GB2298758A

GB2298758A - Transmission line enclosure

Info

Publication number: GB2298758A
Application number: GB9504760A
Authority: GB
Inventors: Che Hung Chan
Original assignee: PRIMA DEV Ltd
Current assignee: PRIMA DEV Ltd
Priority date: 1995-03-09
Filing date: 1995-03-09
Publication date: 1996-09-11
Anticipated expiration: 2015-03-09
Also published as: GB9504760D0; GB2298758B

Description

Loudspeaker Systems and Enclosures The present invention relates to loudspeaker systems, and to loudspeaker enclosures for use in such systems. The invention relates especially to transmission line loudspeaker systems and enclosures.

Loudspeakers are normally mounted to an enclosure in order to prevent sound waves emanating from the rear of the loudspeaker diaphragm from interfering with sound waves emitted from the front of the diaphragm (sound waves from the rear are 1800 out of phase with those from the front, and tend to produce a cancellation effect).

In general, there are three main types of enclosure: infinite baffle, bass reflex and transmission line enclosures.

In the infinite baffle type, the loudspeaker is mounted on an airtight box which completely encloses the rear of the loudspeaker diaphragm. Theoretically, the rear sound waves inside of the box should be completely isolated from the front sound waves outside of the box.

This is never, however, achieved. Also, resonance occurs within the box, and although various measures can be taken to reduce the resonant effects, these are never completely successful, and colouration of the sound from the front of the loudspeaker can occur. Further, accumulated pressure within the closed box modifies the movement of the diaphragm and provides further colouration, as well as making the loudspeaker less efficient.

The bass reflex (or tuned port) system is similar to the infinite baffle system, but includes an especially designed port as an acoustic load. Such systems do not have the problem of accumulated pressure but still suffer from resonance effects.

A transmission line enclosure comprises an open ended passage filled with sound-absorbent material which acts as a lossy transmission line and dissipates the energy of the sound waves as they pass down the line.

Such a transmission line may be formed from baffles mounted in a box to form a tortuous passageway. The rear of the loudspeaker is mounted at one end of the transmission line, and the other end of the line is left open.

Transmission line systems do not have the accumulated pressure or resonance problems associated with the infinite baffle and bass reflex systems, and provide in principle the best form of enclosure.

The present invention relates to improvements in transmission line systems.

Viewed from a first aspect, the invention provides a loudspeaker system comprising a circular loudspeaker unit and a transmission line enclosure of circular cross-section.

The invention also provides a loudspeaker system comprising an elliptical loudspeaker unit and a transmission line enclosure of elliptical cross-section.

Previously, such as in the arrangement mentioned above comprising a box with internal baffles, transmission line systems have employed transmission lines of rectangular cross-section which produce sectional resonance. The use of circular and elliptical transmission lines for circular and elliptical loudspeakers respectively, however, has been found to produce uniformly directed and controlled near field reflections at the rear of the loudspeaker in contrast to prior transmission line and other enclosures.

Furthermore, they tend to minimise vibrations due to the symmetrical dispersion of the rear sound waves.

Preferably, the transmission line has substantially the same cross-sectional size as the diaphragm of the loudspeaker unit at the interface between the unit and the transmission line. This too ensures that less near field reflections are bounced back to the loudspeaker.

Further preferably, the transmission line extends initially directly back from the loudspeaker along the centre axis of the loudspeaker's diaphragm.

The transmission line may be made from ducting of circular or elliptical cross-section. It has been found that conventional drainage pipes are acceptable.

Preferably, the transmission line is bent, and may be folded about itself. This allows the transmission line to take up less space. The bends are preferably smooth and preferably curve gently. This contrasts with the prior art in which the bends are sharp. The use of smooth bends reduces unwanted reflections caused by sharp corners.

Preferably, the transmission line comprises straight sections with smooth bends therebetween.

