Device for generating sound

Abstract

A device for generating sound. The device includes: a first assembly and a second assembly. The first assembly includes a magnet unit and a coil. The magnet unit includes: a permanent magnet; at least one flux guiding element for guiding magnetic flux produced by the permanent magnet in a magnetic circuit. The second assembly, which is configured to move with respect to the first assembly, includes: a slug containing ferromagnetic material for carrying magnetic flux; a resilient suspension member attached to the first assembly, wherein the resilient suspension member is configured to suspend the slug from the first assembly such that the slug is positioned in the magnetic flux of the magnetic circuit. The device further includes an AC current signal source configured to supply an AC current signal to the coil when the device is activated so that magnetic flux generated by the AC current signal in the coil combines with magnetic flux of the magnetic circuit so as to cause the slug and resilient suspension member to vibrate along a movement axis thereby causing the device to generate sound. The device may be included in a A horn apparatus for a vehicle.

Description

[0001] DEVICE FOR GENERATING SOUND

[0002] This application claims priority to GB2308194.6 filed 1 June 2023 and GB2309656.3 filed 27 June 2023.

[0003] Field of the Invention

[0004] The present invention relates to a device for generating sound, e.g. as might be used in a vehicle.

[0005] Background

[0006] Most automotive vehicles are equipped with a horn apparatus for producing an audible warning sound (vehicle horn) to alert other road users. A typical audible warning sound produced by a horn apparatus is easily recognized by people as a vehicle horn and, worldwide, people have grown accustomed to how vehicle horns sound.

[0007] One common type of horn apparatus typically used in a vehicle is an electromechanical apparatus comprising a metal disc under the control of a solenoid. The metal disc is configured to ring and produce sound when a contact between the disc and the solenoid is broken. Typically, the contact between the disc and the solenoid is broken 400-500 times per second. This can also be described as a hammer knocking on the metal disc, leaving the disc to resonate before interrupting the sound by knocking on the disc again. An electromechanical horn apparatus of this type can wear out quickly.

[0008] Another type of horn apparatus sometimes used in a vehicle is configured to electrically switch (e.g. using a MOSFET) the current to the solenoid. Such vehicle horns are typically louder, and more durable because the contact does not need to be mechanically broken. However, these horns are much more expensive than traditional electromechanical vehicle horns.

[0009] Yet another type of horn apparatus used in a vehicle may be pneumatically activated wherein an air-tank is filled with compressed air which is then released along a reed thus forcing the reed to oscillate and produce the audible warning sound. However, the pneumatically activated vehicle horns are only economically viable if the compressed air is already available on the vehicle, for example in heavy goods vehicles, or if the required sound pressure level for the audible warning sound is very high, for example for emergency vehicle horns.

[0010] A horn apparatus as described above may be referred to as a “single tone” horn apparatus, as it will typically produce sound at a single fundamental frequency. The sound produced by such a “single tone” horn apparatus may similarly be referred to as “single tone” vehicle horn sound. A common arrangement is a “dual tone” horn apparatus, which uses two “single tone” horn apparatuses each tuned to provide sound at a different fundamental resonant frequency, wherein the two fundamental resonant frequencies are spaced at minor intervals apart (e.g. 420Hz and 500Hz). The sound produced by such a “dual tone” horn apparatus may be referred to as a “dual tone” vehicle horn sound. A “dual tone” horn apparatus can be useful for producing a more noticeable audible warning sound to the human ear, compared with a “single tone” horn apparatus.

[0011] Another type of horn apparatus used in a vehicle includes a loudspeaker, typically an electrodynamic loudspeaker. Such a horn apparatus may be referred to as an “loudspeaker-based” horn apparatus (in ECE-28 it is referred to as an “electronic horn”). In such examples, the loudspeaker is configured to produce an audible warning sound based on an audio signal, which is typically stored in memory and provided to the loudspeaker via an amplifier. This audio signal may be a recording of an audible warning sound produced by a conventional horn apparatus (e.g. a “dual tone” vehicle horn sound, as described above) and as such, may contain multiple frequencies for playback by the loudspeaker simultaneously. The present inventor has observed that in such an audio signal, the fundamental frequency of the signal is not fixed electronically or mechanically and, therefore, the frequency spectrum of the audio signal may be adjusted freely within the limits of the loudspeaker.

[0012] In some examples, the sound pressure level (SPL) output of a horn apparatus for a vehicle may be increased by including a trumpet-like funnel to shape the spectrum of the audible warning sound to be louder at higher frequencies where the human ear is most sensitive. The inclusion of a funnel may also increase the directivity of the audible warning sound.

[0013] The present inventor has observed that, in some jurisdictions, a loudspeaker for use in a vehicle alerting system is legally required to produce a combined, A-weighted SPL (sound pressure level) in the 1 / 3 octave frequency bands 2khz, 2.5kHz and 3.15kHz to be no less than 105dB measured under anechoic conditions in 2m distance on the principle axis of the device. See, for example, Regulation No 28 of the Economic Commission for Europe of the United Nations (UN / ECE) - “Uniform provisions concerning the approval of audible warning devices and of motor vehicles with regard to their audible signals”, referred to herein as “ECE-28”.

[0014] Slow driving electric vehicles produce too little noise to be noticed by pedestrians which poses a safety issue, particularly in front of schools, at pedestrian crossings or traffic lights. Legislation has been adapted to address this matter by making mandatory the generation of an artificial sound. Acoustic Vehicle Alerting Systems (“AVAS” systems) are known systems which are designed to emit this artificial sound to alert pedestrians to the presence of electric drive vehicles. These include hybrid (HEVs), plug-in hybrid (PHEVs), and full battery electric vehicles (BEVs) travelling at low speeds, especially in the lowest speed range below which the noise generated by rolling tires can no longer be easily heard.

[0015] The present inventor has observed that it may be desirable to use the audible warning device of a vehicle for producing an AVAS sound, typically produced by a dedicated loudspeaker. This would allow vehicle manufacturers to remove the loudspeaker from the vehicle, reducing system complexity, cost, and weight.

[0016] However, the present inventor has observed that a typical acoustic warning device is not suitable for reproducing AVAS sound.

[0017] There are some known examples in which the warning functionality of AVAS is combined with a vehicle horn, for example, see US10406976B2, FR2983025A1 , US2020 / 0070719A1 . However, such examples typically comprise separate loudspeakers configured to produce different frequency ranges requiring multiple amplifier channels or a passive cross-over network. Other example loudspeaker arrangements capable of meeting the required SPL requirements of both a horn apparatus and AVAS system are public address type systems such as PA loudspeakers with large neodymium magnets or very large ferrite magnets. Therefore, these systems generally have a limited bandwidth or require a lot of input power, or they require a large amount of rare earth magnet material, are very heavy, or are very large in size. In many cases it would be impractical or not economically viable to incorporate such systems into a typical automobile.

[0018] When setting up a loudspeaker-based horn apparatus according to the requirements of ECE-28, it must be accompanied by an audio amplifier and an alternating signal to be played back. The present inventor has observed that a traditional loudspeaker design is not suitable for generating the required output and sound signature of traditional horns.

[0019] WO2022 / 128595A1 discloses an audible vehicle warning system arranged for generating a horn signal and comprising a loudspeaker system. The present invention has been devised in light of the above considerations.

[0020] Summary of the Invention

[0021] In a first aspect, the present invention provides: A device for generating sound, including: a first assembly including: a magnet unit including: a permanent magnet; at least one flux guiding element for guiding magnetic flux produced by the permanent magnet in a magnetic circuit; a coil; a second assembly, configured to move with respect to the first assembly, the second assembly including: a slug containing ferromagnetic material for carrying magnetic flux; a resilient suspension member attached to the first assembly, wherein the resilient suspension member is configured to suspend the slug from the first assembly such that the slug is positioned in the magnetic flux of the magnetic circuit; an AC current signal source configured to supply an AC current signal to the coil when the device is activated so that magnetic flux generated by the AC current signal in the coil combines with magnetic flux of the magnetic circuit so as to cause the slug and resilient suspension member to vibrate along a movement axis thereby causing the device to generate sound.

[0022] In this way, the device can generate sound by providing an AC signal to the coil. Moreover, by appropriate choice of AC current signal, the device can be used to generate a sound suitable for use as a vehicle horn (e.g. by supplying the coil with an AC current having a frequency component which corresponds to a resonant frequency of the second assembly - see below for more details), as well as to generate a sound suitable for use as an AVAS warning sound and / or containing speech (e.g. by supplying the coil with an AC current having a broadband current signal - see below for more details). Moreover, by having the resilient suspension member suspend the slug so that the slug is positioned in the magnetic flux of the magnetic circuit, the resilient suspension member need not form part of the magnetic circuit, which provides more design freedom for the construction of the resilient suspension member (which allows it to be constructed in a manner that enhances acoustic performance, without the need to construct it from ferromagnetic material).

[0023] The first assembly may optionally be referred to as a “fixed” assembly, since the first assembly is intended to stay in a fixed position relative to the second assembly when the device is activated.

