GB2068679A - Loudspeaker grilles - Google Patents

Abstract

The invention relates to loudspeaker systems comprising at least one transducer mounted in an enclosure to radiate through an aperture in a wall of the enclosure, said aperture being covered by acoustically transparent material. Usually the acoustically transparent material is mounted on a wooden frame but such a frame causes distortion of the radiated sound. Accordingly the wooden frame is replaced by a metal frame designed to have an acoustically transparent peripheral portion which does not distort the radiated sound to the same extent as a wooden frame. The frame may consist of two parallel parts 12, 13, one of which is attached to a wall of the enclosure by means of pegs locating in corresponding holes 14. Alternatively, if the frame is made of resilient material such as steel wire, it may be held in position by locating a first part of the frame in a recess formed in the enclosure wall. <IMAGE>

Description

SPECIFICATION Improvements in or relating to loudspeaker systems This invention relates to loudspeaker systems.

Most loudspeaker systems include at least one transducer mounted in an enclosure to radiate through an aperture in a wall of the enclosure. From a purely acoustic point of view, the aperture should be open, but, from an aesthic point of view, it is normally necessary to provide a cover over the aperture. Usually such a cover comprises an acoustically transparent, but optically opaque, grill consisting of, for example, a textile material. When a cover is provided, it is necessary to attach it to the enclosure in such a waythat it is spaced outwardly from the wall through which the transducer radiates. In the case of a mid-range of low-frequency range transducer, this spacing is necessary because the transducer diaphragm moves an appreciable distance in and out during operation.The spacing is also required in the case of a hig h-frequency transducer because, in this case, the transducer diaphragm normally protrudes some distance outwardly from the chassis plate by means of which the transducer is mounted in the enclosure. To overcome this problem in the past it has been proposed to mount the transducers some distance behind the front of the wall through which they rotate. In this case it would be possible to attach the cover directly to the wall of the enclosure. However, the arrangement is not satisfactorily because the enclosure wall then interferes with radiation from the transducer since sound waves from the diaphragm impinge on the peripheral surface of the aperture and are reflected into the radiation path from the transducer.These reflections interfere with the directly transmitted sound and modify it in an undesirable fashion.

A second solution which has been adopted is to mount the cover on a wooden frame surrounding the aperture. However, this frame acts in precisely the same way as the peripheral wall of the aperture and causes reflections which distort the transmitted pattern.

Accordingly it is an object of the present invention to provide a loudspeaker system in which the cover is attached to the enclosure in such a way that it has substantially no distorting effect on the radiation from the transducer or transducers.

The invention consists in a loudspeaker system including at least one transducer mounted in an enclosure to radiate through an aperture in a wall of the enclosure, wherein said aperture is covered by acoustically transparent material secured to a frame which serves to support a first portion of said material substantially parallel to, and spaced in front of, said aperture, and wherein said frame, and a second portion of said material, form a peripheral surround extending from said first portion to said wall, the majority of said surround being constituted by said second portion.

The frame may consist of steel wire, the diameter of which is preferably as small as possible commensurate with the necessary strength to support the acoustically transparent material. A wire frame can be constructed to offer a much smaller obstacle to the radiated sound waves than a wooden frame.

Consequently a wire frame causes less degradation of the acoustic pattern of the loudspeaker system than would a wooden frame.

In one embodiment of the invention the frame consists of a first substantially planar part attached to, but spaced from, a second substantially planar part parallel to said first part. The first part is attached to the enclosure so that the second part is spaced outwardly from the wall through which the transducer, or transducers, radiate. The acoustically transparent material which may be, for example, a textile material, is secured to the frame so that it covers the aperture or apertures in the enclosure wall be means of a first portion substantially parallel to the enclosure wall and a second peripheral portion covering the space between the two parts of the frame.

The first part of the frame may be attached to the enclosure in a number of different ways. For example, a number of pegs may be attached to the first part projecting from the first part in the opposite direction to the second part. These pegs may fit in corresponding recesses in the enclosure wall. Alternatively pegs may be provided projecting outwardly from the enclosure wall and these may locate in holes in tabs secured to the first part of the frame.

However, in a preferred system, a recess is provided around the periphery of the enclosure wall. In this embodiment the first part of the frame is dimensioned to fit in this recess and small retaining tabs are provided to keep the said first part in the recess.

In a modification of this embodiment, the frame is made of resilient material and designed so that its shape is initially somewhat distorted in relation to the shape of the recess. The frame is then assembled in position by bending the first part of the frame sufficiently to fit into the recess, and the frame is held in position solely by the resilience of the material. This latter arrangement is particularly suitable in the case of a circular frame which is initally made to be slightly elliptical so that it has to be deformed to fit into a circular recess. In the case of a rectangular frame, if desired the resilience of the material may be used to retain the frame in position in addition to a number of retaining tabs.