The transmission line may comprise a first portion extending directly back from the rear of the loudspeaker, a second portion extending substantially perpendicularly thereto, a third portion extending generally in the opposite direction to the first portion but angled out of the plane of the first and second portions, a fourth portion extending generally in the opposite direction to the second portion to a position past the first portion, a fifth portion extending across the first portion, a sixth portion extending generally in the opposite direction to the fourth portion and a seventh portion extending generally in the direction of the first portion.

The transmission line is preferably tapered along at least some portion thereof.

Preferably, the loudspeaker and transmission line are themselves mounted within a further secondary enclosure. This secondary enclosure may be box-shaped and have the appearance of a conventional infinite baffle loudspeaker enclosure. It may be made from any suitable material, for example plywood or MDF. The secondary enclosure may make the system easier to handle and to place in position. Furthermore, although there is very little vibration from the transmission line because of its circular or elliptical cross-section, any residual vibrations which may exist in the transmission line may be isolated by the air gap between the transmission line and the secondary enclosure.

Preferably, the transmission line incorporates a sound-absorbent material in order to attenuate the rear sound waves. The material may be of any known type of wadding and may be arranged in the transmission line in any known manner. Pure wool is preferred.

In an especially preferred embodiment of the invention, the end of the transmission line distal from the loudspeaker is closed except for a port therein of smaller cross-section than the end of the line. This is in contrast to prior systems which use completely openended transmission lines.

In this embodiment, sound waves are reflected back from the ported end of the transmission line to the loudspeaker along the same path. Thus, the effective transmission line length is about double its physical length, and a transmission line enclosure according to this embodiment may be about half the length of prior transmission line enclosures, thereby saving space.

The port may be formed in a baffle on the end of the transmission line, which is preferably flat faced and preferably lies perpendicular to the axis of the transmission line.

This concept of double travelled, and attenuated, sound waves in the single transmission line is very important in itself, and, viewed from a further aspect, the invention provides a loudspeaker system comprising a loudspeaker and a transmission line enclosure extending from the rear thereof, the end of the transmission line distal from the loudspeaker being closed but having a port therein of smaller cross-section than the transmission line at that end. Viewed from another aspect, the invention provides a loudspeaker system in which sound waves travel along a lossy transmission line to a ported panel and are reflected back through the same route to the loudspeaker. These aspects of the invention may of course incorporate any of the above mentioned features also.

When the line is filled with sound-absorbing material, mid or high frequency sound waves produced from the rear of the speaker will generally be attenuated to a substantial degree by the time that they reach the ported end of the transmission line, and generally only the lowest frequency sound waves will be able to penetrate to the end of the transmission line and be reflected back to the loudspeaker.

As these low frequency sound waves travel about twice the length of the transmission line, they will generally be quite attenuated by the time that they reach the loudspeaker. Even so, the attenuated sound waves which do return may be used to enhance rather than reduce the sound from the front of the loudspeaker, by ensuring that they travel a distance of about one half of a wavelength from the loudspeaker and back. The lengths of the transmission line may be set to provide this travel distance. When calculating the length, it should be borne in mind that the sound waves follow a zig-zag path within the transmission line, and may have reduced velocity in the sound-absorbent material. The effective length of a transmission line may be about 30% more than its physical length.For example, a four foot (120 cm) long ducting may have an effective length of about 10.4 feet (320 cm), and provide an enhancement effect at approximately 54 Hz. Roll-off further down the frequency range may be quite gentle, and so very clean bass sounds may be produced throughout the lowest audible register.

It is preferred for the transmission line to have an overall effective length of about 8 feet (240 cm) or more. An effective length of about 8 feet (240 cm) has been found to provide excellent results in the lowest register. Shorter line lengths are, however, also possible, for example for a bookshelf loudspeaker system. In such cases, reduced space requirements are provided at the expense of sound quality. However, this relates only to the lowest sound register, and a bookshelf system may still provide very clean bass, although not very deep, and all the details in any other frequencies.

