tweeter

Abstract

There is provided a tweeter comprising: a chassis; an annular diaphragm having an outside edge, an inside edge, a front face which faces in a forward direction and a rear face which faces in a rearward direction; a drive unit configured to move the diaphragm along a movement axis in the forward direction and in the rearward direction, wherein the drive unit is located rearwards of the annular diaphragm; an outer suspension attached to the outside edge of the annular diaphragm and attached to the chassis to suspend the annular diaphragm; an inner suspension attached to the inside edge of the annular diaphragm and attached to the chassis to suspend the annular diaphragm; wherein the chassis includes a port which forms a port passage, wherein the inside edge of the annular diaphragm extends around the port passage and the port passage connects an internal space rearwards of the annular diaphragm with an external space forwards of the annular diaphragm; and wherein the drive unit is operable to move the annular diaphragm so as to: cause the front face of the annular diaphragm to produce sound which is emitted from the tweeter in the forward direction, and cause the rear face of the annular diaphragm to produce sound which is emitted from the tweeter in the forward direction via the port passage.

Description

[0001] 008860157

[0002] 1

[0003] TWEETER

[0004] This application claims priority from GB2418788.2 filed 20 December 2024, the contents and elements of which are herein incorporated by reference for all purposes.

[0005] Field of the Invention

[0006] The present invention relates to a tweeter.

[0007] Background

[0008] Tweeters are a known type of loudspeaker for producing sound at high audio frequencies, typically from a few hundred Hertz up to at least the limit of human hearing, around 20 Kilohertz.

[0009] Efforts have been made to increase the output of tweeters across the operational frequency range, e.g. the midband output at the lower end of the tweeter frequency response. This can be done by making use of the rear radiated sound by use of a Helmholtz resonator.

[0010] US10462577B2 and US10469938B2 describe examples of dome tweeters having a port connecting the internal space inside the tweeter (behind the dome) with the external space (in front of the dome) to provide a Helmholtz resonator contributing to the radiated sound.

[0011] The present invention has been devised in light of the above considerations.

[0012] Summary of the Invention

[0013] According to a first aspect, there is provided a tweeter comprising: a chassis; an annular diaphragm having an outside edge, an inside edge, a front face which faces in a forward direction and a rear face which faces in a rearward direction; a drive unit configured to move the diaphragm along a movement axis in the forward direction and in the rearward direction, wherein the drive unit is located rearwards of the annular diaphragm; an outer suspension attached to the outside edge of the annular diaphragm and attached to the chassis to suspend the annular diaphragm (from the chassis); an inner suspension attached to the inside edge of the annular diaphragm and attached to the chassis to suspend the annular diaphragm (from the chassis); wherein the chassis includes a port which forms a port passage, wherein the inside edge of the annular diaphragm extends around the port passage and the port passage connects an internal space rearwards of the annular diaphragm with an external space forwards of the annular diaphragm; and wherein the drive unit is operable to move the annular diaphragm so as to: cause the front face of the annular diaphragm to produce sound which is emitted from the tweeter in the forward direction, and cause the rear face of the annular diaphragm to produce sound which is emitted from the tweeter in the forward direction via the port passage.

[0014] The tweeter according to the first aspect includes a Helmholtz resonator, in the form of the internal space and the port connecting the internal space to the external space in front of the annular diaphragm, which may increase output towards the lower end of the tweeter frequency response.

[0015] By arranging the annular diaphragm to extend around the port passage connecting the internal space and the external space, a spatially efficient arrangement may be provided. US10462577B2 describes a 008860157

[0016] 2 tweeter with a plurality of acoustic ducts connecting the internal space behind the diaphragm with the external space in front of the diaphragm. The acoustic ducts are arranged around the diaphragm such that, for a given external diameter of the diaphragm, even more space is needed to accommodate the acoustic ducts. The arrangement described in US10462577B2 is therefore suitable for comparatively large tweeters.

[0017] By utilising the annular diaphragm with the outer suspension and the inner suspension, stiffness of the diaphragm may not be adversely affected by the port. US10469938B2 describes a dome tweeter with an aperture through the centre of the dome. The inventors consider this arrangement undesirable as it could decreases the stiffness of the dome with a negative impact on the frequency response, and requires a gasket sealing off the gap the aperture and a port structure behind the dome. Moreover, the dome must be carefully aligned around the port structure, making the design unsuitable for cost-effective mass production, e.g. as in the automotive industry.

[0018] According to examples described below, the port may include an internal port portion in the internal space rearwards of the annular diaphragm, or an external port portion in the external space forwards of the annular diaphragm, or both the internal port portion and the external port portion. As such, flexibility in structural arrangement of the tweeter is provided. Moreover, the internal port portion may provide an internal port passage and the external port portion may provide an external port passage. Thus, flexibility in acoustic performance is also provided, by including the internal port passage, or the external port passage, or both the internal port passage and the external port passage.

[0019] The port may include said internal port portion in the internal space rearwards of the annular diaphragm, and the port passage may include said internal port passage formed by the internal port portion.

[0020] The internal port passage may diverge in the forward direction, or may be straight (neither diverge nor converge in the forward direction).

[0021] The chassis may include an internal frame element in the internal space and which includes the internal port portion.

