontal and vertical directions to achieve a desired spread of sound, or as shown and as is preferred it can be convex in a horizontal direction and concave in vertical profile. At higher frequency ranges where the parasitic tweeter 48 begins to operate, i.e. above 5 KHz, the drive unit 18 produces a relatively narrow beam of sound which is directed onto the reflective surface of the sound mirror 14, the tendency for a narrow beam to' be produced becoming increasingly marked towards the upper limit of audibility. It has been found in practice that this narrow beam is very efficiently spread by the sound mirror 14, bearing in mind also the increasingly accurate direction of the beam from the drive unit 18 onto the mirror as frequency rises. The - 13 predictability of the sound distribution from the loudspeaker is increased because there is only a single effective signal source from which sound is radiated onto the mirror 14, the tweeter and the main diaphragm being concentric.

A problem arises in the support of the housing 10 in a cantilever manner above the sound mirror 14 so that its front is unobstructed because if the struts 16 supporting the dome 20 are made mechanically strong enough, they are liable to disturb the pattern of sound from the mirror 14. problem, the housing 10 is substantial struts 16 which In order to overcome this supported by relatively face towards the centre of the drive unit 18 and which as stated above subtend relatively small angles from 5 to 10 degrees, typically about 7 degrees. The angular spacing between the struts can be 40 to 60 degrees, especially degrees. 1 Figure 5 shows diagrammatically an equaliser network which fits beneath the mirror 14 and modifies the incoming signal according to the characteristics o-LE the loudspeaker. The filter selectively decreases frequencies at around 800 Hz and around 5 KHz to suit - 14 the characteristics of this particular speaker unit. The decrease at about 800 Hz is required, in this particular device, because of the spatial relationship between the sound mirror 14 and the drive unit 18, and the attenuation at 5 KHz is necessary, in this example, because of the characteristics of the particular drive unit selected.

A second form of the speaker unit is shown in Figures 6 and 7 and incorporates a drive unit 60 in which the outer ends of both the outer diaphragm 63 and the inner horn-like parasitic tweeter 64 are elliptical in outline. in such an arrangement, for the reasons previously discussed, the major axis 61 is directed at right angles to the line 65 joining the centre of the loudspeaker 60 to the centre of the sound mirror 14, the minor axis of the speaker 60 being aligned, precisely or with some angular deviation, with the line 65.

Claims

CLAIMS:

is 1. A speaker unit comprising a drive unit or speaker which emits sound in a generally vertical direction and an acoustic reflector disposed at a spacing from the speaker for redirecting sound therefrom into a generally horizontal direction, the reflector having a generally conical surface facing the speaker with the apex of the cone nearest the speaker and with the centre of the speaker offset from the axis of the conical surface so as to define a direction for sound leaving the speaker, wherein there is provided a single speaker having concentric first and second diaphra_gms for lower and higher frequencies.
2. A loudspeaker unit according to claim 1, wherein the second diaphragm fits within but is unattached to the first diaphragm, the first and secnd diaphragms being independently driven.
3. A unit according to claim 1, wherein the second diaphragm fits within and is attached to the first diaphragm and a single drive vibrates both diaphragms.

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4. A unit according to any preceding claim, wherein the second diaphragm is arranged to come into operation at frequencies above 5 kHz.
5. A unit according to any preceding claim, wherein the second diaphragm is horn-like and is circular in outline at its inner and outer ends.
6. A unit according to any preceding claim, wherein the first diaphragm is circular in outline at its inner and outer ends.
7. A unit according to any of claims 1 to 4, wherein the second diaphragm is hornlike and is elliptical in outline at its outer end.
8. A unit according to any of claims 1 to 4 and 7, wherein the first diaphragm is elliptical in outline at its outer end.
9. A speaker unit according to any preceding claim, wherein the speaker drive unit fits into -a housing which is closed except for an aperture or apertures defining a Helmholtz resonator.

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10. A speaker unit according to claim 9, wherein the frequency of the Helmholtz resonator is about 50 Hz.
11. A speaker unit according to any of claims 9 or 10, wherein the regions of the housing remote from the drive unit are domed.
12. A speaker unit as claimed in any preceding claim, wherein struts extend between the acoustic mirror and the dome for supporting the dome in spaced relationship above the acoustic mirror, the struts being disposed to a side of the conical surface opposite to the direction from which sound is intended to leave the speaker.
13. A loudspeaker unit according to claim 12, wherein the or each strut has a length greater than its width and is directed towards the centre of the speaker or 1 drive unit.
14. A speaker unit according to claim 12 or 13, wherein the or each strut subtends an angle of 5 to 10 degrees when viewed from the centre of the speaker or drive unit.

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15. A speaker unit according to any of claims 12 to 14, wherein there are two struts.
16. A speaker unit according to claim 15, wherein the angular spacing between the struts, when viewed from the centre of the speaker or drive unit, is 40 to 60 degrees.
17. A speaker unit according to claim 15 or 16, when dependent upon claims 9 or 10, wherein the aperture or apertures defining the Helmholtz resonator exit between the struts.
18. A speaker unit substantially as hereinbefore described with reference to and as illustrated in Figures 1 to 5 or 6 and 7 of the accompanying drawings.
19. A speaker unit having a speaker or drive unit which emits sounds in a generally vertical direction and an acoustic reflector disposed at a spacing from the speaker for redirecting sound therefrom into a generally horizontal direction, the reflector having a generally conical surface facing the speaker with the apex of the cone nearest to the speaker and with the 1 - 19 centre of the speaker offset from the axis of the conical surface so as to define a direction for sound leaving the speaker, wherein a strut or struts extend between the housing and the acoustic reflector, the or each strut being located to an opposite side with respect to the axis of the conical surface fror, the drive unit
20. A speaker unit according to claim 19, wherein the or each strut has a length greater than its width and is directed towards the centre of the drive unit.
21. A speaker unit according to claim 20, wherein each strut has straight sides and a curved end facing towards the speaker, the sides of the struts converging in the direction of the speaker.
22. A speaker unit according to any of claims 19 to 21, wherein the or each strut subtends a angle of 5 to 10 degrees when viewed from the axis of the speaker or drive unit.
23. A speaker unit according to any of claims 19 to 22, wherein there are two struts.
24. A speaker unit according to claim 23, wherein the angular spacing between the struts is 40 to 60 degrees.
25. A speaker unit according to claim 24, wherein the speaker or drive unit is within a closed housing having an aperture or apertures defining a Helmholtz resonator, the or each aperture being located between the struts.

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