Patent Description:
The present invention relates to a loudspeaker including a frame, a diaphragm and a drive unit, the loudspeaker being a subwoofer.

A typical conventional loudspeaker has a frame, a diaphragm and a drive unit for the reproduction of sound. In use, the drive unit causes the diaphragm, which acts as a piston, to move backwards and forwards to generate pressure waves, i.e. sound.

The drive unit typically includes a magnet unit attached to the frame and a voice coil attached to the diaphragm. By energising the voice coil, the magnet unit and the voice coil magnetically cooperate, i.e. magnetically interact, with each other to effect displacement of the combination of the voice coil and the diaphragm to thereby produce sound.

<CIT> describes a magnet movable type speaker and seat for vehicle. <CIT> describes a dual coil moving magnet transducer. <CIT> describes a speaker. <CIT> describes a speaker. <CIT> describes a heat dissipation dome and miniature speaker. <CIT> describes a voice coil loudspeaker. <CIT> describes a moving-magnet type speaker and method of manufacturing the same. <CIT> describes an audio driver assembly, headphone including such an audio driver assembly, and related methods. <CIT> describes a push-pull electromagnetic transducer with increased Xmax. <CIT> describes an electroacoustic transducer.

In contrast to the conventional loudspeaker outlined in the background section above, according to the present invention there is provided a loudspeaker with a drive unit including a translatable magnet unit and a stationary voice coil.

According to a first aspect of the invention, there is provided a loudspeaker according to claim <NUM>.

The loudspeaker may provide an improved structure with fewer and/or simpler component parts, for example removing the need for flexible lead wires and ticking noise that can be associated with flexible lead wires. Moreover, the exemplary loudspeaker may be manufacturable on existing production lines with existing machines, jigs and fixtures.

The loudspeaker includes suspension elements.

The suspension elements include a first suspension element, e.g. a surround, which attaches to the frame at a first landing surface on the frame. The first suspension element may attach directly or indirectly to the diaphragm. In particular, the first suspension element may be secured to an outer edge of the diaphragm.

The suspension elements include a second suspension element, e.g. a damper, which attaches to the frame at a second landing surface on the frame. The second suspension element may, for example, be secured to the translatable part of the drive unit or may be secured to the diaphragm at a location inwardly located with respect to the outer edge of the diaphragm.

The centre of gravity of the translatable part of the drive unit has a position along the movement axis that is between the first landing surface and the second landing surface.

By locating the centre of gravity of the magnet unit between the first landing surface and the second landing surface, rocking may be inhibited. More particularly, the rocking modes of the loudspeaker may be pushed outside of the working frequency range of the loudspeaker.

The second suspension element (e.g. damper) may have a position along the movement axis and arranged to radially extend towards and secure to a magnet unit of the drive unit. This may allow for a loudspeaker with a reduced depth compared with a conventional loudspeaker in which the damper typically is above or below the magnet unit (see e.g. <FIG>).

The second suspension element may extend in a radial direction perpendicular to the movement axis, i.e. may extend in a direction perpendicular to the movement axis.

By arranging the second suspension element to extend perpendicularly to the movement axis, a spatially efficient arrangement may be achieved. This may allow for an improved shallow loudspeaker, particularly where the centre of gravity is located between the landing surfaces, as specified above. The resulting loudspeaker may be both shallow and inhibit diaphragm rocking. This is in contrast to shallow loudspeakers of conventional configuration, which may be particularly prone to diaphragm rocking as a result of the shallow construction.

The diaphragm may have a first radiating surface facing in a forward direction (e.g. away from the frame) and a second radiating surface facing in a rearward direction (e.g. towards the frame).

The translatable part of the drive unit may be located in an aperture through the diaphragm.

An exposed portion of the translatable part of the drive unit may face in the forward direction. Similarly, an inner portion of the translatable part may face in the rearward direction.

The exposed portion may in use dissipate heat generated in the loudspeaker to ambient air. Particularly heat generated as a result of energising the voice coil may be dissipated in this way.

The exposed portion of the translatable part may include structural cooling elements. The structural cooling elements may increase the surface area and thereby improve heat transfer to ambient air. The structural cooling elements may include protrusions or depressions, such as cooling fins or cooling channels.

By providing structural cooling elements on the exposed portion of the translatable part, heat transfer from the translatable part to ambient air may be improved. Particularly where the translatable part includes a thermal conductor with high thermal conductivity (e.g. of at least <NUM> Watts/(metres*Kelvin)), heat generated as a result of operation may be removed more efficiently from the loudspeaker.

The diaphragm may include a thermal conductor with high thermal conductivity (e.g. of at least <NUM> Watts/(metres*Kelvin)). Preferably the thermal conductor is formed from a metal or a metal alloy. More preferably, the thermal conductor is formed from aluminium or an aluminium alloy.

In conventional loudspeakers, heat transfer from the voice coil to the diaphragm may be hindered due to a spatial separation of the voice coil and the diaphragm (e.g. caused by using a voice coil former). By contrast, the loudspeaker according to the present disclosure may improve heat transfer to the diaphragm, for example through the translatable part of the drive unit. Furthermore, where the translatable part of the drive unit also has high thermal conductivity, heat transfer from the loudspeaker may be further improved.

The wire from which the voice coil is formed may have any suitable cross-section. The cross-section of the wire may be circular or non-circular. Suitably, the cross-section of the wire is chosen to increase the fill factor of the voice coil. The wire from which the voice coil is formed may have a rectangular cross-section, optionally a square cross-section. Herein, a square can be understood as a subset of rectangular shapes.

