Patent Description:
A coaxial loudspeaker design offers a compact acoustic arrangement that improves system directivity through the crossover region, by avoiding the off-axis phase cancellation that occurs with discrete, axially offset acoustic sources.

However, it is recognised that coaxial loudspeakers often suffer from a compromised directivity pattern (acoustic response off-axis) across their frequency spectrum. In particular, when a conventional, axisymmetric cone shape is used as the low/mid-frequency part of the coaxial loudspeaker arrangement, the directivity of the high-frequency section is compromised because the axisymmetric low/mid-frequency cone forms the walls of a horn within which the high-frequency sound waves propagate, so an axisymmetric directivity pattern is imposed upon the high-frequency acoustic output. This axisymmetric directivity pattern is generally not optimal for professional loudspeakers. Also, because the angle of the cone neck must be steep in order to ensure good low/mid-frequency performance, the axisymmetrical high frequency directivity pattern often has a beamwidth which decreases with increasing frequency, further compromising the design.

Attempts to improve the directivity of coaxial loudspeakers, whilst maintaining compactness, may introduce further effects that act to impair acoustic performance; for example, by occluding acoustically active elements of the loudspeaker (the low/mid-frequency cone) or diffusing sound in a rudimentary and uncontrolled manner.

It is an aim of the present invention to alleviate at least some of the aforementioned problems. An aspect of the invention is set out in the independent claim and optional features are set out in the dependent claims.

The present invention relates to a coaxial loudspeaker apparatus comprisinα: a first unit. being arranged to propagate sound in a first frequency range; a second unit, being arranged to propagate sound in a second frequency range that is higher than the first frequency range, comprising a first waveguide; a second waveguide arranged to extend substantially in prolongation of the first waveguide; and a third waveguide arranged to extend substantially in prolongation of the second waveguide.

Preferably, the second waveguide is provided on the first unit, and more preferably on the moving parts of the first unit, and still more preferably in such a way that the second waveguide is affixed / bonded to the first unit. Preferably, the first unit comprises a cone for reproducing sound, wherein the second waveguide is provided on the cone, for example in such a way that it is affixed /bonded to, or integral with, the cone.

Advantageously, in this way an enlarged effective waveguide is provided for the second unit, thereby improving efficiency and performance of the second unit.

The second waveguide and the third waveguide are respectively arranged to extend substantially in prolongation of the first waveguide and second waveguide in a downstream direction.

Preferably, the first unit is also arranged to extend substantially in prolongation of the first waveguide in a downstream direction.

The first waveguide and the third waveguide may be fixed. The second waveguide is arranged to move relative to the first waveguide and relative to the third waveguide.

For optimum performance and as though a single waveguide were present, preferably the third waveguide extends substantially continuously from the second waveguide and wherein the second waveguide extends substantially continuously from the first waveguide. Preferably, the first unit also extends substantially continuously from the first waveguide.

So as not to impede operation of the first unit, the second waveguide is arranged to move in unison with the first unit in operation.

Preferably, the first waveguide, second waveguide and/or the third waveguide are coaxial (and/or preferably concentric).

Preferably, the first unit comprises a cone or other sound reproducing diaphragm and wherein the second unit comprises a driver unit that is arranged upstream of the cone (or other sound reproducing diaphragm); in this way occlusion of the first unit by the second unit is reduced.

The first unit may comprise only one, preferably single, cone or other sound reproducing diaphragm. Preferably, the second unit comprises a horn, a dome and/or a cone or other sound reproducing diaphragm.

Preferably, the first waveguide comprises a mouth located at a junction with the second waveguide; a throat located acoustically upstream; and a passage extending between the mouth and the throat.

Preferably, the third waveguide is arranged to extend substantially, and preferably directly, in prolongation of at least a portion of the first unit, preferably in a region where no second waveguide is provided. Preferably, the third waveguide is arranged to extend substantially, and preferably directly, in prolongation of a portion of the first unit and of the second waveguide.

Preferably, the third waveguide comprises a convex-shaped lip, preferably wherein the convex-shaped lip overhangs directly downstream of (preferably "above") the first unit.

In order not to hinder the acoustic performance of the first unit, preferably the third waveguide includes at least one aperture for allowing sound from the first unit to pass through the third waveguide.

