Patent Description:
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. The magnet unit defines a magnetic circuit including an air gap across which magnetic flux is guided and in which the voice coil sits when at rest. 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.

The present inventor has observed that, in accordance with conventional principles, in order to make efficient use of the magnetic circuit when the loudspeaker is in operation, the magnet unit generates the magnetic flux in the air gap such that the associated magnetic flux density is as uniform as possible across said air gap. For example, for small bass loudspeakers, the magnetic flux density is desired to be as uniform as possible across the air gap in order to ensure efficient use of the magnetic circuit, since for these loudspeakers the mass and size of the voice coil may be kept comparatively to inhibit rocking. Thus, in conventional loudspeakers the magnetic flux density may typically drop from an initial <NUM>% to a value of <NUM>%, or maybe as low as <NUM>%, across the voice coil in the air gap.

<CIT> discloses a speaker device and method of manufacturing the speaker device.

<CIT> discloses a concentric tube suspension system for loudspeakers.

<CIT> discloses speaker drivers that include a woofer and a tweeter driven by a first and a second magnetic circuit, respectively, both magnetic circuits sharing a single magnet.

Herein is described a bass loudspeaker that may be viewed as departing from the conventional principles outlined in the background section above. More particularly, the described bass loudspeaker has an air gap wherein the magnetic flux density across the voice coil drops significantly, by <NUM>% or more. The described bass loudspeaker further has improved rocking resistance, which allows for a greater range of voice coil parameters, particularly comparatively large/heavy voice coils, to achieve desired performance of the loudspeaker.

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

By having a magnetic flux density at an outer perimeter of the voice coil is <NUM>% (percent) or less of a magnetic flux density at an inner perimeter of the voice coil, the loudspeaker is able to have a magnet unit which is smaller or lighter, thereby facilitating a lighter overall loudspeaker (even if the coil is made heavier to compensate). By arranging the voice coil and the suspension elements such that the centre of gravity of the voice coil is located between the first landing surface and the second landing surface, rocking (e.g. as caused by having a heavier coil to compensate for reduced flux across the air gap) may be inhibited. More particularly, the rocking modes of the loudspeaker may be pushed outside of the working frequency range of the loudspeaker. As a result, voice coil, e.g. size/weight, may be selected to compensate for the drop in magnetic flux density without compromising desired performance, e.g. force factor.

The bass loudspeaker as described above may be configured to produce sound with frequencies in a bass frequency range. The bass frequency range may include <NUM>-<NUM>, where "Hz" represents the physical unit "Hertz". More preferably, the bass frequency range may include <NUM>-<NUM>. By way of example, the bass frequency range may be <NUM>-<NUM>.

The first suspension element may be provided as a surround. The first suspension element may attach directly or indirectly to the diaphragm. In some examples, the first suspension element may be secured to an outer edge of the diaphragm.

The second suspension element may be provided as a damper (which may be referred to as a "spider"). The second suspension element may attach directly or indirectly to the diaphragm. In some examples, the second suspension element may be secured to the diaphragm at a location inwardly located with respect to the outer edge of the diaphragm.

The voice coil is rigidly connected to the diaphragm such that the diaphragm and the voice coil move together (e.g. "as one") along the movement axis when the loudspeaker is energised. The voice coil may be rigidly connected to the diaphragm directly or indirectly.

The magnetic flux density in the air gap as referenced herein may be a radial magnetic flux density. The radial magnetic flux density may be a magnetic flux density as measured in a direction perpendicular to the movement axis.

If the voice coil is generally circular, the inner perimeter of the voice coil may be referred to as the inner diameter of the voice coil, and the outer perimeter of the voice coil may be referred to as the outer diameter of the voice coil.

A distance between the inner perimeter and outer perimeter in a radial direction (i.e. in a direction perpendicular to the movement axis) may be taken as a winding thickness of the voice coil. Accordingly, the magnetic flux density permeating the voice coil may drop over the winding thickness of the voice coil as specified above.

The extent to which the magnetic flux density drops (by <NUM>% or more) across the voice coil may depend on design considerations, which may vary from loudspeaker to loudspeaker. In most cases, it is thought that the drop may be up to <NUM>%, or even higher. If the drop is up to <NUM>%, then the magnetic flux density at the outer perimeter of the voice coil may be in a range of <NUM>% to <NUM>% of the magnetic flux density at the inner perimeter of the voice coil.

The at least two flux guiding elements may define the air gap as a volume of space between the at least two flux guiding elements. When the diaphragm is at rest such that the voice coil sits in the air gap, the voice coil may be located between the at least two flux guiding elements.

The at least two flux guiding elements may include a washer and may include a yoke, optionally provided as a U-yoke. The permanent magnet may be located between the washer and the yoke.