Since the voice coil is included in the stationary part secured to the frame, the weight of the voice coil is supported by the frame. By contrast, in a conventional loudspeaker as described above, the voice coil is connected to the diaphragm such that an increased weight of the voice coil may render the diaphragm prone to rocking and therefore an increased fill factor may be undesirable.

In some examples, the stationary part of the drive unit includes a voice coil former (in addition to the voice coil), wherein the voice coil is mounted on (e.g. wound on) the voice coil former.

In some examples, the translatable part of the drive unit is the magnet unit.

The magnet unit (or 'magnet system') includes at least one permanent magnet and includes flux guides (e.g. two flux guides). The flux guides are configured to guide magnetic flux provided by the at least one permanent magnet to the air gap.

The at least one permanent magnet has a first mass, the voice coil has a second mass, and the first mass is smaller than the second mass. The first mass may be smaller than the second mass by at least a factor of two.

Thus, by contrast to the conventional loudspeaker described above, the mass of the voice coil may exceed and even greatly exceed the mass of the permanent magnet. Unlike in the conventional loudspeaker, an increase in the mass of the voice coil may not render the diaphragm more prone to rocking since the mass of the voice coil is carried by the frame rather than attached to the diaphragm.

The flux guides include a yoke, optionally provided as a U-yoke, and includes a washer.

The/each flux guide may be made from a material with high thermal conductivity (e.g. of at least <NUM> Watts/(metres*Kelvin)), particularly metal or metal alloy. This may help in the dissipation of heat (in addition to the guiding of flux).

The yoke may comprise a base. The yoke comprises a sidewall. The sidewall may extend from the base.

The thickness of the voice coil in a radial direction perpendicular to the movement axis may be greater than a thickness of the sidewall of the yoke in the radial direction by at least a factor of three, optionally five. The sidewall may have a uniform thickness or a non-uniform thickness. Where the sidewall has a uniform wall thickness, the thickness of the voice coil in the radial direction may be greater than the (uniform) wall thickness in the radial direction, optionally greater by at least a factor of three, optionally five. Where the sidewall has a non-uniform wall thickness, the thickness of the voice coil in the radial direction may be greater than the maximum value of the (non-uniform) wall thickness in the radial direction, optionally greater by at least a factor of three, optionally five.

According to some examples, the magnet unit may comprise one permanent magnet and two flux guides. The two flux guides are a washer and a yoke. The permanent magnet may be located between the washer and the yoke, The washer and the yoke are arranged to define the airgap between the washer and a sidewall of the yoke.

According to some other examples, the magnet unit may comprise two permanent magnets and three flux guides. The three flux guides may be provided as a washer and two yokes. The washer may be located between the two permanent magnets arranged with alike poles facing each other. Each yoke may extend from one of the two permanent magnets to define an airgap between the respective yoke and the washer.

According to some other examples, the magnet unit may comprise one permanent magnet and three flux guides provided as two washers and a tubular yoke, e.g. a cylinder-shaped yoke. The permanent magnet may be located between the two washers. The tubular yoke may extend around the permanent magnet and the two washers to define two airgaps between the tubular yoke and the two washers.

The loudspeaker is provided as a subwoofer configured to produce sound with frequencies in a bass frequency range. The bass frequency range includes <NUM>-<NUM>, more preferably include <NUM>-<NUM>. By way of example, the bass frequency range may be <NUM>-<NUM>.

The loudspeaker as described above may be provided in an enclosure. The enclosure may define an internal volume of up to <NUM> litres in which the loudspeaker is mounted.

In a second aspect of the invention, there may be provided a loudspeaker assembly according to claim <NUM>.

The first voice coil and the second voice coil may be configured to be energised by the same signal. By utilising the same signal, complete cancellation of forces as a result of displacing the first translatable part and the second translatable part may be achieved.

The first loudspeaker and the second loudspeaker may, for example, be provided as <NUM>-inch (<NUM>) subwoofers in an enclosure of approximately <NUM> litres net volume per loudspeaker.

The loudspeaker assembly may include a magnetic shielding member between the first loudspeaker and the second loudspeaker to magnetically shield the magnet units of the first loudspeaker and the second loudspeaker from one another.

By providing the magnetic shielding member, interaction between the magnet unit of the first loudspeaker and the magnet unit of the second loudspeaker may be reduced. In particular, the magnetic shielding member may provide for sufficient magnetic shielding to prevent the magnet units interacting at rest and/or when operating the loudspeaker assembly in normal working range.

The magnetic shielding member may be configured to become saturated with magnetic flux when the magnet unit of the first loudspeaker and the magnet unit of the second loudspeaker approach the magnetic shielding member, thereby causing mutual repulsion of the magnet units.

By including the magnetic shielding and allowing for magnetic saturation thereof, performance of the loudspeakers may be improved and, at the same time, safe operation ensured even when operated at peak power operation as the additional forces experienced by the magnet units may prevent a collision of the translatable parts with the magnetic shielding member or the frame.

The magnetic shielding member may be provided as a sheet of metal or metal alloy, optionally steel.

The invention is defined in the appended independent claim. The dependent claims thereof define preferred embodiments of the invention.

The present invention relates to loudspeakers including a frame, a diaphragm and a drive unit. A detailed discussion of examples of traditional loudspeakers follows, for purposes of illustrating the context in which the present invention has been made, before presenting a detailed discussion of the present invention.