Preferably, the at least one aperture is arranged directly downstream of the first unit, and more preferably wherein every aperture is arranged directly downstream of the first unit.

Preferably, the third waveguide extends, in the downstream direction, laterally away from the second unit.

Preferably, the third waveguide further comprises a peak at a most downstream point of the third waveguide. Preferably, the third waveguide further comprises a trough extending in an upstream direction, laterally away from the peak and from the second waveguide. Preferably, the peak is the most downstream point of the third waveguide that is local to or most proximate to the second unit. Preferably, the trough has a concave profile. Preferably, the third waveguide has a substantially sinusoidal profile.

Preferably, the third waveguide is provided with a smooth and continuous surface, preferably extending at least from a junction between the third waveguide and the second waveguide and extending to the peak.

The at least one aperture may be provided on the third waveguide laterally away from the peak and the second unit, and preferably the at least one aperture is only provided on the third waveguide laterally away from the peak and the second unit.

Optionally, the at least one aperture is provided on the trough, and preferably only on the trough. Preferably, the third waveguide and the second waveguide are separated. Preferably, the third waveguide is arranged such that the third waveguide does not prevent travel of the first unit and/or the second waveguide, preferably by being separated from one another by an appropriate distance.

Preferably, the third waveguide is separated from the second waveguide by a constant distance, preferably across their entire junction.

Preferably, the second waveguide is arranged as a portion of a truncated conical surface. Preferably, there is provided a hollow region between the cone and the second waveguide, preferably wherein the hollow region is entirely enclosed.

Preferably, there is provided a cavity in a region bound by the first unit (and preferably, specifically the cone), the second waveguide and the third waveguide.

Preferably, the third waveguide comprises a flange arranged opposite the second waveguide, thereby arranged to provide an acoustic duct connecting the cavity with a region outside of the cavity and preferably downstream of the second waveguide.

The flange may extend towards the cone and the flange may extend parallel to the second waveguide. Preferably, the acoustic duct (also preferably referred to as an "acoustic duct" or "acoustic channel") is arranged to reduce resonance within the cavity.

Optionally, the second waveguide is less dense than the moving parts (preferably the cone) of the first unit, preferably wherein the second waveguide and/or the first unit is formed from paper, fibreglass, fabric and/or composite materials.

Suitably, the third waveguide is denser than the second waveguide. Preferably there is provided at least two: first waveguides; second waveguides; and/or third waveguides. Optionally, the at least two first waveguides, the at least two second waveguides and/or the at least two third waveguides are arranged on (preferably, meaning "aligned in") a plane that bisects the first unit.

Preferably, the at least two second waveguides and the at least two third waveguides are arranged on opposite lateral sides of the second unit. Optionally, the first waveguide, second waveguide and/or third waveguide are axially asymmetric.

The first waveguide, second waveguide and/or third waveguide may be shaped to produce differential acoustic dispersion. The second unit may comprise an effective mouth provided by a lateral distance between the peaks of the static waveguide.

Preferably, the first unit, and more preferably the cone, is the most downstream sound reproducing member of the apparatus. Preferably, the second unit is the most upstream sound reproducing member of the apparatus.

Preferably, sound reproducing members of the apparatus consist only of the first and second units, for example there are provided only two sound reproducing diaphragm (one diaphragm forming - preferably, part of - the first unit, and the other forming - preferably, part of - the second unit).

Preferably, the first unit is anchored to a part of the apparatus other than the third waveguide, and preferably other than the first waveguide also.

Preferably, the most proximate parts of the first and third waveguide are laterally set apart from one another, preferably by a distance that is at least equal to the lateral extent of the second waveguide.

According to another aspect of the invention, there is provided a loudspeaker incorporating the coaxial loudspeaker apparatus as described above, preferably, wherein the loudspeaker is arranged as a monitor speaker.

According to another aspect of the invention, there is provided a waveguide member for a coaxial loudspeaker comprising a first waveguide and a second waveguide extending substantially in prolongation of the first waveguide, said waveguide member being arranged to extend substantially in prolongation of the second waveguide preferably the waveguide member being the third waveguide described above.

Preferably, the waveguide member is arranged to be fixed relative to the second waveguide, preferably wherein the waveguide member is fixed directly to the coaxial loudspeaker.

