GB2250157A

GB2250157A - Loudspeaker enclosures

Info

Publication number: GB2250157A
Application number: GB9024805A
Authority: GB
Inventors: Stuart Victor Showell
Original assignee: Individual
Current assignee: Individual
Priority date: 1990-11-15
Filing date: 1990-11-15
Publication date: 1992-05-27
Also published as: GB9024805D0

Abstract

A transmission line loudspeaker enclosure comprises a generally tubular member 11 of elongate configuration within which is disposed approximately midway therealong a loudspeaker unit 10, the tubular member 11 being provided internally with sound absorbent material (16, fig 5) and being of a width and depth of the same order of magnitude as the loudspeaker unit 10. <IMAGE>

Description

Loudspeaker Enclosures This invention relates to loudspeaker enclosures.

More particularly the invention relates to loudspeaker enclosures of the kind utilising the transmission line principle to improve reproduction of sound from a loudspeaker unit mounted in the enclosure.

Loudspeaker units of the conventional moving coil kind are usually mounted in an enclosure, the main purpose of which is to isolate the radiation from the back of the cone of the loudspeaker from that from the front, the radiation from the back being 1800 out of phase from the radiation from the front so that it could tend to cancel out the radiation from the front.

At high frequency this effect is minimal for two reasons. Firstly, sound pressure waves take too long to travel between the front and rear of any practical unit because the distance is large compared to the wavelength of the source. Secondly, radiation at high frequency from the front (and back) of the unit out into free space is considerably more efficient. However, at lower frequencies the time of travel of the back projected radiation is less significant so that the cancellation effect becomes more pronounced.

Attempts have been made in the past to contain, eliminate or significantly reduce the radiation from the back of the loudspeaker cone, and these have included loudspeaker enclosures using the transmission line principle (such an enclosure hereinafter being referred to as a transmission line loudspeaker enclosure). In principle a transmission line loudspeaker enclosure comprises a long tube fitted behind the loudspeaker unit, the cross sectional area of the tube being at least the same as the area of the loudspeaker unit cone. The tube has been found to absorb sound effectively provided it contains a sound absorbent material (such as sheeps wool) and is at least as long as one quarter of a wavelength of the sound to be absorbed.Sound having a quarterwavelength more than the length of the tube can emerge from an opening at the end of the tube unabsorbed, but due to its traverse along the tube it will have suffered a time delay itself so that by the time it can interact with the sound from the front of the cone, the phase difference is no longer anything like 1800 so there is less or no cancellation of sound. In practice a typical tube or transmission line for domestic use may be a total of 8 foot long, "folded" by means of partitions within a rectangular enclosure into three sections. This will work satisfactorily for sound radiation of 30 Hz and below.

However, previous transmission line loudspeaker enclosures suffer from the disadvantage that sound, particularly sound at several hundred Hz and above (which is intrinsically more directional and subject to reflection than low frequency sound) is radiated from the back of the cone in all directions, and it is not readily possible to ensure that any reflector provided directs all or at least most of such reflected sound into the transmission line and not back through the loudspeaker cone. In addition, serious difficulties can arise in the reflection of sound round the bends and corners in a "folded" transmission line, and there is a tendency for resonances to be set up in each section.

It is an object of the present invention to overcome or at least substantially reduce the above mentioned problems.

In accordance with one aspect of the present invention there is provided a transmission line loudspeaker enclosure wherein the enclosure comprises a generally tubular member of elongate configuration within which is disposed approximately midway therealong a loudspeaker unit, the tubular member being provided internally with sound absorbent material.

In accordance with another aspect of the present invention there is provided a transmission line loudspeaker enclosure wherein the enclosure comprises a generally tubular member of elongate configuration within which is disposed approximately midway therealong a loudspeaker unit, the tubular member being provided internally with sound absorbent material and being being of a width and depth of the same order of magnitude as the loudspeaker unit.

The loudspeaker enclosure may be disposed in a vertical disposition and may be of an overall length compatible with the height of a room in which it is designed to be fitted, be it an auditorium or a room in a residential building. Thus it may be of the order of 8 feet in total length for use in a room in a residential building.

The depth of the tube may be such that the driving magnet of the loudspeaker is mounted in or adjacent the rear wall thereof, whilst the front peripheral frame of the cone of the loudspeaker is mounted in or adjacent the front wall.

In a preferred embodiment of the invention the width of the tube adjacent the loudspeaker is no greater than the width of the loudspeaker unit.

In circumstances where reflection round bends in the transmission line is not considered to be a significant problem, the loudspeaker enclosure may, on one or both sides of the loudspeaker unit, be "folded" into a labyrinth type formation, if sharply and appropriately angled corners to the unit are provided.

The loudspeaker enclosure of the invention may be of any convenient and appropriate cross section such as rectangular, oval or circular. Preferably at least adjacent the loudspeaker unit the walls of the loudspeaker enclosure are flat to facilitate correct reflection. The cross section may be the same along its length, or may be of a varying cross section, and may decrease away from the loudspeaker unit.