[0024] The second assembly may optionally be referred to as a “moving” assembly, since it moves with respect to the first assembly when the device is activated.

[0025] The “slug” may be understood as a component of the device containing ferromagnetic material. The slug may have potentially any shape. The slug may contain a single material, or may contain multiple materials. However, it is important that the slug contains ferromagnetic material for carrying magnetic flux, so that slug can be influenced by magnetic flux generated by the AC current signal in the coil (noting that the slug will in practice be subject to the reluctance force generated by the combination of the magnetic flux produced by the permanent magnet and the magnetic flux generated by the AC current signal in the coil). In some examples, the slug may be a lump of ferromagnetic metal.

[0026] The resilient suspension member should be able to deform elastically under an applied stress, i.e., so it returns to substantially its original shape when the applied stress is released.

[0027] For avoidance of any doubt, vibration of the slug and resilient suspension member may cause the generation of sound directly (e.g. if the resilient suspension member is a resilient membrane that is configured to generate sound when it is vibrated) or indirectly (e.g. if the resilient suspension member is attached to a further element of the device, such as a rigid membrane, e.g. a rigid cone, whereby the further element is configured to generate sound when it is vibrated by the resilient suspension member).

[0028] Reference herein to the device being “activated” can be understood as referring to the device when an AC current signal is supplied to the coil so that magnetic flux generated by the AC current signal in the coil combines with magnetic flux of magnetic circuit so as to cause the slug and resilient suspension member to vibrate along a movement axis thereby causing the device to generate sound.

[0029] Reference herein to the device being “not activated” or “at rest” can be understood as referring to the device when no current is supplied to the coil and the device is at rest (i.e., the second assembly is not moving with respect to the first assembly).

[0030] In some examples, the resilient suspension member may be configured to suspend the slug from the first assembly in a position in which, when the device is not activated, the magnetic flux of the magnetic circuit biases the slug along the movement axis towards a component of the magnet unit, and the resilient suspension member provides a restoring force which holds the slug away from said component of the magnet unit.

[0031] In this way, the magnetic circuit pre-tensions the resilient suspension member, which is useful for giving the second assembly a desired broadband acoustic behaviour (suitable for producing an AVAS warning sound and / or speech playback) for reasons apparent from the discussion below.

[0032] The component of the magnet unit towards which the slug is biased by the magnetic flux of the magnetic circuit may be a flux guiding element of the magnetic circuit. In some examples, this flux guiding element (towards which the component of the magnet unit is biased) may have a projection towards which the slug is biased by the magnetic flux of the magnetic circuit. If the flux guiding element has a T shape when viewed in cross section along the movement axis (see below), the projection towards which the slug is biased by the magnetic flux of the magnetic circuit may be the upright part (as opposed to the crossbar part) of the T shape.

[0033] The resilient suspension member may be configured to suspend the slug from the first assembly in a position in which, when the device is not activated, the smallest distance between the slug and the component of the magnet unit towards which the slug is biased is below 3mm, more preferably below 2mm, more preferably below 1 mm. In other examples, the component of the magnet unit towards which the slug is biased by the magnetic flux of the magnetic circuit may be the permanent magnet (see e.g. Fig. 12a) - such an arrangement might be suitable for an AC current signal applied to the coil such that the slug avoids making contact with the magnet unit, but might be less suitable for an AC current signal applied to the coil such that the slug repeatedly makes contact with the magnet unit (since this contact might damage the permanent magnet).

[0034] In some examples, the resilient suspension member may be configured to suspend the slug from the first assembly in a position in which, when the device is not activated, the slug changes the direction of magnetic flux of the magnetic circuit entering and leaving the slug.

[0035] This may help the device to achieve a linear force vs current relationship when it is activated, which is useful for giving the second assembly a desired broadband acoustic behaviour (suitable for producing an AVAS warning sound and / or speech playback).

[0036] For example, the resilient suspension member may be configured to suspend the slug from the first assembly in a position in which, when the device is not activated, magnetic flux of the magnetic circuit enters / leaves the slug in one or more first directions and leaves / enters the slug in one or more second directions, wherein the one or more first directions are different from the one or more second directions.

[0037] For example, the one or more first directions may be substantially aligned with the movement axis. The one or more second directions may extend substantially perpendicular (e.g., radially) with respect to the movement axis (see e.g. Fig. 1).

[0038] For avoidance of any doubt, the polarity of the magnetic flux within the magnetic circuit is not believed to be important in this context, i.e., the magnetic flux may enter the slug in the one or more first directions and leave the slug in the one or more second directions, or may enter the slug in the one or more second directions and leave the slug in the one or more first directions. However, a skilled person would appreciate that polarity may be relevant in certain contexts - for example, for an asymmetric AC current signal applied to the coil such that the slug repeatedly makes contact with the magnet unit, the magnet polarity and current asymmetry would preferably be chosen such that the AC current maximises downwards force on the slug (i.e. in a direction towards the magnet unit) rather than maximising upwards force on the slug (i.e. in a direction away from the magnet unit).

[0039] In some examples, the magnet unit may include two flux guiding elements. The two flux guiding elements may be positioned on opposite sides of the permanent magnet (i.e. a first flux guiding element positioned on a first side of the permanent magnet, and a second flux guiding element position on a second side of the permanent magnet, wherein the first side is opposite to the second side).

[0040] In some examples, the two flux guiding elements may each include a planar portion which extends in a plane perpendicular to the movement axis.

[0041] In some examples, a first flux guiding element of the two flux guiding elements may have a T shape, when viewed in cross section along the movement axis. In such examples, the crossbar part of the T shape (as opposed to the upright part of the T shape) may extend in a plane perpendicular to the movement axis. In such examples, a second flux guiding element of the two flux guiding elements may be an annular element, e.g., a washer, which lies in a plane perpendicular to the movement axis.

[0042] In some examples, the permanent magnet may be an annular element which extends around the movement axis. In such examples, the permanent magnet may be configured to generate flux in an axial direction with respect to (i.e., along) the movement axis.

[0043] For avoidance of any doubt, in some examples, the magnet unit may include just one flux guiding element, e.g., in an arrangement in which the permanent magnet generates flux in a radial direction with respect to the movement axis.

[0044] In some examples, the resilient suspension member may be attached to the first assembly via a flexible material.

[0045] The flexible material (via which the resilient suspension member is attached to the first assembly) may help to allow relative movement between the resilient suspension member and the first assembly, e.g. so that an angle formed between the resilient suspension member and the first assembly at a junction between the resilient suspension member and the first assembly is able to vary when the device is in use.

[0046] Attaching the resilient suspension member to the first assembly via a flexible material (rather than via a non-flexible material which clamps the resilient suspension member to the first assembly in a manner that does not allow an angle formed between the resilient suspension member and the first assembly to vary at a junction between the resilient suspension member and the first assembly) has been found to be advantageous because it adds design freedom that can be used to influence the whole modal behaviour of the second assembly, e.g. which may make it easier to provide a fundamental resonant frequency in a frequency range suitable for use as a vehicle horn.

[0047] In some examples, the flexible material which attaches the resilient suspension member to the first assembly may, for example, be a layer of flexible material (e.g. of silicone rubber) formed on the first assembly.

[0048] In some examples, the flexible material which attaches the resilient suspension member to the first assembly may, for example, be a flexible component which is attached to both the resilient suspension member and the first assembly.

[0049] In some examples, the first assembly may include a rigid support component via which the resilient suspension member is attached to the remainder of the first assembly. The support component may extend around the movement axis. The support component may be ring-shaped, if the resilient suspension member is circular. However, the rigid support component may be omitted from the device in other examples, e.g., since the resilient suspension member may attach to the first assembly via a component of the first assembly other than the rigid support component (e.g. a flux guiding element of the magnet unit).

[0050] In some examples, the resilient suspension member may be a resilient membrane configured to generate sound when it is vibrated. In some examples, the resilient membrane may have the form of a disc, e.g. a flat disc. The disc may have a round outer periphery. The disc may be made of metal or of polymer. However, other shapes of resilient membrane (e.g. a squarish membrane - see e.g. Fig. 11), and other materials, are possible. In contrast, traditional horn membranes are often squarish and clamped only on the corners.

[0051] In some examples (e.g., where the resilient suspension member is attached to a rigid membrane configured to generate sound when it is vibrated), the resilient suspension member might not be configured to generate sound when vibrated. For example, the resilient suspension member may contain holes.

[0052] In some examples, the resilient suspension member may be attached to a rigid membrane, e.g., a rigid cone, configured to generate sound when it is vibrated. In such examples, the resilient suspension member might not be configured to generate sound, and thus may, for example, include holes.

[0053] In some examples, the device may include a funnel shaped waveguide configured to receive and direct sound produced by the device. The funnel shaped waveguide may be configured to increase and / or shape the SPL spectrum of sound produced by the device. Such waveguides are commonly used in horn apparatuses for vehicles, with each such waveguide commonly referred to as a “bell”.