[0022] In some examples, the internal frame element may be a single-piece component, e.g. formed by injection moulding.

[0023] The internal port portion may be located exclusively in the internal space, or alternatively the majority of the internal port portion may be located in the internal space and a part of the internal port portion may be outside of the internal space. Likewise, the internal frame element may be located exclusively in the internal space, or alternatively the majority of the internal frame element may be located in the internal space and a part of the internal frame element may be outside of the internal space.

[0024] The inner suspension is attached to the internal port portion. The internal port portion may include an axis-facing surface which faces towards the movement axis and to which the inner suspension may be attached. The axis-facing surface may, in some example, face towards the movement axis and face in the forward direction. For example, the axis-facing surface may be inclined relative to the movement axis so that it faces partially towards the movement axis and partially in the forward direction. By attaching the 008860157

[0025] 3 inner suspension to the axis-facing surface, a spatially efficient arrangement may be provided when compared to, for example, an arrangement whereby the inner suspension was attached to a surface perpendicular to the movement axis. This spatially efficient arrangement may help reduce the amount of radiating surface area lost as a result of the attachment of the annular diaphragm to the internal port portion and the presence of the port passage. Furthermore, this arrangement may help reduce the size of the tweeter.

[0026] The drive unit may include a stationary part, which forms part of the chassis, and a moveable part attached to the annular diaphragm and moveable along the movement axis relative to the stationary part. The internal frame element may include a base which is attached to the stationary part of the drive unit. By attaching the internal frame element to the stationary part, the stationary part may conveniently support the internal port portion. Moreover, where the annular diaphragm is attached to the internal port portion, the stationary part may furthermore support the inside edge of the annular diaphragm.

[0027] The base of the internal frame element may be attached to the stationary part of the drive unit by any suitable means, for example adhesive.

[0028] The stationary part of the drive unit may include a permanent magnet and a flux guide arranged to guide magnetic flux produced by the magnet across an air gap. The magnet may have any suitable shape, e.g. a disk shape or tablet shape. The flux guide may have any suitable shape, e.g. a disk shape or washer shape.

[0029] The stationary part of the drive unit may be configured to provide a flux density of least 1 .2 Tesla in the air gap, optionally of at least 1 .5 Tesla.

[0030] The stationary part of the drive unit may include another permanent magnet to provide a countermagnet. The flux guide may be arranged between the magnet and the countermagnet, with the flux guide for guiding flux produced by the magnet and countermagnet across the air gap. By providing the magnet and countermagnet, this may increase magnetic flux density in the air gap, e.g. to exceed 1 .5 Tesla, to provide a comparatively strong drive unit. A comparatively strong drive unit may be desirable for purposes of driving the Helmholtz resonator and increasing its contribution to the sound emitted by the tweeter.

[0031] The drive unit may be configured to have a motor strength of (BL)A21 R_DC > 3 (product of B and L squared, divided by R_DC exceeds 3). Here, B is the magnetic flux density, L is the length of the coil windings in the air gap, and R_DC is the electrical resistance of the coil windings. The motor strength is a known measure to express the strength of a drive unit.

[0032] The countermagnet may have any suitable shape, e.g. a disk shape or tablet shape. In some examples, the countermagnet has an annular shape, which may provide for a lightweight and cost-effective arrangement and also increased magnetic flux density in the air gap.

[0033] The flux guide may have a forward surface. The forward surface may face in the forward direction, and the countermagnet may be arranged at the forward surface of the flux guide. 008860157

[0034] 4

[0035] The flux guide may have a rearward surface. The rearward surface may face in the rearward direction, and the magnet may be arranged at the rearward surface of the flux guide.

[0036] The countermagnet may have a forward surface. The forward surface may face in the forward direction.

[0037] The countermagnet may have a rearward surface. The rearward surface may face in the rearward direction, and the flux guide may be arranged at the rearward surface of the countermagnet.

[0038] The stationary part of the drive unit may include a recess which faces in the forward direction. The internal frame element may extend into the recess such that the base of the internal frame element abuts a bottom of the recess. By arranging the internal frame element to extend into the recess and abut against the bottom of the recess, support of the internal port portion (and, where attached to the internal port portion, the inner suspension) may be improved. Moreover, assembly of the tweeter may be improved given that the recess may sufficiently locate the internal frame element for accurate positioning of the internal frame element.

[0039] The recess may be formed, in part or entirely, in the flux guide. Likewise, the recess may be formed, in part or entirely, in the countermagnet. In some examples, the recess is formed by both the countermagnet and the flux guide. By providing the recess in the stationary part, a spatially efficient arrangement may be provided since the whole of the internal frame element need not be mounted forwards of the stationary part. Moreover, by providing the countermagnet as part of the stationary part, this spatially efficient arrangement may not adversely affect the strength of the drive unit but, on the contrary, may provide for a stronger drive unit.

[0040] The bottom of the recess may be formed by the flux guide. In some examples, the forward surface of the flux guide may provide the bottom of the recess, such that the recess terminates at the (possibly flat) forward surface of the flux guide. In some examples, the bottom of the recess may be formed in the forward surface of the flux guide, e.g. such that the recess extends into the flux guide.