<FIG> and <FIG> illustrate a traditional loudspeaker <NUM>. In the traditional loudspeaker <NUM>, a voice coil <NUM> is suspended in an airgap <NUM> between an inner steel part <NUM> and outer steel part <NUM> guiding the magnetic flux of a permanent magnet <NUM> through the voice coil <NUM>. A loudspeaker cone <NUM> is connected to the voice coil <NUM> via a voice coil former <NUM>. Two suspension elements are connected to the voice coil former and the cone; a damper <NUM> is connected to the voice coil former <NUM> and a surround <NUM> is connected to the cone <NUM>.

The mass of the voice coil <NUM> is typically small compared to the mass of the permanent magnet <NUM> or may be in the same range. Typically, a moving mass of the loudspeaker, i.e. the total mass displaced in operation, is small compared to the total mass of the loudspeaker including magnet unit and frame (not shown).

The force generating element relative to the frame of the loudspeaker <NUM> are voice coil windings <NUM> (Lorentz Force). The centre of gravity <NUM> of these windings <NUM> is outside of the volume between the two landing surfaces <NUM>, <NUM> on the frame where the damper <NUM> and the surround <NUM> are attached to the frame. This makes the construction prone to rocking of the assembly as a whole which can potentially lead to the coil rubbing against steel parts and damage to the loudspeaker, particularly for heavy coils and/or shallow loudspeaker constructions.

<FIG> shows an alternative traditional construction, also known as an open magnet system, wherein two permanent magnets <NUM> are magnetized in opposite direction and push the flux lines through an in-between washer <NUM>. Alternatively, no washer may be present and the permanent magnets <NUM> are spaced by a non-magnetic element.

In the open magnet system, it is possible to have the centre of gravity of the voice coil between the suspension elements, but such a magnet system may be inefficient as it poses great reluctance to the magnetic flux lines (not shown) due to the comparatively long path through the air. To have acceptable magnet permeance coefficients and useable linearity in terms of the force factor vs displacement of the cone BL(x), the magnets must be tall - and so big in volume and hence expensive. Thin magnets may not be suitable for an open magnet system, because the linear displacement is limited as the windings enter a zone of inverted magnetic flux density for large excursions.

For efficient low frequency reproduction, the moving mass and force factor must be large to counteract the effect of the stiffness from the air volume that a small, closed box poses to a loudspeaker cone. Typically, this large moving mass and large force factor is generated by means of a voice coil having <NUM> to <NUM> layers and a strong, large and expensive rare earth magnet in a low reluctance magnetic circuit requiring thick steel components to guide the magnetic flux. To increase the moving mass often a weight, e.g. a brass dustcap is added. A dustcap or heavy diaphragm may also be used to shift the centre of gravity of the whole moving assembly closer to the two suspensions and so decrease rocking risk in application.

A known way of increasing the moving mass is use of a voice coil winding wire with not a round but a rectangular or square cross-section. This increases the motor strength but increases the rocking risk as most of the moving mass is concentrated away from the suspension elements.

As in a traditional loudspeaker the voice coil is moving, flexible leadwires must connect the voice coil windings with the terminal on the frame. Leadwires are prone to ticking noise, over-stretching during operation, must be connected to the windings, e.g. via a solder connection on the voice coil former, and are generally to be avoided.

For heavy moving mass woofers as described above, the counter-force on the frame and enclosure is large. It is known to mount two loudspeakers on opposing faces of an enclosure, i.e. in back-to-back configuration, to cancel out the net force on the enclosure. However, this leads to a necessary long elongation of the enclosure along the principle axis of the loudspeakers when so mounted due to the depth of the loudspeakers.

For subwoofer loudspeaker of e.g. <NUM> to <NUM> inches (<NUM> to <NUM>) of nominal diameter, the above considerations pose severe limitations for the loudspeaker designer. On the market we hardly see loudspeakers in this range with a moving mass of more than <NUM> (grams) or suitable for use in an enclosure smaller than two litres. However, such a moving mass is necessary to reach an in-box resonance frequency of close to or below <NUM> (Hertz) suitable for subwoofer application. If such loudspeakers are designed, they typically come with a big and expensive magnet system to allow for a reasonable in-box quality factor Qtc <<NUM> controlling the high moving mass making them suitable for music reproduction. The current world market price of rare earths needed for high temperature stable magnets with high remanent flux density such as NdFeB+Dy magnets prohibit such loudspeakers to be widely used in automotive or consumer applications.

Hence, there is a need for an improved loudspeaker. The examples discussed below may provide for one or more of low-cost, lightweight, a shallow (subwoofer) driver with high moving mass but rocking stability, easy to manufacture and suitable for back-to-back mounting for force cancelling operation.

Aspects and embodiments of the present invention will now be discussed with reference to the accompanying <FIG>. The invention is related to low frequency sound reproduction, more specifically to loudspeaker music playback from a small, closed box volume.

<FIG> is a sectional view of an exemplary loudspeaker <NUM>. The loudspeaker <NUM> includes a frame <NUM>, a diaphragm <NUM> suspended from the frame <NUM>, and a drive unit <NUM>.

The drive unit <NUM> has a stationary part <NUM> and a translatable part <NUM>. The stationary part <NUM> is secured to the frame <NUM> and includes a voice coil <NUM>. The translatable part <NUM> is secured to the diaphragm <NUM> and includes a magnet unit <NUM> configured to produce a magnetic field in an air gap <NUM>. When the diaphragm <NUM> is at rest, the voice coil <NUM> sits in the air gap <NUM>.

The loudspeaker <NUM> is operable to energise the voice coil <NUM> to cause the magnet unit <NUM> to move relative to the voice coil <NUM>, along a movement axis <NUM>, thereby moving the diaphragm <NUM> along the movement axis <NUM> to produce sound.

