WO1995028064A1

WO1995028064A1 - A loudspeaker

Info

Publication number: WO1995028064A1
Application number: PCT/GB1995/000824
Authority: WO
Inventors: Indrek Hilpus
Original assignee: Indrek Hilpus
Priority date: 1994-04-12
Filing date: 1995-04-11
Publication date: 1995-10-19
Also published as: AU2145895A; GB9407221D0

Abstract

A loudspeaker enclosure comprises an outer casing and a plurality of internal partitions which serve to define two tortuous paths extending from the rear of an opening in the front wall of the casing which receives the loudspeaker diaphragm and an outlet formed in the casing. Sound waves generated to the front of the loudspeaker diaphragm pass out of the casing through the opening in the front wall of the casing whilst sound waves generated to the rear of diaphragm are conveyed along the tortuous paths and pass out of the casing via the outlet formed therein. The outlet has a cross-sectional area which is substantially equal to the cross-sectional area of the opening in the front wall for the loudspeaker diaphragm. A deflector unit may be provided within the casing opposite the opening for the loudspeaker diaphragm so as to deflect sound waves generated to the rear of the diaphragm away from the diaphragm in a predetermined direction along the tortuous paths so that these waves are not reflected back onto the rear of the diaphragm from any part of the enclosure structure.

Description

"A LOUDSPEAKER"

THE PRESENT INVENTION relates to a loudspeaker and more particularly to an enclosure for a loudspeaker.

A loudspeaker is a device for converting electrical energy into acoustic energy, i.e. sound. A common type of loudspeaker comprises a diaphragm in the form of a laminated paper cone which can be vibrated or moved, with the vibrations being transmitted to the air adjacent the diaphragm as sound waves. The diaphragm is commonly caused to vibrate by means of an electromagnetic drive, one typical arrangement having a movable coil connected to the diaphragm with the electrical input being supplied to this coil as an AC supply and a permanent magnet, around one pole of which the movable coil is located.

In order for a loudspeaker of this type accurately to convert electrical energy to acoustic energy the diaphragm must be allowed to move freely following the path of its coil's electrical input.

The movement of a conventional loudspeaker diaphragm is, however, inhibited by various factors. Firstly it will be appreciated that as the diaphragm vibrates it emits pressure waves to the front as it moves forwards and pressure waves to the rear as it moves backwards. When the diaphragm is mounted in a loudspeaker casing the rearwardly emanating waves are reflected back onto the rear of the diaphragm from the structure of the casing and therefore affect and modify the diaphragm movement so that it cannot perfectly follow the electrical input. Secondly the back and forth movement of the diaphragm creates variations in the internal pressure within the loudspeaker casing which results in a pressure differential across the diaphragm which in turn affects the movement of the diaphragm. Thus, as the diaphragm moves rearwardly in a direction towards the interior of the casing within which it is mounted, it produces a rear wave which increases the pressure within the casing, this increased pressure offering resistance to the rearwards movement of the cone.

It is also well known that at low freguencies the rear waves, which are out of phase by 180 degrees with respect to the front waves emitted by the diaphragm, can cancel out the front waves. In order to address this problem it is common to mount a loudspeaker diaphragm within what is referred to as a "baffle", the baffle representing a partition which isolates the rear waves from the front waves. Mounting the diaphragm in a baffle of this type does not, however, address the other problems discussed above. In general, making the baffle larger enables lower frequencies to be reproduced. For example, mounting a loudspeaker diaphragm in a hole at the centre of a sheet of material measuring approximately 15.24m x 7.62m would enable a frequency as low as approximately 45Hz to be reproduced. It is, of course, totally impractical to incorporate this type of baffle into a loudspeaker for use with a domestic music system or for use whenever there may be space constraints.

Whilst various types of loudspeaker have been proposed which seek to address one or more of the problems discussed above these basic problems are still present in most loudspeaker designs. The present invention seeks to provide an improved loudspeaker and an enclosure therefor which obviates or mitigates the problems discussed above.

In one aspect the present invention provides a loudspeaker enclosure comprising an outer casing, the casing defining a diaphragm opening through one wall thereof and being designed to support a loudspeaker diaphragm and associated drive adjacent said opening such that sound waves generated to the front of the diaphragm by movement thereof pass out of the casing through the opening, the casing incorporating at least one internal partition, the or each partition defining an elongate path within the casing, the path extending between the diaphragm opening and an outlet formed through part of the casing, the path being designed so that in use sound waves generated to the rear of a loudspeaker diaphragm mounted within the enclosure are conveyed from the rear of the diaphragm to the outlet, the outlet through the casing having a cross-sectional area substantially equal to the cross-sectional area of the diaphragm opening.