The yoke may include a base and a sidewall extending from the base. The washer and the yoke may be arranged to define the air gap between the washer and a sidewall of the yoke.

A thickness of the sidewall of the yoke in a direction perpendicular to the movement axis may be smaller than a thickness of the voice coil in the direction perpendicular to the movement axis. Where the sidewall has a uniform wall thickness (in the direction perpendicular to the movement axis), the thickness of the voice coil (in the direction perpendicular to the movement axis) may be greater than the (uniform) wall thickness. Where the sidewall has a non-uniform wall thickness, the maximal value of the (non-uniform) wall thickness (in the direction perpendicular to the movement axis) may be smaller than the thickness of the voice coil (in the direction perpendicular to the movement axis).

The thickness of the sidewall (e.g. as defined above) may be smaller than the thickness of the voice coil by at least a factor of two, more preferably three, e.g. a factor of <NUM>.

In some examples, the thickness of the sidewall (e.g. as defined above) may be smaller than the thickness of the voice coil by a factor of five or more.

An average magnetic flux density within the at least two flux guiding elements may be between <NUM>. 5T and 2T, where "T" represents the physical unit "Tesla", e.g. to avoid/reduce problems caused by saturation of the flux guiding elements.

A separation between the first and second landing surfaces may be defined as a distance between a location on the first landing surface and a location on the second landing surface as measured in direction parallel to the movement axis.

An extent of the voice coil as measured in direction parallel to the movement axis (or 'height' of the voice coil) may be in a range of <NUM>% and <NUM>% of the separation between the first and second landing surfaces as measured in direction parallel to the movement axis. This configuration may enable large linear displacement of the voice coil while effectively inhibit rocking motion.

The magnet unit and the air gap may form a magnetic circuit. The magnetic circuit may provide a substantially closed circuit (or loop) for the magnetic flux that is generated by the permanent magnet and is guided by the at least two flux guiding elements.

The magnetic reluctance of the magnetic circuit is at least <NUM> x <NUM>^<NUM> [<NUM>/H], and in some examples may be <NUM> x <NUM>^<NUM> [<NUM>/H], where "H" represents the physical unit "Henry". By contrast, a conventional magnet unit may have a magnetic reluctance of at most <NUM> x <NUM>^<NUM> [<NUM>/H]. Thus, the magnetic reluctance according to the present disclosure corresponds to, or exceeds, <NUM>% or even <NUM>% of the magnetic reluctance of a more conventional magnetic unit.

The air gap has a magnetic reluctance of at least <NUM> x <NUM>^<NUM> [<NUM>/H]. Thus, the majority of the magnetic reluctance of the magnetic circuit can be attributed to the air gap.

By utilising a magnetic circuit with high magnetic reluctance, and particularly a high-reluctance air gap, it is possible to utilise comparatively small flux guiding elements. Thus, it is possible to reduce the weight of the magnet unit. This weight reduction of the magnet unit may more than compensate for the weight of a large voice coil, meaning that the comparatively high magnetic reluctance of the magnetic circuit enables designing of particularly lightweight loudspeakers. Such considerations may be relevant especially for applications in, for example, the automobile industry.

The permanent magnet may have a smaller mass than the mass of the voice coil. That is to say, the permanent magnet may have a first mass, the voice coil may have a second mass, and the first mass may be smaller than the second mass. The first mass may be smaller than the second mass by at least a factor of two, or even by at least a factor of <NUM>, for example by a factor of <NUM>.

In a conventional loudspeaker, the mass of the permanent magnet may be relatively large compared to the mass of the voice coil, for reasons outlined in the background section above. However, the present invention allows for a comparatively heavy, and hence large/dense, voice coil which may be combined with a smaller/lighter permanent magnet (and hence magnet unit). Accordingly, the combination of a smaller permanent magnet and a larger/denser voice coil may help to achieve desired performance parameters, whilst reducing weight of the loudspeaker and weight of the permanent magnet. In view of the increasing prices for rare earth magnets, this may provide for a more cost-effective configuration.

The diaphragm may comprise a first diaphragm body and a second diaphragm body which is centred with respect to the first diaphragm body (e.g. with respect to a movement axis). The first diaphragm body may also be referred to as an outer diaphragm body and the second diaphragm body may also be referred to as a central diaphragm body.

The outer diaphragm body and the central diaphragm body may be formed integrally. Alternatively, the outer diaphragm body and the central diaphragm body may be formed separately and joined together, e.g. using a suitable adhesive.