The diaphragm <NUM> has a first sound radiating surface <NUM> and a second sound radiating surface <NUM>. The first sound radiating surface <NUM> faces in a forward direction <NUM> (away from the frame <NUM>) and is in use utilised for producing sound. The second sound radiating surface <NUM> faces in a rearward direction <NUM>, i.e. into the frame <NUM>. The forward direction <NUM> and the rearward direction <NUM> are opposite directions parallel to the movement axis <NUM>.

Note that the air gap <NUM> faces in the rearward direction, and which inhibits the accumulation of dust without the need for a dustcap as seen in some conventional loudspeakers.

The frame <NUM> of the loudspeaker <NUM> includes a base portion <NUM> and a rim <NUM>. The base portion <NUM> extends radially outwardly with respect to the movement axis <NUM>. The rim <NUM> extends axially with respect to the movement axis <NUM>, that is at least partly along the movement axis <NUM>. The rim <NUM> of the frame <NUM> is positioned at the periphery of the base portion <NUM> and is positioned radially outwardly of the magnet unit <NUM>.

The stationary part <NUM> of the drive unit <NUM> is secured to the base portion <NUM> of the frame <NUM> while the diaphragm <NUM> and the translatable part <NUM> of the drive unit <NUM> are suspended from the frame <NUM> by means of suspension elements <NUM>, <NUM>. The suspension elements <NUM>, <NUM> are configured to allow movement along the movement axis <NUM>, i.e. in a direction parallel to the movement axis <NUM>, and inhibit movement in a radial direction <NUM> (shown in <FIG>), i.e. in a direction perpendicular to the movement axis <NUM>.

A first suspension element <NUM> is attached to the frame <NUM> at a first landing surface <NUM> defined by the rim <NUM> of the frame <NUM>. The first suspension element <NUM> is provided as a surround secured to an outer edge <NUM> of the diaphragm <NUM>.

A second suspension element <NUM> is attached to the frame <NUM> at a second landing surface <NUM> defined by the rim <NUM> of the frame <NUM>. The second suspension element <NUM> is provided as a damper secured to the translatable part <NUM> of the drive unit <NUM> and extends radially outwardly with respect to the movement axis <NUM>.

<FIG> is a sectional view of part of the loudspeaker <NUM> and shows the diaphragm <NUM>, the drive unit <NUM>, and the suspension elements <NUM>, <NUM>.

The diaphragm <NUM> and the second suspension element <NUM> are secured to the translatable part <NUM> of the drive unit <NUM> such that the centre of gravity <NUM> of the translatable part <NUM>, indicated by a chequer-patterned disk, is located between the first landing surface <NUM> and the second landing surface <NUM>. That is to say, the first landing surface <NUM> and the second landing surface <NUM> are spaced apart along the movement axis <NUM> and the centre of gravity <NUM> of the translatable part <NUM> is located therebetween.

The magnet unit <NUM> is suspended by the second suspension element <NUM>, which in this example provided as a damper, connecting the magnet unit <NUM> to the frame <NUM>. The magnet unit <NUM> is also suspended via the diaphragm <NUM>, in this example provided as a cone, and the first suspension element <NUM>, in this example provided as a rubber surround, which is also connected to the frame <NUM>. Accordingly, the centre of gravity <NUM> of the magnet unit <NUM> is located along the movement axis <NUM> and between the two landing surfaces <NUM>, <NUM> of the suspension elements <NUM>, <NUM> to the frame <NUM>.

The diaphragm <NUM> has an inner edge <NUM>. The inner edge <NUM> defines a diaphragm aperture <NUM>, i.e. bounds the diaphragm aperture <NUM>. The diaphragm aperture <NUM> extends through the diaphragm <NUM>, i.e. extends from the first sound radiating surface <NUM> to the second sound radiating surface <NUM>.

The translatable part <NUM> of the drive unit <NUM> extends through the diaphragm aperture <NUM>. Thus, an exposed portion <NUM> (or 'first portion') of the translatable part <NUM> is located one side of the diaphragm <NUM>, while an inner portion <NUM> (or 'second portion') of the translatable part <NUM> is on the other side of the diaphragm <NUM>. More particularly, the exposed portion <NUM> faces in the forward direction <NUM> and the inner portion <NUM> faces in the rearward direction <NUM>. The exposed portion <NUM> of the translatable part <NUM> is in use located outside of the volume enclosed by the frame <NUM> and the diaphragm <NUM>, i.e. is exposed to ambient air.

Some conventional loudspeakers are mounted with the magnet unit towards the inside of the frame/enclosure. This leads to increased temperature in the enclosure, limiting the power handling of the loudspeaker, especially when also active electronics, such as an amplifier, are present. The exemplary loudspeaker <NUM> has the magnet unit towards the outside of the frame/box which may allow for much better heat radiation. It also allows equipping the translatable part <NUM> with cooling fins, see <FIG>, or using a metal cone (e.g. from Aluminium) as an additional heat sink, as it is directly connected to the translatable part <NUM> while having the airgap <NUM> protected inside the box/frame; away from dust and debris.

<FIG> is a sectional view of the translatable part <NUM>. In this example, the translatable part <NUM> of the drive unit <NUM> corresponds to the magnet unit <NUM>.

The magnet unit <NUM> is in use arranged in the loudspeaker <NUM> such that the air gap <NUM> is open towards the frame <NUM>, as shown in <FIG>, and the voice coil <NUM> extends into the air gap <NUM> in the forward direction <NUM>.

