GB2024563A

GB2024563A - Improvements in and Relating to Loudspeakers

Info

Publication number: GB2024563A
Application number: GB7828237A
Authority: GB
Original assignee: IMF ELECTRONICS Ltd
Current assignee: IMF ELECTRONICS Ltd
Priority date: 1978-06-28
Filing date: 1978-06-28
Publication date: 1980-01-09

Abstract

A multi-unit loudspeaker approximating to the behaviour of a point source of sound includes speaker units handling various frequency bands. One frequency band is handled by one or more units (10) disposed at or about a reference point. Further units (12, 13) handling a frequency band are disposed about the reference point in such a way that their effective combined source of sound is located at the same reference point. Still further units handling sound within a third frequency band can be provided in a similar fashion. <IMAGE>

Description

SPECIFICATION Improvements in and Relating to Loudspeakers The present invention relates to loudspeakers and more particularly to improved arrangements for deploying a plurality of loudspeaker drive units.

A problem in the design of wide-range loudspeakers is that to obtain a wide frequency range, it is generally necessary to use a multiplicity of loudspeaker drive units (hereafter referred to simply as "units"), and it has hitherto proved impossible to ensure that the sound from these separate units integrates into a sound of the desired high quality at the ears of the listener for all reasonable positions of the listener in relation to the loudspeaker. The reason for this difficulty is that each unit occupies a different physical position in space, and the relative times of arrival of the sound at the listener from the various units changes with the position of the listener, resulting in a change in sound quality.This problem affects not only the sounds travelling straight from the units to the listener, but also those components of sound reflected from each point on the walls of the listening room. There is evidence that inferior quality of such reflected sounds can prove to have a detrimental effect on the subjective listening quality of a loudspeaker, even if the quality of the direct sounds reaching the listener is, by itself, adequate.

In order to ensure that the sounds from several units handling different frequency ranges in a loudspeaker arrive at the ears of a listener at the same time, it has previously been proposed that the units be placed vertically one above the other, with the smaller or shallower units placed further back than the larger units, so that the effective source positions of sounds for the various units lie in a vertical line. Figures 1 and 2 show two such previously proposed arrangements designed to achieve this aim, where the crosses mark the effective source of sound for each of the two units, U1 and U2.

The problem with such an arrangement is that the sounds only arrive at the ears of the listener at precisely the same time provided that the listener is equidistant from the acoustic centres of the several units. In practice, this means that the ears of the listener must be located within a horizontal plane perpendicular to the vertical line joining the units. Should the listener move outside this plane, or should he hear reflections of sounds emitted from the speaker in non-horizontal directions, then relative delays between units will be heard, with consequent deleterious effect on quality.

A second arrangement attempts to compensate for the relative time delays between units by using electronic time delays to feed some of the units, but again such an artifice will only work for a limited range of listening positions where the relative delays between units match those of the electronic delays. For other listening positions, and for reflected sounds, a deterioration of quality will occur.

There is described hereinafter an arrangement to mitigate or substantially overcome the problem of relative time delays between units handling different frequency ranges varying with listening position. Such an arrangement seeks substantially to approximate the behaviour of a point source of sound, so that all frequency components of the sound reach the listener at substantially the same time, irrespective of his/her position.

The preferred embodiment of the invention enables a multiple drive unit loudspeaker to approximate to a point source of sound by providing a source of sound of a first frequency band at a point and a plurality of drive units disposed about said point for producing a source of sound of a second frequency band at said point.

Preferably the source of sound of the first frequency band is a single drive unit located at said point but it may comprise a further plurality of drive units disposed about said point for producing the source of sound of the first frequency band at said point.

One or more further sources of sound of other frequency bands may be provided by locating further pluralities of drive units of the appropriate frequency response in a similar way such that the further sources of sound also coincide with said point.