The outer diaphragm body 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 central diaphragm body may include an annular wall which extends around the voice coil, the annular wall extending along the movement axis between a first end and a second end of the central diaphragm body. The first end may be further forwards along the movement axis than the second end. The outer diaphragm body may be connected to the annular wall at a location between the first end and the second end of the central diaphragm body, such that the second end of the annular wall is separated from the location where the outer diaphragm is connected to the annular wall.

The outer diaphragm body may be connected to the first suspension element, e.g. at an outer edge of the outer diaphragm body. The second suspension element may be connected to the annular wall at or towards the second end of the central diaphragm body.

The central diaphragm body may provide an improved structure for attaching a suspension element. In particular, the second end of the annular wall of the diaphragm body provides for a free end (i.e. the second end) extending away from the outer diaphragm body (in a rearward direction) and to this free end the suspension element may be attached. Thus, positioning of the suspension element may be improved and rocking inhibited.

Where the outer diaphragm body and the central diaphragm body are provided as separately formed bodies, this may provide an improved structure for purposes of assembly. In particular, this may facilitate improved installation of lead wires during assembly since the outer diaphragm body may be installed in a subsequent manufacturing step (i.e. after the central diaphragm body), thereby providing ease of access.

If the outer diaphragm body and the central diaphragm body are formed separately, these may be joined together by mating corresponding surfaces. For example, the outer diaphragm body may have an inner circumferential surface inclined relative to the movement axis, and the annular wall of the central diaphragm body may have an outer circumferential surface inclined relative to the movement axis. The inner circumferential surface and the outer circumferential surface may be inclined relative to the movement axis by substantially the same angle and secured together, e.g. using a suitable adhesive.

The or each angle may be in a range of <NUM> degrees and <NUM> degrees relative to the movement axis, preferably between <NUM> degrees and <NUM> degrees relative to the movement axis.

The indicated ranges of angles may improve joining of the outer diaphragm body and the central diaphragm body, where these are formed separately, and may improve positioning of the second end of the annular wall (where the annular wall is straight) for purposes of attaching the second suspension element.

The central diaphragm body may include a signal track to transmit an electrical signal to or from the voice coil. The signal track may extend from a location on the outer circumferential surface of the annular wall of the central diaphragm, along the annular wall and towards the voice coil. The location from which the signal track extends may be at or towards the second end of the annular wall. A first solder pad may be provided on the central diaphragm body at the location from which the signal track extends. Here, 'at or towards the second end of the annular wall' may be understood to mean that this location is at the second end of the annular wall or between the second end and where the outer diaphragm body attaches to the annular wall.

In some examples, the signal track may extend to a second solder pad on a wall of the central diaphragm body which is proximate to the voice coil, e.g. to facilitate easy connection of the signal track to the voice coil. This second solder pad may be on the annular wall of the central diaphragm body or another wall of the central diaphragm body. Preferably, the second solder pad is on a wall of the central diaphragm body that can be accessed after the central diaphragm has been secured to the second suspension element, since this can facilitate connecting the voice coil to the second solder pad during installation. In some examples, the second solder pad is provided on an inner annular wall defining an aperture through the central diaphragm body. For example, the second solder pad may be on a face of the inner annular wall which faces towards the movement axis. Hence, the second solder pad may be reachable through the aperture.

Thus, the central membrane body may be utilised for purposes of signal transmission to and/or from the voice coil, replacing lead wires for part of signal transmission. This may decrease the risk of ticking lead wire noise and may improve installation of lead wires during assembly, which conventionally may be cumbersome and difficult.

An axial extent of the central diaphragm body along the movement axis may be greater than the separation of the landing surfaces along the movement axis.

The first end of the central diaphragm body may have a first extent in a direction perpendicular to the movement axis. The second end of the central diaphragm body may have a second extent in the direction perpendicular to the movement axis. The first extent may be smaller than the second extent.

The first end of the central diaphragm body may be closed. Additionally or alternatively, a dust cap may be located over the first end of the central diaphragm body.

By closing the central diaphragm body, either by providing a closed first end or covering the central diaphragm body with a dust cap, ingress into the loudspeaker may be prevented in part or even entirely.

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

Also disclosed herein is a bass loudspeaker including: a frame; a diaphragm suspended from the frame by at least a first suspension element and a second suspension element, wherein the first suspension element is attached to the frame at a first landing surface on the frame and the second suspension element is attached to the frame at a second landing surface on the frame; a magnet unit secured to the frame, wherein the magnet unit includes a permanent magnet and at least two flux guiding elements configured to guide magnetic flux across an air gap; a voice coil rigidly connected to the diaphragm; wherein the loudspeaker is operable to energise the voice coil to cause the voice coil to move relative to the magnet unit along a movement axis, thereby moving the diaphragm along the movement axis to produce sound; wherein the diaphragm comprises an outer diaphragm body and a central diaphragm body; wherein the central diaphragm body includes an annular wall which extends around the voice coil, the annular wall extending along the movement axis between a first end and a second end of the central diaphragm body; wherein the outer diaphragm body is connected to the annular wall at a location between the first end and the second end, such that the second end of the annular wall is separated from the location where the outer diaphragm is connected to the annular wall; wherein the first suspension element is connected to the outer diaphragm body; and wherein the second suspension element is connected to the annular wall at or towards the second end of the central diaphragm body.