The magnet unit <NUM> includes a permanent magnet <NUM>, a (magnetic) washer <NUM> and a (magnetic) yoke <NUM>. The permanent magnet <NUM>, the washer <NUM>, and the yoke <NUM> are axially symmetric about the movement axis <NUM>, though other arrangements are possible.

The permanent magnet <NUM> is provided as a rare earth magnet. The permanent magnet <NUM> has a mass which is smaller than the mass of the voice coil <NUM>. In this example, the mass of the voice coil <NUM> is greater than the mass of the permanent magnet <NUM> by a factor of two, i.e. the mass of the voice coil <NUM> is two times greater than the mass of the permanent magnet <NUM>.

The washer <NUM> and the yoke <NUM>, which in this example is provided as a U-yoke, are configured to guide magnetic flux generated by the permanent magnet <NUM> to the air gap <NUM> between the washer <NUM> and the yoke <NUM>.

The yoke <NUM> has a base <NUM> and a sidewall <NUM> projecting from the base <NUM>. The base <NUM> extends in the radial direction <NUM>, while the sidewall <NUM> extends axially with respect to the movement axis <NUM>.

As the suspension elements <NUM>, <NUM> are mounted on the outside of the sidewall <NUM> of the yoke <NUM>, the centre of gravity <NUM> of the moving, force generating element - in this case the magnet unit <NUM> - can be between the second suspension element <NUM> (damper) and the first suspension element <NUM> (surround), which may lead to excellent rocking stability and pure, axial motion. Moreover, having the suspension radially adjacent to the yoke <NUM> may allow for very shallow designs, enabling back-to-back force-cancelled operation in a small box as described with reference to <FIG>.

The sidewall <NUM> of the yoke <NUM> has a (uniform) thickness bounded radially, i.e. in the radial direction <NUM>. The voice coil <NUM> also has a (uniform) thickness bounded radially. The thickness of the voice coil <NUM> is greater than the thickness of the sidewall <NUM> of the yoke <NUM>. A ratio of the thickness of the voice coil <NUM>, i.e. the "winding thickness", over the thickness of the sidewall <NUM> of <NUM>:<NUM> or even <NUM>:<NUM> is preferred. A traditional speaker of same size may have a ratio as little as <NUM>:<NUM>.

The loudspeaker <NUM> makes use of an exceptionally big voice coil using many layers in a magnetic circuit. However, as the air-gap is much wider as compared to a traditional loudspeaker to accommodate the big voice coil, the total reluctance in the magnetic circuit increases so much that the cross-sections of the flux guiding washer and yoke parts can be thin.

This new configuration leads to a comparably high voice coil mass and comparably low magnet unit mass. Accordingly, has been found beneficial to fix the voice coil <NUM> to the frame <NUM> and let the magnet unit <NUM> be free to oscillate by means of two or more suspension elements <NUM>, <NUM> as then more of the total mass of the loudspeaker <NUM> is active and moving.

The drive unit <NUM> leads to high moving mass at low overall driver mass. A ratio of moving mass to total mass of up to <NUM>:<NUM>, preferably up to <NUM>:<NUM>, is possible. Also, a ratio of magnet mass to voice coil winding mass of <NUM>:<NUM>, preferably up to <NUM>:<NUM>, can be reached. This leads to surprisingly lightweight subwoofers with low resonance frequency in box.

<FIG> is a sectional view of an exemplary loudspeaker assembly <NUM> according to the present disclosure. The loudspeaker assembly <NUM> includes a first loudspeaker <NUM> and a second loudspeaker <NUM> as described above and arranged in a back-to-back configuration, facing into opposite directions.

The frames <NUM> of the loudspeakers <NUM> are joined at their respective base portions <NUM>. In this example, the base portions <NUM> are formed separately and joined by means of adhesive, it is also envisaged that the base portions <NUM> could be formed integrally with one another. The resulting configuration of the loudspeaker assembly <NUM> is such that the base portions <NUM> serve as a divider between the volumes enclosed by each loudspeaker <NUM>.

The rims <NUM> of the loudspeakers <NUM> extend in opposite direction from the respective base portion <NUM> of the frame <NUM>.

The diaphragms <NUM> are located on opposite sides of the loudspeaker assembly <NUM>.

The drive units <NUM> of the loudspeakers <NUM> are located between the diaphragms <NUM>. The translatable parts <NUM> of the drive units <NUM> are moveable along the movement axis <NUM>, which is common to both loudspeakers <NUM>.

The loudspeaker assembly <NUM> is operable to energise the voice coil <NUM> of the first loudspeaker <NUM> and the voice coil <NUM> of the second loudspeaker <NUM> in order to cause the magnet unit <NUM> of the first loudspeaker <NUM> and the magnet unit <NUM> of the second loudspeaker <NUM> to move along the movement axis <NUM>, thereby moving the diaphragms <NUM> of both loudspeakers <NUM> and producing sound. In this example, the voice coils <NUM> of the loudspeakers <NUM> are configured to receive the same signal to cause the voice coils <NUM> to be energised.

The loudspeakers <NUM> are provided as <NUM>-inch (<NUM>) subwoofers for use up to <NUM> in an enclosure (closed box) of approximately <NUM> litres net volume per loudspeaker <NUM>. In <FIG>, the two loudspeakers <NUM> are mounted back-to-back in a closed box volume of <NUM> litres in total, designed for a nominal stroke of +-<NUM> (millimetres).

The washer <NUM> of each loudspeaker <NUM> has a thickness of <NUM>, which may effectively guide the magnetic flux through the voice coil windings. The thickness of the sidewall <NUM> of the yoke <NUM> adjacent to half the height of the voice coil windings is <NUM>. In this example, the sidewall <NUM> has uniform thickness.

