Loudspeakers

Abstract

A loudspeaker comprising: an acoustic diaphragm having front and rear surfaces, the acoustic diaphragm in use being driven so as to vibrate and radiate acoustic waves from its front surface in a forward direction away from the loudspeaker and from its rear surface in a rearward direction, and a drive unit located rearwardly or to the front/outside of the diaphragm, there being at least one open duct leading in a rearward direction away from the diaphragm, in which the at least one open duct has a cross-sectional area which decreases in the rearward direction, and in which acoustic waves radiated from the rear surface of the diaphragm pass through the open duct before contacting a front surface of an acoustic metamaterial absorber located generally behind the drive unit and immediately to the rear of the duct.

Claims

1. A loudspeaker comprising: i. an acoustic diaphragm having front and rear surfaces, the acoustic diaphragm in use being driven so as to vibrate and radiate acoustic waves from its front surface in a forward direction away from the loudspeaker and from its rear surface in a rearward direction, and ii. a drive unit, and iii. at least one open duct leading through the drive unit in a rearward direction away from the diaphragm and having an opening at its rearward end, in which the at least one open duct has a cross-sectional area extending in the rearward direction, in which the cross-sectional area decreases along at least part of the rearward direction, and in which acoustic waves radiated from the rear surface of the diaphragm pass through substantially all of the open duct before contacting a front surface of an acoustic metamaterial absorber located generally outside and to the rear of the duct, and immediately to the rear of the decreasing cross-sectional area, and in which at least a part of the or each open duct tapers conically towards the front surface of the acoustic metamaterial absorber.

2. The loudspeaker according to claim 1 in which the front surface of the acoustic metamaterial absorber is located at the opening at the rearward end of the or each open duct.

3. The loudspeaker according to claim 1 in which the metamaterial behind the opening at the rearward end of the or each open duct has a size perpendicular to the front-rear direction, greater than the size of the opening at the rearward end of the or each open duct.

4. The loudspeaker according to claim 1, wherein the length of the metamaterial in the front-rear direction is less than its size perpendicular to the front-rear direction.

5. The loudspeaker according to claim 1 in which the metamaterial comprises a plurality of narrow channels adapted to dissipate acoustic energy, and in which at least a part of each channel perpendicularly away from the opening at the rearward end of the or each open duct is aligned perpendicularly to the front-rear direction.

6. The loudspeaker according to claim 1 in which the cross-sectional area of the or each open duct tapers or decreases linearly in a rearward direction to the opening at its rearward end.

7. The loudspeaker according to claim 1 in which the drive unit and the at least one open duct extend in a rearward direction, away from the diaphragm, the front surface of the acoustic metamaterial absorber being located generally to the rear of the drive unit.

8. The loudspeaker according to claim 1 in which the acoustic impedance of the acoustic metamaterial absorber substantially matches the characteristic acoustic impedance of acoustic waves radiated from the rear surface of the diaphragm at the point they contact the surface of the acoustic metamaterial absorber.

9. The loudspeaker according to claim 1 in which at least a part of the or each open duct has walls which taper inwardly in a curve towards the front surface of the acoustic metamaterial absorber.

10. The loudspeaker according to claim 1 comprising a plurality of open ducts, in which each duct leads to a separate acoustic metamaterial absorber.

11. The loudspeaker according to claim 1 in which the or each open duct has a constant cross-sectional shape.

12. The loudspeaker according to claim 1 in which the at least one open duct comprises an annular duct.

13. The loudspeaker according to claim 1 in which the or each open duct contains sound absorbent material.

14. A method of designing a loudspeaker comprising an acoustic diaphragm having front and rear surfaces, the acoustic diaphragm in use being driven so as to vibrate and radiate acoustic waves from its front surface in a forward direction away from the loudspeaker and from its rear surface in a rearward direction, a drive unit, and at least one open duct leading through the drive unit in a rearward direction away from the diaphragm and having an opening at its rearward end in which the at least one open duct has a cross-sectional area extending in the rearward direction, in which the cross-sectional area decreases conically along at least part of the rearward direction, and in which acoustic waves radiated from the rear surface of the diaphragm pass through substantially all of the open duct before contacting a front surface of an acoustic metamaterial absorber located generally outside and to the rear of the duct, and immediately to the rear of the decreasing cross-sectional area in which one or more of the length, one or both end areas, the resonance frequency and the resonant strengths of the or each open duct are adjusted so as to allow the acoustic impedance of the acoustic metamaterial absorber substantially to match the characteristic acoustic impedance of acoustic waves radiated from the rear surface of the diaphragm at the point they contact the surface of the acoustic metamaterial absorber.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The invention will now be described by way of example and with reference to the accompanying figures, in which;

(2) FIGS. 1a and 1b show in cross-section prior art high frequency drivers from coaxial drivers;

(3) FIG. 2 shows an embodiment of a loudspeaker arrangement in accordance with the invention, and

(4) FIG. 3 shows a schematic view in cross-section of another loudspeaker arrangement in accordance with the invention.

