GB2059221A - Improvements in or relating to loudspeaker systems - Google Patents

Abstract

The invention is concerned with loudspeaker systems of the kind including a bass transducer in an enclosure (1), a mid-range transducer in a housing (3), and a treble transducer in a housing (5). To prevent distortion of the radiation pattern by reflections from walls or the like adjacent to the loudspeaker system, the housings for the mid-range and treble transducers are enclosed in a self-supporting bonnet (8) consisting of an acoustically absorbent material such as polyester foam or polyurethane foam. <IMAGE>

Description

SPECIFICATION Improvements in or relating to loudspeaker systems This invention relates to loudspeaker systems of the kind which include a plurality of transducers each designed to radiate a particular range of audio frequencies and connected to the output of an audio-frequency amplifier through a frequencydividing network.

Usually such systems include a bass transducer (commonly referred to as a 'woofer') mounted in an enclosure to radiate through an aperture in a front wall of the enclosure. In the past, most such systems have also included a mid-range transducer and a treble transducer (commonly referred to as a 'tweeter') mounted in the same enclosure to radiate through further apertures in the front wall of the enclosure. However, it has recently been found that improved results can be obtained by mounting the mid-range and treble transducers outside the enclosure containing the bass transducer. In particular, attempts have been made to design the mid-range and treble transducers and their housings in such a way that they radiate at least the majority of the frequencies for which they are designed substantially equally in all azimuthal directions.To a large extent, the radiation pattern is also substantially constant in all directions above the horizontal.

However, since the mid-range and treble transducerr will normally be mounted on top of the bass enclosure, downward radiation is limited by the presence of the enclosure, and is normally also further limited by the provision of a soundabsorbing material between each of the mid-range and treble transducers and the top of the bass enclosure.

The above system can produce excellent results in an acoustically favourable environment. However, in the case of loudspeaker systems designed for domestic use, it is normally necessary for the system to be mounted in relatively close proximity to at least one wall of a room. Such walls are usually acoustically reflective and, accordingly, distort the radiation pattern of the mid-range and treble transducers.

Accordingly, it is an object of the present invention to reduce the effect of this distortion on the sound produced by the system.

From one aspect the invention consists in a loudspeaker system including a bass transducer mounted in an enclosure to radiate through an aperture in a front wall of the enclosure, and mid-range and treble transducers mounted in separate housings to radiate through apertures in respective front walls of said housings, wherein said housings are mounted outside said enclosure and are substantially devoid of discontinuities which would act as secondary sound sources, and wherein said housings, apart from the front walls thereof, are surrounded by acoustically absorbent material.

Preferably the housing for said mid-range transducer is mounted on top of said enclosure and the housing for the treble transducer is mounted on top of the housing for the mid-range transducer. The housing forthe mid-range transducer may be separated from the enclosure by an acoustically absorbent mat and similarly the housing for the treble transducer may be separated from the housing for the mid-range transducer by a further acoustically absorbent mat. The housing for the treble transducer may be, for example, generally spherical in shape, or may be generally in the form of a right circular cylinder having an axial length substantially equal to its diameter. In the latter case however, the housing should be constructed without sharp edges at the ends of the cylinder.The housing for the mid-range transducer is preferably substantially cylindrical having a polygonal crosssection. Once again, the housing is constructed without sharp edges at the front and back since such sharp edges would act as secondary sound sources.

Preferably the acoustically absorbent material is an expanded synthetic resin material such as polyester foam or polyurethane foam. The foam material is preferably relatively thick so that it is substantially self-supporting. Thus, the acoustically-absorbent material may be constructed as a self-supporting bonnet which will fit over the treble and mid-range transducers. The internal shape of the bonnet will correspond generally to the external shape of the housings of the treble and mid-range transducers. It is to be understood that the housing for the treble transducer will be very much smaller than the housing for the mid-range transducer and, accordingly, the treble housing may be located in a recess in the top of the bonnet. Thus, in this case, the upper surface of the treble housing will not be covered by acoustically-absorbent material.This arragement is normally acceptable because any upward radiation from the treble transducer will not normally impinge on a nearby reflective surface.

If desired, the acoustic bonnet may be surrounded by a cover consisting of generally acoustically transparent material.