The permanent magnet <NUM> is <NUM> (millimetres) in diameter and <NUM> in height and has a weight of <NUM> (grams). This is considered to be an exceptionally small and lightweight magnet for a subwoofer with this application.

The moving mass of each loudspeaker <NUM>, i.e. the mass of the translatable part <NUM> of each loudspeaker <NUM>, is approximately <NUM>.

The voice coil <NUM> of each loudspeaker <NUM>, wound on a voice coil former <NUM> fixed to the respective frame <NUM>, has <NUM> winding height along the movement axis <NUM>, at a winding thickness of <NUM> in the radial direction <NUM>. The weight of the voice coil windings is <NUM>.

These parameters lead to a ratio of magnet weight to coil weight of <NUM>:<NUM>. The ratio of the voice coil winding thickness to the thickness of the sidewall <NUM> of the yoke <NUM> is <NUM>:<NUM>.

The frame <NUM> of each loudspeaker <NUM>, which in this example is made from plastic, weighs <NUM>.

The total mass of each loudspeaker is below <NUM>, while the total mass of the loudspeaker assembly <NUM> is below <NUM>, with a total moving mass of <NUM>. In operation, <NUM> of moving mass per loudspeaker <NUM> are moving in opposite directions leading to no net force on the enclosure. Thus, it may be possible to achieve the same or comparable maximum output and resonance frequency from the loudspeaker assembly <NUM> with a volume of <NUM> litres as a comparatively heavy, high-performance single <NUM>-inch (<NUM>) subwoofer in a volume of approximately <NUM> litres. Such a great decrease in package and reduction of vibration of the enclosure may allow subwoofer placement in positions where the sheer size previously prohibited subwoofer application. This can be e.g. close to the bottom of the A-style or between the foot wells in a car cabin.

<FIG> and <FIG> are sectional views of the drive units <NUM> of the loudspeaker assembly <NUM>, illustrating the magnetic flux in different configurations of the drive units <NUM>. In <FIG>, the drive units <NUM> are shown at rest, i.e. the voice coils <NUM> are not energised and the translatable parts <NUM> of each drive unit <NUM> are located in at corresponding rest positions. In <FIG>, the translatable parts <NUM> are displaced towards each other. That is to say, displacement is negative in <FIG>.

With the two drive units <NUM> mounted back-to-back, there is an additional force acting on the magnet units <NUM>. Due to the large airgaps <NUM> in the individual magnet units <NUM> with alike poles facing each other, the individual leakage fluxes of the magnet units <NUM> may push the magnet units <NUM> apart. To mitigate this effect at rest position and during the normal working range of the loudspeakers <NUM>, a magnetic shielding member <NUM> is added. In this example, the magnetic shielding member <NUM> is provided as thin sheet of mild steel of <NUM> diameter and <NUM> thickness.

The magnetic shielding member <NUM> is provided on the junction between the two loudspeaker frames <NUM>, i.e. where the base portions <NUM> are joined (see <FIG>). The magnetic shielding member <NUM> shields the two magnet units <NUM> from each other. According to the present example, the magnetic shielding member <NUM> is configured to shield up to the point where the magnet units <NUM> get very close to each other, exceeding normal working range, e.g. at approximately -<NUM> displacement. In this example, this is the limit for normal operation and displacements beyond that are not intended.

The magnetic shielding member <NUM> is configured so that beyond the nominal displacement the magnetic shielding member <NUM> fully saturates by the leakage flux of the two magnet units <NUM>. For example, the thickness of the sheet of mild steel may be chosen such that full saturation occurs accordingly.

When the magnetic shielding member <NUM> is fully saturated, the two magnet units <NUM> push each other apart through the magnetic shielding member <NUM>. In this example, this may be creating up to -50N (Newtons) of force relative to the frame <NUM> (or 100N relative to each other, with reference to the magnet units <NUM>). This additional force may make it difficult or even impossible for the magnet units <NUM> to hit the frame <NUM> for negative displacements even when operated at peak power and so may ensure safe operation even during peak power operation.

<FIG> show graphs illustrating performance parameters of the loudspeaker assembly <NUM>.

<FIG> illustrates the forces acting on the translatable part <NUM> relative to the frame <NUM>. The solid line represents the restoring force of the entrapped air volume acting on the cone, <NUM>, 108cm2. The dotted line represents the restoring force of the mechanical suspension elements. The dashed line represents the restoring force of the magnetic interaction between the magnet units.

<FIG> illustrates the force factor BL(x) for each drive unit <NUM> relative to the respective frame <NUM>.

The entrapped air inside the enclosure, i.e. closed box, acts as additional, axial stiffness on the diaphragm <NUM> (provided as a cone) of each loudspeaker <NUM>. For a displacement of +-<NUM> of an effective radiating surface area of 108cm2 (square centimetres) on <NUM> litres, this leads to a force of up to -150N and +130N respectively. In comparison, the force of the suspension elements <NUM>, <NUM> is small with up to +-25N.

As the loudspeakers are joined back-to-back, the net force on the base portion <NUM> of each individual frame <NUM> is nil and allows the frame <NUM> to be thin and lightweight. The force factor vs displacement of each drive unit <NUM> is symmetric and drops to <NUM>% relative to the rest position at +-<NUM>, leading to low distortion over a wide displacement range.

Having the magnetic shielding member <NUM> close to magnetic saturation at approx. 6Teslas allows keeping the inductance so low that it may have little influence on the frequency response in the working range up to <NUM>. In fact, the higher inductance compared to a traditional loudspeaker with fewer windings, and consequently lower inductance, leads to decreased higher order distortions due to the decreasing output for higher frequencies above <NUM>.