[0054] In some examples, the slug may have a curved profile when viewed in cross section along the movement axis, e.g., to help guide flux between the slug and a flux guiding element of the magnet unit (which may be a planar flux guiding element when viewed in cross section along the movement axis).

[0055] In some examples, the device may include a spring located between the slug and the magnet unit (e.g. a first flux guiding element of the magnet unit), wherein the spring is configured to bias the slug away from the magnet unit. The spring may be a coil spring or a compliant member, for example. The spring may be useful for adjusting the fundamental resonant frequency of the second assembly.

[0056] In some examples, the AC current signal source may be configured to supply: a first AC current signal to the coil which is configured to cause the slug and resilient suspension member to vibrate along the movement axis such that the slug avoids making contact with the magnet unit thereby causing the device to generate a first sound.

[0057] In some examples, the AC current signal source may be configured to supply: a second AC current signal to the coil which is configured to cause the slug and resilient suspension member to vibrate along the movement axis such that the slug repeatedly makes contact with the magnet unit thereby causing the device to generate a second sound.

[0058] In some examples, the AC current signal source may be configured to supply: a first AC current signal to the coil which is configured to cause the slug and resilient suspension member to vibrate along the movement axis such that the slug avoids making contact with the magnet unit thereby causing the device to generate a first sound; and / or a second AC current signal to the coil which is configured to cause the slug and resilient suspension member to vibrate along the movement axis such that the slug repeatedly makes contact with the magnet unit thereby causing the device to generate a second sound. For avoidance of any doubt, in some examples the AC current signal source may be configured to generate just a first AC current signal or just a second AC current signal (in which case the labels “first” and “second” may be omitted). In other examples, the AC current source may be configured to generate both the first AC current signal and the second AC current signal (typically at different times).

[0059] The first AC current signal may be a broadband current signal containing multiple frequencies across the range 300Hz and 3kHz, e.g., as might be useful for producing an AVAS warning sound and / or speech playback. In this case, the first AC current signal may be referred to as a “broadband” AC current signal. The first sound may thus be a broadband sound, which may be suitable for use as an AVAS warning sound and / or for containing speech, for example.

[0060] The first sound may meet design requirements for use as an AVAS warning sound or other broadband sound containing speech.

[0061] The first sound may be a sound suitable for use as a vehicle horn, in which case the first AC current signal may be referred to as a “horn generating” AC current signal, instead of being referred to as a first AC current signal. The first sound may meet an ECE standard for a vehicle horn (e.g. ECE-28).

[0062] Where the first sound is intended for use as a vehicle horn, the first AC current signal may have a frequency component which corresponds to a resonant frequency (e.g. a fundamental resonant frequency) of the second assembly.

[0063] Where the first sound is intended for use as a vehicle horn, the first AC current signal may have a square wave profile which is useful because it has a low crest factor. But other forms of signal are equally possible and may be chosen according to design requirements, e.g. sine waves, triangular waves, and sawtooth waves. The exact form of the first AC current signal when used to generate a sound suitable for use as a vehicle horn is not thought to be particularly critical.

[0064] The second AC current signal may have a frequency component which corresponds to a resonant frequency (e.g. a fundamental resonant frequency) of the second assembly, in which case the second AC current signal may cause the slug and resilient suspension member to vibrate at the resonant frequency of the second assembly such that the slug repeatedly makes contact with the magnet unit thereby causing the device to generate the second sound.

[0065] The second AC current signal may have a square wave profile, which is useful because it has a low crest factor. But other forms of signal are equally possible and may be chosen according to design requirements.

[0066] The second sound may be a sound suitable for use as a vehicle horn, in which case the second AC current signal may be referred to as a “horn generating” AC current signal, instead of being referred to as a second AC current signal. The second sound may meet an ECE standard for a vehicle horn (e.g. ECE-28). The second moving assembly may be configured to have a resonant frequency (preferably a fundamental resonant frequency) in the range 150Hz - 600Hz, more preferably 250Hz - 500Hz, more preferably 400Hz - 520Hz.

[0067] A resonant frequency (preferably a fundamental resonant frequency) in this range allows the device to produce a sound which is able to mimic a traditional “single tone” vehicle horn sound when it is driven to vibrate at the fundamental resonant frequency of the second assembly (e.g. by supplying a “horn generating” AC current signal to the coil at this frequency).

[0068] Herein, a fundamental resonant frequency of a given assembly can be understood as the lowest resonant frequency of that assembly.

[0069] The second moving assembly may be configured to have a fundamental resonant frequency in the range 150Hz - 600Hz, more preferably 250Hz - 500Hz, more preferably 400Hz - 520Hz, preferably with higher resonant frequencies being aligned with a mode of a funnel shaped waveguide included in the device, which may help with efficiency and producing a “horn like” sound signature. For example, the second resonant frequency (mechanical resonance) of the second moving assembly, e.g. at 800Hz may be aligned with the second longitudinal (acoustical) resonance of the funnel shaped waveguide at 800Hz and so create a strong 2nd order component when playing a 400Hz signal. .The AC current signal source may include one or more amplifiers. The AC current signal source may include an AC current signal for producing an AVAS sound.

[0070] In a second aspect, the invention may provide: A horn apparatus for a vehicle, the horn apparatus including: a horn activation mechanism (e.g. a button) operable by a user of the vehicle; a device according to the first aspect of the invention; wherein the AC current signal source is configured to supply, when the horn activation mechanism is operated, a horn generating AC current signal to the coil so that magnetic flux generated by the AC current signal in the coil combines with magnetic flux of the magnetic circuit so as to cause the slug and resilient suspension member to vibrate along a movement axis thereby causing the device to generate sound suitable for use as a vehicle horn.

[0071] For avoidance of any doubt, the horn generating AC current signal might be either: an AC current signal which is configured to cause the slug and resilient suspension member to vibrate along the movement axis such that the slug avoids making contact with the magnet unit thereby causing the device to generate a first sound (e.g. an AC current signal having properties corresponding to the “first” AC current signal discussed above); or an AC current signal which is configured to cause the slug and resilient suspension member to vibrate along the movement axis such that the slug repeatedly makes contact with the magnet unit thereby causing the device to generate a second sound (e.g. an AC current signal having properties corresponding to the “second” AC current signal discussed above);. Here we note that the present inventor has found that it is possible to generate an authentic and loud horn sound using an AC current signal having properties corresponding either to the “first” AC current signal (for which the slug avoids making contact with the magnet unit) or the “second” AC current signal (for which the slug makes contact with the magnet unit) as described above.

[0072] If the horn generating AC current signal has properties corresponding to the “first” AC current signal (for which the slug avoids making contact with the magnet unit) as described above, then the second aspect of the invention may provide in a first set of examples: A horn apparatus for a vehicle, the horn apparatus including: a horn activation mechanism (e.g. a button) operable by a user of the vehicle; a device according to the first aspect of the invention; wherein the AC current signal source is configured to supply, when the horn activation mechanism is operated, a horn generating AC current signal to the coil which is configured to cause the slug and resilient suspension member to vibrate along the movement axis such that the slug avoids making contact with the magnet unit thereby causing the device to generate a sound suitable for use as a vehicle horn.

[0073] In the first set of examples of the second aspect of the invention, the skilled person would understand that the horn generating AC current signal may take a variety of forms as discussed previously.

[0074] If the horn generating AC current signal has properties corresponding to the “second” AC current signal (for which the slug makes contact with the magnet unit) as described above, then the second aspect of the invention may provide in a second set of examples: A horn apparatus for a vehicle, the horn apparatus including: a horn activation mechanism (e.g. a button) operable by a user of the vehicle; a device according to the first aspect of the invention; wherein the AC current signal source is configured to supply, when the horn activation mechanism is operated, a horn generating AC current signal to the coil so that magnetic flux generated by the AC current signal in the coil combines with magnetic flux of magnetic circuit so as to cause the slug and resilient suspension member to vibrate along a movement axis such that the slug repeatedly makes contact with the magnet unit thereby causing the device to generate sound suitable for use as a vehicle horn.

[0075] In the second set of examples of the second aspect of the invention, the horn generating AC current signal may have a frequency component which corresponds to a resonant frequency (e.g. a fundamental resonant frequency) of the second assembly, in which case the AC current signal may cause the slug and resilient suspension member to vibrate at the resonant frequency of the second assembly such that the slug repeatedly makes contact with the magnet unit thereby causing the device to generate the sound suitable for use as a vehicle horn.

[0076] In some examples, the AC current signal source may be further configured to supply a broadband AC current signal to the coil so that magnetic flux generated by the AC current signal in the coil combines with magnetic flux of magnetic circuit so as to cause the slug and resilient suspension member to vibrate along a movement axis such that the slug avoids making contact with the magnet unit thereby causing the device to generate a broadband sound.