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

In a traditional loudspeaker, the depth of the box is limited by the depth of the loudspeaker itself. There are known loudspeakers that are particularly designed to be shallow, such as those described in <CIT>, <CIT>, <CIT>. In <CIT>, a loudspeaker is described with an undulated membrane including a V-shaped cone; a lower suspension part is positioned next to a magnet system to decrease rocking motion of a moving assembly of the loudspeaker. In <CIT>, a similar loudspeaker is described which uses a tube suspension system. In <CIT>, yet another loudspeaker is described with a lower suspension adjacent to a magnet system of the loudspeaker. Each of the aforementioned loudspeakers is configured to provide magnetic flux of approximately uniform flux density across the air gap of the respective loudspeaker. As set out in the background section above, this corresponds to conventional principles of loudspeaker design. It may therefore be a departure from said conventional principles that a loudspeaker is described herein which includes a magnet unit of comparatively non-uniform flux density.

Also, each of the aforementioned loudspeakers makes use of a drive unit with a comparatively small/lightweight voice coil and a comparatively large/heavy, low-reluctance magnetic magnet system. This may also correspond to conventional principles of loudspeaker design. By contrast, the magnet unit of the loudspeaker described herein is relatively lightweight and has a comparatively high reluctance, while the voice coil is comparatively large/heavy.

Compared to the aforementioned known loudspeakers, the loudspeaker described herein may provide a combination of numerous advantages. For example, the loudspeaker described herein is shallow, lightweight and uses a comparatively small amount of rare earth material in the magnet unit. Moreover, it is easy to build with traditional machinery and can be built without increasing the part count compared to a traditional loudspeaker.

<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 translatable part <NUM> and a stationary part <NUM>. The translatable part <NUM> is secured to the diaphragm <NUM> and includes a voice coil <NUM>. The stationary part <NUM> is secured to the frame <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 voice coil <NUM> to move along the movement axis <NUM> relative to the magnet unit <NUM>. The voice coil <NUM> is rigidly connected to the diaphragm <NUM>, such that the voice coil <NUM> and the diaphragm <NUM> move together. When causing the voice coil <NUM> to move along the movement axis <NUM>, the diaphragm <NUM> also moves along the movement axis <NUM>, thereby producing sound.

More particularly, 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 in use is 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>. A direction perpendicular to the movement axis <NUM> is also referred to as a radial direction <NUM>.

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

The first suspension element <NUM> and the second suspension element <NUM> are secured to the diaphragm <NUM> such that the centre of gravity of the translatable part <NUM> is located between the first landing surface <NUM> and the second landing surface <NUM>. More particularly, the voice coil <NUM> is configured to sit in the air gap <NUM> when the diaphragm <NUM> is at rest, with the centre of mass of the voice coil <NUM> having a position along the movement axis <NUM> that is between the first landing surface <NUM> and the second landing surface <NUM>.

<FIG> and <FIG> are sectional views of parts of the loudspeaker <NUM>. <FIG> shows the diaphragm <NUM>, the voice coil <NUM>, and the suspension elements <NUM>, <NUM>. <FIG> shows the voice coil <NUM> and the magnet unit <NUM>.

The voice coil <NUM> has an axial extent along the movement axis <NUM> and a radial extent perpendicular to the movement axis <NUM>. The axial extent of the voice coil <NUM>, which is also known as a height of the voice coil <NUM>, is a distance between a pair of ends <NUM> of the voice coil <NUM>. The radial extent of the voice coil <NUM>, which is also known as a winding thickness of the voice coil <NUM>, is a distance between an inner perimeter <NUM> of the voice coil <NUM> and an outer perimeter <NUM> of the voice coil <NUM>.

The height of the voice coil <NUM> is approximately <NUM>% of the separation between the first landing surface <NUM> and second landing surface <NUM> as measured in a direction parallel to the movement axis <NUM>.

The magnet unit <NUM> includes a permanent magnet <NUM>, a (magnetic) washer <NUM> and a (magnetic) yoke <NUM>. The permanent magnet <NUM> is provided as a rare earth magnet and may comprise more than one structural element. 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 the magnetic flux generated by the permanent magnet <NUM> to the air gap <NUM> between the washer <NUM> and the yoke <NUM>. In particular, the washer <NUM> and the yoke <NUM> guide the magnetic flux across the air gap <NUM>.