Preferably the path is of substantially uniform cross-section over its length between the diaphragm opening and the outlet.

Conveniently a plurality of paths are defined within the casing between the diaphragm opening and the outlet.

Advantageously paths of differing lengths are defined within the casing. Preferably the or each path is at least approximately 2.6m in length from the diaphragm opening to the outlet.

Conveniently the or each path is filled with a sound absorbing material.

Advantageously a plurality of partitions are provided within the casing, the partitions being disposed substantially parallel to each other and defining a tortuous path between the diaphragm opening and the outlet.

Preferably turning guides are provided at the corners of the tortuous path, the turning guides having sound reflecting surfaces configured to guide sound waves around corners in the tortuous path.

Conveniently the turning guides are formed from marble dust and a resin binder.

Advantageously the turning guides each define arcuate sound reflecting surfaces.

Preferably the total cross-sectional area of the or all of the paths defined within the casing is substantially equal to the cross-sectional area of the diaphragm opening.

Conveniently a deflector unit is mounted within the casing opposite the diaphragm opening, the deflector unit having a contoured surface directed towards the diaphragm opening which, in use, deflects sound waves generated to the rear of the diaphragm in a predetermined direction along the path or one of the paths within the casing. Advantageously the casing defines a loudspeaker enclosure according to any one of the preceding claims wherein the casing defines a plurality of diaphragm openings, the casing being designed to support a plurality of loudspeaker diaphragms adjacent respective openings, the outlet through the casing having a cross-sectional area substantially equal to the sum of the cross-sectional areas of the diaphragm openings.

In a second aspect this invention provides a loudspeaker enclosure comprising an outer casing, the casing defining a diaphragm opening through one wall thereof and being designed to support a loudspeaker diaphragm and associated drive adjacent said opening such that sound waves generated to the front of the diaphragm by movement thereof pass out of the casing through the opening, the casing incorporating at least one internal partition, the or each partition defining an elongate path within the casing between the diaphragm opening and an outlet formed in part of the casing, the path being designed so that in use sound waves generated to the rear of the diaphragm are conveyed from the rear of the diaphragm to the outlet, the enclosure incorporating a deflector unit mounted within the casing opposite the diaphragm opening, the deflector unit defining a contoured surface directed towards the diaphragm opening which, when a diaphragm is mounted adjacent the opening and is in use, deflects sound waves generated to the rear of the diaphragm away from the rear of the diaphragm in a predetermined direction along the path or one of the paths defined within the casing so that the waves are not reflected back onto the rear of the diaphragm from any part of the enclosure. Preferably the contoured surface of the deflector unit deflects waves in a plurality of predetermined directions.

Conveniently the contoured surface of the deflector unit deflects waves in two substantially opposite directions along two predetermined paths defined by the partitions within the casing.

The deflector unit may be mounted upon an internal partition within the casing.

Alternatively the deflector unit may be constituted by part of an internal partition within the casing.

This invention also provides a loudspeaker comprising a loudspeaker enclosure as described above.

In order that the present invention may be more readily understood and so that further features thereof may be appreciated the invention will now be described by way of example with reference to the accompanying drawings in which:

FIGURE 1 is a cut away perspective view of a loudspeaker enclosure in accordance with one aspect of this invention;

FIGURE 2 is a vertical cross-sectional view taken on the line II-II of Figure 1;

FIGURE 3 is an enlarged view showing part of the internal structure of the enclosure of Figures 1 and 2 with a loudspeaker diaphragm mounted in the enclosure; FIGURE 4 is a view corresponding to Figure 2 but showing a slightly modified arrangement;

FIGURE 5 is also a view corresponding to Figure 2 but showing a further modified arrangement;

FIGURE 6 is a schematic perspective view of another embodiment of a loudspeaker enclosure in accordance with one aspect of this invention;

FIGURE 7 is a cross-sectional view through the enclosure of Figure 6 with a loudspeaker diaphragm mounted in the enclosure.

FIGURE 8 is a schematic, partial view of a modified version of the loudspeaker enclosure of Figures 1 to 3;

FIGURE 9 is a cross-sectional view through part of the structure of Figure 8; and

FIGURE 10 is a view, from the front, of part of Figure 8 with items omitted for clarity of illustration

Referring initially to Figures 1 and 2 of the accompanying drawings a loudspeaker enclosure comprises an outer casing 1 having a front wall 2 a rear wall 3, side walls 4, 5 a top wall 6 and a base 7. The casing 1 is of generally parallelepipedal form and may be approximately lm in height, 350mm wide and 450mm deep. These dimensions are, of course, merely illustrative of one particular example of a preferred enclosure design.