The waveguide member may include at least one aperture for allowing sound from the coaxial loudspeaker to pass through the third waveguide. The waveguide member may comprise a peak at a most downstream point of the third waveguide; and a trough at a recession, in an upstream direction, laterally away from the peak and from the second waveguide.

Optionally, the waveguide member is provided in two separate parts, preferably wherein the parts are identical. Suitably, the waveguide member is provided with a flange for forming an acoustic channel with the second waveguide when the waveguide member is in situ such that it extends substantially in prolongation of the second waveguide.

Further features of the invention are characterised by the dependent claims. The invention extends to the apparatus as herein described and/or as illustrated with reference to the accompanying drawings.

In this specification the word 'or' can be interpreted in the exclusive or inclusive sense unless stated otherwise. Furthermore, features implemented in hardware may generally be implemented in software, and vice versa. Any reference to software and hardware features herein should be construed accordingly.

The invention extends to a coaxial loudspeaker apparatus as described herein and/or substantially as illustrated with reference to the accompanying drawings.

The present invention is now described, purely by way of example, with reference to the accompanying diagrammatic drawings, in which:-.

<FIG> show various views and examples of a coaxial loudspeaker <NUM>.

In more detail, <FIG> shows a front view of the coaxial loudspeaker <NUM> comprising: a low/mid-frequency unit in the form of a low/mid-frequency cone <NUM> or diaphragm; a high-frequency unit comprising a fixed waveguide <NUM> coaxial to the low/mid-frequency cone <NUM>; and a moving waveguide <NUM>.

Coaxial loudspeakers comprising a moving waveguide associated with a low/mid-frequency cone and a fixed waveguide associated with a high-frequency unit are described in the PCT application <CIT>.

A principal acoustic downstream direction <NUM> is shown in <FIG>, and this term is used throughout preferably to refer to a direction in which sound propagates away from the front of the coaxial loudspeaker <NUM>, wherein the axis of the downstream direction <NUM> is coaxial to the low/mid-frequency cone <NUM> (herein referred to as the "cone" <NUM>) and to the high frequency fixed waveguide. The term "upstream direction" as used herein preferably opposes the downstream direction <NUM>. The term "off-axis" preferably refers to points that are perpendicularly offset from the axis of the downstream direction <NUM>.

The term "lateral direction" is preferably applied to connote a direction perpendicular to the axis of the downstream direction <NUM>.

In the examples shown throughout <FIG>, the fixed waveguide <NUM> of the high frequency unit is in the form of a fixed high frequency horn adapted to propagate pressure waves in the form of sound in a higher frequency range than the low/mid-frequency unit <NUM>.

The fixed waveguide <NUM> extends from a throat <NUM> (as shown in <FIG>), via a passage <NUM>, to a mouth <NUM>.

The fixed waveguide <NUM> therefore defines a passage for channelling sound (in particular, sound in a higher frequency range than the sound reproduced by the cone <NUM>, for example at <NUM>-<NUM>, and more preferably at <NUM>-<NUM>). The fixed waveguide <NUM> is shown as a differential acoustic dispersion horn and is composed of fibreglass, plastic or aluminium.

In more detail, the horn mouth <NUM> of the fixed waveguide <NUM> interfaces with two moving waveguides <NUM> at a junction <NUM>, such that when the cone <NUM> is at rest there is substantially no discontinuity between the surface of the moving waveguide <NUM> and the fixed waveguide <NUM>. However, in use, when the cone <NUM> is vibrating the moving waveguide <NUM> moves together with the cone <NUM> whilst the fixed waveguide <NUM> remains substantially at rest (as further described with reference to <FIG>). The motion of the moving waveguide <NUM> and cone <NUM> is achieved by coupling the moving waveguide <NUM> to the cone <NUM> or by forming the moving waveguide <NUM> into, or integrally with, the cone <NUM>. The moving waveguide <NUM> can therefore be considered to be a 'moving' waveguide as, in use, it is non-static.

The coaxial loudspeaker <NUM> further comprises a static waveguide <NUM>, which is stationary, such that the cone <NUM> and the moving waveguide <NUM> move relative to the static waveguide, as is also the case with the fixed waveguide <NUM>. In operation, the static waveguide <NUM> and the fixed waveguide <NUM> do not therefore move relative to one another. The static waveguide <NUM> is arranged coaxial to the fixed waveguide <NUM> and moving waveguide <NUM>.