One method of performing the invention will now be described with reference to the accompanying diagrammatic drawings, in which: Figure lisa side view of a loudspeaker system in accordance with the invention; Figure 2 is a front view of the loudspeaker system illustrated in Figure 1; and Figure 3 is a polar diagram showing radiation patterns of one particular loudspeaker system.

The system illustrated includes a bass enclosure 1 having an aperture 2 in its front wall. A bass transducer (not shown in the drawings) is mounted in the enclosure to radiate through the aperture 2.

Mounted on top of the enclosure lisa housing 3 for a mid-range transducer. This transducer is mounted within the housing 3 to radiate through an aperture 4 in the front wall thereof. Mounted on top of the housing 3 is a further housing 5 for a treble transducer which is arranged to radiate through an aperture 6 in the front wall of the housing 5. A faring 7 is provided between the housings 3 and 5 to ensure smooth radiation of sound from the treble transducer. It will also be seen that the front and rear walls of the housing 3 are chamfered to avoid any sharp edges which might act as secondary sound sources.

In order to reduce sound radiation from the sides and rear of the housings 3 and 5, these housings are covered by a self-supporting bonnet 8 consisting of acoustically-absorbent material such as polyester foam or polyurethane foam. The bonnet 8 is provided with a recess 9 to accommodate the housing 5.

Thus it will be seen that radiation from the midrange transducer is substantially prevented in all directions except the forward direction, while radiation from the treble transducer is only permitted in the forward and upward directions. As already mentioned, any upward radiation from the treble transducer will not normally impinge on a nearby reflective surface.

In the arrangement shown, downward radiation from the mid-range transducer is substantially prevented by the enclosure 1, and downward radiation from the treble transducer is substantially prevented by the housing 3. However, if desired, an acoustically-absorbent mat may be provided between the housing 3 and the enclosure 1, and a further mat may be provided between the housings 5 and 3.

It will be seen that the thickest part of the bonnet 8 is located behind the mid-range and treble transducers since radiation in this direction is most likely to give rise to distortion of the radiation pattern. The next thickest part of the bonnet is located at the sides, and the thinnest part is located at the top.

Figure 3 shows the radiation pattern of one particular loudspeaker system at a frequency of 4kHz. The dash-dotted line 10 shows the radiation pattern without the bonnet and the solid line 11 shows the radiation pattern with the bonnet in position. It will be seen that, at this particular frequency, the radiation in the reverse direction, i.e.

at 1800, is considerably reduced when bonnet is in position. It will also be seen that there is an appreciable reduction in the sideways direction, i.e.

at 90" and 270w. Somewhat similar reductions are produced at other frequencies, particularly above 4kHz.

Claims (10)

1. A loudspeaker system including a bass transducer mounted in an enclosure to radiate through an aperture in a front wall of the enclosure, and mid-range and treble transducers mounted in sepa rate housings to radiate through apertures in respective front walls of said housings, wherein said housings are mounted outside said enclosure and are substantially devoid of discontinuities which would act as secondary sound sources, and wherein said housings, apart from the front walls thereof, are surrounded by acoustically absorbent material.

2. A loudspeaker system as claimed in Claim 1, wherein the acoustically absorbent material is an expanded synthetic resin material.

3. A loudspeaker system as claimed in Claim 2, wherein the synthetic resin material is polyester foam.

4. A loudspeaker system as claimed in Claim 2, wherein the synthetic resin material is polyurethane foam.

5. A loudspeaker system as claimed in any of the preceding Claims, wherein the acoustically absorbent material is substantially self-supporting.

6. Aloudspeaker system as claimed in Claim 5, wherein the housing for the mid-range transducer is mounted on the upper surface of the enclosure, the housing for the treble transducer is mounted on the upper surface of the mid-range transducer housing, and wherein the acoustically absorbent material is formed into a self-supporting bonnet which fits over the said housings.

7. A loudspeaker system as claimed in Claim 6, wherein the treble transducer housing is located in a recess in the top of the bonnet such that the upper surface of the treble transducer housing is not covered by acoustically absorbent material.

8. A loudspeaker system as claimed in any of Claims 5 to 7, wherein the bonnet is surrounded by a removable cover consisting of substantially acous ticallytransparent material.

9. A loudspeaker system substantially as hereinbefore described with reference to, and as illustrated in, the accompanying diagrammatic drawings.

10. Any features of novelty, taken singly or in combination, of the loudspeaker system as hereinbefore described with reference to the accompanying diagrammatic drawings.