The front wall 2 of the casing defines a circular opening 8 at a position centrally with respect to the width of the front wall 2 and having its centre approximately 750mm from the base 7 i.e. approximately three quarters of the distance up the height of the front wall 2. The opening 8 is the main sound outlet for the enclosure and is dimensioned so as to have a diameter approximately equal to the outer or largest diameter of a cone-shaped loudspeaker diaphragm which would be mounted in position adjacent the internal surface of the front wall 2 of the casing such that movement of the diaphragm causes sound waves to be generated from the front of the diaphragm, the sound waves passing out through the opening 8.

Internally the casing 1 is provided with a plurality of partitions which serve to define two serpentine or tortuous paths 9, 10 within the casing, each path 9, 10 extending between the back of the opening 8 and a further opening or outlet 11 in the base 7. The outlet 11 extends centrally across the base 7 of the casing and is of rectangular form. In the preferred embodiment illustrated in the drawings the internal partitions consist of five parallel, equally spaced partitions 12, 13, 14, 15, 16 which stand upright within the casing, parallel to the front wall 2 and the rear wall 3 and extending between the opposed side walls 4, 5.

The two outermost partitions 12, 13 and the central partition 14 are interconnected at their upper edges by a further partition 17 which lies in a horizontal plane parallel to the top wall 6 of the casing. The three partitions 12, 13, 14 and the horizontal partition 17 which interconnects them are supported within the casing 1 by the side walls 4, 5 to which they are connected. The partitions 12, 13 are supported within the casing so that they are spaced inwardly of the front wall 2 and the rear wall 3 respectively. Similarly the horizontal, partition baffle 17 is positioned so that it is spaced from the top wall 6 of the casing. The free ends of the outermost partitions 12, 13 are spaced from the base 7 of the casing whilst the free end of the central partition 14 extends downwardly to the level of the base 7 in the centre of the opening or outlet 11.

The two remaining partitions 15, 16 each extend vertically upwardly from the base 7 of the casing at positions centrally between a respective outermost partition 12, 13 and the central partition 14. The upper ends of the partitions 15, 16 are spaced from the horizontal partition 17 which interconnects the upper ends of the three remaining partitions 12, 13, 14. Thus, the partitions 15, 16 may be considered to be interdigitated with the partitions 12, 13, 14 in such a way that the partitions define the serpentine or tortuous paths 9, 10 which wind their way around the partitions.

It will be appreciated from the description given above and from the drawings that one path 9 extends upwardly from the back of the opening 8 along the inside of the front wall 2 before extending horizontally along the inside of the top wall 6 and then extending downwardly along the inside of the rear wall 3. The path 9 is then diverted through 180 degrees so as to pass between the rear, outermost partition 13 and the partition 16 before again being diverted through 180 degrees so as to pass between the central partition 14 and the partition 16 as it extends to the outlet 11. The other path 10 extends downwardly from the back of the opening 8 along the inside of the front wall 2 of the casing before turning through 180 degrees to pass between the front, outermost partition 12 and the partition 15. The path is then diverted through 180 degrees again so as to extend between the central baffle 14 and the partition 15 as it extends to the outlet 11.

As explained above when a loudspeaker diaphragm is mounted within the casing 1 adjacent the opening 8 not only will it generate sound waves which pass out through the opening 8 to the front of the casing, but it will also generate sound waves which are projected rearwardly of the diaphragm towards the interior of the casing. Figure 3 illustrates, in a schematic form, a loudspeaker diaphragm 18 and an associated drive unit 19 mounted on the front wall 2 of the casing and also illustrates, again in a schematic form, sound waves 20 being generated to the rear of the diaphragm 18. The diaphragm 18 and its drive 19 are wired to an AC power supply.

Opposite the opening 8 and to the rear of the diaphragm 18 a deflector unit 21 is mounted on the partition 12 so that a forward region of the deflector is located within the space defined between the baffle 12 and the front wall 2 of the casing. The deflector unit is designed so as to deflect all of the rear waves 20 which emanate from the rear of the diaphragm 18 in a vertical direction, with those rear waves 20 which emanate from the upper half of the diaphragm 18 being deflected vertically upwardly along the path 9 and those waves 20 which emanate from the lower half of the diaphragm 18 being deflected vertically downwardly along the path 10.