The moving waveguide <NUM> is bonded to the conic surface of the cone <NUM> and the voice-coil former <NUM> and is configured effectively to modify the shape of the frontal face of the cone <NUM>, at least, to improve the output of the high frequency output of the horn. Likewise, the static waveguide <NUM> is provided for the same reason.

The fixed waveguide <NUM>, moving waveguide <NUM> and static waveguide <NUM> form a continuation of the horn resulting in a larger effective horn <NUM> (albeit in three separate, but complementary, parts) having an effective mouth <NUM> that is defined by the lateral distance between the most downstream points of the static waveguide. The size of the effective horn <NUM>, both in terms of its extent downstream and its lateral extent, is therefore greater than where only a fixed waveguide <NUM> and a moving waveguide <NUM> is provided.

In order to continue the first unit (specifically, for example, the cone <NUM>, which forms part of - or is - the first unit), the static waveguide <NUM> is arranged to extend the shape of the moving waveguide <NUM>, which is in turn is arranged to extend the shape of the fixed waveguide <NUM>.

In order to prevent excessive acoustic reflection between the cone <NUM> and the static waveguide <NUM>, which would greatly interfere with the low frequency output, it is desirable to minimise the extent to which the static waveguide <NUM> occludes the cone <NUM>. That is, it is preferable to minimise the extent to which the static waveguide extends over and/or into the volume bound by the cone <NUM>. As a result, the junction <NUM> between the static waveguide <NUM> and the moving waveguide <NUM> is arranged above, but close to, the outer suspension <NUM> (preferably also referred to as a "surround") of the cone <NUM>. At the same time, the static waveguide <NUM> is arranged not to interfere with the travel of the cone <NUM>.

Simultaneously, in order to prevent excessive acoustic reflection between the moving waveguide <NUM> and the static waveguide <NUM>, which would greatly interfere with the high frequency output, it is desirable to minimise the distance between these waveguides at junction <NUM>.

In order for the static waveguide <NUM> to remain stationary relative to the cone <NUM> and to the moving waveguide <NUM>, it is fixed to the cabinet <NUM> within which the coaxial loudspeaker <NUM> is arranged. The static waveguide can be removed.

<FIG> shows a cross-section of the coaxial loudspeaker <NUM> along the line "A" indicated in <FIG>. The magnet assembly is not shown for conciseness.

At the junction of the fixed waveguide and the moving waveguide <NUM>, the gradients of each waveguide are substantially identical, as demonstrated by the tangent coincident upon the terminal ends of both the fixed waveguide <NUM> and the moving waveguide <NUM>, where the angle of the tangent is α.

At the junction of the moving waveguide and the static waveguide <NUM>, the gradients of each waveguide are also substantially identical, as demonstrated by the tangent coincident upon the terminal ends of both the moving waveguide <NUM> and the static waveguide <NUM>, where the angle of the tangent is β. In the example shown, β>α in order to provide an extended horn that increases non-linearly in width in the downstream direction <NUM>.

The moving waveguide <NUM> is designed to minimise its effect on the operation of the cone <NUM>, such as the desirable cone break-up (effectively de-coupling the outer area of the cone <NUM> from the central area which reduces the piston diameter at high frequencies and increases the beamwidth of the output audio, compared to a rigid piston cone of the same size as the cone <NUM>).

To minimise detriment to the oscillation of the cone <NUM>, the moving waveguide <NUM> is made as lightweight as possible. Moving waveguides that are too massive, although beneficial to the high frequency output, interfere with the operation of the cone. As a result, the effective extension to the horn, by means of the presence of the moving waveguide, is limited by the desire to balance enlargement of the horn and unimpeded operation of the cone.

The static waveguide <NUM> comprises a lateral supporting rib <NUM>.

<FIG> shows a cross a cross-section of the coaxial loudspeaker <NUM> along a line laterally offset from the cross section line "A"; in this view, the supporting rib <NUM> is no longer visible.

<FIG> shows the loudspeaker apparatus <NUM> in the cross-sectional view shown in <FIG> when the cone <NUM> is in a state where it is being driven by the low/mid-frequency voice coil <NUM>. The cone <NUM> is therefore shifted in the downstream direction <NUM> relative to its rest position <NUM>, as the moving waveguide <NUM> is free to move relative to the fixed waveguide <NUM>.