[0077] In some examples, the horn apparatus may include: a first device according to the first aspect of the invention, wherein the second assembly of the first device is configured to have a resonant frequency (preferably a fundamental resonant frequency) in the range 150Hz - 600 Hz; a second device according to the first aspect of the invention, wherein the second assembly of the second device is configured to have a resonant frequency (preferably a fundamental resonant frequency) in the range 150Hz - 600 Hz which is different from the resonant frequency of the second assembly of the first device; wherein the AC current signal source is configured to supply, when the horn activation mechanism is operated: the coil of the first device with a first “horn generating” AC current signal which is configured to cause the slug and resilient suspension member to vibrate along the movement axis thereby causing the device to generate a sound suitable for use as a vehicle horn; the coil of the first device with a second “horn generating” AC current signal which is configured to cause the slug and resilient suspension member to vibrate along the movement axis thereby causing the device to generate a sound suitable for use as a vehicle horn.

[0078] For avoidance of any doubt, the first horn generating AC current signal and the second horn generating AC current signal may each be either: an AC current signal which is configured to cause the slug and resilient suspension member to vibrate along the movement axis such that the slug avoids making contact with the magnet unit thereby causing the device to generate a first sound (e.g. an AC current signal having properties corresponding to the “first” AC current signal discussed above); or an AC current signal which is configured to cause the slug and resilient suspension member to vibrate along the movement axis such that the slug repeatedly makes contact with the magnet unit thereby causing the device to generate a second sound (e.g. an AC current signal having properties corresponding to the “second” AC current signal discussed above).

[0079] In some examples (e.g. where the first and second horn generating AC current signal have properties corresponding to the “second” AC current signal discussed above), the horn apparatus may include: a first device according to the first aspect of the invention, wherein the second assembly of the first device is configured to have a resonant frequency (preferably a fundamental resonant frequency) in the range 150Hz - 600 Hz; a second device according to the first aspect of the invention, wherein the second assembly of the second device is configured to have a resonant frequency (preferably a fundamental resonant frequency) in the range 150Hz - 600 Hz which is different from the resonant frequency of the second assembly of the first device; wherein the AC current signal source is configured to supply, when the horn activation mechanism is operated: the coil of the first device with a first “horn generating” AC current signal configured to cause the slug and resilient suspension member of the first device to vibrate along the movement axis such that the slug repeatedly makes contact with the magnet unit thereby causing the first device to generate a second sound (the first “horn generating” AC current signal may, for example, have a frequency component which corresponds to the resonant frequency of the second assembly of the first device, and which may cause the slug and resilient suspension member of the first device to vibrate at the resonant frequency of the second assembly of the first device such that the slug of the first device repeatedly makes contact with the magnet unit of the first device so as to generate a first sound suitable for use as a vehicle horn); the coil of the second device with a second “horn generating” AC current signal configured to cause the slug and resilient suspension member of the second device to vibrate along the movement axis such that the slug repeatedly makes contact with the magnet unit thereby causing the second device to generate a second sound (the second “horn generating” AC current signal may, for example, have a frequency component which corresponds to the resonant frequency of the second assembly of the second device, and which may cause the slug and resilient suspension member of the second device to vibrate at the resonant frequency of the second assembly of the second device such that the slug of the second device repeatedly makes contact with the magnet unit of the second device so as to generate a second sound suitable for use as a vehicle horn).

[0080] The first and second “horn generating” AC current signals referenced here may each have properties as described above with reference to the “second AC current signal”, for example.

[0081] In this way, the horn apparatus may be able to generate a sound which mimics a traditional “dual tone” vehicle horn sound.

[0082] The AC current signal source of the horn apparatus may further be configured to supply a broadband AC current signal (e.g. containing multiple frequencies across the range 300Hz and 3kHz to the coils of the first and second devices, e.g., as might be useful for producing an AVAS warning sound and / or speech playback) to the coil of the first device and the coil of the second device. The broadband AC current signal is preferably configured to cause the slug and resilient suspension member of each of the first and second devices to vibrate along the movement axis such that the slug avoids making contact with the magnet unit thereby causing the first and second devices to generate a broadband sound, e.g., suitable for use as an AVAS warning sound and / or for containing speech.

[0083] The “broadband” AC current signal may have properties as described above with reference to the “first AC current signal”, for example.

[0084] In a third aspect, the invention may provide: a vehicle including the horn apparatus of the second aspect of the invention.

[0085] In a fourth aspect, the invention provides: A device for generating sound, including: a first assembly including: at least one flux guiding element; a coil; a second assembly, configured to move with respect to the first assembly, the second assembly including: a permanent magnet; a resilient suspension member attached to the first assembly; wherein the resilient suspension member is configured to suspend the magnet from the first assembly such that the magnet is positioned to form a magnetic circuit in which the at least one flux guiding element guides magnetic flux produced by the permanent magnet; an AC current signal source configured to supply an AC current signal to the coil when the device is activated so that magnetic flux generated by the AC current signal in the coil combines with magnetic flux of the magnetic circuit so as to cause the permanent magnet and resilient suspension member to vibrate along a movement axis thereby causing the device to generate sound.

[0086] Such a device (see e.g. Fig. 12b) provides similar benefits to the device according to the first aspect of the invention, and may be particularly suitable for use when operated in a manner in which the permanent magnet avoids making contact with the first assembly.

[0087] Thus, in some examples, the AC current signal source may be configured to supply: an AC current signal to the coil which is configured to cause the slug and resilient suspension member to vibrate along the movement axis such that the permanent magnet avoids making contact with the first assembly thereby causing the device to generate a first sound.

[0088] This AC current signal supplied by the AC current signal source of the fourth aspect of the invention may have any of the features of the “first AC current signal” described in relation to the first aspect of the invention.

[0089] For example, the AC current signal may be a broadband current signal containing multiple frequencies across the range 300Hz and 3kHz, e.g., as might be useful for producing an AVAS warning sound and / or speech playback. In this case, the first AC current signal may be referred to as a “broadband” AC current signal. The sound generated by the device may thus be a broadband sound, which may be suitable for use as an AVAS warning sound and / or for containing speech, for example. The sound may meet design requirements for use as an AVAS warning sound or other broadband sound containing speech, for example.

[0090] For example, the sound generated by the device may be a sound suitable for use as a vehicle horn, in which case the AC current signal may be referred to as a “horn generating” AC current signal. The first sound may meet an ECE standard for a vehicle horn (e.g. ECE-28).

[0091] In some examples, the second assembly may include an additional flux guiding element (e.g. slug) for guiding magnetic flux produced by the permanent magnet in the magnetic circuit. In some examples, the magnet is of neodymium, which provides a good power to weight ratio which may be useful for generating loud sounds (e.g. suitable for use as a vehicle horn) without requiring contact between the permanent magnet and the first assembly.

[0092] The invention includes the combination of the aspects and preferred features described except where such a combination is clearly impermissible or expressly avoided.

[0093] Summary of the Figures

[0094] Embodiments and experiments illustrating the principles of the invention will now be discussed with reference to the accompanying figures in which:

[0095] Fig. 1 is a partial cross section through a device for generating sound, taken along a movement axis of the device.

[0096] Fig. 2 is a cross section through the device of Fig. 1 , wherein the device has been modified to include a funnel configured to receive and direct sound produced by the device.

[0097] Fig. 3 shows reluctance force vs coil current for the device shown in Fig. 1 .

[0098] Fig. 4 shows electrical impedance vs frequency for the coil of the device shown in Fig. 1 .

[0099] Fig. 5 shows the electroacoustic transfer function in dBSPL / V of the device of Fig. 1 at 1 m distance under anechoic conditions.

[0100] Fig. 6 shows a horn apparatus for a vehicle, including two devices of the type shown in Fig. 1 , tuned to have different fundamental resonant frequencies.

[0101] Fig. 7 shows an example complex waveform that might be reproduced by the device of Fig. 1 .

[0102] Fig. 8 shows another example complex waveform that might be reproduced by the device of Fig. 1

[0103] Fig. 9 shows a device for generating sound similar to that of Fig. 1 , in which the shape of the disc and slug has been modified.

[0104] Fig. 10 shows a device for generating sound similar to that of Fig. 1 , in which the disc is attached to a rigid cone configured to generate sound when it is vibrated.

[0105] Fig. 11 shows another example device for generating sound similar to that of Fig. 1 , in which a spring configured to bias the slug away from the magnet unit has been incorporated.

[0106] Fig. 12a shows a partial cross section of a device for generating sound, taken along a movement axis of the device.

[0107] Fig. 12b shows a device for generating sound similarto that of Fig. 12a, in which the position of the magnet has been modified. Detailed Description of the Invention

[0108] 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.

[0109] Fig. 1 is a partial cross section through a device 100 for generating sound, taken along a movement axis 102 of the device 100. The view shown in Fig. 1 is only partial, because only the part of the device 100 to the right of a movement axis 102 is shown (the part to the left of the movement axis 102 can be seen e.g., in Fig. 2). In the example shown in Fig. 1 , the device 100 is circular, with the components shown in Fig. 1 all extending around the movement axis 102.

[0110] Fig. 2 is a cross section through the device 100 of Fig. 1 , wherein the device 100 has been augmented to include a funnel shaped waveguide 170 configured to receive and direct sound produced by the device 100.