The size of the port in the end of the transmission line should be set to ensure that frequencies less than a set amount cannot escape and will be reflected back along the ducting. This frequency may be set for example, at a value above which sounds are in any case substantially reduced due for example to the physical limitations of the loudspeaker unit itself. Thus, as a woofer speaker generally starts to roll-off at about 2 kHz, and the signal is substantially reduced at about 5 kHz, the port may be set at about 3 cm, in view of the 3.4 cm half wavelength of a 5 kHz wave.

Where the transmission line is enclosed in a further secondary enclosure, the port is preferably defined in a part of the secondary enclosure itself.

The enclosure may be of a generally U-shaped construction, with an extended bridge portion, in which case the port opening end of the transmission line faces the same direction as the front of the loudspeaker.

Alternatively, the transmission line may be formed so that the port faces the opposite direction to the loudspeaker, e.g. the transmission line may comprise a first portion extending back from the loudspeaker, a second portion extending generally perpendicularly thereto, and a third portion extending generally perpendicularly to the second portion in a direction away from the loudspeaker.

The invention also extends to transmission line enclosures for use in any of the above systems.

Embodiments of the present invention will now be described, by way of example, with reference to the accompanying drawings, in which: Fig. 1 is a longitudinal cross-section through a first embodiment of a system in accordance with the invention; Fig. 2 is a cross-section of the ducting of Fig. 1 transverse to the loudspeaker axis; Fig. 3 shows an enclosure for a loudspeaker system in accordance with an embodiment of the present invention; Fig. 4 is a cross-section through ducting according to a further embodiment of the invention; Fig. 5 shows a speaker system in accordance with a still further embodiment of the present invention; Fig. 6 shows a cutaway view of the speaker system of Fig. 5; Fig. 7 shows an alternative transmission line enclosure for use in the embodiment of Figs. 5 and 6; and Figs. 8 and 9 show schematically typical preferred dimensional relationships of the radii and length of transmission lines which may be used, for example, in bookshelf and floor standing systems respectively.

Referring to Figs. 1 and 2, a circular loudspeaker 1 is mounted within a transmission line ducting 2 of circular cross-section. The ducting is sized to be of substantially the same diameter as the loudspeaker 1.

By using ducting of circular cross-section, near field reflections are reduced. Also, because of the symmetry of the ducting, vibrations in the ducting are reduced.

This produces a better quality sound from the loudspeaker 1.

In order to save space, the ducting 2 may be bent and may be wrapped about itself as for example shown in Fig. 3, in which the transmission line 2 includes a first portion 2a extending directly back from the rear of the loudspeaker, a second portion 2b extending substantially perpendicularly thereto, a third portion 2c extending generally in the opposite direction to the first portion 2a but angled out of the plane of the first and second portions, a fourth portion 2d extending generally in the opposite direction to the second portion 2b to a position past the first portion 2a, a fifth portion 2e extending across the first portion 2a, a sixth portion 2f extending generally in the opposite direction to the fourth portion 2d and a seventh portion 2g extending generally in the direction of the first portion 2a.

Bends 3 in the ducting 2 are smooth and curve gently, so as to reduce any unwanted reflections directly back to the loudspeaker. To provide optimum results, the transmission line should have no sharp edges, and should be smooth along its length.

As a compromise between space and sound quality, the ducting 2 may include tapered sections, for example portions 2a and 2b.

Experiments have shown that conventional pipes used for drainage provide acceptable transmission lines.

Fig. 4 shows ducting of elliptical cross-section, for use with loudspeakers of elliptical shape.

As, for example, shown in Figs. 5 and 6, systems comprising a loudspeaker and ducting as described may themselves be mounted in a further secondary enclosure 4. This allows the systems to be more easily handled and placed. Also, the gap between the ducting 2 and walls of the enclosure 5 provides further insulation, so that any vibrations of the waveguide 2 which may exist are further isolated.