When displaced in the downstream direction <NUM>, the moving waveguide <NUM> approaches, but stays clear of, the static waveguide <NUM>; this is in order not to affect negatively the cone's freedom to travel, which is achieved by providing a suitable clearance between the static waveguide and the moving waveguide. Even at maximal downstream displacement, the motion of the moving waveguide is not blocked by the static waveguide. For example, the clearance is preferably <NUM> to <NUM>, and more preferably <NUM> to <NUM>, at rest.

The shape of the moving waveguide <NUM> is based on a truncated conical surface. The moving waveguide is hollow; a cavity <NUM> is therefore present between the cone <NUM> and the moving waveguide <NUM> (as best seen in <FIG>).

<FIG> shows a coaxial speaker <NUM> without a static waveguide <NUM>, but only comprising the moving waveguide <NUM> and the fixed waveguide <NUM>.

A hollow region - cavity <NUM> - is provided, as defined by the region bound by the static waveguide <NUM>, the moving waveguide <NUM> and the cone <NUM>, as best seen in <FIG>.

As best seen in <FIG>, <FIG> and <FIG>, the static waveguide <NUM> is arranged to mimic the lateral curvature of the moving waveguide <NUM> at its junction <NUM> with the moving waveguide.

There are provided, as part of the coaxial loudspeaker <NUM>, two static waveguides <NUM>, two moving waveguides <NUM>, and two fixed waveguides <NUM>.

In regions where there is no moving waveguide provided on the cone <NUM>, the static waveguide <NUM> is instead shaped to continue the curvature of the cone, thereby enlarging the cone in order to improve its efficiency.

In order to improve acoustic output, in particular in order to output a more consistent frequency response and SPL (Sound Pressure Level) over a given space, the coaxial loudspeaker <NUM> is arranged to provide differential dispersion.

The differential dispersion of sound is provided selectively to modify, in the downstream direction <NUM>, the lateral throw pattern from the coaxial loudspeaker. In particular, the fixed waveguide <NUM>, moving waveguide <NUM> and static waveguide <NUM> are shaped to effect differential dispersion of sound from the horn by modifying the sound propagating surfaces.

In order to provide differential dispersion, the coaxial loudspeaker <NUM> is provided with axially asymmetric (about the downstream axis <NUM>) waveguides.

In more detail, for example the fixed waveguide <NUM> and the moving waveguide <NUM> are lobed <NUM> (as best seen in <FIG>). Relative to the circular base of the cone <NUM> (e.g. which covers the area bound by the downstream terminus of the cone), the static waveguide <NUM>, the fixed waveguide <NUM> and the moving waveguide <NUM> cover an angular region of the imaginary circular base of less than <NUM>°, and preferably between <NUM>° to <NUM>°, and more preferably <NUM>° to <NUM>°, that is the waveguides do not cover the entire face of the cone <NUM>.

In this way, the present loudspeaker arrangement reduces high frequency beaming (where high frequency dispersion reduces as the frequency increases) and provides a - desirably - narrow beamwidth across the whole output of the high frequency unit. Compared to conventional coaxial speakers, the provision of an enlarged effective horn having differential dispersion as described above provides, not only a more efficient horn, but also more spectrally consistent high frequency dispersion across a broader frequency range, in particular towards the lower end of the acoustic spectrum. That is, the static waveguide <NUM> extends the bandwidth of directivity control of the coaxial loudspeaker <NUM> to a lower frequency.

For example, effective differential dispersion down to between <NUM>, or even <NUM>, is achieved by means of the coaxial loudspeaker <NUM>.

In the examples shown in <FIG>, the fixed <NUM>, moving <NUM> and static <NUM> waveguides are shaped in order to produce a substantially uniform rectangular coverage pattern by outputting wide lateral (preferably horizontally) coverage close to the loudspeaker. In this way, the consistency in frequency response and SPL - both close to the loudspeaker and further away - is more uniform than that provided by conventional loudspeakers. The improved coverage as a result of differential dispersion also means that such loudspeakers provide improved coverage using fewer loudspeakers.

As a result, it is desirable to provide as large a static waveguide <NUM> as possible, but without impeding the cone <NUM>, in order to maximise the size of the effective horn mouth <NUM>. To this end, the static waveguide is available to extend from the moving waveguide to any size in the lateral and/or downstream directions.