[0111] A traditional car horn apparatus (not shown) is powered by DC current and by its very nature is only capable of reproducing a single tone by means of forced mechanical oscillation. Whereas the device 100 of Figs. 1 and 2 is powered by an AC current signal and can generate both sound for use as a vehicle horn and sound for use as AVAS warning sound.

[0112] The device 100 includes a first (“fixed”) assembly 110 including a magnet unit 112 and a coil 130. The magnet unit 112 includes a permanent magnet 114, a first flux guiding element 116 and a second flux guiding element 118. The flux guiding elements 116, 118 are for guiding the magnetic flux produced by the permanent magnet 114 in a magnetic circuit having flux cpM.

[0113] In this example, the permanent magnet 114 is a ferrite ring magnet configured to generate flux in an axial direction with respect to the movement axis 102. In this example, the first flux guiding element 116 has a T shape when viewed in cross section along the movement axis 102 (see Fig. 2), and the second flux guiding element 118 is a washer which lies in a plane perpendicular to the movement axis 102. The washer 118 extends around the movement axis 102, and has a hole at its centre which is aligned with the movement axis 102 (in which a slug 150 is positioned by a resilient suspension member 160 - see below).

[0114] The first flux guiding element 116 and second flux guiding element 118 are similar to the T-yoke and washer found in conventional loudspeakers having a ‘T-yoke’ design.

[0115] The coil 130 is part of the first (“fixed”) assembly 110, and in this example is attached directly to the first flux guiding element 116. The coil 130 extends around the movement axis 102, and hence around the upright part of the T shape of the first flux guiding element 116.

[0116] In this example, the first (“fixed”) assembly 110 includes a rigid support component which extends around the movement axis 102 and takes the form of a rigid support ring 120 which sits on top of the washer 118.

[0117] The device 100 also includes a second (“moving”) assembly 140, configured to move with respect to the first (fixed”) assembly 110. The second assembly 140 includes: a slug 150 containing ferromagnetic material for carrying magnetic flux; and a resilient suspension member 160 attached to the first assembly 110, wherein the resilient suspension member 160 is configured to suspend the slug 150 from the first assembly 110 such that the slug 150 is positioned in the magnetic flux of the magnetic circuit.

[0118] In this example, the resilient support member 160 takes the form of a disc 160, which serves as a resilient membrane configured to generate sound when it is vibrated.

[0119] In this example, the slug 150 is a lump of low carbon steel.

[0120] As shown in Fig. 1 (with the device 100 not activated), the flux lines representing the flux <f>Mof the magnetic circuit in this cross-section exit the permanent magnet 114 axially (i.e., in a direction parallel to the movement axis 102) and are guided through the washer 118 radially (i.e., in a direction perpendicular to the movement axis 102) towards the slug 150. The flux lines change direction in the slug 150 (owing to the position in which the slug 150 is suspended in the magnetic circuit by the disc 160) and exit the slug 150 axially and are guided back to the magnet 114 via the first flux guiding component 116. With the north pole of the magnet 114 being upwards in this example, the flux lines go through the magnetic circuit counter-clockwise as indicated by the arrow on the flux <f>Min Fig. 1 .

[0121] As shown in Figs. 1 and 2 (with the device 100 not activated), the disc 160 suspends the slug 150 from the first assembly 110 in a position in which the magnetic flux of the magnetic circuit biases the slug 150 along the movement axis 102 towards the upright part of the T shape of the first flux guiding element 116 of the magnet unit 112, and the disc 160 provides a restoring force which holds the slug 150 away from said component of the magnet unit 112. In this way, the magnetic circuit pre-tensions the disc 160. Fig. 2 also shows an AC current signal source 180. The AC current signal source 180 is configured to supply an AC current signal to the coil 130 via wiring 182 when the device 100 is activated so that magnetic flux generated by the AC current signal in the coil 130 combines with magnetic flux of the magnetic circuit so as to cause the slug 150 and resilient suspension member 160 to vibrate along a movement axis 102 thereby causing the device 100 to generate sound. The wiring 182 is here shown as reaching the coil 130 via the first flux guiding element 116, but other wiring paths are equally possible.

[0122] In particular, when the AC current signal is supplied to the coil 130 (e.g., when the device 100 is activated), it generates a magnetic flux < >c, which combines with the magnetic flux <f>Mof the magnetic circuit. Depending on the instantaneous current direction of the AC current signal, the static magnetic flux present in the flux guiding elements 116, 118 is either enhanced or weakened. That is, when the instantaneous current exits the plane shown in Fig. 1 towards the reader, the coil flux is counter-clockwise and strengthens the total flux, when the current enters the into plane depicted in Fig. 1 (away from the reader) the generated flux is clockwise and weakens the field (Right-Hand Rule of magnetic flux around a current carrying wire).

[0123] The slug 150 and disc 160 are subject to the magnetic reluctance force FRas a result of the magnetic flux <f>Mbridging the airgap between the slug 150 and disc 160, and hence are axially attracted to the upright part of the T shape of the first flux guiding element 116. The radial component of the reluctance force FRgenerated between the slug 150 and the second flux guiding element 118 is cancelled in an axisymmetric arrangement. When the device 100 is not activated, i.e., with only the static magnetic flux <f>Mpresent, the disc 160 is pre-tensioned by said axial force.

[0124] When supplying the coil 130 with current of either positive or negative polarity the static magnetic flux is modulated and gives way to a linear force vs current relationship. When supplying the current with a “broadband” AC current signal of audible frequency and which is configured to cause the slug 150 and resilient disc 160 to vibrate along the movement axis 102 such that the slug 150 avoids making contact with the magnet unit 112, the slug 150 and disc 160 will consequently vibrate with this frequency and a first sound suitable for use as an AVAS warning sound and / or for containing speech will be generated (in this example, as a result of the vibrating disc 160).

[0125] To achieve a high reluctance force FRand strong force dependence on the coil 130 current, the distance between slug 150 and T-yoke core portion 116 is preferably small and in the range below 3mm, more preferably below 2mm, more preferably below 1 mm. This small distance is useful because when supplying a suitably large AC current signal to the coil 130, it helps allow the slug 150 to hit the upright part of the T shape of the first flux guiding element 116 during downwards motion when the flux is increased. This behaviour is particularly observed when a “horn generating” AC current signal having a square wave profile at the fundamental resonant frequency (drum mode) of the pre-tensioned second (“moving”) assembly 140 is supplied to the coil 130, causing a second sound suitable for use as a vehicle horn to be heard. This second sound has a harsh impact quality, and it is increases in loudness, the higher the magnitude of the “horn generating” AC current signal that is supplied to the coil 130. As this hitting of a central slug 150 connected to a disc 160 is similar to the sound generating mechanism in a traditional car horn apparatus, the spectrum of the sound is very comparable to a traditional car horn. The fundamental resonant frequency of the second (“moving”) assembly 140 may be tuned to be in the range of a traditional car horn, e.g., approximately between 250Hz and 500Hz, more preferably in the range of 400Hz to 520Hz.

[0126] Tuning the frequency of the moving second (“moving”) assembly 140 can be achieved, for example, by varying the shape, mass, and thickness of the disc 160 and the mass of the slug 150. There is considerable design freedom in choosing the material from which the disc 160 is made, and also in the size and shape of the disc 160 (which does not need to be a disc at all, noting that it may include holes and / or have a non-disc-like shape).

[0127] The present inventor has also found that when the AC current signal source 180 supplies an AC current signal to the coil which is configured to cause the slug and resilient suspension member to vibrate along the movement axis such that the slug avoids making contact with the magnet unit, the device is nonetheless able to generate a sound loud enough for use as a vehicle horn. In other words, the present inventor found that the slug striking the first assembly is not a pre-requisite to generating a sound loud enough for use as a vehicle horn. This means there are potential examples in which a sound suitable for a vehicle horn can be generated by devices in which there is not repeated contact between slug and first assembly (see Figs. 12a and 12b, below). In the example device 100 depicted in Fig. 1 , the disc 160 is a metal disc, is round and has a 75mm diameter, and is formed from 0.3mm thick powder coated spring steel. The disc 160 is attached to the rigid support ring 120 via a thin flexible layer of silicone rubber (not shown), which allows relative movement between the disc 160 and the rigid support ring 120, so that an angle formed between the disc 160 and the rigid support ring 120 at a junction between the disc 160 and the rigid support ring 120 is able to vary when the device 100 is in use. There is considerable design freedom in choosing the material from which the disc 160 is made, as it does not need to be part of the magnetic circuit, so it can be ferromagnetic or not.

[0128] As such, a wide range of resilient materials configured to deform elastically (i.e. not subject to creep, i.e. not subject to plastic deformation over time) may be suitable for use as the disc 160, as a change in pretension or distance between slug 150 and T-yoke core 116 can be used to alter the fundamental resonant frequency of the second assembly 140 and the required voltage for hitting the first flux guiding element 116 with the slug 150. The resilient material used for the disc 160 may include a metal or a polymer, for example. The disc 160 doesn’t have to be round or flat, or be made of any specific material, so there is lots of design freedom that can be varied according to application requirements.