It will be appreciated that rear waves 20 are generated from all positions around the cone shaped diaphragm 18 and the surface of the deflector unit 21 is so configured as to deflect all of these waves along one or other of the paths 9, 10 in a vertical direction, as viewed in Figures 2 and 3 of the drawings. Thus the outer surface of the deflector unit 21 which is directed towards the diaphragm 18 is of a complex configuration and the vertical cross-section shown in Figure 3 represents the cross- sectional shape of the deflector unit 21 at one particular position only. Thus a horizontal cross-section taken through the centre of the deflector unit 21 will be of a different shape, but the outer surface of the deflector unit would again ensure that all of the rearwardly emanating waves 20 are deflected in a vertical direction along one of the paths 9, 10. The deflector unit 21 is located as close as possible to the rear of the diaphragm 18.

It will be appreciated that the deflector unit 21 re-directs the sound waves which emanate from the rear of the diaphragm so that they are not reflected back onto the rear of the diaphragm and do not in any way impede the free movement of the diaphragm. The unit may be formed of marble dust and resin which provides good sound reflection with minimum sound absorption. Whilst in Figure 3 the unit is shown as being formed integrally with one of the partitions in the enclosure, it could be formed as a separate component which is mounted on the front of the partition 12, or the unit could be constituted by one or more of the partitions within the enclosure. Thus, for example, if the diaphragm 18 is mounted in an opening at one top corner of the casing 1 then the corner between the partitions 12, 17 or 13, 17 could constitute a deflector, albeit of slightly reduced efficiency. In the arrangement of Figure 3, a rear region of the deflector projects into the paths defined between the partition 12 and a further rearwardly disposed partition. This particular arrangement is designs for use in another embodiment to be described below and for the purposes of t"he embodiment of Figures 1 and 2 it is only really the part of the deflector unit which is mounted to the front of the partition 12 (i.e. to the left in Figure 3) which is of relevance.

Following deflection by the deflector unit 21 the rear waves 20 follow one of the paths 9, 10 before exiting the casing by way of the outlet 11. The sound waves are guided along the tortuous paths 9, 10 by way of guides 22 provided at each of the turning points in each path 9, 10, the guides each defining an arcuate reflecting surface to reflect the sound waves 20 around the bends in the path. The guides 22 at each turning point in each of the paths 9, 10 comprise blocks which are "moulded" into the corners around which the sound waves need to be guided, the blocks being formed of a high density particulate material such as sand or marble dust bound by an appropriate resin, the resulting composition providing very good sound reflection qualities with minimum sound absorption. Instead of defining arcuate reflecting surfaces the guides 22 could define linear reflecting surfaces but it is believed that arcuate surfaces will provide better reflection and guiding of the sound waves.

The partitions 12, 13, 14, 15, 16 and the walls of the casing itself may be formed of medium density fibre board. It is envisaged that the partitions will have a thickness of approximately 9 or lOmm, although the front, outermost partition 12 may be of greater thickness in order to provide a rather more rigid support for the deflector unit 21. Thus the front, outermost partition 12 may be twice the thickness of the remaining partitions.

The partitions are so arranged within the casing that each of the paths 9, 10 is of constant cross-section as it extends tortuously from the back of the opening 8 to the outlet 11. The cross-sectional area of the outlet 11 is equal to the sum of the cross-sectional areas of the paths 9 and 10 and is also equal to the cross-sectional area of the opening 8 (and to the frontal area of a cone- shaped loudspeaker diaphragm which is to be mounted adjacent the opening 8) . Each of the paths 9, 10 has a cross-sectional area which is egual to 50 percent of the cross-sectional area of the opening 8 and 50 percent of the frontal area of a loudspeaker diaphragm to be mounted adjacent that opening. Whilst in the specific embodiment which has been described and illustrated two paths 9, 10 are defined, it would be possible to conceive of arrangements whereby a greater number of paths extend from the rear of the opening 8 to the outlet 11. It is, however, of importance that the sum of the cross-sectional areas of the paths which extend between the back of the opening 8 and the outlet 11 is equal to the cross-sectional area of the opening 8 and to the frontal area of the loudspeaker diaphragm which is to be mounted within the enclosure.

This particular arrangement of the partitions and the relationship between the size of the outlet 11 and the opening 8 ensures that the pressure within the paths 9, 10 does not increase relative to the pressure outside of the casing as could happen, for example, if the paths become smaller in cross-section over their length which would result in back pressure acting upon the rear of the loudspeaker diaphragm 18 which would hamper the free movement of the diaphragm and thereby detract from its sound reproduction qualities. If, on the other hand, the paths 9, 10 were to increase in cross-section along their length then this could result in the rear waves 20 increasing in amplitude when they pass along the paths 9, 10 rather than diminishing, as is desired, so that they have either diminished completely or almost completely by the time they reach the outlet 11.

The paths 9, 10 are filled with a sound absorbing material such as BAF wadding which is commonly used for sound absorption purposes. Each path 9, 10 may be considered as equivalent to an open-ended pipe having a sound source at one end and extra wadding is provided in the paths 9, 10 at those points where antinodes would arise at resonance in order to provide additional damping.