In order to minimise the effects of resonance within the cavity <NUM>, the static waveguide <NUM> is provided with a flange <NUM> proximate to its junction with the moving waveguide.

The flange <NUM> complies, in the lateral and downstream directions, with the shape of the static waveguide <NUM>. The flange <NUM> extends towards the cone, parallel to the moving waveguide <NUM> thereby forming, at the junction <NUM> of the static waveguide and the moving waveguide, an acoustic duct (preferably, also referred to as an "acoustic channel"). The acoustic duct reduces the effect of the cavity resonance in the cavity <NUM>, and as a result the consistency of the SPL of the high frequency output of the coaxial drive unit is maintained.

In another example, as shown in <FIG>, the static waveguide <NUM> comprises, beyond its most downstream point <NUM> - its peak - a drop-off, in which the static waveguide <NUM> recedes upstream with increasing lateral distance from the cone <NUM>. A substantially concave-shaped trough <NUM> is formed beyond the peak <NUM>. Overall, the shape of the static waveguide <NUM> is therefore substantially sinusoidal.

The trough <NUM> is provided in order further to improve acoustic dispersion, particularly at high frequencies, by providing a beneficial diffraction effect and/or by delaying high frequency wave propagation relative to low frequency wave propagation, which prevents excessively wide throw of high-frequency components.

The acoustic effect of the trough <NUM> is described in more detail with reference to <FIG>.

As shown in <FIG> and <FIG>, the static waveguide <NUM> overhangs directly above the cone <NUM>.

In order to minimise acoustic occlusion of the cone <NUM>, the static waveguide <NUM> is provided with apertures <NUM> through which sound from the cone <NUM> is able to pass. The apertures <NUM> are arranged directly downstream of the cone <NUM> and outer suspension <NUM>, and primarily adjacent the periphery of the cone.

In another example, as shown in <FIG>, but unlike <FIG>, the apertures are arranged only beyond (laterally) the peak <NUM> of the static waveguide <NUM>.

Since, the static waveguide <NUM> is arranged to improve high frequency output from the horn, in this example the static waveguide is desirably provided with a continuous surface <NUM> - that is, without apertures - in order to guide high frequency output.

Apertures <NUM> are therefore instead provided on a surface of the static waveguide that is not arranged directly to guide high-frequency output, that is beyond the peak <NUM> of the static waveguide <NUM>, such as within or around the trough <NUM>.

For optimal packing density, the apertures <NUM> are hexagonal-shaped and dimensioned not to impede low frequency output.

<FIG> and <FIG> show an example of the coaxial loudspeaker <NUM> in which the static waveguide <NUM> is provided without a trough (such that the entirety of the static waveguide <NUM> continues the shape of the moving waveguide). The exemplary coaxial loudspeaker shown in <FIG> is provided with apertures <NUM>.

<FIG> show the coaxial loudspeaker <NUM> applied to a monitor (or a foldback) loudspeaker. The diameter of the cone <NUM> is preferably <NUM> to <NUM>, and more preferably <NUM> to <NUM>.

<FIG> show a preferred form of static waveguide <NUM>, in isolation and as applied to the cone <NUM>.

The static waveguide <NUM> is formed from a solid core, for example injection moulded foam that is clad such that it is non-resonant and does not act as a physical low pass filter.

<FIG> show perspective underside views of the coaxial loudspeaker <NUM>.

In <FIG>, the cone <NUM> has been removed in order to aid understanding of the cavernous shapes of the moving waveguide <NUM> and the static waveguide <NUM>. The supporting rib <NUM> of the static waveguide is also shown.

<FIG> shows the coaxial loudspeaker from the same view as in <FIG>, but with the addition of the cone <NUM>. The fixed waveguide <NUM> and static waveguide <NUM> are also shown. The static waveguide <NUM> is provided as two separate, but identical, parts.

In more detail, <FIG> and <FIG> show the acoustic effect of providing the static waveguide with a trough <NUM>.

With reference to <FIG>, the plots show spatial SPL with frequency from a coaxial loudspeaker <NUM> without a trough, but all remaining factors being the same; in this case at mid-frequencies, in particular in the region of <NUM> to <NUM>, a spatially wide (laterally) acoustic pattern is thrown.