[0129] In the example device 100 depicted in Fig. 1 , the slug 150 is a cylinder that is 10mm in diameter and from mild steel, just like the first and second flux guiding elements 116, 118. The distance between slug 150 and first flux guiding element 116 may be tuned such that hitting of the first flux guiding element 116 occurs when the coil 130 is supplied with a square wave with a peak value of approximately 40V. Other ferromagnetic materials (e.g. a ferromagnetic stainless steel) could optionally be used for the slug 150.

[0130] In the example device 100 depicted in Fig. 1 , a front-facing side of the disc 160 is configured to radiate sound into a funnel shaped waveguide as depicted in Fig. 2, which for example may be a 33cm long folded horn with throat diameter 14mm and an elliptical mouth with dimensions 8cm by 6cm. These parameters are chosen so as to be tuned for improving the sound generated by the second movement assembly 140 (tuning a funnel shaped waveguide for resonance of a vehicle horn is well understood in the art and documented in standard textbooks, so need not be described further herein).

[0131] To increase the electroacoustic conversion efficiency, particularly for higher frequencies, or add acoustical resonances, particularly for lower frequencies, the device 100 can be fitted with a funnel shaped waveguide 170 (“horn”), e.g., as shown schematically in Fig. 2. Here we note that the dimensions of the device 100 can be in the same range as traditional car horns and can be just as easy to manufacture and very similar in cost. In fact, the device of Fig. 2 is arguably simpler than a conventional car horn apparatus, as the signal does not need to be interrupted as in an electromechanical horn and no electronics is needed as in the electronic horns using a MOSFET circuit. As there is no mechanical making and breaking of the contact, the device 100 can readily be constructed to withstand well beyond 100k cycles just like an electronic horn but without the additional complexity.

[0132] The AC current signal source 180 may include one or more audio amplifiers 185. For example, the AC current signal source 180 may include an amplifier 185 of a kind that is already used in vehicles for AVAS playback for supplying the “broadband” AC current signal to the coil 130, and another more powerful amplifier 185’ for supplying the second AC current signal to the coil 130 (noting that a more powerful AC current signal would typically be needed to produce a sound suitable for use as a vehicle horn). In other examples, the AC current source may include a single amplifier 185 that is used to supply both the first AC current signal and the second AC current signal. This single amplifier 185 may in some examples be more powerful than a typical amplifier currently used for AVAS playback, in order to produce a second AC current signal that is powerful enough to produce a sound suitable for use as a vehicle horn (in other examples, additional power may be provided by using more windings on the coil or a more powerful permanent magnet - these parameters may all be varied according to design requirements).

[0133] Here we note that traditional (solenoid-based) car horns typically do not use an amplifier, but instead draw DC current from the car battery. However, the power requirements of a device 100 as shown in Fig. 1 would be comparable to the power requirements of a traditional (solenoid-based) car horn when used to produce sound of similar SPL.

[0134] Here we also note that the device 100 of Fig. 1 is able to be constructed to be very robust. In particular, unlike in a conventional loudspeaker, flexible tinsel wire (“lead wire”) is not needed to supply the coil 130 with an AC current signal, since the coil 130 is part of the first (fixed) assembly 110. Moreover, the coil 130 is well protected within the motor system. The disc 160 can be made to be comparably thick and robust and can be equipped with a coating capable of withstanding the elements, noting that the device 100 would typically be mounted with only the disc 160 exposed to the outside of the car (the internals would thus be well protected by the thick disc). The disc 160 may use high force at comparably high moving mass which may help achieve a low fundamental resonant frequency (e.g. between 250Hz and 500Hz) without the need for a large back volume typically associated with loudspeakers capable of low frequency reproduction.

[0135] Fig. 3 shows the attraction force (Reluctance Force [N]) between the first flux guiding element 116 and the slug 150 is -19N when no current is applied, with negative force values representing a downwards force of the slug 150 towards the first flux guiding element 116. When applying -1A to the coil 130 the force is -31 N, when applying +1A the force is -9.6N. Through the fact that the flux lines enter the slug 150 radially and exit axially there is a substantially linear relationship between coil 130 current and attraction force up to approximately 1 ,5A. Moreover, in this example the device 100 is useable up to +-2A with acceptable distortion from the motor system. For this device 100, increasing current beyond 2A is not preferred as it may lead to the permanent magnet 114 being demagnetised. At 3A the flux of the coil 130 has the same magnitude but opposite polarity as the flux of the permanent magnet 114 and there is no net force on the second (slug 150 and disc 160) assembly 140. Were currents higher than 3A adopted, this would “overpower” the magnet 114 entirely and lead to negative reluctance again (i.e., attraction between the slug 150 and the first flux guiding element 116). For negative currents the flux of the permanent magnet 114 and the coil 130 add up and the maximum achievable reluctance force is in practice limited by the saturation of the flux guiding components 116, 118 (since when the flux guiding components 116, 118 are saturated an increase in current would only lead to a small increase in flux and hence force) and the maximum current through the coil. Here we note that use of the slug 150 attached to the resilient suspension member (disc) 160 provides the advantage that the resilient suspension member 160 does not need to carry flux, which means there is more design freedom in tuning the resilient suspension member 160 (e.g. through selection of appropriate materials etc) to provide the second assembly 140 with a desired fundamental resonant frequency.

[0136] Since the device 100 is an AC device, it needs to be matched with an amplifier 185. Fig 4 shows the magnitude of the electrical input impedance of the coil 130 (AC resistance, Ohms) vs frequency. From an RDC of 3.4 ohms the impedance rises mainly following the shape of an inductor with 7.4mH leading to 50 Ohms at 1 kHz. Despite a coil 130 with only two-hundred windings the inductance is high due to the high permeability of the non-saturated flux guiding elements 116, 118 surrounding the coil 130 almost entirely. The reluctance for the AC flux generated by the coil 130 is mainly defined by the magnet 114 height as the ferrite magnet itself has a permeability close to 1 . The high reactive resistance of the coil 130 as shown by Fig. 4 demonstrates that the high voltages needed to drive the device 100 of Fig. 1 won’t need to draw high power, which is an advantage.

[0137] Note that the coil 130 has two-hundred windings in this example, but the number of windings may be changed according to design requirements.

[0138] Fig. 5 shows the electroacoustic transfer function in dBSPLA / of the device at 1 m distance under anechoic conditions. The curve shows broadband output between 300Hz and 3kHz suitable for AVAS output and speech playback (telephone band), albeit coloured by the peaky response (which is not as smooth as might be ideal, but is able to provide reasonable performance at low cost). The device 100 is intended to be used with an amplifier (for example, the amplifier 185) with a peak output voltage of 45- 48V as nowadays available in electric vehicles. Two of the devices 100 shown in Fig. 1 , but tuned to have different fundamental resonant frequencies (see discussion of Fig. 6, below), and mounted on the same bracket fed by the same AC-signal will add up their output by 6dB and are suitable for AVAS output. Ideally, the peaks and dips of the frequency responses of the two devices 100 would be interleaved such that at each frequency band one of the two devices 100 is strong whereas the other one is weak and so the overall transfer function is smoothed.

[0139] When applying a 48V 280Hz square wave - the frequency corresponding to the first peak in the transfer function as shown in Fig. 5, i.e., corresponding to the fundamental resonant frequency of the second assembly 140 of the device 100 used to create the transfer function shown in Fig. 5, the slug 150 hits the first flux guiding element 116 with this frequency and creates a horn like sound with an alarm-like characteristic. In 2m distance 103dB(A) are reached. Two of these devices 100 tuned differently, through adding up their output by +3dB, would be able to meet the output requirement of ECE-R28 just like a pair of traditional horns.

[0140] Fig. 6 shows a horn apparatus 200 for a vehicle, the horn apparatus including: a horn activation mechanism 190 (e.g. a button) operable by a user of the vehicle; a first device 100a similar to that shown in Fig. 2, wherein the second assembly of the first device 100a is configured (“tuned”) to have a fundamental resonant frequency in the range 150Hz - 600 Hz; a second device 100b similar to that shown in Fig. 2, wherein the second assembly of the second device 100b is configured (“tuned”) to have a fundamental resonant frequency in the range 150Hz - 600 Hz which is different from the fundamental resonant frequency of the second assembly of the first device 100a; wherein the AC current signal source 180 is configured to supply, when the horn activation mechanism 190 is operated: the coil of the first device 100a with a first “horn generating” AC current signal having a frequency which corresponds to the fundamental resonant frequency of the second assembly of the first device 100a (which may cause the slug and membrane of the first device 100a to vibrate at the fundamental resonant frequency of the second assembly of the first device 100a (optionally such that the slug of the first device 100a repeatedly makes contact with the magnet unit of the first device 100a) so as to generate a first sound suitable for use as a vehicle horn); the coil of the second device 100b with a second “horn generating” AC current signal having a frequency which corresponds to the fundamental resonant frequency of the second assembly of the second device 100b (which may cause the slug and membrane of the second device 100b to vibrate at the fundamental resonant frequency of the second assembly of the second device 100b (optionally such that the slug of the second device 100b repeatedly makes contact with the magnet unit of the second device 100b) so as to generate a second sound suitable for use as a vehicle horn).