[0041] The aforementioned countermagnet may be provided as an annular countermagnet arranged to extend around the recess. Thus the annular countermagnet may bound the recess, e.g. in a direction perpendicular to the movement axis. The base of the internal frame element may extend through the annular countermagnet.

[0042] The internal frame element may include a plurality of ribs between the base and the internal port portion. The ribs may be arranged to radiate outwards relative to the movement axis. For example, the ribs may extend in a radial direction perpendicular to the movement axis.

[0043] The internal frame element, e.g. the ribs of the internal frame element, may abut the forward surface of the annular countermagnet. By arranging the internal frame element to abut against the annular countermagnet, support of the internal port portion (and, where attached to the internal port portion, the inner suspension) may be improved.

[0044] The internal space rearward of the annular diaphragm may include an outer region which surrounds the internal frame element. The internal frame element includes a plurality of radial passages which connect 008860157

[0045] 5 the internal port passage and the outer region of the internal space. The radial passages may extend in a radial direction perpendicular to the movement axis.

[0046] The radial passages may be formed between the ribs of the internal frame element. As such, the internal frame element may provide for a spatially efficient arrangement for supporting the internal port portion (and, where attached to the internal port portion, the inner suspension) against the stationary part, and also for connecting the internal port passage and the outer region of the internal space.

[0047] The port may include said external port portion in the external space forwards of the annular diaphragm, and the port passage may include said external port passage formed by the external port portion. By providing the external port portion, the length of the port passage, as measured along the movement axis, may be conveniently extending. This may provide for increased sensitivity of the tweeter over a wide bandwidth. In some examples, the port passage may include the internal port passage which is extended by the external port passage. In some examples, the port passage may include the external port passage (and not include the internal port passage) to provide the port passage with desired length.

[0048] The chassis may include an external frame element arranged forwards of the front face of the annular diaphragm. The external frame element may be, for example, part of a frame or attached to the frame. The external frame element may include the external port portion.

[0049] The external port passage may diverge in the forward direction.

[0050] The external frame element may include a front-face waveguide arranged forwards of the annular diaphragm and configured to boost sound pressure level of sound generated by the front face of the annular diaphragm. By providing the front-face waveguide and the diverging port passage, SPL of sound produced by the front face and the rear face of the diaphragm may be emitted and boosted. More particularly, the front-face waveguide may boost one or more frequency bands above a frequency band mainly affected by the port (i.e. the Helmholtz resonator), such that a comparatively wide range of frequency bands may be boosted.

[0051] The tweeter may be operable to generate sound of a frequency at which the sound pressure level is boosted by the front-face waveguide and by the port. That it so say, in some examples there may be an overlap of the boost by the front-face waveguide and the boost by the port (i.e. the Helmholtz resonator) such that sound generated at a frequency may be boosted by both the front-face waveguide and the port.

[0052] The internal port portion may be configured to boost the sound pressure level of sound in a first frequency range. The external port portion and the front-face waveguide may be configured to boost the sound pressure level of sound in a second frequency range.

[0053] The first frequency range may be the range from 800 Hz to 3150 Hz.

[0054] The second frequency range may be the range from 1600 Hz to 16000 Hz.

[0055] The first frequency range may include at least some frequencies which are below the second frequency range. The second frequency range includes at least some frequencies which are above the first frequency range. As such, the first frequency range and the second frequency range may be different but 008860157

[0056] 6 may partially overlap. For example, the frequency ranges may overlap from in a range from 1600 Hertz to 3150 Hertz. Below, said range of overlap is also referred to as the boost overlap range of frequencies.

[0057] The internal port portion and the combination of the external port portion and the front-face waveguide may collectively boost SPL over a boost overlap range of frequencies. The boost overlap range of frequencies may be from 1600 Hz to 3150 Hz. By suitably configuring the combination of the external port portion and the front-face waveguide, e.g. increasing the waveguide size, the lower limit of the boost overlap range may be decreased to a value lower than 1600 Hertz.

[0058] The tweeter may define a Helmholtz resonator with a tuning frequency between 500 Hertz and 2 Kilohertz.

[0059] The internal port portion and the combination of the external port portion and the front-face waveguide may collectively boost SPL at a boost overlap frequency. The boost overlap frequency may correspond to three times the tuning frequency of the Helmholtz resonator.

[0060] The inner suspension and the outer suspension may be formed from a same material.

[0061] The inner suspension and the outer suspension may each be a roll suspension.

[0062] The annular diaphragm may have a Young’s modulus of at least 2 gigapascals. In some examples, the annular diaphragm may have a Young’s modulus of at least 20 gigapascals.

[0063] The drive unit may have a mass of 80 grams or less.

[0064] The tweeter may have a mass of 100 grams or less.

[0065] The invention includes the combination of the aspects and preferred features described except where such a combination is clearly impermissible or expressly avoided.

[0066] Summary of the Figures

[0067] Embodiments and experiments illustrating the principles of the invention will now be discussed with reference to the accompanying figures in which:

[0068] Figure 1 is a sectional perspective view of a tweeter.

[0069] Figure 2 is a cross-sectional view of the tweeter.

[0070] Figure 3 is an exploded view of the tweeter.

[0071] Figure 4 is a perspective view of an internal frame element of the tweeter.

[0072] Figure 5 is a cross-sectional view of part of the tweeter.

[0073] Figure 6 is a perspective view of an external frame element of the tweeter.

