GB2273847A - Loudspeaker having acoustic mirror. - Google Patents

Abstract

A loudspeaker unit is described for motor car and other vehicle sound systems in which a drive unit 25 and frustoconical sound mirror 31 with their axes relatively offset produce a wide but directional distribution of mid range and high frequency sound into the interior of the vehicle. The sound mirror when viewed in plan can have lateral edges subtending an angle of 90 or 120 degrees (figs 4, 6) and the loudspeaker units can be assembled into groups or clusters (figs 9, 13). The loudspeaker unit may also be used with computers, televisions etc. <IMAGE>

Description

SOUND OUTPUT DEVICE This invention relates to a sound output device or speaker unit which may be incorporated into a stereo audio system for close personal listening, for example in a motor road vehicle such as a private motor car.

Another possible use for the sound output device of the invention is where it is desired to produce high quality sound at a relatively close proximity to the user e.g. when the sound is generated from a personal computer.

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 sound waves in a vertical direction and an acoustic reflector which is disposed above the diaphragm and reflects the sound waves 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 center axis of the conical surface of the acoustic reflector is offset from the center 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.Further aspects of sound output system using generally conical sound mirrors are disclosed in EP-A-0320270 and EP-A-0409360. A device of this kind for reproducing sound in a domestic enironment is described in our international application number W092/07449.

In one aspect of this invention is based on the realisation that the ability to produce a wide distribution of sound using a compact sound source and a conical or part conical sound mirror, with the axes of the sound source and the sound mirror offset from one another has advantages in audio speakers for private motor cars and other road vehicles. In particular such loudspeakers which by incorporation of a sound mirror produce a widened polar and preferably also vertical distribution of reflected sound can be used to provide a more realistic sound stage in front of the listener than is achievable by conventional door mounted stereo loudspeakers.

It has been realised that speakers provided with sound mirrors can be of particular benefit when used to provide personal stereo listening for an individual occupant of a seat within a motor vehicle. For this purpose it is desirable to provide pairs of speakers on the front dashboard of an automobile individually directing sound towards a listener who is occupying either the driver's or passenger's seat. It is not practical to provide a pair of forwardly facing loudspeakers on the front of the dashboard because the minimum diameter of a driver produces an acceptable sound quality is about 85 mm (3 inches) and the space needed for a pair of these driver units is simply not available on the dashboard of an ordinary automobile.

This difficulty is overcome, according to an aspect of the invention by providing a dashboard for an automobile or other passenger road vehicle, said dashboard having a top surface, at least one pair of openings in the top surface in which loudspeaker units are received, each loudspeaker unit comprising an upwardly facing drive unit and a sound mirror for directing the sound towards the front seats of the vehicle.

The sound mirror may be planar, but is preferably frustoconical or a segment of a cone. A frustoconical reflector can give rise to a soft focus beam of mid range and high frequency sound which is better in the relatively harsh environment of an automobile interior which readily gives rise to sound reflections and is relatively confined so that the reflected signal reaches the user within a relatively short time delay relative to the direct signal from the loudspeaker, giving rise to confusion of the stereo image. The laterally and optionally vertically dispersed sound from a conical sound mirror reduces unpredictable behaviour due to multiple reflections of an undispersed sound beam.

The dashboard may have sound mirrors arranged to direct beams of sound in a relatively wide distribution towards the occupants of both the driver's and passenger's seats but preferably it has at least one pair of loudspeaker units arranged to direct mid range and high frequency sound towards the head of an occupant of the driver's or passenger's seat, or to pairs of loudspeakers respectively directing mid and high frequency sound towards the head of an occupant of the driver's seat and the passenger's seat. Each drive unit is fitted into the dashboard at a generally horizontal attitude or rearwardly inclined at a small acute angle, and a recess in the dashboard may be provided to receive each drive unit.

The invention in another aspect provides a speaker unit having a drive unit and the sound mirror supported in spaced relationship from the drive unit, the sound mirror having a conical surface for redirecting sound therefrom into an intended listening direction with the apex direction of the cone facing towards the drive unit and with the center of the drive unit being offset from the axis of the sound mirror in a direction defining an intended listening direction, and wherein the sound mirror is generally frustoconical. With this arrangement the height of the unit can be made relatively small which is a desideratum for vehicle use.

Advantageously the sound mirror occupies a sector of the cone which preferably a 90 degree sector but which in other applications could occupy a different fraction of a circle e.g. a 120 degree sector. The above speaker unit can be used in automobile and other vehicles as aforesaid, but can be used in other environments e.g. in a home or office where personalised listening and a dispersed soft focus sound beam is required. For vehicle use the sound mirror is normally disposed above the drive unit, referring to the normal attitude in use. For other uses e.g. domestic use the sound mirror may be disposed above or below the drive unit.

