GB2316846A - Loudspeaker horn divided by restrictor element - Google Patents

Abstract

A loudspeaker has an acoustic restrictor 5 in the horn 3 mounted in front of a transducer 2. The shape of the restrictor is such that, in one direction mounting parts 26 of the restrictor span the width of the horn and subdivide it into two discrete acoustic channels over opposed profiled surfaces of the restrictor, whilst in the perpendicular direction the restrictor has the usual "bullet" cross-section.

Description

LOUDGPEAKER The present invention relates to a loudspeaker.

Loudspeaker designs can be categorised according to the way in which the acoustic energy from the transducer is coupled into the ambient air. In one such category which includes "horn-loaded" loudspeakers, the transducer does not drive the external air directly, but via a closed-sided acoustic channel; the transducer radiates into the channel from one end towards the other, open end. The geometry of the channel provides an opportunity to influence the coupling of the acoustic energy from the transducer into the ambient air. Probably the best known and longest established example of this is the "exponential horn" where the interior walls of the enclosure which define the sides of the channel flare towards the open end of the channel following an exponential curve. In its simplest, theoretical, form, the horn "throat" is rotationally symmetric; practical implementations as found in sound reinforcement and public address systems often have a rectangular cross-section, with one or both pairs of side walls flaring towards the horn mouth.

The advantages and disadvantages of exponential horns are well known: the electrical to acoustic efficiency of transduction is very high, but the horn is inconvenient to fabricate and subject to acoustic colouration.

Another approach to the implementation of an acoustic channel whose cross-sectional area varies along the length of the channel is disclosed in British Patent No.

1592246 (Isaac). In this prior proposal, a progressive increase in the cross-sectional area of the acoustic channel is achieved not by flaring the external walls of the channel but by the presence in the channel of a suitable profiled, co-axial "restrictor" element (so-called because it restricts the cross-sectional area of the channel which is occupied by air). This restrictor element tapers, in the fashion of an aerofoil cross-section, from its widest cross-section in the region of the channel in front of the loudspeaker cone to a rounded point in the mouth of the channel. The rear face of the element, which faces towards the loudspeaker cone and central pole piece of the loudspeaker, is frustoconical; the restrictor element is mounted in the enclosure by means of lateral spars which extend across the channel at the widest point of the restrictor element.

A more recent proposal for a restrictor element is disclosed in British Patent No. 2270606 (Andrews & Newsham).

In this design, the main part of the restrictor element is asymmetrical in cross-section, overall resembling the shape of an axe head. The restrictor element includes a rearward projection through the voice coil form of the loudspeaker to the central pole piece of the loudspeaker. This rearward projection serves to mount the restrictor element in the acoustic channel.

In both the Isaac and Andrews & Newsham prior art proposals, the restrictor element essentially sits in the acoustic channel surrounded on all sides by air which provides the acoustic path from the loudspeaker to the outside of the loudspeaker enclosure. Further, in both these proposals, the restrictor element is an elongate object which is, in the illustrated embodiments, exclusively supported and mounted relative to the remainder of the enclosure towards the inner end of the element. Although it is possible in both designs to contemplate additional points of support closer to the mouth of the acoustic channel, the present invention seeks to provide an alternative design.

According to the present invention there is provided a loudspeaker comprising: a loudspeaker horn defining an interior volume which is acoustically open at one end and is acoustically closed at its other end and at all sides; an electro-acoustic transducer arranged to radiate acoustic energy into the closed end of the channel; and a restrictor element located in said interior volume, the restrictor element having a rear portion and a front portion, the rear portion being located towards the closed end of the volume and being spaced in a direction towards the open end from the transducer and the front portion extending at least the majority of the distance along the length of the interior volume from the rear portion to the open end, the front portion having, in a first plane of cross-section, an acoustic restrictor profile and, in a second plane of cross-section orthogonal to the first plane, a cross-section which spans the corresponding cross-section of the interior volume so as: a) to mount the restrictor element, via the lateral edges of its front portion, along opposed regions of the side walls of the interior volume; and b) to subdivide the interior volume into two acoustic channels leading over opposed faces of the profile from the rear portion of the restrictor element to the opposite end of the front portion of the restrictor element.