Features and advantages of the present invention will become apparent from the following description of an embodiment thereof given by way of example with reference to the accompanying drawings in which: Figures 3A to 3F show various arrangements of loudspeaker drive units; Figure 4 is a side sectional representation of one embodiment of the invention; and Figure 5 is a front perspective view of another embodiment of the invention.

An approximation to point-source behaviour at a given reference point is achieved by using two or more identical units placed symmetrically about the reference point. The term "symmetrical about" means that the configuration of identical units is unchanged either by reflection in a given plane through the reference point or by rotation through a given angle about a line through the reference point. Thus Figures 3A to 3F show six symmetrical arrangements of identical units. Such arrangements will appear to approximate the behaviour of a point source of sound, the point source being located at the reference point, shown in each case, marked with a cross.

Three features of the preferred embodiment will now be described which contribute to the approximation of a point source: (1) More than one type of unit should be used, and the plurality of units of each type should be symmetrical about the same reference point.

(2) The reference point should be substantially at the middle point of the effective acoustic positions of all units of any given type.

(3) The distance between the effective acoustic position of a unit and the reference point should preferably be not more than about two wavelengths in air of the highest frequency which that unit is called upon to handle with substantial intensity.

It should be noted that as far as can be ascertained at present, such arrangements only apply to units that have an effective acoustic position, i.e. that act as though sounds originate from a single point for listeners over most of the normal listening area. This requirement does not apply to speakers using units whose dimensions are a number of wavelengths in lengths and width such as are encountered in some large-diaphragm electrostatic loudspeakers. Such large-diaphragm units have no effective source position.

The reason why all types of unit should be in symmetrical configurations about the same reference point is that the effective point position of the total loudspeaker should be at the reference point. Feature (2) above ensures that the effective point-source position of the plurality of units be at the reference point, and feature (3) ensures that the plurality of units is sufficiently small in size to form effectively an approximation to a point source. By this means, all frequencies will appear to emanate from one source, whatever the position of the listener, and wherever acoustic reflections take place.

As mentioned above, the source of sound of one frequency band may preferably be provided by only one unit located at the desired reference point, as illustrated in Figure 3F. Clearly, with several types of units sharing the same reference point, it is physically possible for only one of the types to be situated actually at the reference point so that the other types of unit must occur two or more times each. Additionally there is no requirement that the symmetry of the arrangement of units of each type need be the same symmetry as of units another type.

Figure 4 illustrates a two-way loudspeaker design, using two types of unit, according to the invention. Here the smaller treble unit 10 is placed with its acoustic centre A situated near the vertex of a V-shaped baffle 11, and to either side are two identical larger low-frequency or bass units 12, 13 symmetrically disposed about the treble unit 11. The V-shaped baffle helps pull the effective acoustic centre of the physically deeper bass units 12, 13 forward into the same vertical plane as that of the treble unit 11. If the acoustic centres of the bass units 12, 1 3 are designated by B, C respectively, their effective acoustic centre will be located half way along the line B, C. This can then be made to coincide with the acoustic centre A of the treble unit 11, and this will then be the reference point of the system.

The configurations of loudspeakers can be such that their acoustic centres lie in a vertical line or in a horizontal line. In addition, the inwardoriented V of the bass units 12, 13 helps to increase the dispersion of sound towards the upper limit of their frequency range over a wider listening area than would otherwise occur with units mounted on a single flat baffle. For best results, it is desirable that the units be placed as close together as is reasonably possible bearing in mind their sizes and construction, so that at the upper frequency end of the range of bass units, the speakers are not more than a very few wavelengths apart.Ideally, the separation between the acoustic centres of the two bass units should not be more than one wavelength in air at the highest frequency at which the bass units operate, although often practical considerations will force a separation of perhaps two or three wavelengths to be used. Any symmetrical arrangement of identical drive units may be used such as those shown in Figures 3A to 3F. These are then combined with similar or other symmetrical arrangements of mutually identical drive units covering a different range in such a way that the reference points of the various arrangements substantially coincide.