The drive unit preferably has concentric first and second diaphragms for lower and higher frequencies, and these may be independently driven by respective voice coils, or the second diaphragm may be attached to the first diaphragm to be vibrated by a single voice coil and be of a horn-like shape arranged at high frequencies to direct sound in a relatively concentrated beam onto the audio mirror (a so called parasitic tweeter). The second diaphragm that may be arranged to effectively come into operation at frequencies above 2khz, and in particular above 5khz.

The drive unit is most conveniently circular, but it may of course, also be elliptical. In this arrangement, the frustoconical diaphragm is preferably arranged to occlude the whole of the second or higher frequency diaphragm and a major part but less than the whole, of the first or lower frequency diaphragm.

Again for the reduction of the overall height of the sound mirror it is preferred that the curvature of the sound mirror when viewed in profile should progressively increase with increasing distance from the front plane of the drive unit.

In the 90 or 120 degree versions of the aforesaid loudspeaker in particular, there is the possibility of assembling them into clusters, for example for fitting as a pair at the mid position at the front of the vehicle so as to direct sound respectively onto the driver's and passenger's seats where individual pairs of speakers are provided for these seats.

The invention further resides in the use as a speaker, or as one of a pair of stereo speakers, in a private motor car or other road vehicle of a loudspeaker in which a sound mirror reflects mid range and high frequency sound from the drive unit to produce a widened but directional polar distribution of sound into the interior of the vehicle.

Various embodiments of the invention will now be described, by way of example only with reference to the accompanying drawings in which; Figure 1 is a diagrammatic plan view of the drivers side passenger space of a representative relatively large motor car and Figure 2 is a diagrammatic side elevation of that part of the passenger space; Figure 3 and 4 are a diagrammatic vertical section and diagrammatic plan of a drive unit for fitting into the vehicle of figures 1 and 2; Figures 3 and 4 are a diagrammatical vertical section and a diagrammatic plan view of the major components of a drive unit for use in the production of widely distributed stereophonic sound in automobiles; Figures 5 to 8 are respectively a vertical section, a plan view, a front view and a side view of a loudspeaker unit for fitting into the dashboard of a motor vehicle;; Figure 9 is a diagrammatic view of two of the units of figures 5 to 8 grouped together; Figures 10 and 11 are respectively a front view and a plan view of a second form of loudspeaker unit for use with a personal computer or other item of domestic sound reproduction equipment; Figure 12 shows a method of supporting the unit of figure 11; Figure 13 shows a group of the loudspeaker units of figures 10 and 11; and Figure 14A-H shows various loudspeaker arrangements in an automobile interior.

The present invention is particularly concerned with audio speakers for fitting into passenger motor cars, but the speakers can also be fitted for example into the driving cab of buses and heavy goods vehicles and into the passenger compartments of buses and other passenger vehicles. However, the first embodiment of this invention will be further described with particular reference to private motor cars for which that embodiment is primarily intended.

In figure 1 there is shown a driver's side seat space generally indicated by the reference numeral 10 of a passenger motor vehicle the major dimensions of which are defined by a drivers seat 12, a lower edge 14 of a windscreen, a drivers door inner surface 16 and a vehicle longitudinal mid line 18. A dashboard generally indicated by the reference numeral 19 has a pair of wide dispersion loudspeakers 20 so as to provide stereo sound at the driver's position. In the embodiment illustrated, the loudspeakers 20 are 570 mm apart, and the driver sits with his ears approximately 900 mm behind the speakers 20. The driver's ears may typically be about 250 mm above the level of the speakers 20 so that sound coming from the speakers 20 needs to pass upwardly at an angle of about 150 to reach the drivers ears as shown in figure 2.The speakers subtend an angle of about 350 at the listening position. The loudspeakers 20 are mounted at a minimum 100 mm clearance from the front screen 14. With this arrangement, and using wide dispersion loudspeakers incorporating audio mirrors, it is possible to give to the listener a relatively realistic stereo sound stage. The sound from such a pair of wide imaging loudspeakers on the dashboard can be made better than the sound from a pair of door mounted loudspeakers such as is common in motor vehicles at the present time and which are not only awkward to fit but can give inappropriate polar directivity or dispersion, no clear stereo image, and are not adjustable to take account of the differing listening positions of different individuals driving the car.

In a preferred form of the invention, there is associated with the passenger seat in the vehicle a further pair of loudspeakers like the loudspeakers 20.

These can comprise two individual loudspeakers at the left and right hand ends of the dashboard and a grouped pair of loudspeakers adjacent the mid line 18 of the vehicle.

It is also possible using wide dispersion loudspeakers to have a single pair of loudspeakers at the left hand end or right hand end of the dashboard, the loudspeakers adjacent the mid line 18 being dispensed with. This reduces the number of individual loudspeaker units that have to be fitted, but compromises to some degree the stereo image perceived by the driver and passenger of the vehicle.