[0074] Figure 7 is a cross-sectional view of the external frame element.

[0075] Figure 8 is a plot of magnetic flux density generated by a drive unit of the tweeter. 008860157

[0076] 7

[0077] Figure 9 is a graph showing an impedance curve of the tweeter.

[0078] Figure 10 is a graph illustrating a frequency response of the tweeter.

[0079] Figure 11 is a graph illustrating a total harmonic distortion of the tweeter.

[0080] Figure 12 is a cross-sectional view of another tweeter.

[0081] Figure 13 is a cross-sectional view of another tweeter.

[0082] Detailed Description of the Invention

[0083] Aspects and embodiments of the present invention will now be discussed with reference to the accompanying figures. Further aspects and embodiments will be apparent to those skilled in the art. All documents mentioned in this text are incorporated herein by reference.

[0084] In recent years, there have been efforts to increase the midband output of tweeters by making use of the rear radiated sound by means of a Helmholtz resonator.

[0085] US10469938B2 and US10462577B2 describe dome tweeters with a port connecting the internal volume behind the dome with the external volume in front of the dome and so have the Helmholtz resonator contribute to the radiated sound, even when the tweeter is mounted in a baffle.

[0086] US10462577B2 describes a tweeter with large ferrite motor system, an insulation ring (“diaphragm frame”) and a faceplate with aligned holes through both and so creating ports. This solution is useful for large tweeters, e.g. Hi-Fi applications, but may not be applicable to compact tweeters as used, for example, in pro-audio applications utilising large arrays of tweeters or in space constrained automotive applications.

[0087] US10469938B2 describes a port in the centre of a dome tweeter. This arrangement decreases the stiffness of the dome with a negative impact on the frequency response and requires a gasket sealing off the gap between the fixed port and the aperture in the dome. The dome must be carefully aligned around the port making the design unsuitable for low-cost mass production as required, e.g. for large quantities as required by the automotive market.

[0088] The tweeter described below is of the ring radiator type. Ring radiators may generally include two suspensions and typically a stiff diaphragm between them. The inner and outer suspensions attached to the W-shaped diaphragm (or “V-shaped” diaphragm, with a V-shape on both sides of the diaphragm) are often on the same plane perpendicular to the main radiation axis. EP0065883A2 is an example of such a type of tweeter. By driving the diaphragm at an annular valley of the diaphragm a comparably large diaphragm with pistonic behaviour over a wide frequency range can be achieved. Ring radiators may be preferred by some listeners due to the larger, annular diaphragm and associated directivity it provides.

[0089] Figures 1 , 2 and 3 show a tweeter 100. Figure 1 is a sectional perspective view of the tweeter 100, while Figure 2 is a cross-sectional view of the tweeter 100, and Figure 3 is an exploded view of the tweeter 100. 008860157

[0090] 8

[0091] The tweeter 100 includes a chassis 110, an annular diaphragm 120, and a drive unit 130 rearwards of the diaphragm 120. The drive unit 130 includes a moveable part, which here is provided as a voice coil 140 attached to the diaphragm 120, and a stationary part, which here is provided as a magnet unit 150.

[0092] The chassis 110 includes a frame 112.

[0093] The magnet unit 150, which is attached to the frame 112 and forms part of the chassis 110, is configured to generate magnetic flux and to guide the magnetic flux across an air gap. The air gap may be annular, and the magnetic flux density may be guided across in a direction perpendicular to the movement axis 101.

[0094] The voice coil 140 includes voice coil windings 142 arranged on a voice coil former 144. The voice coil former 144 includes a plurality of apertures 146 through the voice coil former 144.

[0095] The tweeter 100 is operable to energise the drive unit 130 to cause movement of the voice coil 140 relative to the magnet unit 150 along a movement axis 101 in a forward direction 102 and a rearward direction 103. Suitably, the voice coil 140 and the magnet unit 150 magnetically interact with each other to effect the movement of the voice coil 140. The diaphragm 120 and the voice coil 140 are joined together to define a moveable assembly 160, such that they move together ‘as a unit’ when the voice coil 140 is caused to move. When causing the moveable assembly 160 to move, the diaphragm 120 acts as a piston and produces sound.

[0096] As seen in Figures 1 and 2, the diaphragm 120 in this example is W-shaped. As noted above, a W- shaped diaphragm is sometimes called V-shaped when referring to half of the diaphragm (e.g. in view of rotational symmetry such that revolution around the movement axis 101 yields the W-shape).

[0097] In some examples, the annular diaphragm 120 has Young’s modulus of at least 2 gigapascals. Suitable materials for fabrication of a diaphragm with such a stiffness may include, for example, plastics as known in the art. In some examples, the annular diaphragm 120 has a Young’s modulus of at least 20 gigapascals. Suitable materials for fabrication of a diaphragm with such a stiffness may include, for example, glass fibres and metals, as known in the art.

[0098] Other suitable materials for the diaphragm may, as are known in the art, may include paper, glass, carbon fibre, graphene, and graphene oxide, and may be chosen depending on the desired Young’s modulus.