Methods of performing the invention will now be described with reference to the accompanying diagrammatic drawings in which: Figure 1 is a front view of a loudspeaker enclosure with a cover mounted on a wooden frame; Figure 2 is a front view of a loudspeaker enclosure with the cover mounted in accordance with a first embodiment of the invention; Figure 3 is a side view of a portion of the enclosure illustrated in Figure 1; Figure 4 is a side view of a second embodiment of the invention; Figure 5 is a scrap view illustrating a portion of the embodiment illustrated in Figure 4; Figure 6 is a sectional view of the portion ill ustrated in Figure 5 taken on the line VI-VI; Figure 7 is a front view of a third embodiment of the invention;; Figure 8 is a view to a reduced scale ofthe cover and mounting frame used in the embodiment of Fig ure7; Figure 9 is a graph showing the effect of the cover and mounting frame of Figure 2 on the on-axis fre quency response of the high-frequency transducer; and Figure 10 illustrates the effect of the cover and mounting frame of Figures 1 and 3 on the on-axis frequency response of the hig frequency transducer.

Figures 1 and 3 illustrate a conventional louds peakersystem having a medium- and low-frequency transducer, the mounting plate and diaphragm of which can be seen at 1 and 2 respectively in Figure 3, and a high-frequency transducer, the mounting plate and diaphragm of which can be seen at3 and 4 in Figure 3.

The low-frequency diaphragm 2 is shown in Figure 3 at the maximum forward position of its normal travel. The two transducers are covered partlyto protect the diaphragms and partly for aesthetic effect by a cover 5 consisting of acousticailywrans- parent material such as a textile material. To avoid contact between this material and the diaphragms, the cover is mounted on a wooden frame 6 which is held in position by means of four studs as indicated, for example, at 7. As can be seen, particularly from Figure 3, the cover material is wrapped round the frame as indicated at8 and glued in position before the frame is attached to the front wall of the enclosure.As can be seen from Figure 3, sound waves emanating from the loudspeaker diaphragms will impinge on the inner edge, and in particular on the sharp corners of the inner edge, of the frame. These sound waves are reflected off, and diffracted by, the wooden frame in such a way thatthe axial sound pressure/frequency response of the loudspeaker is made less uniform.

The result of the reflections on the axial frequency response is indicated in Figure 10 of the drawings in which the broken line shows the measured frequency response of the high-frequency transducer with the cover and its mounting frame removed, while the full line shows the measured frequency response of the same high-frequencytransducer with the cover and wooden frame in positio As can be seen, the sound pressure level is increased at some frequencies by the presence of the cover and frame and is reduced at other frequencies. Thus the overall frequency response of the system is considerably distorted by the presence of the cover and frame.

It is to be understood that in the particular system which was tested to produce the results shown in Figure 10, the cross-sectional area of the wooden frame was reduced as far as possible commensurate with the necessary strength to support the covering material.

Figure 2 illustrates one embodiment of the invention in which the distortion of the frequency response is reduced bythe use of a frame made of steel instead of wood for supporting the cover.

Because of the greater strength of the steel com pared with wood, the cross-sectional dimensions of the frame can be considerably reduced. The frame then offers a much smaller obstacle to the radiating sound waves with a consequent reduction in the degradation of the acoustic performance.

The frame illustrated in Figure 2 includes a rear subframe 11 consisting of round edge mild steel strip and a forward sti'bframe 12 consisting of steel rod The two subframes are inter-connected by braces 13 brazed or weldedto the strip and the rod.

Fixing lugs 14 are alswelded to the strip 11. These lugsare offset so thattlia holes 14 are contained in a pFarre approximately midway between the plates of the-two subframes. Thefameis held in position on the front of the enclosuieby rneans of pegs project- - ing from the front of the enclosure irr- registration with the holes in the lugs14. These pegs may if desired be combined witlrthe means for holding the bass transducer or transducers in posFtion. They are provided with a shoulder writhe like so that the lugs can be pushed over therm and then held in position by the shoulder. The cover material 5 is wrapped round the strip 11 and may beheld in position by means ofg[ue or spring steel clips or both glue and clips.

As can be seen the arrangement shown in Figure 2 does not present any sharp edges orsubstantial solid areas to reflect the sound waves radiated by the transducers. The radiated sound can pass through the gap between the subframes 11 and 12 and the total cross-sectional area of the two subframes, the supporting braces and the mounting lugs is very much smaller than that of the wooden frame shown in Figure 1.

Figure 9 shows the effect of the cover and frame illustrated in Figure 2 using the same enclosure and transducers as in the case of the prior art arrange mentillustrated in Figure 1.The broken line in Figure 9 showsthe relation between frequency and sound pressure level with the cover and its mounting frame removed, while the solid line shows the response when the cover and frame illustrated in Figure 2 are fitted. Itwill be seen that the distortion introduced by fitting the cover is less than that indicated by Figure 10 which relates to the prior art arrangement shown in Figure t.