The overall length of the path from the rear of the opening 8 to the outlet 11 dictates the lowest frequency which is reproducible and the shortest length which is considered acceptable for either of the paths 9, 10 is approximately 2.6m in order to provide sound reproduction at a frequency of approximately 35/36 Hertz. Of course a longer length of path could be used in order to provide sound reproduction at even lower frequencies. It will be appreciated from the accompanying drawings that the paths 9, 10 in the preferred embodiment are unequal in length. It has been found that with this arrangement the loudspeaker gives the impression that sounds of different frequencies emanate from different areas or locations.

With the preferred arrangement described above, the majority of the sound waves emanating from the rear of the loudspeaker diaphragm are directed away from the diaphragm along predetermined paths such that they cannot be reflected back onto the diaphragm. In addition there is virtually no pressure differential across the diaphragm because the rear waves are conveyed from the loudspeaker diaphragm along passages designed so as not to allow any build up of back pressure within the enclosure. The casing and the internal partitions act in a manner equivalent to a single very large baffle forming a partition between the front and rear of the loudspeaker diaphragm. The enclosure might therefore be regarded as a 'folded baffle type' enclosure. The overall result is that the diaphragm can move much more freely in response to an electrical input than is conventionally the case and can provide improved sound reproduction over a wider range of frequencies.

It is to be appreciated that whilst one basic embodiment has been described and illustrated in Figures 1 to 3 it is possible to make many modifications to the described arrangement. In Figure 4 an arrangement is illustrated which is very similar to that previously described but where the two paths 9, 10 combine into a single path having a cross-sectional area which is approximately equal to the cross-sectional area of the opening 8 after having initially started as two separate paths adjacent the opening 8 with each path having a cross- sectional area equal to approximately 50 per cent of the area of the opening 8. In all other respects the arrangement shown in Figure 4 is very similar to that previously described with the loudspeaker diaphragm mounted adjacent the opening 8 and a deflector mounted very close to the rear of the loudspeaker diaphragm in order to deflect rear waves vertically along one or other of the paths 9, 10. As can be seen from the drawing the path 10 extends down the front of the loudspeaker casing before turning through 180 degrees to extend upwardly again where, as it turns through 180 degrees again, it joins with the path 9 which has extended upwardly from the loudspeaker diaphragm at the front of the casing before turning through 180 degrees. The two paths combine and then extend as a single path of constant cross-sectional area around further partitions to an outlet in the base of the casing adjacent the rear wall thereof. With this design the desired minimum path length of approximately 2.6 metres would be specified for the path which commences as path 9 since the overall path which commences as path 10 is, of course, of greater length. As mentioned above each of the paths which commence as paths 9 and 10 will have nodes and antinodes at positions along its length and the partitions defining the paths are arranged such that a node in one path occurs at the same position or very close to an antinode in the other path so that there is a cancellation or at least partial cancellation of the waves originating in the two separate paths.

The design shown in Figure 4 requires fewer internal partitions than the arrangement shown in the previous Figures and the cancellation or reduction of resonance in the rear waves enable the use of materials for the construction of the enclosure which can be more readily produced than if there were no cancellation or reduction of the resonant peaks in the rear wave. Thus, for example, this particular design may enable the use of relatively thin plastics mouldings for the partitions and the casing of the enclosure.

Figure 5 illustrates a further variation on the basic design. In this arrangement the opening 8 for the loudspeaker diaphragm is formed in the top wall 6 of the casing and the deflector unit 21 serves to deflect rear waves horizontally along two paths 9, 10 which then extend in a tortuous manner around partitions to an outlet located centrally in the base 7. Again the internal construction of the casing is very similar to that previously described. This design is provided with a further deflector unit 25 which is located externally of the casing. The deflector unit 25 is positioned on the top wall 6 of the casing^' and has a downwardly directed conical portion 26 which is disposed above the opening 8 for the loudspeaker diaphragm and which serves to deflect sound emanating from the front of the diaphragm in a substantially horizontal direction. It is envisaged that the arrangement would be designed such that the sound is deflected substantially horizontally at ear level.