DETAILED DESCRIPTION OF THE EMBODIMENTS

(5) FIG. 1a shows a high-frequency driver with a 25.4 mm diameter diaphragm using a large central vent tube/duct filled with dense acoustical wadding. FIG. 1b shows a high-frequency driver with a 25.4 mm diameter diaphragm using a 120 mm long, exponentially tapering duct which is also filled with dense acoustical wadding.

(6) FIG. 2 shows in cross-section a tweeter 20 forming part of a coaxial driver with a highly effective arrangement according to this invention. The conical duct 24 through the drive unit 26 connecting the 25.4 mm diameter diaphragm 10 to the front surface 30 of the acoustic metamaterial 28 results in a spherical contracting acoustical wave with radius 146.4 mm at the front surface 30 of the metamaterial 28. The characteristic acoustical impedance of this wave is a close match to the impedance of the metamaterial described in WO 2018/047153 when a design frequency of 600 Hz is used. The impedance match in this case is not perfect and only over a limited bandwidth but it is enough that the reflection issue is almost totally solved to the extent that it is not a limiting factor in the tweeter performance.

(7) A tapering duct is also very practical for a number of reasons: 1. Commonly dome-shaped diaphragms are used on high-frequency units and the concave side tends to radiate the rear wave. This type of diaphragm can be made to generate a close to ideal spherical wave over a wide bandwidth when connected to an appropriate tapering duct (see for example U.S. Pat. No. 8,094,854B2) 2. The required entrance area of the metamaterial absorber is reduced by the tapered duct and this reduces the size of the metamaterial absorber fairly substantially. 3. The tapered duct occupies less space than a straight duct and makes it easier to accommodate this into a loudspeaker design where other parts are competing for space.

(8) A conical duct is a good choice since it carries a spherical acoustic wave in a single parameter fashion, and consequently there is no diffraction and minimal reflection as the wave propagates in the duct. Other tapered ducts with curved walls could equally be used and provided the radius of the acoustical wave where the duct joins the metamaterial is correct an impedance miss-match could be largely avoided; this can be achieved by ensuring that the cross-sectional area of the duct decreases linearly in the direction of the metamaterial, particularly as the duct approaches the front surface of the metamaterial. In some cases such an arrangement may give preferable results or a more practical geometry; for example, the part of the duct immediately behind the diaphragm could be enlarged so as to provide an acoustic volume before the duct begins to taper.

(9) FIG. 2 shows that the metamaterial 28 not only extends axially in a rearward direction (to the left as shown) behind the duct 24, but also that it extends radially from the XX axis to a substantially greater extent than the radius of the conical duct 24. For the metamaterial 28 shown to be most effective, the narrow acoustic channels (not shown) forming the metamaterial have at least a part of their lengths oriented radially (or with a substantially radial component); this allows the axial dimensions of the loudspeaker to be kept small. As in WO 2018/047153, the radial parts of the channels may be folded, so as to incorporate channels of greater overall length within a short axial distance.

(10) The metamaterial 28 is shown as having a front surface 30 which extends across the open rear end of the duct 24; this front surface may be formed by the ends of the structural walls which form the narrow channels, so that there is a physical, albeit discontinuous, surface extending across the open end of the duct 24. Alternatively, and so as to facilitate the directing of acoustic waves along radially-directed channels, there may be a concavity, or “interface volume”, (i.e. an empty volume—not shown, but extending to the left of the right hand broken vertical line in the drawing) at the front of the metamaterial where it meets the rear end of the open duct 24; the inner surface of this interface volume is shaped to have at least a part facing outwardly radially or substantially radially so as to direct acoustic waves in or approaching a radial direction. The interface volume could for example, be part spherical, domed or even cylindrical (provided that there is always at least a solid rear boundary 31 to the metamaterial 28 (at the left hand broken vertical line in the drawing); the significant design element of this interface volume is that its impedance matches the end of the conical duct. Accordingly, it should be understood that reference herein to the “front surface” of the metamaterial embraces not only cases where there is a physical albeit discontinuous surface of metamaterial structure extending radially across the open rear end of the duct 24, but also where there is only a virtual surface extending radially across the open rear end of the duct 24 (i.e. where there is an interface volume within that part of the metamaterial immediately adjacent the open rear end of the duct 24). Where there is such an interface volume and only a virtual front surface to the metamaterial adjacent the duct, the front surface of the metamaterial outside the interface volume/the open rear end of the duct seals against the rear structure of the tweeter 20 as shown to prevent acoustic energy from travelling other than through the narrow channels—to be dissipated therein.