Afurtherembodirnentofthe invention is illustrated in Figures 4, 5 and 6. In this case the front wall of the enclosure consists of a synthetic resin material such as structural polystyrenefoam and it is then a simple matter two provide a recess 21! around the periphery of the moulding. Inthis embodiment the strip subframe 11 is replaced; by a subframe 22 con sisting of the same type of rod as the subframe 12.

The two subframesare again inter-connected by short lengths of rod 13. The cover material 5 is again wrapped round the rear subframe and held in position by glue or spring steel clips In this embodiment it is unnecessary to provide any fixing lugs becausethe frame may be held in position simply by taking advantage of the resilience of the frame material Preferably a number of protrusions (not shown) are provided in the moulding, projecting a short distance over the inner edge of the peripheral recess 21. The subframe 22 is distorted sufficiently to clear these protrusions and is then allowed to spring back under the protrusions into the peripheral recess 21. The width and depth of the peripheral recess may both be equal to the diameter of the rod used for the subframe 22.

Yet another embodiment of the invention is illustrated in Figures 7 and 8 of the drawings. In this particular instance the enclosure is in the form of an octagonal cylinder and the cover5 is mounted on a circular frame. This once again consists of a rear subframe 31 and a forward subframe 32, interconnected by braces 33. A circular recess 34 is formed in the front wall of the enclosure moulding.

The circular subframe 30 is retained in the recess 34 simply by the resilience of the material of the subframe. The subframe is initially formed so that it is a slightly elliptical shape as indicated by the full line 35 in Figure 8. It is then necessary to distort the frame to the circular shape illustrated by the broken line 36 so that it can be fitted in the recess 34. Due to the resilience of the steel, the subframe will grip the periphery of the recess and hold the frame in position.

Claims (18)

1. A loudspeaker system including at least one transducer mounted in an enclosure to radiate through an aperture in a wall of the enclosure, wherein said aperture is covered by acoustically transparent material secured to a frame which serves to support a first portion of said material substantially parallel to, and spaced in front of, said aperture, and wherein said frame, and a second portion of said material, form a peripheral surround extending from said first portion to said wall, the majority of said surround being constituted by said second portion.

2. A loudspeaker system as claimed in Claim 1 wherein the frame includes a first substantially planar part attached to the enclosure and a second substantially planar part spaced from and parallel to said first part.

3. A loudspeaker system as claimed in Claim 2 wherein an acoustically transparent material is secured to the frame so that it covers the aperture or apertures in the enclosure wall by means of a first portion substantially parallel to the enclosure wall and a second peripheral portion covering the space between the said two parts of the frame.

4. A loudspeaker system as claimed in any of the preceding Claims wherein the frame is made of steel.

5. A loudspeaker system as claimed in any of the preceding Claims wherein the frame consists of wire.

6. A loudspeaker system as claimed in Claim 2 or Claim 3 wherein the first part of the frame consists of metal strip material while the second part consists of wire.

7. A loudspeaker system as claimed in Claim 6 wherein the second part of the frame is attached to the first part of the frame by means ofwire braces.

8. A loudspeaker system as claimed in Claim 2, Claim 3, Claim 6 or Claim 7 wherein the first part of the frame is attached to the enclosure by means of a number of pegs co-operating with corresponding apertures.

9. A loudspeaker system as claimed in Claim 8 wherein the pegs project from the first part and fit in apertures in the enclosure wall.

10. A loudspeaker system as claimed in Claim 8 wherein the pegs project outwardly from the enclosure wall and locate in holes in tabs secured to the first part of the frame.

11. A loudspeaker system as claimed in any of Claims 2 to 5 wherein a recess is provided around the periphery of the enclosure wall and the first part of the frame is dimensioned to fit in this recess.

12. A loudspeaker system as claimed in Claim 11 wherein retaining tabs are provided to keep the first part of the frame in the recess.

13. A loudspeaker system as claimed in Claim 11 or Claim 12 wherein the frame is made of resilient material and wherein the first part of the frame is shaped so that it is held in position in the recess by the resilience of the material.

14. Aloudspeakersystem as claimed in Claim 13 wherein the first part of the frame is circular when in position in the recess.

15. A loudspeaker system as claimed in Claims 11 to 14 wherein said recess is formed in a synthetic resin material moulding constituting at least a part of the enclosure.

16. A loudspeaker system as claimed in Claim 15 wherein the synthetic resin material is structural polystyrene foam.

17. A loudspeaker system substantially as hereinbefore described with reference to and as illustrated in Figure 2 or any of Figures 4to 8 ofthe accompanying diagrammatic drawings.

18. Any features of novelty taken singly or in combination of the loudspeaker system hereinbefore described with reference to Figure 2 or any of Figures 4to 8 of the accompanying diagrammatic drawings.