The loudspeaker enclosure may be provided with simple open ends at each end, or may be provided with reflectors, such as conical reflectors or, in the case of rectangular section enclosures for example, reflector plates disposed at an appropriate angle (such as 450) to the axis of the tube.

The loudspeaker unit may be a full range unit or a bass unit, or may be in some circumstances a mid or even a high frequency loudspeaker unit.

In order that the invention may be more readily understood embodiments thereof will now be described by way of example with reference to the accompanying drawings in which: Figures 1 and 2 illustrate schematically previous arrangements of transmission line loudspeaker enclosures.

Figure 3 is a schematic isometric illustration of one form of loudspeaker enclosure in accordance with the invention; Figure 4 is a schematic isometric illustration of another form of loudspeaker enclosure according to the invention; Figure 5 is a sectional schematic side elevation of the central portion of the loudspeaker enclosure of Figure 3; and Figure 6 is a sectional schematic rear elevation of the loudspeaker enclosure of Figure 3.

Referring to Figures l and 2, there is illustrated known forms of transmission line loudspeaker enclosures 1 incorporating loudspeaker units 2 within a rectangular enclosure. In each case a reflector 3,4 which may be large as shown in Figure 1 at 3 or small as shown in Figure 2 at 4 is disposed with the intention of deflecting sound radiation from the back of the cone 5 of the loudspeaker unit down into a convuluted transmission line defined by baffles 6 and ending in an outlet 7 at the base of the respective enclosure.As can be seen the desired path of the sound radiation from the back of the loudspeaker down into the transmission line can be followed by some radiation as indicated in dotted line 8, whereas other radiation, indicated in solid line 9 is in fact reflected back through the loudspeaker such as to mix to a greater or lesser extent with the sound radiation from the front of the loudspeaker cone 5.

The purpose of the present invention is to overcome or at least substantially reduce such problems associated with the production of most frequencies of sound radiation, and involves ensuring that sound radiation from the rear of the cone is disposed of quickly and efficiently without back pressure or reflections back onto the loudspeaker cone.

As shown in Figures 3, 5 and 6 a forward facing moving coil loudspeaker unit 10 of, say, 6 inch outside diameter, and, say, a 4 inch cone diameter is mounted generally centrally of an elongate rectangular section tubular enclosure 11 which may be, say, 8 foot in total height. The width of the enclosure 11 is made as small as possible to enable the loudspeaker unit to be mounted therein, and the depth is chosen such that the cross sectional internal area of the enclosure 11 is of the order of half the area of the loudspeaker cone 12. By this arrangement the rear of the loudspeaker cone drives two columns of air each being half of its own area. As can be seen from Figure 5 this desirable practical consideration incidentally and normally necessitates the drive unit magnet 13 protruding through the rear surface of the enclosure 11.As prelricusly stated the enclosure extends upwards and downwards as far as possible consistent with standing the unit in the room, possibly against a wall.

In operation, if a signal of low frequency is fed to the loudspeaker unit, low frequency sound radiation is produced from the front and rear of the loudspeaker cone.

As far as that from the rear is concerned, because there is no directionality in the sound emitted from the cone and it is not easily reflected, air is being simply pumped up and down the two halves of the tube which behave as normal transmission lines.

As the frequency of the signal rises, the sound radiated from the cone acquires an increasingly directional character and is more readily reflected, as for example, its wavelength falls to approximately four times the physical dimensions of the body radiating or reflecting it. These properties come into play as the frequency enters the audio mid band range, and at these frequencies and above sound is radiated increasingly perpendicularly from the surfaces of the cone 12.

However, with the arrangement of the enclosure 11 walls closely proximate to the cone 12 of a loudspeaker unit 10, it is to be appreciated that the sound radiation is in practice always reflected away from the cone and up or down the tube or transmission lines away from the loudspeaker unit.

In a model 8 foot high, because each section of the line is only 4 foot long, it might be expected that the unit would only work well down to a wavelength of 4 foot x 4 i.e. 16 foot, that is a wavelength of about 70 Hz, with a rapid reduction in efficiency below that. For the following reasons, the unit can radiate satisfactorily dowry to about 40 Hz or less:: (a) The standard wool packing in transmission line loudspeakers not only absorbs sound but slows down its velocity in air by a factor of 10 to 30%, depending on its density for example, making the line behave acoustically as if it were correspondingly longer; (b) Any bass radiation not absorbed, because its wavelength is too long, is still isolated from the primary radiation from the front of the cone 12 by a free air path of at least an additional 4 foot (instead of the shorter distance if the line was "folded" as it is in existing designs) giving a greater effective phase shift;; (c) If the enclosure is sited so that its ends are at the junctions of walls with floor and ceiling, or better still at corners, the bass radiation escaping from its ends is radiated (because of the adjacent presence of the wall or walls and floor or ceiling) into a one-quarter or a one-eighth portion of the nominal free space respectively, concentrating the radiation and increasing its power. In addition the diverging internal room surfaces will act as rudimentary "horns" which increase the efficiency of the radiating process.