<FIG> show another example of a drive unit <NUM>. <FIG> is a sectional view of the drive unit, while <FIG> is a perspective view of the drive unit <NUM>. For ease of assembly, the magnet unit of <FIG> above may be preferable, consisting of only three separate component parts. However, alternative configurations of magnet units with other advantages are envisaged.

The drive unit <NUM> is similar to the drive unit <NUM> described above and detailed description of similar parts is omitted.

The drive unit <NUM> comprises a stationary part <NUM> and a translatable part <NUM>. The stationary part <NUM> comprises a voice coil <NUM>. The translatable part <NUM> comprises a magnet unit <NUM>.

The magnet unit <NUM> includes two permanent magnets <NUM> arranged with alike poles facing each other. The two permanent magnets <NUM> are fixed to a washer <NUM> pushing the flux radially outwards. Two U-yokes <NUM> are used to guide the flux around the voice coil windings and radially through the voice coil windings. This arrangement may allow for a voice coil with smaller inner diameter as the permanent magnet volume can be distributed over two permanent magnets <NUM> acting in parallel on the same voice coil windings.

In this example, the voice coil <NUM> is fixed to the frame <NUM> radially through arms <NUM> protruding the slits <NUM> in the U-yokes. The arms <NUM> protruding the slits <NUM> are also helpful for leadouts <NUM> of the voice coil <NUM>. The term "leadout" is understood to refer to wiring which is not flexible, unlike leadwires which are understood to be flexible.

The described arrangement may be less shallow than the arrangement described with reference to <FIG>, but still similar in height to a traditional loudspeaker. Also, the rocking resistance may be outstanding as the suspension elements can be spaced further apart than in a traditional loudspeaker.

<FIG> illustrate the magnetic properties of the drive unit <NUM> of <FIG>.

The symmetric arrangement of the permanent magnets <NUM> results in a symmetric BL(x) curve with almost no leakage flux despite the comparatively large airgap. Also, it is possible to introduce a gap <NUM> between the sidewalls <NUM> (or 'outside portions') of the U-yokes <NUM> to decrease the moving mass and cost of the parts as the flux lines will anyway take the lowest reluctance path entering sidewalls <NUM> under an angle and not strictly radial.

Compared to a traditional open magnet system or a U-yoke magnet system with a countermagnet, the permeance coefficient of both permanent magnets <NUM> is high as they are both loaded by the magnetic circuit with U-yokes <NUM> with comparably low overall reluctance. Also, the large surface area of both U-yokes <NUM> may act as very effective heatsink. Both, the good heat dissipating properties of this design and the high permeance coefficient allow the use of low-cost permanent magnets, e.g. Neodymium magnets, with low demagnetization resistance.

<FIG> and <FIG> show an exemplary loudspeaker <NUM> comprising the drive unit <NUM> described with reference to <FIG>. More particularly, <FIG> is a broken-away perspective view of the loudspeaker <NUM>, while <FIG> is a sectional view of the loudspeaker <NUM>.

The loudspeaker <NUM> is similar to the loudspeaker <NUM> described above with reference to <FIG>. A detailed description of corresponding parts is omitted.

The loudspeaker <NUM> comprises a frame <NUM>, the diaphragm <NUM>, and the drive unit <NUM>. A stationary part <NUM> of the drive unit <NUM> is secured to the frame <NUM>. The diaphragm <NUM> and the translatable part <NUM> of the drive unit <NUM> are suspended from the frame <NUM>. In this example, a third suspension element <NUM> is provided that is secured to the translatable part <NUM> of the drive unit <NUM>.

The drive unit <NUM> extends from the front of the loudspeaker <NUM> to the rear of the loudspeaker <NUM>. More particularly, the drive unit <NUM> extends from the diaphragm <NUM> along the movement axis <NUM>, all the through a rear aperture <NUM> in the third suspension element <NUM>. The third suspension element <NUM> is provided in a base aperture <NUM> of the base portion <NUM> of the frame <NUM>. The translatable part <NUM> protrudes in the rearward direction <NUM> from the rear aperture <NUM>.

In operation, the rear of the loudspeaker <NUM> as defined by the second suspension element and the translatable part <NUM> also moves when the loudspeaker <NUM> is operated.

<FIG> is a sectional view of another exemplary drive unit <NUM>. The drive unit <NUM> is similar to the drive units <NUM>, <NUM> and detailed description of like parts is omitted.

The drive unit <NUM> has a stationary part <NUM> and a translatable part <NUM>. The translatable part <NUM> includes a magnet unit <NUM>, comprising a single permanent magnet <NUM>, configured to produce a magnetic field in an air gap <NUM>.

The stationary part <NUM> comprises a first voice coil <NUM> and a second voice coil <NUM>. The voice coils <NUM>, <NUM> sit in the air gap <NUM> when the diaphragm is at rest. Hence, the drive unit utilises one permanent magnet and two voice coils.

The permanent magnet <NUM> is located between two washers <NUM> that are used for guiding the flux lines through to both voice coil windings with a cylinder-shaped magnetic yoke <NUM> adjacent to the outside of the voice coils <NUM>, <NUM>. The cylinder-shaped magnetic yoke <NUM> does not have a base portion <NUM> extending in the radial direction <NUM>, as described with reference to the yoke <NUM>. Instead, the cylinder-shaped magnetic yoke <NUM> extends only axially.