The invention will be further described by way of non-limitative example with reference to the accompanying drawings, in which: Figure 1 is a vertical cross-section through one embodiment of the invention taken at its horizontal midpoint; and Figures 2 is a horizontal cross-section taken through the embodiment of Figure 1 at its vertical mid-point.

The loudspeaker 1 is intended for use in sound reproduction and sound reinforcement applications and, as such, need not have a frequency response which covers the entire audio spectrum; rather, it may be used in conjunction with other loudspeakers covering other parts of the spectrum.

The loudspeaker 1 comprises four principal components, namely:- a loudspeaker transducer 2; a sound-radiator horn 3; a closure member 4, which forms a sealed airtight enclosure around the rear of the transducer 2; and an acoustic restrictor 5 mounted in the throat of the horn 3.

All of these components are described in greater detail below. It will be see from Figures 1 and 2 that each of them is coaxial with the central front-rear axis 6 of the loudspeaker 1.

It will be noted that in the form shown, the loudspeaker does not have the conventional box-shaped wooden or fibre-board casing, but one may be provided in applications where the protection of such a casing is required; alternatively, for permanent installations, the loudspeaker may have an external rigid frame e.g. of steel.

The horn 3 is fabricated from suitable rigid material e.g. reinforced fibreglass and defines an interior volume 7 which increases in cross-sectional area from its inner end 8 to its open mouth 9; the interior volume 7 is closed at its sides by the vertical side walls 10a,10b of the horn 3 and, at its top and bottom, by the horizontal top and bottom walls, lla,llb of horn 3. As illustrated, the side walls 10a,10b flare continuously and linearly from the inner end 8 to the open mouth 9, while the top and bottom walls lla,llb start parallel to one another at the inner end of the horn and begin to flare linearly about half way along its length. The invention is of course not limited to linearly flared walls, and is equally applicable to non-linear flares as well as combinations of linear and non-linear flares.

The transducer 2 is mounted, e.g. by bolts to the rear face of the horn 3 so that, in operation, its conical diaphragm 12 radiates acoustic energy into the inner end 8 of the interior volume 7, driven by a voice coil 13 wound on a conventional cylindrical annular former.

The walls of the horn 3 are shaped so that the width and height of the interior volume (and consequently its crosssectional area in a plane perpendicular to the loudspeaker axis) increase in the direction from the transducer 2 towards the open mouth 9 of the horn. The rate of increase in crosssectional area changes (i.e. increases) where the top and bottom walls 10a,10b start to flare. The cross-section of this volume is conveniently at least approximately rectangular.

The restrictor element 5 is mounted within the horn 3 and is supported at its vertical, flat side edges by the side walls 10a,10b of the horn. Restrictor element 5 has a front part 21 and a rear part 22. It will be seen from Figure 1 that the front part 21 spans the entire width of the interior volume of the insert. The effect of this is twofold. Firstly, it enables the mounting of the element 5 to the remainder of the loudspeaker to be achieved by securing the flat lateral side faces 23a,23b of the restrictor element to corresponding, abutted surface portions of the side walls 10a,10b of the horn 3; this securement may be by any suitable means e.g. screw- or bolt fittings, adhesive, etc. Secondly, the restrictor element 5 divides the interior volume of the horn 3 into two discrete acoustic channels, one above, and the other below, the front part 21 of the restrictor element; these discrete acoustic paths extend over the majority of the distance from the region of the loudspeaker transducer 2 to the open mouth of the enclosure, and allow for the opportunity to tailor their acoustic impedances by suitable profiling of the front part 21 of the restrictor element 5 having regard to the profiles of the inner walls of the insert.

It will be seen from Figure 1 and 2 that the front part 21 has a central core 25 which decreases (not necessarily linearly) in height from the rear of part 21 towards its outer edge 24; as viewed from above the outer edge 24 is convexly curved.

In Figure 1 the variation of the height of the restrictor element 5 is such that the upper and lower surfaces of element 5 are convexly curved rather than flat.

It will be appreciated that suitable profiling of the inner walls of the horn 3 and the upper and lower surfaces of the element 5 enable the designer to adjust the rate of change of cross-sectional area of the two acoustic channels along their length.

