GB2248996A - Speaker assembly - Google Patents

Abstract

A loudspeaker has a diaphragm 24 and a horn 26 driven by a common drive unit 28 so as to move together. The horn 26 acts as a parasitic tweeter for reproducing high frequency sound. Sound is directed from the diaphragm 24 and horn 26 onto a reflective surface 16 of a preferably conical sound mirror 14, and a sound redirecting member having vanes 45A to 45D is provided below the loud speaker to direct sound at high frequencies, 15kHz or above, towards the sound mirror 14 so as to increase the intensity of high frequency sound in the listening direction and/or to reduce the intensity of high frequency sound in unwanted directions. <IMAGE>

Description

SPEAKER ASSEMBLY This invention relates to a speaker assembly comprising a tweeter and an audio mirror disposed in the path of sound from the tweeter.

Sound output systems including a speaker for radiating sounds in a generally vertical direction which may be upwards or downwards and a generally conical or part conical sound mirror disposed in the path of the sound to cause the reflected sound to be radiated in a desired listening direction e.g. in a generally horizontal direction, the mirror preferably being curved in an upward direction to cause divergence of the reflected sound in the vertical direction are known e.g. from EP-A-0320270.

We have measured the polar distribution of sound radiated from a speaker unit as aforesaid at various frequencies and have noted that there may be fall off in sound intensity radiated in the intended listening direction at the higher range of frequencies to be output by the speaker e.g. at frequencies of 15 to 20kHz.

The problem is solved by providing a speaker assembly comprising a tweeter and an audio mirror disposed in the path of sound from the tweeter as aforesaid to cause reflected sound to be radiated in a predetermined listening direction, in which means is disposed between the tweeter and the mirror for directing high frequency sound from the tweeter onto the mirror.

The invention also comprises a speaker assembly as aforesaid in which one or more vanes is disposed between the tweeter and the mirror and is directed towards the surface of the mirror.

The problem has been observed, and therefore the invention is particularly concerned, in relation to speaker assemblies in which there is a so called parasitic tweeter i.e. a loudspeaker having a diaphragm and a horn within the diaphragm and arranged for movement with it, the horn acting as a parasitic tweeter in response to electrical signals in the high frequency part of the audible range of frequencies fed to the speaker, and serving as the tweeter of the speaker assembly.

The invention will be further described, by way of example only, with reference to the accompanying drawings in which: Figure 1 is diagrammatic partly sectional side view of a speaker assembly according to the invention; Figure 2a to 2f are plots showing the polar distribution of radiated sound intensity measured in a generally horizontal plane level with the sound mirror at the various frequencies and with no means provided between the tweeter and the sound mirror for directing high frequency sound onto the mirror; Figure 3 is a diagram of a speaker and part of the sound mirror showing a mechanism by which high frequency sound may fail to reach the sound mirror; Figure 4 is a diagramatic underneath view of the tweeter and a sound re-directing member supported beneath it; and Figure 5 is a plot showing the intensity distribution with angle of high frequency sound radiated from the audio mirror of the speaker assembly according to the invention.

In figure 1 there is shown a speaker unit comprising a cabinet 10 housing a mid and high frequency loud speaker 12 arranged with its axis or sound emitting direction vertically downwards as shown. A sound mirror 14 which is symmetrical about its axis and has a reflecting wall 16 of smoothly curving upwardly concave profile is supported from the cabinet 10 by a depending post 18. The axis 20 of the sound mirror 14 is offset from the axis of center line 22 of the loudspeaker 12, the intended listening direction being a horizontal direction generally level with the sound mirror 14 and in or adjacent the plane passing through the axes 20, 22. The curvature in an upward direction of the reflective wall 16 of the sound mirror serves to cause divergence of the reflected sound in a vertical direction.

The speaker unit has a diaphragm 24 whose overall depth is 24 mm and which has a central or dome area of diameter approximately 26mm, and the diaphragm 24 curves smoothly outwards towards its mouth which is of diameter approximately 90mm. Located centrally within the diaphragm 24 and arising from the same line as the back end of the diaphragm is a horn 26 which is flared forwardly and outwardly as shown to reach a maximum diameter of about 55mm at its mouth end. The diaphragm 24 and horn 26 are moved together by a common driver unit diagrammatically illustrated by the reference numeral 28.

The distribution of sound produced by a speaker generally as shown in figure 1 was measured in the plane of the sound mirror 14 for a range of frequencies, and the results are shown in figures 2a to 2f. In figure 2a where the sound being detected was at 460 Hz a smoothly varying distribution of sound was produced all around the speaker assembly, with the intensity of sound predominating in the listening direction. Figure 2b shows a measurement at 5kHz and shows a high intensity in the listening direction with relatively little fall off up to angles of 30 degrees to either side of the listening direction but relatively little rearwardly reflected sound.Figure 2c shows a measurement taken at 12.5kHz and again shows the greatest intensity of sound in the listening direction, but the sound falls off away from the listening direction to a minimum and a substantial intensity appears at right angles to the listening direction. In figure 2d which shows a measurement at 15kHz the pattern is similar but with the sound in the listening direction being of similar intensity to the sideways sound. At 17.5kHz (figure 2e) the greatest intensity is in lobes that appear generally to the sides of the listening direction and the same pattern of sound distribution appears in an even more pronounced manner in figure 2f which shows the distribution at 20kHz.