[0099] The annular diaphragm 120 has an outside edge 121 and an inside edge 122. An outer suspension 172 is attached to the outside edge 121 and is attached to the chassis 110 to suspend the annular diaphragm 120. The outer suspension 172 is attached to the frame 112. An inner suspension 174 is attached to the inside edge 122 of the annular diaphragm 120 and attached is to the chassis 110 to suspend the annular diaphragm 120.

[0100] The relative terms “inner” and “outer” are used, as is common, with respect to the movement axis 101 . That is to say, the inner suspension element 174 is closer to the movement axis 101 , while the outer suspension element 172 is farther from the movement axis 101 . 008860157

[0101] 9

[0102] The annular diaphragm 120 has a front face 123 which faces in the forward direction 102 and a rear face 124 which faces in the rearward direction 103.

[0103] The chassis 110 further includes a port 180 which forms a port passage 190. The inside edge 122 of the annular diaphragm 120 extends around the port passage 190 and the port passage 190 and connects an internal space 104 with an external space 105. The internal space 104 is rearwards of the annular diaphragm 120 and is “internal” to the tweeter 100, i.e. is enclosed by the chassis 110 and the diaphragm 120. The external space 105 is “external” to the tweeter 100, i.e. is outside of the tweeter 100 and, in particular, is forwards of the annular diaphragm 120.

[0104] The drive unit 130 is operable to move the annular diaphragm 120 so as to cause the front face 123 of the annular diaphragm 120 to produce sound which is emitted from the tweeter 100 in the forward direction 102, and cause the rear face 124 of the annular diaphragm 120 to produce sound which is emitted in the forward direction 102 via the port passage 190.

[0105] The tweeter 100 further includes an internal frame element 200 and an external frame element 300.

[0106] Figure 4 is a perspective view of the internal frame element 200. The internal frame element 200 is injection mouldable for industrial-scale manufacturing. The internal frame element 200 shown in Figure 4 is injection mouldable using an open-close direction coinciding with the movement axis 101.

[0107] The internal frame element 200 is arranged in the internal space 104 inside the tweeter 100 and includes an internal port portion 210. The port passage 190 includes an internal port passage 212 formed by the internal port portion 210. The internal port passage 212 extends from the aperture in the annular diaphragm 120 into the internal space 104.

[0108] The inner suspension 122 is attached to the internal port portion 210. More particularly, the internal portion 210 includes an axis-facing surface 214 which extends around the movement axis 101 and faces towards the movement axis 101 and faces in the forward direction 102. In particular, the axis-facing surface 214 is inclined relative to the movement axis 101 so that it faces partially towards the movement axis 101 and partially in the forward direction. The inner suspension 122 is attached to the axis-facing surface 214 of the internal port portion 210 using adhesive.

[0109] The internal frame element 200 includes a base 220 and ribs 230 arranged between the base 220 and the internal port portion 210.

[0110] The ribs 230 radiate outwards relative to the movement axis 101 in a radial direction perpendicular to the movement axis 101 . A plurality of radial passages 232 is formed by the ribs 230, with one passage 232 between adjacent ribs 230. The radial passages 232 allow passage of air between the internal space 104 bounded by the frame 112, the diaphragm 120, the magnet unit 150, and the suspensions 172, 174 and the external space 105 in front of the diaphragm 120.

[0111] The passages 232 extend generally in the radial direction, i.e. away from the movement axis 101 , and are arranged to connect an outer region 106 of the internal space 104 to the port passage 190. The outer region 106 surrounds the internal frame element 200. 008860157

[0112] 10

[0113] The aforementioned apertures 146 in the voice coil former 144, which also extend in the radial direction, further connect the internal space 104, and in particular the outer region 106, to the port passage 190.

[0114] By means of the passages 232 and the apertures 146, sound generated across the rearward face 124 of the diaphragm 120 may propagate towards the port passage 190 and be emitted therefrom.

[0115] Figure 5 is a cross-sectional view of part of the tweeter 100. In particular, Figure 5 shows the frame 112 and the magnet unit 150.

[0116] The magnet unit 150 includes a magnet 151 and a countermagnet 152 to generate the magnetic flux, as well as a flux guide 153 and a yoke 154 to guide the magnetic flux across the air gap. The flux guide 153 is arranged between the magnet 151 and the countermagnet 152, and the magnets 151 , 152 and flux guide 153 are mounted inside the yoke 154.

[0117] The magnet 151 and the countermagnet 152 are rare earth magnets and each may comprise more than one structural element. The flux guiding elements 153, 154 are provided as a (magnetic) U-yoke 154 and a (magnetic) washer 153.

[0118] The flux guide 153 has a forward surface 156 which faces in the forward direction 102. The countermagnet 152 attached to the forward surface 156 of the flux guide 153 using adhesive.

[0119] The flux guide 153 has a rearward surface 157 which faces in the rearward direction 103. The magnet 151 attached to the rearward surface 157 of the flux guide 153 using adhesive.

[0120] The countermagnet 152 is ring-shaped or annular, with an aperture extending through the countermagnet 152. The countermagnet 152 has a forward surface 158 which faces in the forward direction 102. The ribs 230 of the internal frame element 200 abut the forward surface 158.

[0121] The yoke 154 is provided as a U-yoke with an annular wall upstanding from a base. In some examples, a rearward face of the base is concavely shaped, such that the base is thinner towards the movement axis, and is thicker away from the movement axis. Such a configuration of the yoke 154 may provide for weight savings without adversely affecting performance of the drive unit 130.