Various other speaker arrangements are possible within the invention. Thus there can be a woofer incorporated at the mid line 18 of the vehicle at the rear of the passenger compartment conventiently on the shelf behind the passenger seats. Alternatively, or in addition a pair of left and right hand mid range and high frequency speakers can also be provided at the rear of the passenger compartment. As a further refinement the left and right hand loudspeakers can be grouped in pairs associated with each passenger seat with or without a woofer between them. These possibilities are shown in figure 14A to H.

The working components of an individual wide imaging speaker for use in a vehicle as described above are diagrammatically shown in figures 4 to 7. A full range drive unit 25 of diameter about 85 cm has a main cone 27 and a parisitic tweeter 29. A sound mirror 31 is supported in closely spaced relationship above the drive unit 25. It is shaped as a segment of a frustum of a cone with, in this instance lateral edges 33 directed at right angles to one another, an outer edge 35 which at one position approximately overlies the main cone 27 as shown and is of radius about 115 mm, and an inner curved edge 37 of radius about 43 cm that terminates adjacent to and only a few millimetres above the tip of the parasitic tweeter 29.The sound mirror 27 therefore covers the parasitic tweeter 29 entirely, and a major portion of the main cone 27, but there is a small segment 39 of the main cone which is not covered by the sound mirror 37. The applicants believe that of the mid range and high frequency sound for which the mirror 31 is effective, the uncovered region 39 will bring about only a loss of about 10% in the efficiency of the sound mirror whilst enabling a significant reduction to be made in the axial distance which the sound mirror 31 extends away from the front plane 41 of the drive unit 25. Furthermore the sound which is not reflected by the mirror 31 is of relatively low frequency and therefore less directional, so that this non-reflected portion of the sound makes little difference to the perceived stereo image.

For use in automobiles it is important that the "height" of the sound mirror should be as small as possible to minimise its physical and visual obtrusiveness. When viewed in profile the sound mirror can have a double curvature or more complex curvature with the radius of curvature progressively decreasing with increasing axial distance from the drive unit 25.

In the embodiment shown in figure 4 the sound mirror 31 has a first region 43 which is closest to the drive unit 25 and has a radius of curvature of 174 mm and a second region 45 which has a radius of curvature of 87 mm. This increasingly steep curvature again reduces the overall height of the sound mirror 31. The curvature of the mirror 31 may, of course, change continuously rather than in distinct regions.

Using the principles of geometrical acoustics, which may give some guidance as to the vertical distribution of sound and high frequencies, it will be perceived from figure 3 that the sound mirror 31 will produce reflected high frequency sound at angles from about -7 to +20 degrees relative to the front plane 41. This range of angles will provide a good intensitive sound upto 20 degrees upwards from the horizontal which will in most vehicles cover the expected height of the ears of most drivers or passengers when they are sitting normally. The drive unit 25 may be mounted in the dashboard with its front plane 41 directed generally horizontally, or the front plane may be inclined rearwardly towards the passenger compartment at an angle of e.g. 10 degrees.The unit fits into a recess opening to the top of the dashboard, which in many vehicles is relatively uncluttered, and does not take up valuable space at the face of the dashboard which is presented to the driver. Because it may be supplied as a unit encasing the drive unit and sound mirror, it is rapid and convenient to fit. It is desirable that the material of which the sound mirror 31 is made should be selected so as to minimise the risk of injury to passengers. Thus the mirror 31 may be a low pressure polyurethane molding.

Using a sound mirror 31 of the profile illustrated in figure 4, it is desirable that the lateral edges 33 of the sound mirror should be plain and free of any sound reflective service. The reason is that if, for example vertical walls are provided at the edges 33,, the sound reflected from these walls can interfere with the sound reflected from the mirror 31 itself which can produce anomalies in the polar distribution of sound intensity especially along the intended listening direction 45 which is defined by a line passing through the axis 47 of the sound mirror 31 and the axis 49 of the drive unit 25. It will be noted that when viewed in plan as in figure 4 the lateral edges 33 are in this embodiment approximately tangential to the drive unit main cone 27, and this arrangement provides a desirable combination of sound directivity and compactness.

A possible practical form of loudspeaker incorporating the drive unit 25 and sound mirror 31 is shown in figures 5 to 8. The drive unit 25 is mounted in a molded plastics case 50 having a front aperture 52 through which sound from the drive unit 25 can reach the mirror 31, the aperture 52 being partly occluded by the sound mirror 31. The case 50 can define with the rear portion of the sound mirror 25 and internal space 52 having a volume of e.g. from 0.5 to 1.5 litres. One or more reflex loading ports may open into the space 52 which may then act as a bass frequency Helmhotz resonator as known in the art. The sound mirror 31 is covered by a top cover 54 and may be protected by a front grille 56 (fig 7).