An inner portion of the magnet unit <NUM>, i.e. the permanent magnet <NUM> and the washers <NUM>, is mechanically connected with an outside of the yoke <NUM> by at least one non-magnetic member <NUM> (dashed lines in <FIG>), ensuring the flux lines are guided through the voice coil windings. In this example, the non-magnetic member <NUM> is provided as an annular bracket.

The described arrangement is symmetric and may provide all the advantages of allowing the fixation of the suspension elements and diaphragm to the outside of the magnet unit. The described arrangement further has the benefit that the voice coils <NUM>, <NUM> are connected in such a way (or wound with opposite winding direction) that their inductance partially cancels, which may improve performance particularly at higher frequencies.

<FIG> shows the magnet unit <NUM> of <FIG>, but provided with optional cooling features.

The magnet unit <NUM> is as described above with reference to <FIG>. However, the exposed portion <NUM> is provided with structural cooling elements <NUM>, <NUM> to increase the surface area of the exposed portion <NUM>. In this example, the structural cooling elements <NUM>, <NUM> are provided as cooling fins <NUM> and as cooling channels <NUM>.

In view of the exemplary drive units <NUM>, <NUM>, <NUM>, the person skilled in the art can easily imagine further examples where the magnetic flux is split into dominant portions by means of one or several washers (split-washer design) or gaps or dominant winding portions introduced in the voice coil winding (split-winding) to shape the resulting BL(x) curve trading off linearity vs force factor at rest position.

<FIG> shows an automobile <NUM>. Any exemplary loudspeaker as described above may be installed in the automobile <NUM>. In this example, the loudspeaker <NUM> described above with reference to <FIG> is provided between the footwells <NUM> of the automobile <NUM>. Other locations are also envisaged, such as at or towards the bottom of an A-style.

The exemplary loudspeakers described above may be manufacturable using existing tooling. The exemplary loudspeakers make use of traditional loudspeaker components built into a single frame, allowing production on existing production lines with existing machines, jigs and fixtures.

Each loudspeaker can be easily built independently with a single jig inserted at the bottom of the frame, first aligning the magnet unit parts towards each other and with respect to the frame. Adding the damper and cone assembly is simple and just like traditional loudspeaker building. In fact, having the cone connected to the magnet unit does not require a dustcap anymore and the coil working without flexible leadwires may make the build easier than that of a traditional loudspeaker.

The voice coil of the drive unit is made from square wire, i.e. wire with a square cross-section. As the use of rectangular wire does not shift down the centre of gravity of the moving assembly (as the coil is fixed to the frame anyway) it is a convenient way of increasing the motor strength without any downsides except cost. Also, the filling factor for a winding from square wire is substantially higher than that of a traditional helical winding using round wire. With ever increasing Neodymium raw material prices such "exotic" coils are becoming more and more viable as an alternative when trading off cost between components for same performance.

The voice coil of the drive unit may be made from any suitable material. In this example, the wire from which the voice coil is formed is made out of copper. Suitable metals or metal alloys, such as aluminium, are also envisaged.

The diaphragm is formed from aluminium, which is a thermal conductor with high thermal conductivity.

The diaphragm, i.e. cone, is made from <NUM> thick aluminium and so effectively acts as heat sink giving the loudspeaker excellent power handling capability.

The translatable part of the drive unit includes a thermal conductor with high thermal conductivity (e.g. of at least <NUM> Watts/(metres*Kelvin)), heat generated as a result of operation may be removed more efficiently from the loudspeaker. Suitably, the washer and the yoke are made of steel, although other materials are possible. As steel parts are guiding the flux lines away from the magnet the height of the magnet is independent from the motor system linearity unlike in an open magnet system (cf. <FIG>; requires a high magnet in order to avoid flux line cancellation in voice coil). This allows for the usage of a small volume of high-grade Neodymium, e.g. N55 which typically yields the lowest cost.

Some or all of the following aspects may be present in the examples described above:.

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 as claimed in diverse forms thereof.

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 scope of the invention as defined by the appended claims.

Claim 1:
A loudspeaker (<NUM>) including:
a frame (<NUM>), a diaphragm (<NUM>) suspended from the frame and a drive unit (<NUM>);
wherein the drive unit has a stationary part (<NUM>) secured to the frame and a translatable part (<NUM>) secured to the diaphragm;
wherein the translatable part of the drive unit includes a magnet unit (<NUM>) which includes at least one permanent magnet (<NUM>), a washer (<NUM>) and a yoke (<NUM>), wherein the magnet unit is configured to produce a magnetic field in an air gap (<NUM>) between the washer and a sidewall (<NUM>) of the yoke, wherein the at least one permanent magnet, the washer and the yoke are configured to guide magnetic flux provided by the at least one permanent magnet to the air gap;
wherein the stationary part of the drive unit includes a voice coil (<NUM>) configured to sit in the air gap when the diaphragm is at rest;
wherein the at least one permanent magnet has a first mass, the voice coil has a second mass, and the first mass is smaller than the second mass;
wherein the loudspeaker is operable to energise the voice coil to cause the magnet unit to move along a movement axis (<NUM>) relative to the voice coil, thereby moving the diaphragm along the movement axis to produce sound;
further including a first suspension element (<NUM>) attached to the frame at a first landing surface (<NUM>) on the frame and a second suspension element (<NUM>) attached to the frame at a second landing surface (<NUM>) on the frame, wherein the centre of gravity of the translatable part of the drive unit has a position along the movement axis that is between the first landing surface and the second landing surface;
wherein the loudspeaker is provided as a subwoofer configured to produce sound with frequencies in a bass frequency range.