Whilst the designs described above might all be referred to as cabinet-like enclosures, it is to be appreciated that the enclosure embodying the present invention may take many different shapes and forms whilst still incorporating the concept of a "folded baffle". Thus, Figures 6 and 7 illustrate a loudspeaker having an enclosure which is generally disc or saucer-shaped. The enclosure is therefore circular in plan view and, relative to its diameter, of shallow depth. The enclosure comprises a casing having an upper disc 30 and a lower disc 31 which is of the same form as the upper disc and which is interconnected to the upper disc around its outer periphery by way of a short peripheral wall 32. As can best be seen in Figure 7 of the drawing the upper disc 30 and the lower disc 31 are not flat but are both angled slightly to the horizontal and the two discs are arranged one above the other in alternate orientations such that there is a relatively large spacing between the two discs in a central region thereof and a shallow or narrow spacing between the discs at their outer peripheries where they are interconnected by the wall 32. The upper disc 30 and the lower disc 31 are each provided with a centrally located respective circular opening 33, 34, the opening 33 in the upper disc 30 serving to receive a loudspeaker diaphragm 35 and corresponding to the opening 8 in the front of the casing of the arrangement shown in Figure 1. The opening 34 in the lower disc 31 constitutes an outlet by way of which pressure waves generated to the rear of the diaphragm 35 mounted within the casing may escape therefrom and therefore corresponds to the outlet 11 in the base of the arrangement shown in Figure 1. Internally the casing is provided with a partition 36 which is in the form of a flat disc located centrally between the upper disc 30 and the lower disc 31 with its outermost periphery spaced slightly from the wall 32. The partition 36 has a central opening 37 within which a rearmost region of the diaphragm 35 and the associated drive unit 38 are accommodated. The central region of the partition 36 also acts as a support for a deflector unit 39 comparable to the deflector unit 21 of the arrangement shown in Figures 1 to 3. Indeed the cross- sectional view of Figure 3 gives a more detailed view of the assembly in the central region of Figure 7. It will be appreciated that rear waves from the diaphragm 35 are deflected, by the deflector unit 39 so that they travel radially away from the diaphragm in all directions towards the periphery of the enclosure. Initially the rear waves pass along the space defined between the upper disc 30 and the central partition 36. At the outermost edge of the enclosure the peripheral wall 32 is formed with turning guides 40 of the same form as the turning guides 22 in the previously described arrangements, the turning guides serving to turn the rear waves through 180 degrees so that they then pass along the space defined between the lower disc 31 and the central partition 36 in a direction back towards the centre of the enclosure. On its underside adjacent the central opening 37 the partition 36 is formed with a turning guide/deflector 41 which serves to turn the rear waves through 90 degrees so that they pass axially out through the opening 34.

It is to be appreciated that the drawings of Figures 6 and 7 are not to scale and the overall diameter of the enclosure would be very much greater than that illustrated in order to provide a path from the rear of the diaphragm 35 to the outlet 34 which is of a sufficient minimum length. Thus, the overall diameter relative to the depth of the enclosure would be approximately five times that illustrated in Figure 7.

The arrangement of Figures 6 and 7 is therefore again of a "folded baffle" type in which the cross- sectional area of the path followed by the rear waves remains constant between the rear of the diaphragm 35 and the outlet 3. The cross-sectional area of the path at any point is, of course, equal to the multiple of the circumference of the enclosure at any one point and the depth of the space between the partition 36 and the upper or lower disc 30, 31. Thus, although the space between the partition 36 and the upper and lower discs 30, 31 diminishes towards the outer periphery of the enclosure, the circumference increases in this direction so that the cross-sectional area remains constant. The area of the opening 34 which forms the outlet is, as in the embodiments described above, equal to the area of the opening 33 which receives the loudspeaker diaphragm 35. The outlet opening 34 and the opening 33 are also equal in area to the cross- sectional area of the path which is followed by the rear waves.

As suggested above the overall diameter of the enclosure of Figures 6 and 7 would be very large in order for the arrangement to accommodate a typical loudspeaker diaphragm and in order to reduce the overall diameter of the arrangement it would be possible for the enclosure to be modified in the manner as shown in dotted lines in Figure 7. In this modified arrangement the enclosure comprises a plurality of disc-like units of the same form which are stacked on top of each other and interconnected in their central region. Thus, the opening 33 in the upper disc 30 of each unit would be connected to the outlet 34 of a further unit with the central region of all of the units incorporating a support 42 which defines deflectors or turning guides equivalent to the guide 41 so as to cause rear waves to be re-directed along the paths defined by the stacked units. The left hand side of Figure 7 is provided with arrows to indicate the path which would be followed by the rear waves in such a "stacked" arrangement. This construction would enable the overall length of the path between the rear of the diaphragm 35 and the final outlet from the enclosure to be kept constant or even increased without increasing the overall diameter of the assembly but by simply increasing its depth.