[0122] The internal frame element 200 includes a base 220 which is attached to the magnet unit 150. More particularly, the magnet unit 150 includes a recess 155 which faces in the forward direction 102. The recess 155 is bounded by the countermagnet 152, which is annular and extends around the movement axis 101. The recess 155 extends into the forward surface 156 of the flux guide 153 and terminates at a bottom of the recess 155. The base 220 of the internal frame element 200 extends through the annular countermagnet 152 and into the flux guide 153 and abuts against the bottom of the recess 155.

[0123] The tweeter 100, including in particular the internal port passage 212 and the comparatively strong drive unit 130, may allow for increased output towards the lower end of the tweeter frequency response. In particular, a strong drive unit, as described here, may lead to a substantial increase in the low-end output. The surprisingly lightweight drive unit 130 is using a comparably small main magnet in combination with a ring-shaped countermagnet leading to a high flux density in the airgap of more than 1 .6 Tesla. 008860157

[0124] 11

[0125] Figures 6 and 7 show the external frame element 300, which forms part of the chassis 110. Figure 6 is a perspective view of the external frame element 300, while Figure 7 is a cross-sectional view.

[0126] The external frame element 300, which may also be referred to as a front plate, is arranged forwards of the front face 123 of the annular diaphragm 120. The external frame element 300 is attached to the frame 112 using any suitable means, e.g. mechanical means such as clipped onto the frame 112.

[0127] The external frame element 300 extends the length of the port and is proximal to portions of the diaphragm, in particular in the area at the valley of the V-shaped diaphragm.

[0128] The external frame element 300 includes an external port portion 310. The external port portion 310 is arranged outside of the internal space 104, although some parts of the external port portion 310 may extend into the internal space 104.

[0129] The port passage 190 includes an external port passage 312 formed by the external port portion 310. The external port passage 312 extends the port passage 190 outside of the internal space 104. The external port passage 312 extends from the aperture in the annular diaphragm 120 in the forwards direction 102 towards the external space 106.

[0130] The external port passage 312 diverges in the forward direction 102.

[0131] The external frame element 300 includes a front-face waveguide 320 arranged forwards of the annular diaphragm 120 and configured to boost sound pressure level of sound generated by the front face 123 of the annular diaphragm 120. For example, the front-face waveguide 320 may be arranged close to the front face 123 of the diaphragm 120 to create a low pass filter, boosting in a frequency band and exhibiting a roll-off above said frequency band.

[0132] A suitable configuration of the external frame element 300 allows for a boost of the frequency band above the band mainly affected by the Helmholtz resonator. The highest frequencies can be shaped by designing the diaphragm and the associated break-up behaviour of the diaphragm and suspensions. These three effects together may lead to a very compact, yet surprisingly powerful tweeter with low distortion and high output capability while following the traditional manufacturing methods for ring radiators.

[0133] In some examples, the inner suspension 174 is clamped between the front plate (and in particular the external port portion 312) and the inner suspension 174. Such clamping may be in addition to the use of adhesive for attaching the inner suspension 174 to the axis-facing surface 214.

[0134] Below, with reference to Figures 8 to 11 , plots indicating the performance of the tweeter 100 discussed with reference to Figures 1 to 7 are provided. The tweeter 100 has the following dimensions.

[0135] The external frame element 300 has an outside diameter of 55 millimetres.

[0136] The voice coil 140 has a voice coil diameter of 32 millimetres.

[0137] The V-shaped diaphragm has a thickness of 0.03 millimetre and is formed from an aluminium-magnesium alloy. 008860157

[0138] 12

[0139] The suspensions 172, 174 are formed from coated fabric and include tangential pleats for increased stiffness.

[0140] The magnet unit 150 has a total motor system weight of just 80 grams: The magnet 151 weighs 18 grams. The countermagnet 152 weighs 8 grams. The flux guide 153 and the yoke 154 together weigh 54 grams.

[0141] Figure 8 shows a half cross-section of the motor system with the simulated flux lines and associated flux density. The countermagnet is magnetized with opposite polarity relative to the main magnet. The countermagnet with its ring shape has a comparably large height to diameter ratio allowing it to operate at a comparably high permeance coefficient and so decreasing the risk for demagnetization as compared to a thin full disc magnet (having identical weight, remanent flux density and coercivity) at this position. The countermagnet helps to increase the flux density in the air gap, in some cases dramatically, despite its comparatively small weight contribution. This total motor system weight of just 80g is exceptionally low for a tweeter with the provided bandwidth and output capability. The force factor of the motor system is 4.3 Tesla-metres at 4 ohm nominal coil impedance, which may be considered exceptionally high for a tweeter. It is known in the field of woofers, that a high force factor is preferred for driving a Helmholtz resonator allowing controlled frequency band extension and a smooth roll-off without overshoot around the tuning frequency. The same reasoning applies here for this ported tweeter.

[0142] Figure 9 shows the impedance curve of the tweeter 100 in comparison with the same tweeter when the port passage 190 is blocked. The working of the port passage 190 is clearly visible, showing the tuning frequency between the two peaks at 850 Hertz.