It would be quite possible to make the lines longer in a higher ceilinged room, or by folding them back along the entire length of the tube, but this would involve makina the enclosure much bulkier.

The loudspeaker enclosure illustrated in Figure 4 is a modification of that shown in Figure 3 in that, in region 15 outside the central region 14 accommodating the louspeaker unit itself, the transmission lines changes configuration from rectangular to circular section on the practical basis that cylindrical shapes are much less prone to vibration than flat panels, so that the arrangement of Figure 4 should be better at minimising colouration of the sound from secondary panel resources than that of Figure 3.

The walls of the loudspeaker enclosure, especially near the loudspeaker unit 10 itself, should be made of a material as hard and stiff as possible, such as wood (natural or reconstituted), plastics material, metal, or perhaps most satisfactorily of all, of a ceramic material.

Further along the transmission line the walls do not need to be so stiff. The arrangements of both Figures 3 and 4, and particularly the arrangement of Figure 4, are well suited to mass production techniques from good quality material such as plastics, metal, wood or ceramics, which may be selectively used for all or part of the enclosure to minimise vibrations of the walls of the enclosure. The outer transmission line regions 15 may be detachable by screw or other fittings (not shown), from the central region 14 around the loudspeaker unit 10.

The enclosures of both Figures 3 and 4 are filled with a wool filling 16.

In the arrangement of both Figures a high frequency tweeter loudspeaker unit could be coupled to the main loudspeaker unit and mounted on the front face of the enclosure (not shown).

The wool filling 16 may be at a usual recommended density of, for example, 8 to 12 ounce per cubic foot.

It is to be appreciated that the cross sectional dimensions of the enclosures can be scaled up or down to accommodate any particular size of loudspeaker unit, and an enclosure accommodating a 12 inch loudspeaker unit could still be most effective for frequencies as high as it can reproduce.

It is to be understood that the foregoing is merely exemplary of transmission line loudspeaker enclosures in accordance with the invention and that modifications can be readily be made thereto without departing from the true scope of the invention.

Claims

CLAIMS:

1. A transmission line loudspeaker enclosure wherein the enclosure comprises a generally tubular member of elongate configuration within which is disposed approximately midway therealong a loudspeaker unit, the tubular member being provided internally with sound absorbent material.

2. A transmission line loudspeaker enclosure as claimed in Claim 1 wherein the tubular member is of a width and depth of the same order of magnitude as the loudspeaker unit.

3. A transmission line loudspeaker enclosure as claimed in Claim 1 or 2 wherein, in use. the loudspeaker enclosure is disposed vertically and is of an overall length compatible with the height of a room in which it is designed to be fitted.

4. A transmission line loudspeaker enclosure as claimed in Claim 3 wherein the loudspeaker enclosure is of the order of 8 feet in total length.

5. A transmission line loudspeaker enclosure as claimed in any one of the preceding Claims wherein the depth of the tube is such that the driving magnet of the loudspeaker is mounted in or adjacent the rear wall thereof, whilst the front peripheral frame of the cone of the loudspeaker is mounted in or adjacent the front wall.

6. A transmission line loudspeaker enclosure as claimed in any one of the preceding Claims wherein the width of the tube adjacent the loudspeaker is no greater than the width of the loudspeaker unit.

7. A transmission line loudspeaker enclosure as claimed in any one of the preceding Claims wherein the loudspeaker enclosure is on at least one side of the loudspeaker unit folded into a labyrinth type formation with sharply and appropriately angled corners to the unit.

8. A transmission line loudspeaker enclosure as claimed in any one of the preceding Claims wherein at least adjacent the loudspeaker unit the walls of the loudspeaker enclosure are flat to facilitate correct reflection.

9. A transmission line loudspeaker enclosure as claimed in any one of the preceding Claims wherein the cross-sectional dimensions of the enclosure decrease away from the loudspeaker unit.

10. A transmission line loudspeaker enclosure as claimed in any one of the preceding Claims wherein the loudspeaker enclosure is provided with reflectors at its ends.

11. A transmission line loudspeaker enclosure as shown in and as hereinbefore described with reference to the accompanying drawings.

Amendments to the claims have been filed as follows

11. A transmission line loudspeaker according to any one of the preceding Claims wherein the loudspeaker enclosure is rectangular, oval or circular in crosssection.

12. A transmission line loudspeaker according to any one of the preceding Claims wherein the enclosure has walls arranged closely proximate to the cone of the loudspeaker unit such that substantially all sound radiation from the rear surface of the cone is reflected away from the cone and up or down the tubular member away from the loudspeaker unit.

13. A transmission line loudspeaker enclosure as shown in and as hereinbefore described with reference to the accompanying drawings.