(11) In FIG. 3 an alternative loudspeaker arrangement 320 is shown which is in accordance with the invention, in which the drive unit 326 is located forwardly and radially outside the diaphragm 310. The diaphragm 310 is curved in the opposite direction to that shown in FIG. 2, so that its concave surface radiates sound in the direction of arrow A towards the listener, this sound passing through passages in a phase plug 336, leaving the driver opening 334 and passing through acoustic horn 332. In this arrangement, the duct 324 extending rearwardly of the diaphragm 310 is initially curved in profile, and initially it enlarges in cross-sectional area, before curving inwardly and tapering towards the metamaterial 328, the front surface of which 330 is located at the end of the duct 324. There is a plug 340 located inside the duct 324 and having an outer profile which is curved so as to interact with the curved walls 342 of the duct 324 so that the cross-sectional area of the open part of the duct (i.e. the area between the walls 342 and the plug 340) decreases linearly along the XX axis (and the duct in the axial distance between the plug 340 and the front surface 330 of the metamaterial 328 is conical); this arrangement means that the open duct 324 shown in FIG. 3 is effectively “conical” along most of its axial length.

(12) As in the arrangement of FIG. 2, in FIG. 3 the metamaterial 328 has narrow acoustic channels (not shown) forming the metamaterial which have at least a part of their lengths oriented radially (or with a substantially radial component), and/or they may comprise an interface volume as described above.

(13) It will of course be understood that many variations may be made to the above-described embodiment without departing from the scope of the present invention. For example, the embodiment above is described as having one or more circular, conical ducts; however, the invention applies equally to non-circular arrangements, such as oval, elliptical or race track shaped (figure of eight, or triangular/square/polygonal with rounded corners), or any shape being symmetrical in one or two orthogonal directions lying in the general plane perpendicular to the front-rear axis A, as well as combinations of such arrangements and/or shapes. The duct(s) may be conical, with straight walls, or the walls may be curved (e.g. exponentially, elliptical, hyperbolic or parabolic). Conical ducts may be right cones or oblique cones. There may be an annular arrangement of several ducts, which may be parallel, or arranged as a tapering or an enlarging right cone or oblique cone. Where several ducts are provided, there may be separate and/or different acoustic metamaterials provided at the rear end of each different duct. The metamaterial could intrude into a duct, such that the front surface of the metamaterial extends forwardly inside the duct, a short distance forward of its rearward end; this might be for acoustic reasons, or to help accurately locate the metamaterial relative to the duct (such as where there are multiple ducts, the metamaterial might be shaped with protrusions to engage with the rearward ends of some or all of the ducts. Different types of metamaterials may be combined in an embodiment, and the multiple elements forming the metamaterial may repeat or they may be different in shape, dimension or structure. In the drawn embodiment of FIG. 2 there is an empty volume between the rear of the diaphragm 10 and the front surface of the metamaterial; this volume is formed from the volume of the conical duct 24 through the drive unit 26 and from the acoustic volume behind the diaphragm 10. In some embodiments it might be beneficial to enlarge the size of the empty volume, such as by increasing the size of the volume behind the diaphragm, and/or by enlarging the initial part of the tapering duct, as in FIG. 3. It may be that the initial part of the duct, immediately behind the diaphragm, increases in cross-sectional area for a short rearward direction before the duct reduces in cross-sectional area for the remainder of the rearward direction towards the metamaterial. The or each tapering duct may comprise portions which taper conically in combination with portions which taper in a curved profile, provided that the tapering of the duct in the vicinity of the front surface of the metamaterial is conical as described above.

(14) Where different variations or alternative arrangements are described above, it should be understood that embodiments of the invention may incorporate such variations and/or alternatives in any suitable combination.