As can be seen in figure 9, the units can be neatly grouped into pairs e.g. for fitting to either side of the mid line 18 of the vehicle.

An alternative miniature speaker for providing stereo sound in a small scale domestic environment e.g. when playing games or generating sound on a personal computer or in an office workstation or computerised games machine is shown in figures 10 and 11. Again there is used a drive unit 25 of about 80 to 90 mm diameter and a sound mirror 31 having a similar geometrical relationship to that indicated above. The sound mirror and drive unit are built into a casing 60 which is generally square in front view and quarter round in plan and has a perforated grille 62 at its front face.The rear edge of the case 60 is formed with an arcuate or otherwise suitably shaped cut out 64 so that it can be fixed either singly (fig 12) or in groups (fig 13) to a mounting pole 66 which may, as in figure 12 be supported on a stand 68 or may extend or depend from a wall or ceiling mounted bracket, depending upon the users requirements.

It will be appreciated that various modifications may be made to the embodiments described above without departing from the invention, the scope of which is defined in the appended claims. For example, in addition to the arrangement shown in figure 14 a front sound stage can be provided for rear passengers in a motor car by providing a pair of stereo loudspeakers directed rearwardly to the sides of the two front seats, or from the sides of each front seat. Again these speakers will use an off-center audio mirror to provide a widely dispersed but nevertheless directional distribution of high frequency and mid range sound. In the rear compartment the main direction of distribution of the sound may be horizontal rather than inclined, and the locations of the speakers can be selected depending upon the internal structure of the vehicle not to interfere to much with the sound for the front passengers. It is also possible if desired to use these speakers to provide surround sound. Although various dimensions have been given in the foregoing description, these dimensions are by way of example only.

Claims (17)

1. A speaker unit having a drive unit and a sound mirror supported in spaced relationship from the drive unit, the sound mirror having a conical surface for redirecting sound therefrom into an intended listening direction with the apex direction of the cone facing towards the drive unit, and with the center of the drive unit being offset from the axis of the sound mirror in a direction defining an intended listening direction, wherein the sound mirror is generally frustoconical.

2. A unit according to claim 1, wherein the sound mirror occupies a sector of a cone

3. A unit according to claim 2, wherein the sector is a 90 degree sector.

4. A unit according to any preceding claim, wherein the drive unit has concentric first and second diaphragms for lower and higher frequencies and the sound mirror occludes the second diaphragm and less than the whole of the first diaphragm.

5. A unit according to any preceding claim, wherein when the unit is viewed in plan, the lateral and front edges of the sound mirror are approximately tangential to the outer edge of the first diaphragm.

6. A speaker unit according to any preceding claim, wherein the sound mirror is curved when viewed in profile, with the degree of curvature increasing progressively with increasing distance from the drive unit.

7. A cluster comprising two or more units as claimed in any preceding claim.

8. A passenger vehicle having a pair of speaker units as claimed in any preceding claim associated with a seat in the vehicle to provide a stereo phonic sound image to a person occupying that seat.

9. A passenger vehicle having a pair of seats and a pair of speaker units as claimed in any of claims 1 to 7 arranged to provide stereophonic sound to both occupants of the pair of seats.

10. A passenger vehicle having a pair of seats and two pairs of speaker units as claimed in any of claims 1 to 7 each providing stereophonic sound individually to a person occupying each seat.

11. A passenger road vehicle having a driver and passenger seat and a dashboard in front of those seats, wherein at least one pair of loudspeakers having a sound mirror to produce a wide distribution of sound therefrom is fitted to the dashboard of the vehicle.

12. A motor road vehicle having a stereo sound system the speakers of which have sound mirrors to produce a wide distribution of sound therefrom.

13. A dashboard for an automobile or other passenger road vehicle, said dashboard having a top surface, at least one pair of openings in the top surface beneath which loudspeaker units are received, each loudspeaker unit comprising an upwardly facing drive unit and a sound mirror extending above the top service of the dashboard for directing the sound towards the front seats of the vehicle.

14. A dashboard according to claim 13, wherein the loudspeaker units direct mid and high frequency sound towards the head of an occupant of the driver or passenger seat.

15. A dashboard according to claim 13, comprising two pairs of loudspeaker units respectively directing mid and high frequency sound to the head of an occupant of the driver's seat and an occupant of the passenger seat.

16. A dashboard according to any of claims 13 to 15, wherein the or each drive unit is generally horizontal or rearwardly inclined at a small acute angle.

17. A speaker unit or a pair of them, as claimed in any of claims 1 to 6 connected to a computer, a workstation, a computerised games machines or a television.