The side regions of the restrictor element 5 flare laterally outwardly in the directions towards the open mouth of the enclosure and have a vertical cross-section of greatest thickness (height) in the region where the horn walls lla,llb start to flare and taper in thickness towards their leading (25a) and trailing (27a) edges and towards the central core and outer lateral edges of the restrictor element 5. This shape is shown at 26 projected onto the plane of Figure 1.

The rear part 22 of the restrictor element 5 is frusto-conical and faces towards the diaphragm and central pole piece of the loudspeaker transducer 2 but with a substantial space in between it and both of them so as to leave a volume of air to be driven by the transducer cone and also to ensure that there is no possibility of the vibrating cone coming into contact with the restrictor element 5.

It will be noted that the rear part 22 is entirely free of any direct attachment to any part of the transducer 2 though, of course, both are indirectly attached since they are both mounted on the horn 3.

It will be noted that in this embodiment the restrictor element is mirror symmetrical about its longitudinal axis in both horizontal and vertical planes and that this axis is coincident with that of the horn 3 and that of the transducer 2. These symmetries are for convenience of design and assembly (i.e. it does not matter which way up the element 5 is fitted in the horn 3) and are not essential to the invention in its broadest aspect. On the contrary it is possible to contemplate variants in which the profiles of the upper and lower surfaces of the element 5 differ from one another to provide different audio characteristics (e.g. in terms of frequency or phase response or directionality) for the two separate acoustic channels.

The restrictor element 5 may be made of any suitable, rigid material (e.g. fibre glass or plastics). It may be made by moulding or any other technique and may be solid or hollow (though if hollow, it may be desirable to fill it with an acoustically deadening material).

The closure element 4 provides a sealed chamber to the rear of the diaphragm of the transducer 2 and defines a volume of air that can be varied (by the designer) to achieve different audio response characteristics. This volume can be wholly or partly filled with acoustically absorbent material such as BAF wadding.

Various modifications within the scope of the invention as defined in the appended claims will be apparent to those skilled in the art. In particular, the invention is not limited to the specific shape, configuration or profiling of the restrictor element 5 as exemplified in the embodiment described above.

Claims (8)

1. A loudspeaker comprising: a loudspeaker horn defining an interior volume which is acoustically open at one end and is acoustically closed at its other end and at all sides; an electro-acoustic transducer arranged to radiate acoustic energy into the closed end of the channel; and a restrictor element located in said interior volume, the restrictor element having a rear portion and a front portion, the rear portion being located towards the closed end of the volume and being spaced in a direction towards the open end from the transducer and the front portion extending at least the majority of the distance along the length of the interior volume from the rear portion to the open end, the front portion having, in a first plane of cross-section, an acoustic restrictor profile and, in a second plane of cross-section orthogonal to the first plane, a cross-section which spans the corresponding cross-section of the interior volume so as: a) to mount the restrictor element, via the lateral edges of its front portion, along opposed regions of the side walls of the interior volume; and b) to subdivide the interior volume into two acoustic channels leading over opposed faces of the profile from the rear portion of the restrictor element to the opposite end of the front portion of the restrictor element.

2. A loudspeaker according to claim 1, wherein the lateral edges of the front portion of the restrictor have flat faces which butt against corresponding surface portions of the walls of the interior volume.

3. A loudspeaker according to claim 1 or 2, wherein the edge of the restrictor which faces out of the open end of the interior volume is convexly curved in said second plane.

4. A loudspeaker according to claims 1, 2 or 3, wherein the restrictor element comprises a central core and a pair of opposed side regions extending outwardly therefrom in the second plane to said lateral edges.

5. A loudspeaker according to claim 4, wherein the cross-sections of the central core in the first and second planes both taper in the direction towards the open end of the horn.

6. A loudspeaker according to claims 4 or 5, wherein each of the side regions has, in a respective plane parallel to the first plane a cross-section which is of maximum width in the longitudinal central region of the side region and tapers therefrom towards the front and rear edges of the side region.

7. A loudspeaker according to any one of the preceding claims, wherein the cross-sections of the front part of the restrictor element in the first and second planes are each symmetrical about the line of intersection of those planes.

8. A loudspeaker constructed and arranged to operate substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.