The magnet unit <NUM> is arranged in the loudspeaker <NUM> such that the air gap <NUM> faces in the forward direction <NUM>, and receives the voice coil <NUM> extending in the rearward direction <NUM>.

The magnet unit <NUM> and the air gap <NUM> form a magnetic circuit <NUM>. The magnetic circuit <NUM> provides a closed loop for the magnetic flux that is generated by the permanent magnet <NUM> and is guided by the two flux guiding elements <NUM>, <NUM> across the air gap <NUM>. In this example, the air gap <NUM> has a magnetic reluctance of <NUM> x <NUM>^<NUM> [<NUM>/H] and the magnetic circuit <NUM> has a total magnetic reluctance slightly greater than the magnetic reluctance of the air gap <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>.

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 a comparatively large voice coil <NUM> using many layers in the magnetic circuit <NUM>. As the air gap <NUM> is wider to accommodate the larger voice coil <NUM> as compared to a traditional loudspeaker, the total reluctance in the magnetic circuit <NUM> increases so much that the cross-sections of the flux guiding washer <NUM> and yoke <NUM> can be thin.

This new configuration leads to a comparably high voice coil mass and comparatively low magnet unit mass. Yet the overall mass of the drive unit <NUM> may be lower than, for example, for the aforementioned known loudspeakers. 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 bass loudspeaker with low resonance frequency in box.

The permanent magnet <NUM>, the washer <NUM>, and the yoke <NUM> are axially symmetric about the movement axis <NUM>, though other arrangements are possible.

<FIG> and <FIG> illustrate the magnetic flux generated by the magnet unit <NUM>. In particular, <FIG> shows a sectional view of the drive unit <NUM> and illustrates magnetic flux lines generated by the magnet unit <NUM>, while <FIG> is a graph showing the magnetic flux density across the voice coil <NUM> ("COIL ID" to "COIL OD", i.e. inner diameter/perimeter <NUM> to outer diameter/perimeter <NUM>).

In <FIG>, the magnetic flux follows curved paths spreading out in the air gap <NUM> across the voice coil <NUM>. Correspondingly, the magnetic flux density decreases across the voice coil <NUM>. <FIG> shows the radial flux density Br over the winding thickness of the voice coil <NUM>, which shows a substantial drop typically undesired in loudspeaker design. In a typical loudspeaker the magnetic flux density may be almost constant and may drop to approximately <NUM> to <NUM>% (percent) of the initial value. By contrast, here the radial flux density drops by at least <NUM>% over the winding thickness of the voice coil <NUM>. In particular, the magnetic flux density at the outer perimeter <NUM> of the voice coil <NUM> is approximately <NUM>% of the magnetic flux density at the inner perimeter <NUM> of the voice coil <NUM>. In other words, the magnetic flux density drops by approximately <NUM>% from the inner perimeter <NUM> to the outer perimeter <NUM> of the voice coil <NUM>.

The comparatively large decrease in magnetic flux density is related to the comparatively high reluctance of the magnet unit <NUM> of the loudspeaker <NUM>. The reluctance can be estimated by calculations derived from simulations using the Finite Element Method of the static magnetic circuit. The magnetomotive force <IMG> (F_m) of the magnet in the circuit is calculated by multiplying the average magnetic field strength Hav,m (H_av,m) inside the magnet times the height of the magnet hm (h_m). The resulting unit is Amperes [A].

The total magnetic flux ϕB (Phi_B) through a magnetic circuit is estimated by integration of the magnetic flux density penetrating an open surface. The integration is carried out over a surface Sm (Sm), which is typically a cylindrical surface at the top or bottom surface of the magnet perpendicular to the magnet's magnetization direction. The magnetic flux density at the magnet is strictly axial, such that the radial component can be neglected. The resulting unit is Weber [Wb].

Dividing the magnetomotive force <IMG> by the magnetic flux ϕB yields the reluctance <IMG> (R_m) of the magnetic circuit. The resulting unit is one over Henry [<NUM>/H].

It has been found that having a reluctance above <NUM> x <NUM>^<NUM> [<NUM>/H] (or "<NUM>. 5E6 <NUM>/H"), preferably above <NUM> x <NUM>^<NUM> [<NUM>/H] (or "3E6 <NUM>/H") may lead to highly efficient loudspeakers of low weight. By contrast, carrying out the same calculations on magnet systems of traditional loudspeakers shows that their reluctance may not exceed <NUM> x <NUM>^<NUM> [<NUM>/H] (or "<NUM>. 5E6 <NUM>/H").