A particularly preferred feature of the embodiment of Figs. 5 and 6 is that the end of the transmission line 2 distal from the loudspeaker 1 has a port opening 5 of smaller cross-section than that of the transmission line ducting 2. Sound waves reaching this end are therefore reflected back along the ducting 2, so that the ducting 2 provides a transmission line of effectively double the ducting length. This causes a considerable saving in space and a reduction in the size of the speaker system.

In this embodiment, sound waves from the rear of the speaker 1 pass along the ducting 2 and are attenuated by sound absorbent wadding 6, preferably pure wool. The waves are then reflected back from the port 5 to the speaker 1.

Mid and high frequency sounds are substantially attenuated by the wadding 6, and only the lowest frequency sounds are reflected back. The length of the ducting is preferably set so that these lowest frequency sound waves travel a distance of about half a wavelength by the time they return to the loudspeaker, so that any energy remaining in the sound waves enhances rather than cancels the sound from the front of the loudspeaker 1.

When calculating the length of the ducting 2, it should be borne in mind that the sound waves follow a zig-zag path in the ducting as shown in Fig. 1, and have reduced velocity in the wadding 6. The effective length of the ducting is therefore greater than its physical length, typically, about 30% more than its physical length.

In order to provide an enhancement effect at about 54 Hz, which is about the frequency of the waves which return to the loudspeaker with any strength, a four foot (-120 cm) long ducting 2 may be used, which gives an effective travel length of about 10.4 feet (-320 cm).

The port 5 provides pressure equalisation, and is sized to reflect back waves of frequencies below a set frequency above which the waves produced by the loudspeaker are quite weak. For example a general woofer speaker may start to roll off at about 2 kHz, and signals above about 5 kHz will be minimal. Therefore, there is no need to reflect signals of 5 kHz or greater, in which case a 3 cm diameter port could be used (a 5 kHz signal having a half wavelength of about 3.4 cm).

The system of Figures 5 and 6 is shown with only the woofer speaker 1 mounted in the transmission line ducting 2, and with a separate tweeter speaker 7 mounted merely on the secondary enclosure 4. There is no reason, however, why a coaxial full range loudspeaker could not be used instead of a woofer. This would achieve the same performance, whilst providing a point source result.

The above are only preferred embodiments of the present invention, and various modifications thereto are of course envisaged. For example, the transmission line 2 of Figs. 5 and 6 is generally U-shaped with an elongate bridge portion, in which case the port 5 faces in the same direction as the loudspeaker 1.

Alternatively, however, a transmission line according to Fig. 7 may be used, in which the ported end of the transmission line faces in the opposite direction to the loudspeaker 1. In this case, the transmission line comprises a first portion 2a extending back from the loudspeaker, a second portion 2b extending generally perpendicularly thereto, and a third portion 2c extending generally perpendicularly to the second portion 2b in a direction away from the loudspeaker.

A further alternative is shown in Fig. 8, in which the transmission line has no straight sect ion, such a shape being suitable, for example, for a bookshelf type loudspeaker system.

Figs. 8 and 9 show some examples of preferred dimensions and dimensional relationships for transmission lines which may be appropriate for bookshelf and floor standing systems respectively.

In Fig. 8, the speaker unit may typically have a 5" (12.5 cm) diameter, and the minimum ducting diameter to cover the speaker cutout may then be about 4.5" (11.2 cm). It is preferred for r to be about 2" (5 cm), although this is not critical, in which case, R will be 6.5" (16.2 cm). R should be about one half of the length 1 (+ about 15%).

In Fig. 9, the enclosure is similar to that of Fig.

8, but includes a straight section of length m, so that its total height is m + 1. Typically, the tube diameter may need to be about 8" (20 cm) to enclose the speaker cutout, and the desired enclosure height may be about 36" (90 cm). This would require a straight section length of about 16" (40 cm) for an r of about 2" (5 cm) (m = 36 - R.2 = 36 - (2 + 8).2).