It is apparent from figures 2a to 2f therefore, that although the speaker shown in figure 1 gives a satisfactory intensity of sound in the listening direction in the mid frequency range, at frequences above 15kHz there is a falling off of intensity in the listening direction and unwanted intensity at right angles to the listening direction.

Although the invention does not depend for its operation on the correctness or otherwise of this theory, it is believed that at frequencies above 15kHz the parasitic tweeter acts as a wave guide with the result that the sound follows the surface of the parasitic tweeter and does not follow the expected direction of propagation. For example, sound of frequency 17.5kHz has a wavelength of just under 20mm which matches the height of the parasitic tweeter which then acts as a regulator or wave guide.At frequencies below 15kHz the main sound intensity is in an axial direction as indicated by the arrow 40 in figure 3, but at intensities of e.g. of about 17.5kHz or above, the main sound intensity is tangential to the rim of the horn 26 and is directed at an angle to the axis 22 as indicated by the arrows 42, sound at high frequencies may therefore fail to reach the reflective surface 16 of the sound mirror, and it is believed that this phenomenon is at least partly responsible for the falling off in intensity of high frequency reflected sound in the listening direction and/or the unwanted intensities in other directions.

Because the axis 22 of the speaker 12 is offset from the axis 20 of the sound reflector 14, a sideways going sound is more likely to hit the audio mirror which is believed to be the reason of why the side lobes of figures 2e and 2f appear.

The problem is remedied, according to the invention by attaching a regulator in front of the loud speaker 12 to redirect sound of high frequency so that its main intensity continues to hit the audio mirror. For this purpose of a series of concentric vanes 45a to 45d of circular or other convenient shape when viewed in plan is attached to the housing 10 beneath the loud speaker 12. The vanes may conveniently be rectangular when viewed in section with their long side wall directed parallel to the axial direction and they may be of depth about lOmm. They may be made of any convenient material, for example metal, so called "dumped metal" which is a sandwich of metal with an elastomeric material, or they may be made of a rigid plastics material.As best seen in figure 4 the diameter of the vane 45C is slightly larger than the mouth diameter of the horn 26, with a spacing when viewed in underneath plan as in figure 4 of about 6mm therebetween. The spacing between the rim of the horn 26 and the plane containing the inner ends of the vanes 26 is lOmm.

In figure 5 there is shown a measured polar distribution of sound intensity with the sound re-directing member containing vanes 45A to 45D in place, and it is apparent that the distribution of the reflected sound is generally more uniform. The intensity of reflected sound is increased both in the intended listening direction itself and at angles close to the intended listening direction.

Modifications may be made to the form of the invention described above without departing from the invention, the scope of which is defined in the appended claims.

The loudspeaker may face upwardly rather than downwardly, the sound mirror 14 then being inverted.

The sound re-directing member may not include vanes that are circular in plan and, for example, it may comprise intersecting vanes that form a grid. The sound reflector could be of other shape e.g. a simple cone rather than a concave shape. The cone 24 is preferably conical but other outlines e.g. an elliptical outline are not excluded.

Claims (10)

1. A speaker assembly comprising a tweeter and an audio mirror disposed in the path of sound from the tweeter to cause reflected sound to be radiated in a predetermined direction, wherein means is disposed between the tweeter and the mirror for directing high frequency sound from the tweeter onto the mirror.

2. An assembly according to claim 1, comprising a loudspeaker having a diaphragm and a horn within the diaphragm and arranged for movement with it, the horn serving as the tweeter of the speaker assembly and acting as a parasitic tweeter in response to high frequency signals fed to the speaker.

3. An assembly according to claim 2, wherein the horn is outwardly curved or flared in a direction towards the front of the speaker.

4. An assembly according to any preceding -claim, wherein the sound directing means comprises one or more surfaces located between the rim of the horn and the mirror.

5. An assembly according to claim 4, wherein the sound directing means includes a surface which conforms to and is slightly larger than the rim of the horn and is positioned adjacent to the rim of horn.

6. An assembly according to claim 5, wherein the spacing in front view between the rim of the horn and the surface of the directing means is 5 to 15mm and the directing means is 7 to 15mm in front of the surface of the horn.

7. An assembly according to any preceding claim, wherein the sound mirror has a generally conical shape.

8 An assembly according to claim 7, wherein the reflective wall of the sound mirror is of smoothly curving concave profile.

9. An assembly according to claim 7 or 8, wherein the tweeter is offset from but faces generally parallel to the axis of the sound mirror.

10. A speaker assembly substantially as hereinbefore described with reference to and as illustrated in figures 3 and 4 of the accompanying drawings.

ll. A speaker assembly comprising a tweeter, an audio mirror and a sound re-directing member between the tweeter and the audio mirror.