In this example, the flux guiding elements <NUM>, <NUM> are made from steel, such that the reluctance of the flux guiding elements can be neglected as the relative permeability µr of steel is >> <NUM> and all reluctance can be assigned to the air gap <NUM>. Having such a large reluctance air gap <NUM>, the radial flux density is not constant in the air gap <NUM>, as already discussed above.

<FIG> and <FIG> illustrate the diaphragm <NUM>. The diaphragm <NUM> includes an outer diaphragm body <NUM> (or 'cone') and a central diaphragm body <NUM> (or 'sub-cone').

In <FIG>, the outer diaphragm body <NUM> is shown. The outer diaphragm body <NUM> has an outer edge <NUM> and an inner edge <NUM>. The inner edge <NUM> bounds a central aperture <NUM> through the outer diaphragm body <NUM>. When assembled, the central diaphragm body <NUM> is received into the central aperture <NUM>.

The outer edge <NUM> of the outer diaphragm body <NUM> is connected to the first suspension element <NUM>. In this example, the inner edge <NUM> of the outer diaphragm body <NUM> is connected to the central diaphragm body <NUM> since the diaphragm bodies <NUM>, <NUM> are formed separately.

The inner edge <NUM> of the outer diaphragm body <NUM> is provided at an angle relative to the movement axis <NUM>. In this example, the angle is in the range of <NUM> degrees to <NUM> degrees. Thus, an upstanding edge portion at an angle to the movement axis <NUM> is provided.

In <FIG>, the central diaphragm body <NUM> is shown. The central diaphragm body <NUM> has a forward end <NUM> and a rearward end <NUM>. The forward end <NUM> and the rearward end <NUM> are opposite ends of the central diaphragm body <NUM> delimiting a lengthwise extent of the central diaphragm body <NUM>. The forward end <NUM> faces in the forward direction <NUM> and the rearward end <NUM> faces in the rearward direction <NUM>.

The central diaphragm body <NUM> includes an outer annular wall <NUM> and an inner annular wall <NUM>. The annular walls <NUM>, <NUM> are concentrically arranged around the movement axis <NUM>. In this example, the inner annular wall <NUM> and the voice coil <NUM> are sequentially arranged along the movement axis <NUM>, and the outer annular wall <NUM> encloses both the inner annular wall <NUM> and the voice coil <NUM>.

The inner annular wall <NUM> extends towards the voice coil <NUM> such that mechanical and/or electrical connection may be made with the voice coil <NUM> or, where provided, a voice coil former.

The inner annular wall <NUM> bounds an aperture <NUM> extending through the central diaphragm body <NUM>.

The central diaphragm body <NUM> and the outer diaphragm body <NUM> are joined at a location <NUM> (see <FIG>) where the outer annular wall <NUM> receives the outer diaphragm body <NUM>. The location <NUM> is between the forward end <NUM> and the rearward end <NUM> of the central diaphragm body <NUM>. Accordingly, the outer annular wall <NUM> has a free end <NUM> (see <FIG>) in the rearward direction <NUM>. The second suspension element <NUM> is connected to the outer annular wall <NUM> and, in particular, the free end <NUM> of the outer annular wall <NUM>.

The outer annular wall <NUM> of the central diaphragm body <NUM> is under a steep angle relative to the movement axis <NUM>. In this example, this angle is in the range of <NUM> degrees to <NUM> degrees, allowing for a small inside diameter of the second suspension element <NUM>. This may ensure clearance above the yoke <NUM>, allowing for large axial displacement of the voice coil <NUM> and the overall moving assembly.

The inner edge <NUM> of the outer diaphragm body <NUM> and the outer annular wall <NUM> of the central diaphragm body <NUM> are provided at similar steep angles relative to the movement axis <NUM>, e.g. within <NUM> degrees of each other, as this may improve bonding of the diaphragm bodies <NUM>, <NUM>. Further, this design allows for undulation of the diaphragm <NUM>, resulting in a substantial stiffening of the whole downwards portion of outer diaphragm body <NUM> and the central diaphragm body <NUM> while allowing enough clearance to the bottom of the central diaphragm body <NUM> for leadwire connection.

<FIG> shows the central diaphragm body <NUM> and the voice coil <NUM>. The central diaphragm body <NUM> is utilised for transmitting a leadwire signal to and from the voice coil <NUM>. More particularly, the central diaphragm body <NUM> includes a signal track <NUM>, e.g. a copper strip, for guiding a signal to or from the voice coil <NUM>. In practice a minimum of two signal tracks <NUM> may be provided.