The following table gives some values of m for a combination of speaker diameters and enclosure heights in inches:

Enclosure 36 48 60 Speaker Height Diameter 8 16 28 40 6 20 32 44 5 22 34 46

Claims

Claims 1. A loudspeaker system comprising a circular loudspeaker unit and a transmission line enclosure of circular cross-section.
2. A loudspeaker system comprising an elliptical loudspeaker unit and a transmission line enclosure of elliptical cross-section.
3. The loudspeaker system of claim 1 or 2, wherein the transmission line enclosure has substantially the same cross-sectional size as the diaphragm of the loudspeaker unit at the interface between the unit and the transmission line.
4. The loudspeaker system of claim 1, 2 or 3, wherein the transmission line extends initially directly back from the loudspeaker along the centre axis of the loudspeaker's diaphragm.
5. The loudspeaker system of any preceding claim, wherein the transmission line is made from ducting of circular or elliptical cross-section.
6. The loudspeaker system of any preceding claim, wherein the transmission line is smoothly bent.
7. The loudspeaker system of any preceding claim, wherein the transmission line comprises straight sections with smooth bends therebetween.
8. The loudspeaker system of claim 7, wherein the transmission line comprises a first portion extending directly back from the rear of the loudspeaker, a second portion extending substantially perpendicularly thereto, a third portion extending generally in the opposite direction to the first portion but angled out of the plane of the first and second portions, a fourth portion extending generally in the opposite direction to the second portion to a position past the first portion, a fifth portion extending across the first portion, a sixth portion extending generally in the opposite direction to the fourth portion and a seventh portion extending generally in the direction of the first portion.
9. The loudspeaker system of any preceding claim, wherein the transmission line is tapered along at least some portion thereof.
10. The loudspeaker system of any preceding claim, wherein the loudspeaker and transmission line are mounted within a further secondary enclosure.
11. The loudspeaker system of claim 10, wherein secondary enclosure is box-shaped.
12. The loudspeaker system of any preceding claim, wherein the transmission line incorporates a soundabsorbent material.
13. The loudspeaker system of any preceding claim, wherein the end of the transmission line distal from the loudspeaker is closed except for a port therein of smaller cross-section than the end of the line.
14. The loudspeaker system of claim 13, wherein the transmission line enclosure is of a general U-shaped construction with an extended bridge portion.
15. The loudspeaker system of claim 13, wherein the transmission line enclosure comprises a first portion extending back from the loudspeaker, a second portion extending generally perpendicularly thereto, and a third portion extending generally perpendicularly to the second portion away from the loudspeaker.
16. The loudspeaker system of claim 13, 14 or 15, wherein the transmission line enclosure is mounted within a secondary enclosure, and wherein the port is formed in the wall of the secondary enclosure.
17. The loudspeaker system of any of claims 13 to 16, wherein the transmission line enclosure is of such a length that sound waves reflected back to the loudspeaker are substantially in phase with those emitted from the front thereof.
18. The loudspeaker system of any of claims 13 to 17, wherein the port is sized so that frequencies less than a set frequency corresponding to a value above which the loudspeaker output is low are reflected back.
19. The loudspeaker system of any of claims 13 to 18, wherein the port is about 3 cm in diameter.
20. A loudspeaker system comprising a loudspeaker and a transmission line enclosure extending from the rear thereof, the end of the transmission line distal from the loudspeaker being closed but having a port therein of smaller cross-section than the transmission line at that end.
21. A loudspeaker system in which sound waves travel along a lossy transmission line enclosure to a ported panel and are reflected back through the same route to the loudspeaker.
22. The loudspeaker system of claim 20 or 21, including any of the features of claims 1 to 19.
23. An enclosure for a loudspeaker according to the loudspeaker enclosure of the system of any preceding claim.
24. Loudspeaker systems substantially as hereinbefore described with reference to the figures.