The concept of a "folded baffle type" enclosure may readily be used not only with an enclosure receiving a single loudspeaker diaphragm but also with an enclosure provided with several loudspeaker diaphragms such as a base, a mid range and a high frequency diaphragm. Figures 8, 19 and 10 of the drawings illustrate, in schematic form, an arrangement for an enclosure designed to receive three loudspeaker diaphragms. The arrangement is similar in construction to that shown in Figures 1 to 3 of the drawings, but the front wall of the casing is, of course, provided with three openings 45, 467, 47 for receiving bass, mid range and high frequency diaphragms respectively. Internally the enclosure is again provided with partitions which serve to define a tortuous path from the rear of each diaphragm to an outlet. The bass diaphragm and the mid range diaphragm are each associated with a deflector unit which serves to deflect rear waves along the predetermined path within the enclosure. The high frequency diaphragm does not generate rear waves which are sufficient to cause problems and therefore does not require the provision of a deflector unit. In order to accommodate the deflector units associated with the bass diaphragm and the mid range diaphragm and to provide for paths along which rear waves from these diaphragms can be carried away from the rear of the diaphragm, a sub-partition 48 is mounted within the passage defined between the front wall of the casing and the first internal partition (reference No. 12 in Figure 2) . This sub partition 48 extends over only a short part of the depth of the passage between the front wall 8 and the first internal partition 12, namely that part which is behind the openings 45, 46, 47. The sub-partition 48 defines an aperture 49 which is aligned with the opening 45 for the base diaphragm so that the bass diaphragm can extend through the sub-partition 48 to be mounted relative to a deflector unit provided on the first internal partition 12 in the manner as described above in relation to Figures 1 to 3. For ease of description of the reference numerals used in Figures 1 to 3 are used in relation to the arrangement of Figures 8, 9 and 10 for those items which are common.

As mentioned above the bass diaphragm 18 and its associated drive 19 together with its deflector unit 21 are mounted within the enclosure casing in the same manner as described in relation to Figures 1 to 3. The mid range and high frequency diaphragms and their drives and the deflector unit for the mid range diaphragm are mounted on the sub-partition 48. In order to allow the rear waves generated by the mid range diaphragm to pass around the bass diaphragm and the high frequency diaphragm a series of four moulded channels 50 are defined by the sub-partition 48 as can be seen in Figure 10 of the drawings. The channels extend around the sides of the bass and high frequency diaphragms as seen from the front. At the upper and lower edges of the sub-partition 48 the moulded channels 50 extend into the main path defined between the front wall 8 of the casing and the first internal partition 12 whereupon the rear waves follow the same paths as shown in the arrangement of Figures 1 to 3. As with the other arrangements illustrated the cross-sectional area of the rear wave paths is related to the area of the opening which receives the diaphragms giving rise to the rear waves. Thus, the cross-sectional area of each of the four channels 50 which convey the rear waves away from the mid range diaphragm is equal to 25 per cent of the area of the opening 46 whilst the cross-sectional area of the vertical paths above and below the bass diaphragm between the sub¬ partition 48 and the first main internal partition 12 is equal to 50 per cent of the area of the opening 45 within which the bass diaphragm is received. Where the paths followed by the rear waves from the bass diaphragm and the mid range diaphragm combine, the total cross-sectional area of the combined paths is equal to the sum of the cross- sectional areas of the openings 45 and 46. In an arrangement where, for example, the bass diaphragm is three times larger than the mid range diaphragm and the opening 45 is three times larger than the opening 46, the cross- sectional area of each of the moulded channels 50 is equal to 6.25 per cent of the sum of the frontal areas of the bass and mid range diaphragms whilst the passages above and below the bass diaphragm between the sub-partition 48 and the first internal partition 12 each have a cross-sectional area equal to 37.5 per cent of the sum of the frontal areas of the bass and mid range diaphragms. As with the other designs described above the cross-sectional area of the paths leading from the rear of the diaphragms to the outlet remains constant and the outlet has an area equal to the sum of the frontal areas of the bass and mid range diaphragms. It will be appreciated that further modifications may be made to the arrangements described above without departing from the scope of the present invention as defined by the appendant claims.

Claims

CLAIMS :

1. A loudspeaker enclosure comprising an outer casing, the casing defining a diaphragm opening through one wall thereof and being designed to support a loudspeaker diaphragm and associated drive adjacent said opening such that sound waves generated to the front of the diaphragm by movement thereof pass out of the casing through the opening, the casing incorporating at least one internal partition, the or each partition defining an elongate path within the casing, the path extending between the diaphragm opening and an outlet formed through part of the casing, the path being designed so that in use sound waves generated to the rear of a loudspeaker diaphragm mounted within the enclosure are conveyed from the rear of the diaphragm to the outlet, the outlet through the casing having a cross-sectional area substantially equal to the cross-sectional area of the diaphragm opening.