Figure 5 illustrates a more complex configuration for a 3-way loudspeaker design, using 3 types of unit, one for the bass, one for the midrange and one for the treble. Here there is a single treble unit 20 placed with its acoustic centre at a central reference position, pointing forward. Two midrange units 21, 22 are located one to either side of the treble unit 20 on a Vshaped baffle arrangement 23, 24, so that the reference point lies exactly half-way between their respective acoustic centres. Two bass units 25, 26 are placed on a second V-shaped baffle arrangement 27, 28, this time above and below the tweeter, again so that the midpoint of their respective acoustic centre lies substantially as the reference point.In this way, it is possible to place all 5 units in close proximity to one another and to ensure that all 5 acoustic centres of the individual units lie in the same vertical plane. Clearly it is possible to rotate the speaker by 900 so that the bass units lie to either side of the treble unit and the midrange units are above and below.

Furthermore, while it is economically advantageous to use one unit at the reference point rather than a combination symmetrically disposed about it, there is no necessity to do so.

However, if one unit is used in this way, it need not necessarily be the treble unit as described above.

The cross-over networks used to connect the various speakers to an electrical sourcesignai are not discussed here, since the same considerations in designing such networks apply to speakers according to this invention as to conventional multi-unit speakers, with the exception that the symmetry of the designs according to this invention simplifies the design of the cross-over networks since the listening quality of the speaker varies less with listener position than for conventional designs.

It is also worth noting that for every low frequencies (below 100Hz approximately, depending on loudspeaker size), the wavelength of sound in air becomes sufficiently large that a separation of the effective acoustic source of the extreme bass and the reference point will be acceptable provided that such a separation does not exceed half of one wavelength. It is thus permissible to depart from the symmetry requirements of this invention for the very lowest frequencies, by the use for example of ports or acoustic labyrinths for the very lowest frequencies that are not disposed symmetrically about the reference point. Nevertheless, this comment does not exclude the possibility of using a symmetric labyrinth or port arrangement about the reference point.

Claims

1. A multiple transducer loudspeaker comprising means effectively producing at a location a source of sound within a first frequency band, and a plurality of transducers disposed about said location so as to produce a further source of sound within a second frequency band at said location.

2. A loudspeaker according to claim 1 wherein said sound producing means comprises a single first frequency band transducer disposed with its acoustic centre at said location.

3. A loudspeaker according to claim 1 wherein said sound producing means comprises a plurality of first frequency band transducers disposed about said location so as to produce a source of sound at said location.

4. A loudspeaker according to claim 1 wherein said sound producing means comprises an acoustic horn or port disposed at said location, said acoustic port being arranged to radiate sound within said first frequency band.

5. A loudspeaker according to claim 1, 2 or 3 further including a plurality of acoustic horns or ports disposed about said location, said plurality of acoustic horns or ports being arranged to radiate sound within a further frequency band so as to produce a source of sound within the further frequency band at said location.

6. A loudspeaker according to any one of the preceding claims further including a plurality of third frequency band transducers disposed about said location for producing a third source of sound within a third frequency band at said location.

7. A loudspeaker according to any one of the preceding claims wherein the distance between said location and the acoustic centre of each transducer of at least one of said transducer pluralities does not exceed two wavelengths in air of the highest frequency of the respective transducer frequency band.

8. A Joudspeaker according to any one of the preceding claims wherein each transducer of at least one of said transducer pluralities is disposed on a respective side of the vertex of a V-shaped baffle.

9. A loudspeaker according to claim 1 wherein said plurality of transducers are symmetrically disposed about said location.

1 0. A loudspeaker according to claim 1 or claim 9 wherein each of said transducers is disposed at a distance from said location not exceeding two wavelengths of sound at the highest frequency included within said second frequency Dand.

11. A multiple transducer loudspeaker substantially as hereinbefore described with reference to Figures 3A to 3F, 4 and 5 of the accompanying drawings.