[0143] Figure 10 shows frequency response measurements at 2 Volts (corresponding to 1 Watt nominal into 4 Ohms) in 1 metre distance in an infinite baffle. The dotted line shows the frequency response of the tweeter 100 without the external frame element 300 and with the port passage 190 blocked. The solid curve shows the frequency response of the tweeter 100 without the external frame element 300 (and with the port passage 190 open). The dashed line shows the frequency response of the tweeter 100 (i.e. including the external frame element 300 and with the port passage 190 open), showing both the effect of the port passage elongation and the portions of the external frame element 300 being proximal to the diaphragm 140 leading to a boost over a wide frequency band. The average sensitivity of the tweeter between 1 .5 Kilohertz and 15 Kilohertz is 99dB while its weight is below 100 grams.

[0144] The tuning frequency of the tweeter 100 is 850 Hertz (which corresponds to the minimum between the two peaks of the solid line in Figure 9). The internal frame element 200 (i.e. the Helmholtz resonator as provided by the internal port passage 212) boosts frequencies in the range from 800 Hertz to around 5 Kilohertz. The external frame element 300 (including the front-face waveguide 320 and the external port portion 310) boosts frequencies in the range from 1 .8 Kilohertz to 12 Kilohertz. In combination, there is provided a boost over a wide frequency range. More particularly, both the internal frame element 200 and the external frame element 300 provide a boost from the range from around 1 .8 Kilohertz to around 5 Kilohertz. 008860157

[0145] 13

[0146] The aforementioned range from 1 .8 Kilohertz to 5 Kilohertz may be referred to as the boost overlap range over which both the internal frame element 200 and the external frame element 300 provide a boost. A boost overlap frequency equal to three times the tuning frequency (i.e. 2.4 kHz), at which both the internal frame element 200 and the external frame element 300 provide a boost, may also be defined.

[0147] Figure 11 shows a total harmonic distortion measurement of the tweeter 100 at 9 Volt RMS input voltage. The total harmonic distortion is below 10% from 900Hz onwards and dominated by second-order harmonics, which by at least some listeners are not perceived negatively. Third-order order harmonics may be considered negligible, with values substantially below 1 % above 1 ,5kHz with an average sound pressure level (SPL) between 1 ,5kHz and 15kHz of 112dB.

[0148] In summary, tweeter 100 may provide for a low weight, high SPL, high midband output, low distortion, compact tweeter that can be built with traditional manufacturing processes.

[0149] Figure 12 is a cross-sectional view of another tweeter. The tweeter of Figure 12 is identical to the tweeter 100 discussed above but does not include the external frame element 300. Performance of the tweeter of Figure 12 is indicated by the solid line in Figure 10.

[0150] Figure 13 is a cross-sectional view of another tweeter. The tweeter of Figure 13 is identical to the tweeter 100 discussed above but does not include the internal frame element 200. As such, the inner suspension is attached to the external frame element instead, e.g. to a surface which faces away from the movement axis and faces in the rearwards direction. However, the inner suspension could be attached to both the internal frame element and the external frame element (where both are provided). It may be preferable to attach to the internal frame element 200 (where this is provided) for ease of assembly.

[0151] The tweeters described above all have countermagnets, but more generally a drive unit without countermagnet may be utilised. For example, a standard 3-piece U-yoke or T-yoke magnet unit could be used. Such could be used in combination with a split air gap or a split winding to increase the motor linearity where this is desired.

[0152] In some examples, foam - or even activated carbon - may be added to the internal space 104, e.g. to provide some damping or increase the acoustic volume leading to a lower tuning frequency.

[0153] The tweeters described above all have V-shaped diaphragms, but more generally other shapes may be used, e.g. one of the portions of the diaphragm could be curved. Also, the diaphragm could use a different material inside the voice coil as compared to outside the voice coil. Also, the suspensions could be at different positions (as measured along the movement axis), or could be made from different material. The inner portion could be shorter or the angles could differ.

[0154] The features disclosed in the foregoing description, or in the following claims, or in the accompanying drawings, expressed in their specific forms or in terms of a means for performing the disclosed function, or a method or process for obtaining the disclosed results, as appropriate, may, separately, or in any combination of such features, be utilised for realising the invention in diverse forms thereof. 008860157

[0155] 14

[0156] While the invention has been described in conjunction with the exemplary embodiments described above, many equivalent modifications and variations will be apparent to those skilled in the art when given this disclosure. Accordingly, the exemplary embodiments of the invention set forth above are considered to be illustrative and not limiting. Various changes to the described embodiments may be made without departing from the spirit and scope of the invention.

[0157] For the avoidance of any doubt, any theoretical explanations provided herein are provided for the purposes of improving the understanding of a reader. The inventors do not wish to be bound by any of these theoretical explanations.

[0158] Any section headings used herein are for organizational purposes only and are not to be construed as limiting the subject matter described.

[0159] Throughout this specification, including the claims which follow, unless the context requires otherwise, the word “comprise” and “include”, and variations such as “comprises”, “comprising”, and “including” will be understood to imply the inclusion of a stated integer or step or group of integers or steps but not the exclusion of any other integer or step or group of integers or steps.