[0141] The first and second “horn generating” AC current signals may have the properties of the “horn generating” AC current signal described above. When such AC current signals are supplied to the devices 100a, 100b, the sound generated can convincingly mimic the sound of a solenoid-based dual tone vehicle horn apparatus as used in many vehicles today (even if the slugs of the two devices 100a, 100b do not repeatedly make contact with the magnet units).

[0142] However, when supplying the two devices 100a, 100b with a “broadband” AC current signal as described above, e.g., with the spectral energy distributed over a wide frequency range, such as pitch shifted noise used forAVAS applications, the slug 150 does not hit the T-yoke 116 and the two devices 100a, 100b can be used just like a traditional loudspeaker.

[0143] Automotive manufacturers are used to mounting a set of two devices 100a, 100b just like the device 100 shown in Figs. 1 and 2 on a bracket in a vehicle. However, the sound produced by the device 100 can be considerably more complex that that produced by pure mechanical resonance, by supplying a “broadband” AC current signal as described above. This makes the invented device very useful and flexible for many applications beyond the use as a vehicle warning system, e.g., in alarm systems where next to a siren tone also speech reproduction is required.

[0144] Some potential modifications of the device 100 shown in Fig. 1 will now be discussed. Not all modern DC horns are designed to have the slug 150 connected to the disc 160 hit an anvil during each cycle. Some horns are generating a smoother sound by having the slug merely attracted periodically.

[0145] Accordingly, a smoother horn sound could also be generated by the device 100 of Fig. 1 by using a different signal in place of the first AC current signal. For example, instead of using square wave AC current signal of fixed frequency, it would be possible to use another signal type. Viable alternatives include triangular or sawtooth signals. Indeed, since the device 100 is capable of producing sound based on an AC current waveform having any signal, also a more complex waveform can be played back, with or without intentionally having the slug 150 hit the T-yoke 116.

[0146] Fig. 7 shows one such complex waveform, in the form of a cycle of a square wave with changing frequency within the cycle (which repeats every 1 / 400 seconds). Such a wave form contains multiple frequencies with a particular spectral distribution and is able to convincingly mimic a traditional horn with high harmonics content (without the slug striking the magnet unit), which shows that a fixed frequency signal is not required to produce a sound suitable for use as a vehicle horn.

[0147] Fig. 8 shows yet another such complex waveform suitable for playback. In this case, the waveform is more rounded and sounds smoother. It is particularly suitable for a design where the combination of voltage and distance between slug 150 and first flux guiding element 116 does not result in periodic hitting during operation.

[0148] Yet more complex waveforms can be designed to have the spectral distribution align with the resonances of the disc 160 and (if present) the funnel-shaped waveguide, e.g. for maximum output and alarming character.

[0149] The resonances of the resilient suspension member 160 (in this example disc) can be influenced by changing its shape.

[0150] Fig. 9 shows a device 300 for generating sound similar to that of Fig. 1 , in which the shape of the disc 360 and slug 350 has been modified.

[0151] In more detail, in the device 300 of Fig. 9, the disc 360 has been modified to have a new shape with a non-flat cross-section leading to different eigenmodes of the disc 360 and consequently different sound when excited by the slug 350.

[0152] In the device 300 of Fig. 9, the slug 350 has been given a curved profile when viewed in cross section along the movement axis 302. More specifically, the slug 350 has been adapted to have a funnel shape which curves outwardly towards a flat (i.e. non-T shaped) first flux guiding element 316 to increase the opposing surface areas of slug 350 and first flux guiding element 316 to increase the attraction force. Fig. 9 is intended to demonstrate that the resilient suspension member 360 (in this example, a disc) can be made non-round, e.g., oval or square, or indeed follow an entirely arbitrary shape. The disc 360 can be clamped on the outside diameter (rather than be attached to the first assembly 310 via a flexible material, not shown) or it could be supported by a flexible material (as in the device 100 of Fig. 1) or sandwiched by one or more flexible members (not shown). The disc 360 could also have a portion that extends radially beyond a portion on which it is attached to the first assembly 310 via the flexible material, which may add another set of resonances. Neither the thickness nor the material of the disc 260 is required to be constant, and indeed it is observed by the inventors that any mechanical system with its resonances can be excited by the slug 350. Fig. 10 shows a device 400 for generating sound similar to that of Fig. 1 , in which the disc 460 is attached to a rigid cone configured to generate sound when it is vibrated.

[0153] In more detail, the device 400 has been modified by having a rigid membrane 465 attached to the resilient suspension member 460. In this example, the rigid membrane 465 forms a cone, a base of which is closed and attached to a centre of the resilient suspension member 460. As such, the rigid membrane 465 is configured to, when the device 400 is activated, vibrate as a result of the vibrating resilient suspension member 460, and thus generate sound. For avoidance of any doubt, the rigid membrane 465 may be adjusted to have a shape other than a cone, in other examples.

[0154] The rigid membrane 465 of the present example may be suspended from a frame (not shown) by a suspension element, depicted here as a roll suspension 467.

[0155] In the device 400 of Fig. 10, the resilient suspension member 460 need not be used to generate sound, since its primary purpose in this example is to generate the vibrations which cause the rigid membrane 465 to generate sound. Accordingly, the resilient suspension member 460 may in this example be equipped with one or more holes (not shown).

[0156] Sound generated by a front face of the rigid membrane 465 (i.e., the concave face of the cone 465 facing away from the magnet unit 414) may radiate towards a grille that further shapes the transfer function and spectrum of the sound, and may be suitable for use by a vehicle as a vehicle horn and / or for AVAS playback.

[0157] Sound generated by a rear face of the rigid membrane 465 (i.e. the convex face of the cone 465 facing towards the magnet unit 414) may radiate into a closed back-volume (not shown). In other examples however, the rear face of the rigid membrane 465 may instead radiate into an open back volume wherein the sound may escape out of one or more sides of the device 400.

[0158] The device 400 described in Fig. 10 is very efficient at the fundamental resonance followed by a frequency range of lower efficiency followed by a second, strong resonance. These two resonances can be aligned with the signal to the driving coil for an overall high output convincingly mimicking the sound of a simple tone disc DC horn. See, for example, the teaching given in WO2022 / 128595A1 .

[0159] Fig. 11 shows another example device 500 for generating sound similar to that of Fig. 1 , in which a spring 552 configured to bias the slug away from the magnet unit has been incorporated.

[0160] In Fig. 11 , the (electric) coil is omitted / hidden to show the spring 552, though in use the device 500 would include an (electric) coil similar to that of the example device 100 of Fig. 1 , in order to provide a working device.

[0161] In the example device 500 of Fig. 11 , the resilient disc 560 has a squarish shape and is clamped at four holes corresponding to “corners” of the squarish shape.

[0162] The spring 552 that has been included in the example device 500 is (at least partly) located between the slug 550 and the (first flux guiding element 516 of) the magnet unit 510. In this example, the spring 552 is a coil spring which coils around the slug 550. By choosing the spring rate and pretension of the spring 552, the resonance frequency of the second assembly of the device 500 can be influenced. The first assembly of the device 500 may be configured to allow pretension of the spring to be adjusted, e.g. by allowing adjustment of the length of the spring (not shown) and so adjust the fundamental resonant frequency of the second assembly. The spring 552 thus allows additional design freedom in the design of the device 500.

[0163] Although in the example depicted in Fig. 11 the spring 552 is implemented as a coil spring, in other examples the spring 552 may be implemented as a compliant member (e.g. a rubber disc) located between the slug 550 and the (first flux guiding element 516 of) the magnet unit 510 (not shown). In such examples, the compliant member may partially or completely fill the gap between the magnet unit 510 and the slug 550. Where the compliant member does not fill the gap, it may serve to dampen the impact of the slug on the first flux guiding member 516 of the magnet unit 510 during striking operation. Where the compliant member does fill the gap, it may be pre-tensioned by the static reluctance force and the amount of compression may be altered by powering the coil with an AC current.

[0164] Adding a spring 552 or compliant member as described above may be useful for altering the modal behaviour, transfer function and damping of the second assembly of the device 500.

[0165] Fig. 12a and Fig. 12b show partial cross-sections of further examples of devices 500, 600 for generating sound.

[0166] The devices 500, 600 of Figs. 12a and 12b include the same components that are present in the device 100 of Fig. 1 , though several of these components have been rearranged / modified. These differences are described in more detail below in relation to Fig. 12a.

[0167] The device 500 of Fig. 12a includes a first (“fixed”) assembly 540, which includes a permanent magnet 514, a first flux guiding element 516, a second flux guiding element (i.e., a washer) 518, and a coil 530, In this example, the first flux guiding assembly 516 forms a ‘U’ shape when viewed along the movement axis 502. The second flux guiding element 518 is formed in a plane that is perpendicular to the movement axis 502 and is attached to the top portion of the ‘U’ shape of the first flux guiding assembly 516.