With reference to <FIG>, the plots show spatial SPL with frequency for a coaxial loudspeaker <NUM> as described with reference to, and as shown in, <FIG> and <FIG>, that is, in which a trough <NUM> is provided. The presence of the troughs facilitates, at a mid-frequency range of <NUM> to <NUM>, an improved lateral distribution of SPL, that is, more regular distribution of sound is thrown at the - crucial - mid-frequencies.

In yet more detail, <FIG> shows further spatial SPL plot with frequency, in which the horizontal axis indicates lateral distance (in one direction), in the listening plane, from the centre of a coaxial loudspeaker. The vertical axis indicates downstream distance. Each change of shade from white to black represents a 3dB reduction in SPL. Each increment represents <NUM>. The SPL plots show half of the lateral listening plane, the other half is expected to be substantially symmetrical.

<FIG> shows output from a coaxial loudspeaker that comprises no static waveguide, but that includes fixed and moving waveguides, for example as per the coaxial speaker described with reference to <FIG>. In this example, the directivity control reduces (coverage width increases) at <NUM> and ceases below <NUM>.

<FIG> shows output from a coaxial loudspeaker that comprises static, moving and fixed waveguides, but in which the static waveguide does not comprise a trough, for example as per the coaxial speaker described with reference to <FIG> and <FIG>. In this example, the directivity control is extended down to <NUM>, but there is some widening of the coverage area at <NUM> and <NUM> compared to <NUM> as shown in <FIG>.

<FIG> shows the output from a coaxial loudspeaker that comprises a static waveguide having a trough <NUM>, and also comprising moving and fixed waveguides, for example as per the coaxial speaker described with reference to <FIG> and <FIG>. In this example, the directivity control is extended down to <NUM> and the width of the coverage area is more consistent compared with the higher frequencies than the outputs shown in <FIG>.

In an alternative example, the static waveguide is provided as a separate and standalone structure that can be arranged to extend the moving waveguide without coupling to the loudspeaker cabinet.

Although the coaxial loudspeaker arrangement is shown as applied to a monitor loudspeaker (also referred to as a foldback loudspeaker) in <FIG>, this arrangement is available to be applied to any form of coaxial loudspeaker.

In one alternative, the shape of the waveguides is determined by extrapolating the curvature of the immediately preceding (in the direction towards the horn) waveguide.

The loudspeaker apparatus <NUM> is arranged to affect the output of the loudspeaker apparatus <NUM> differentially across the output frequency spectrum (for example, such that a non-axisymmetric high frequency coverage pattern is output) and/or the output SPL with position relative to an axis perpendicular to the downstream direction <NUM>. In one example, the shape of the static waveguide <NUM>, moving waveguide <NUM> and/or the fixed waveguide <NUM> is adapted to achieve any desired manipulation of sound from the cone and/or high frequency unit, and so take any suitable form suitable for this purpose. For example, the fixed waveguide <NUM> is a differential acoustic dispersion horn. Other types of horn, such as, but not limited to, constant directivity, diffraction slot horns, multicell, radial, sectoral, bi-radial and twin Bessel horns are also used. The geometry of the cone <NUM> takes the form of a straight and/or curved, e.g. convex, (truncated) cone.

In one example, the high frequency unit does not comprise a compression driver, but a convex dome (or a direct radiating dome) instead, preferably with a suitable phase corrector (also known as a phase plug) mounted into a fixed horn.

As shown in <FIG>, the moving waveguide <NUM> forms a pocket <NUM> with the cone, rather than an enclosed cavity (as per <FIG>).

It will be understood that the present invention has been described above purely by way of example, and modifications of detail can be made within the scope of the invention as defined by the appended claims.

Claim 1:
A coaxial loudspeaker apparatus comprising:
a first unit, comprising a sound reproducing diaphragm, being arranged to propagate sound in a first frequency range;
a second unit, comprising a driver unit that is arranged upstream of said sound reproducing diaphragm, being arranged to propagate sound in a second frequency range that is higher than the first frequency range, further comprising a first waveguide;
a second waveguide arranged to extend substantially in prolongation of the first waveguide (<NUM>); and
a third waveguide (<NUM>) arranged to extend substantially in prolongation of the second waveguide (<NUM>);
wherein the second waveguide is arranged to move relative to the first waveguide and relative to the third waveguide; and
wherein the second waveguide is arranged to move in unison with the sound reproducing diaphragm of the first unit in operation.