[0160] It must be noted that, as used in the specification and the appended claims, the singular forms “a,” “an,” and “the” include plural referents unless the context clearly dictates otherwise. Ranges may be expressed herein as from “about” one particular value, and / or to “about” another particular value. When such a range is expressed, another embodiment includes from the one particular value and / or to the other particular value. Similarly, when values are expressed as approximations, by the use of the antecedent “about,” it will be understood that the particular value forms another embodiment. The term “about” in relation to a numerical value is optional and means for example + / - 10%.

[0161] References

[0162] A number of publications are cited above in order to more fully describe and disclose the invention and the state of the art to which the invention pertains. Full citations for these references are provided below. The entirety of each of these references is incorporated herein.

[0163] US10462577B2

[0164] US10469938B2

[0165] EP0065883A2

Claims

00886015715Claims:1 . A tweeter comprising: a chassis; an annular diaphragm having an outside edge, an inside edge, a front face which faces in a forward direction and a rear face which faces in a rearward direction; a drive unit configured to move the diaphragm along a movement axis in the forward direction and in the rearward direction, wherein the drive unit is located rearwards of the annular diaphragm; an outer suspension attached to the outside edge of the annular diaphragm and attached to the chassis to suspend the annular diaphragm; an inner suspension attached to the inside edge of the annular diaphragm and attached to the chassis to suspend the annular diaphragm; wherein the chassis includes a port which forms a port passage, wherein the inside edge of the annular diaphragm extends around the port passage and the port passage connects an internal space rearwards of the annular diaphragm with an external space forwards of the annular diaphragm; and wherein the drive unit is operable to move the annular diaphragm so as to: cause the front face of the annular diaphragm to produce sound which is emitted from the tweeter in the forward direction, and cause the rear face of the annular diaphragm to produce sound which is emitted from the tweeter in the forward direction via the port passage.

2. The tweeter of claim 1 , wherein the port includes an internal port portion in the internal space rearwards of the annular diaphragm; wherein the port passage includes an internal port passage formed by the internal port portion.

3. The tweeter of claim 2, wherein the inner suspension is attached to the internal port portion.

4. The tweeter of claim 3, wherein the internal port portion includes an axis-facing surface which faces towards the movement axis, and wherein the inner suspension is attached to the internal port portion at the axisfacing surface.

5. The tweeter of any preceding claim, wherein the chassis includes an internal frame element in the internal space; and wherein the internal frame element includes the internal port portion.008860157166. The tweeter of claim 5, wherein the drive unit includes a stationary part, which forms part of the chassis, and a moveable part attached to the annular diaphragm and moveable along the movement axis relative to the stationary part; the internal frame element includes a base which is attached to the stationary part of the drive unit.

7. The tweeter of claim 6, wherein the stationary part of the drive unit includes: a magnet; a countermagnet; and a flux guide arranged between the magnet and the countermagnet, the flux guide for guiding flux produced by the magnet and countermagnet across an airgap.

8. The tweeter of claim 7, wherein the stationary part of the drive unit includes a recess which faces in the forward direction; and wherein the internal frame element extends into the recess such that the base of the internal frame element abuts a bottom of the recess.

9. The tweeter of claim 8, wherein the flux guide forms the bottom of the recess; and wherein the bottom of the recess is formed in a forward surface of the flux guide element.

10. The tweeter of any one of claims 8 to 9, wherein the stationary part of the drive unit includes an annular countermagnet which extends around the recess; and wherein the base of the internal frame element extends through the annular countermagnet.11 . The tweeter of claim 10, wherein the internal frame element includes a plurality of ribs between the base and the internal port portion; and wherein the ribs abut a forward surface of the annular countermagnet.

12. The tweeter of any one of claims 3 to 11 , wherein the internal space includes an outer region which surrounds the internal frame element; and wherein the internal frame element includes a plurality of radial passages which connect the internal port passage and the outer region of the internal space.0088601571713. The tweeter of any preceding claim, wherein the port includes an external port portion in the external space forwards of the annular diaphragm; and wherein the port passage includes an external port passage formed by the external port portion.

14. The tweeter of claim 13, wherein the chassis includes an external frame element arranged forwards of the front face of the annular diaphragm; and wherein the external frame element includes the external port portion.

15. The tweeter of any one of claims 13 to 14, wherein the external port passage diverges in the forward direction.

16. The tweeter of any one of claims 13 to 15, wherein the external frame element includes a front-face waveguide arranged forwards of the annular diaphragm and configured to boost sound pressure level of sound generated by the front face of the annular diaphragm.

17. The tweeter of claim 16, wherein the tweeter is operable to generate sound of a frequency at which the sound pressure level is boosted by the front-face waveguide and by the port.

18. The tweeter of claim 16 or 17, wherein the internal port portion is configured to boost the sound pressure level of sound in a first frequency range; the external port portion and the front-face waveguide are configured to boost the sound pressure level of sound in a second frequency range; and wherein the first frequency range includes at least some frequencies which are below the second frequency range, and the second frequency range includes at least some frequencies which are above the first frequency range.

19. The tweeter of any preceding claim, wherein the drive unit includes a magnet unit configured to provide a flux density in an air gap of least 1 .2 Tesla.

20. The tweeter of any preceding claim, wherein the inner suspension and the outer suspension is formed from a same material.21 . The tweeter of any preceding claim, wherein the diaphragm has a Young’s modulus of at least 2GPa.

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