[0168] The first flux guiding element 516 and second flux guiding element 518 are similar to the U-yoke and washer found in conventional loudspeakers having a ‘U-yoke’ design.

[0169] In this example, the magnet 514 is attached to a central portion of the first flux guiding element 516 and proximate to a slug 550 (see below). This magnet 514 produces a magnetic flux which travels in a circuit through the first flux guiding element 516, the washer 518 and the slug 550. Preferably, the magnet 514 is made from neodymium, which provides the magnet 514 with a good power to weight ratio, but the magnet 514 may instead be made out of an alternative ferromagnetic material. The coil 530 is also attached to the first flux assembly 516 in this example and forms a ring that extends around the magnet 514.

[0170] The first assembly 510 further includes a rigid support component which extends around the movement axis 502 and takes the form of a rigid support ring 520 which sits on top of the washer 518. The device 500 also includes a second (“moving”) assembly 540, configured to move with respect to the first assembly 510. The second assembly 540 includes: a slug 550 containing ferromagnetic material for carrying magnetic flux; and a resilient suspension member 560 attached to the first assembly 510 via the rigid support ring 520, wherein the resilient suspension member 560 is configured to suspend the slug 550 from the first assembly 510 such that the slug 550 is positioned in the magnetic flux produced by the magnet 514.

[0171] In this example, the slug 550 forms a ‘U’ shape structure which extends through a hole in the washer 518, with the resilient suspension member 560 being attached to the slug 550 at a top portion of this ‘U’ shape.

[0172] The device 500 operates in a similar manner to the devices of previous embodiments, in that supplying a suitable AC current signal to the device 500, and more specifically to the coil 530, will cause the second (“moving”) assembly to move (vibrate) relative to the first (“fixed”) assembly in a direction parallel to the movement axis 502. Preferably, the AC current signal cause movement (vibration) of the second assembly 540 relative to the first assembly 510 in a direction parallel to the movement axis 502 without causing the slug 550 to contact the magnet 514, since the magnet 514 may be unsuitable for withstanding repeated striking by the slug.

[0173] Fig. 12b shows a similar device 600 to the device 500 shown in Fig. 12a, in which a permanent magnet 614 is a part of a second ("moving") assembly 640 rather than a first (“fixed”) assembly 610. In this example, the magnet 614 is attached to a flux guiding element (referred to here as a “slug") 650 which is suspended from the first assembly 640 via a resilient suspension member 660. However, the present invention is not limited to this scenario, and the skilled person would understand that the magnet 614 may instead be attached to the second assembly 640 in several alternative ways (with or without a slug 650).

[0174] The device 600 operates in a similar manner to the devices of previous embodiments, in that supplying a suitable AC current signal to the device 600, and more specifically to the coil 630, will cause the second (“moving”) assembly to move (vibrate) relative to the first (“fixed”) assembly in a direction parallel to the movement axis 602. Preferably, the AC current signal cause movement (vibration) of the second assembly 640 relative to the first assembly 610 in a direction parallel to the movement axis 602 without causing magnet 614 to contact the flux guiding element 616 of the first assembly, since the magnet 614 may be unsuitable for withstanding repeated striking of the first assembly.

[0175] 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.

[0176] 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. 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.

[0177] Any section headings used herein are for organizational purposes only and are not to be construed as limiting the subject matter described.

[0178] 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.

[0179] 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%.

[0180] References

[0181] 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.

[0182] • “Uniform provisions concerning the approval of audible warning devices and of motor vehicles with regard to their audible signals” (“ECE-28”)

[0183] • US10406976B2

[0184] • FR2983025A1

[0185] • US2020 / 0070719A1

[0186] • WO2022 / 128595A1

Claims

Claims:1 . A device for generating sound, including: a first assembly including: a magnet unit including: a permanent magnet; at least one flux guiding element for guiding magnetic flux produced by the permanent magnet in a magnetic circuit; a coil; a second assembly, configured to move with respect to the first assembly, the second assembly including: a slug containing ferromagnetic material for carrying magnetic flux; a resilient suspension member attached to the first assembly, wherein the resilient suspension member is configured to suspend the slug from the first assembly such that the slug is positioned in the magnetic flux of the magnetic circuit; an AC current signal source configured to supply an AC current signal to the coil when the device is activated so that magnetic flux generated by the AC current signal in the coil combines with magnetic flux of the magnetic circuit so as to cause the slug and resilient suspension member to vibrate along a movement axis thereby causing the device to generate sound.

2. A device according to claim 1 , wherein the resilient suspension member is configured to suspend the slug from the first assembly in a position in which, when the device is not activated, the magnetic flux of the magnetic circuit biases the slug along the movement axis towards a component of the magnet unit, and the resilient suspension member provides a restoring force which holds the slug away from said component of the magnet unit.

3. A device according to any previous claim, wherein the resilient suspension member is configured to suspend the slug from the first assembly in a position in which, when the device is not activated, the slug changes the direction of magnetic flux of the magnetic circuit entering and leaving the slug.

4. A device according to any previous claim, wherein the magnet unit includes two flux guiding elements positioned on opposite sides of the permanent magnet.

5. A device according to any previous claim, wherein the resilient suspension member is attached to the first assembly via a flexible material6. A device according to any previous claim, wherein the resilient suspension member is a resilient membrane configured to generate sound when it is vibrated.

7. A device according to any previous claim, wherein the resilient suspension member is attached to a rigid membrane configured to generate sound when it is vibrated.

8. A device according to any previous claim, wherein the device includes a funnel shaped waveguide configured to receive and direct sound produced by the device.

9. A device according to any previous claim, wherein the device includes a spring located between the slug and the magnet unit, wherein the spring is configured to bias the slug away from the magnet unit.

10. A device according to any previous claim, wherein the AC current signal source is configured to supply: a first AC current signal to the coil which is configured to cause the slug and resilient suspension member to vibrate along the movement axis such that the slug avoids making contact with the magnet unit thereby causing the device to generate a first sound11. A device according to claim 10, wherein the first AC current signal is a broadband current signal containing multiple frequencies across the range 300Hz and 3kHz.

12. A device according to any previous claim, wherein the AC current signal source is configured to supply: a second AC current signal to the coil which is configured to cause the slug and resilient suspension member to vibrate along the movement axis such that the slug avoids making contact with the magnet unit thereby causing the device to generate a first sound.

13. A device according to claim 12, wherein the second AC current signal has a frequency component which corresponds to a resonant frequency of the second assembly so as to cause the slug and resilient suspension member to vibrate at the resonant frequency of the second assembly such that the slug repeatedly makes contact with the magnet unit thereby causing the device to generate the second sound.

14. A device according to any previous claim, wherein the second moving assembly is configured to have a resonant frequency in the range 150Hz - 600Hz.

15. A horn apparatus for a vehicle, the horn apparatus including: a horn activation mechanism operable by a user of the vehicle; a device according to any previous claim; wherein the AC current signal source is configured to supply, when the horn activation mechanism is operated, a horn generating AC current signal to the coil so that magnetic flux generated by the AC current signal in the coil combines with magnetic flux of magnetic circuit so as to cause the slug and resilient suspension member to vibrate along a movement axis thereby causing the device to generate sound suitable for use as a vehicle horn.

16. A horn apparatus according to claim 15, wherein the AC current signal source is further configured to supply a broadband AC current signal to the coil so that magnetic flux generated by the AC current signal in the coil combines with magnetic flux of magnetic circuit so as to cause the slug and resilient suspension member to vibrate along a movement axis such that the slug avoids making contact with the magnet unit thereby causing the device to generate a broadband sound.

17. A horn apparatus according to claim 15 or 16 that includes: a first device according to any of claims 1 to 14, wherein the second assembly of the first device is configured to have a resonant frequency in the range 150Hz - 600 Hz; a second device according to any of claims 1 to 14, wherein the second assembly of the second device is configured to have a resonant frequency in the range 150Hz - 600 Hz which is different from the resonant frequency of the second assembly of the first device; wherein the AC current signal source is configured to supply, when the horn activation mechanism is operated: the coil of the first device with a first horn generating AC current signal configured to cause the slug and resilient suspension member of the first device to vibrate along the movement axis thereby causing the first device to generate a second sound; the coil of the second device with a second horn generating AC current signal configured to cause the slug and resilient suspension member of the second device to vibrate along the movement axis thereby causing the second device to generate a second sound.

18. A device for generating sound, including: a first assembly including: at least one flux guiding element; a coil; a second assembly, configured to move with respect to the first assembly, the second assembly including: a permanent magnet; a resilient suspension member attached to the first assembly; wherein the resilient suspension member is configured to suspend the magnet from the first assembly such that the magnet is positioned to form a magnetic circuit in which the at least one flux guiding element guides magnetic flux produced by the permanent magnet; an AC current signal source configured to supply an AC current signal to the coil when the device is activated so that magnetic flux generated by the AC current signal in the coil combines with magnetic flux of the magnetic circuit so as to cause the permanent magnet and resilient suspension member to vibrate along a movement axis thereby causing the device to generate sound.

19. A device according to any previous claim, wherein the magnet is of neodymium.

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