The signal track <NUM> extends from a radially outer face <NUM> of the outer annular wall <NUM> towards the voice coil <NUM>. Suitably, the signal track <NUM> connects a pair of solder pads <NUM> for making a connection with a leadwire <NUM>, at one end of the signal track <NUM>, and the voice coil <NUM>, at the other end of the signal track <NUM>. A first solder pad <NUM> is provided on the radially outer face <NUM> of the outer annular wall <NUM>, the first solder pad <NUM> being located towards the rearward end <NUM>. A second solder pad <NUM> is provided on a radially inner face <NUM> of the inner annular wall <NUM>.

In this example, the signal track <NUM> extends along a radially inner face <NUM> of the outer annular wall <NUM> and along a radially outer face <NUM> of the inner annular wall <NUM>.

Utilisation of the signal track <NUM> may improve ease of assembly of the loudspeaker <NUM>. The signal track <NUM> may be provided as a thin self-adhesive copper strip glued along the central diaphragm body <NUM>, along the inside or outside of the central diaphragm body <NUM>. This may enable easy connection to the voice coil <NUM> at the top and to the leadwire <NUM> at the bottom by means of a solder connection or electrically conductive glue. The flexible leadwires <NUM> may form an arc through the air or be connected to the second suspension element <NUM>, e.g. by stitching or gluing or even be in-woven.

All electrical connections from the voice coil <NUM> to the signal track <NUM>, the leadwire <NUM> and a leadwire terminal <NUM> may be carried out while the components are easily accessible without the presence of the outer diaphragm body <NUM> or the need to flip the loudspeaker <NUM>. Electrical connection of the voice <NUM> may be carried out through the aperture <NUM> of the central diaphragm body <NUM>.

As for the aforementioned known loudspeakers, it is noted that all three loudspeaker designs may suffer from comparatively complicated guidance of the leadwires connecting the voice coil windings to the terminal. <CIT> allows for large displacement as the inside diameter of the lower suspension is close to the outside diameter of the U-yoke, but the construction and lead wire guidance with the tubular element and coupling feature to the cone may be cumbersome. In the cases of <CIT> and <CIT>, the part of the membrane that brings the connection surface to the damper next to the U-yoke is under a shallow angle resulting in a large inner diameter of the damper decreasing the maximum excursion capability.

<FIG> shows another exemplary loudspeaker <NUM>. The loudspeaker <NUM> includes two loudspeakers <NUM>, as described above, mounted in a back-to-back configuration in a closed box volume of <NUM> litres, each designed for a nominal stroke of +-<NUM> (millimetres). Each loudspeaker <NUM> is provided as <NUM>-inch woofer for use up to <NUM> (Hertz) in a closed box of as little as <NUM> litres net volume.

As described above, the magnet unit <NUM> consists of only three pieces: the U-yoke <NUM>, the permanent magnet <NUM> (provided as a disc) and the washer <NUM>. The permanent magnet <NUM> is <NUM> in diameter and <NUM> in height and has a weight of <NUM> (grams). This may be exceptionally little for a woofer with this application. The voice coil <NUM> has <NUM> winding height at a winding thickness of <NUM>. The weight of the windings is <NUM>. To guide the flux effectively through the windings, the washer <NUM> has a thickness of <NUM> and the U-yoke has a wall thickness (measured adjacent to half the height of the voice coil windings) of <NUM>. These parameters lead to a ratio of magnet weight to coil weight of <NUM>:<NUM>. The ratio of winding thickness to U-yoke thickness is <NUM>:<NUM>.

In <FIG>, two of these loudspeakers <NUM> are mounted back-to-back in a closed box of <NUM> litres total net volume allowing each loudspeaker <NUM> to act onto a closed volume of <NUM> litres. Due to the small size, the box does not need excessive internal ribbing or stiffening elements which would otherwise increase the necessary outer dimensions and gross volume. The magnet units <NUM> are mounted back-to-back, mitigating the effect of any leakage flux towards the centre surface where the U-yokes touch, as all flux is forced back into the flux guiding steel allowing the cross section to be minimal.

As the loudspeakers <NUM> are joined back-to-back, the net force on the rear portion of each individual frame is nil and allows it to be thin and lightweight. The force factor vs displacement of each motor system is symmetric and drops to <NUM>% relative to the rest position at +-<NUM> leading to low distortion over a wide displacement range. Having the steel flux guiding elements close to magnetic saturation at approx. 6T allows keeping the inductance so low that it has 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>.

The moving mass of the loudspeaker <NUM> is approximately <NUM>, the sum of the suspension and box stiffness adds up to 15N/mm resulting in <NUM> in-box resonance frequency. The total mass of the loudspeaker <NUM> is below <NUM>.