2. A loudspeaker enclosure according to Claim 1 wherein the path is of substantially uniform cross-section over its length between the diaphragm opening and the outlet.

3. A loudspeaker enclosure according to Claim 1 or Claim 2 wherein a plurality of paths are defined within the casing between the diaphragm opening and the outlet.

4. A loudspeaker enclosure according to Claim 3 wherein paths of differing lengths are defined within the casing.

5. A loudspeaker enclosure according to any one of Claims 1 to 4 wherein the or each path is at least approximately 2.6m in length from the diaphragm opening to the outlet.

6. A loudspeaker enclosure according to any one of Claims 1 to 5 wherein the or each path is filled with a sound absorbing material.

7. A loudspeaker enclosure according to any one of the preceding claims wherein a plurality of partitions are provided within the casing, the partitions being disposed substantially parallel to each other and defining a tortuous path between the diaphragm opening and the outlet.

8. A loudspeaker enclosure according to Claim 7 wherein turning guides are provided at the corners of the tortuous path, the turning guides having sound reflecting surfaces configured to guide sound waves around corners in the tortuous path.

9. A loudspeaker enclosure according to Claim 8 wherein the turning guides are formed from a high density particulate material and a resin binder.

10. A loudspeaker enclosure according to Claim 8 or Claim 9 wherein the turning guides each define arcuate sound reflecting surfaces.

11. A loudspeaker enclosure according to any one of Claims l to 10 wherein the total cross-sectional area of the or all of the paths defined within the casing is substantially equal to the cross-sectiόnal area of the diaphragm opening.

12. A loudspeaker enclosure according to any one of the preceding claims wherein a deflector unit is mounted within the casing opposite the diaphragm opening, the deflector unit having a contoured surface directed towards the diaphragm opening which, in use, deflects sound waves generated to the rear of the diaphragm in a predetermined direction along the path or one of the paths within the casing.

13. A loudspeaker enclosure according to any one of the preceding claims wherein the casing defines a plurality of diaphragm openings, the casing being designed to support a plurality of loudspeaker diaphragms adjacent respective openings, the outlet through the casing having a cross- sectional area substantially equal to the sum of the cross- sectional areas of the diaphragm openings.

14. A loudspeaker enclosure comprising an outer casing, the casing defining a diaphragm opening through one wall thereof and being designed to support a loudspeaker diaphragm and associated drive adjacent said opening such that sound waves generated to the front of the diaphragm by movement thereof pass out of the casing through the opening, the casing incorporating at least one internal partition, the or each partition defining an elongate path within the casing between the diaphragm opening and an outlet formed in part of the casing, the path being designed so that in use sound waves generated to the rear of the diaphragm are conveyed from the rear of the diaphragm to the outlet, the enclosure incorporating a deflector unit mounted within the casing opposite the diaphragm opening, the deflector unit defining a contoured surface directed towards the diaphragm opening which, when a diaphragm is mounted adjacent the opening and is in use, deflects sound waves generated to the rear of the diaphragm away from the rear of the diaphragm in a predetermined direction along the path or one of the paths defined within the casing so that the waves are not reflected back onto the rear of the diaphragm from any part of the enclosure.

15. A loudspeaker enclosure according to Claim 14 wherein the contoured surface of the deflector unit deflects waves in a plurality of predetermined directions.

16. A loudspeaker enclosure according to Claim 15 wherein the contoured surface of the deflector unit deflects waves in two substantially opposite directions along two predetermined paths defined by the partitions within the casing.

17. A loudspeaker enclosure according to any one of Claims 13, 14 or 15 wherein the deflector unit is mounted upon an internal partition within the casing.

18. A loudspeaker enclosure according to Claim 13, 14 or 15 wherein the deflector unit is constituted by part of an internal partition within the casing.

19. A loudspeaker enclosure substantially as herein described with reference to and as shown in Figures 1 to 3 of the accompanying drawings.

20. A loudspeaker enclosure substantially as herein described with reference to and as shown in Figures 1 to 3 as modified by Figure 4 of the accompanying drawings.

21. A loudspeaker enclosure substantially as herein described with reference to and as shown in Figures 1 to 3 as modified by Figure 5 of the accompanying drawings.

22. A loudspeaker enclosure substantially as herein described with reference to and as shown in Figures 6 and 7 of the accompanying drawings.

23. A loudspeaker enclosure substantially as herein described with reference to and as shown in Figures 8, 9 and 10 of the accompanying drawings.

24. A loudspeaker comprising a loudspeaker enclosure in accordance with any one of the preceding claims.

25. A loudspeaker substantially as herein described with reference to and as shown in any one of the accompanying drawings.

26. Any novel feature or combination of features disclosed herein.