It is worth appreciating the fact that for two drivers back-to-back the total mass of the drivers is below <NUM> of which <NUM> are moving in opposite directions leading to no net force on the cabinet. The mass of the whole assembly of two drivers in box is below <NUM>.

Such small size and weight and absence of any vibration of the cabinet allows placement in positions where the sheer size previously prohibited the application. This can be e.g. close to the bottom of the A-style or between the foot wells in a car cabin in a high power kick-bass application.

One can easily appreciate the possibilities for manufacturing in view of the above disclosure. For example, leadwire guidance with subsequent placement of a dustcap may be improved. In <FIG>, the loudspeakers <NUM> are shown with separate dustcaps <NUM>. This may allow a traditional build starting from frame and magnet system and adding the dustcap <NUM> last. Alternatively, the dustcap is integrated in the central diaphragm body <NUM> (see <FIG>) allowing the pre-assembly with the voice coil <NUM> and installation of the signal track <NUM> before inserting into the magnet unit and centering via a jig. The diaphragm <NUM> has a central portion that covers the top of the voice coil and so acting as a dustcap. In this case, the whole loudspeaker may be manufactured around an alignment jig initially inserted into the frame which is finally replaced by the magnet unit. Using this approach, the part count may not increase relative to a traditional loudspeaker while offering substantial benefits in terms of performance, as set out above.

Also shown in <FIG> is a leadwire fixed to the second suspension element <NUM> at multiple locations. One could also imagine the leadwire being partially fixed to the underside of the diaphragm <NUM> and from there stretching an arc through the air towards the terminal <NUM>. This may decrease the risk of ticking leadwire noise as compared to an arc that starts at the bottom of the central diaphragm body <NUM>.

The voice coil <NUM> can have one (as described above) or multiple windings being driven from multiple amplifier channels via separate pairs of signal tracks <NUM> and leadwires <NUM>. Also, the winding wire of the voice coil <NUM> can have a round cross-section or rectangular cross-section. These winding wire types allow for a higher conductor ratio compared to the volume taken up by the windings (fill factor) and are particularly useful in the described loudspeaker with large coil volume. The winding wire material is any suitable conductor material; preferably Copper but also be Aluminium or a mixture of the two, such as Copper-Clad Aluminium wire.

Both the outer diaphragm body <NUM> and the central diaphragm body <NUM> can be made from a material with high heat conductivity (e.g. Aluminium) effectively acting as a heat sink as they are connected proximal to the voice coil <NUM> without a long voice coil former. In this case, the signal tracks <NUM> are insulated from the outer diaphragm body <NUM> and the central diaphragm body <NUM>. One can also imagine soldering one electrical connection to the central diaphragm body <NUM> and one connection to the outer diaphragm body <NUM>; in this case, electrical insulation between the diaphragm bodies <NUM>, <NUM> may be provided by the adhesive between them. The leadwires <NUM> are then also connected to the outer diaphragm body <NUM> and the central diaphragm body <NUM>, respectively. Of course, also a combination is possible where e.g. only one connection is done via a signal track <NUM> and the other via an electrically conductive central diaphragm body <NUM>.

In fact, it is also possible to guide the leadouts of the winding wire or the coil along the top surface of the central diaphragm body <NUM> towards the bottom of the central diaphragm body <NUM> for connection to the leadwires. This is particularly interesting if the diaphragm <NUM> has an integrated dustcap and covers the wires completely. The electrical connection along the central diaphragm body <NUM> can also be carried out by means of an insert moulded conductor in case the central diaphragm body <NUM> is injection moulded. Even a thin copper layer with only a strip separating the conducting surfaces added to an otherwise insulating central diaphragm body <NUM> by means of vapor deposition is an option. As one can see, there are many ways to implement the basic concept of the electrical connection being guided along the central diaphragm body <NUM> and to the inside diameter of the second suspension element <NUM> and below the diaphragm <NUM>.

<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 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.

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.

In fact, the scope of the invention is defined in the appended claims.

Claim 1:
A bass loudspeaker (<NUM>) including:
a frame (<NUM>);
a diaphragm (<NUM>) suspended from the frame (<NUM>) by at least a first suspension element (<NUM>) and a second suspension element (<NUM>), wherein the first suspension element (<NUM>) is attached to the frame (<NUM>) at a first landing surface (<NUM>) on the frame (<NUM>) and the second suspension element (<NUM>) is attached to the frame (<NUM>) at a second landing surface (<NUM>) on the frame (<NUM>);
a magnet unit (<NUM>) secured to the frame (<NUM>), wherein the magnet unit (<NUM>) includes a permanent magnet (<NUM>) and at least two flux guiding elements (<NUM>, <NUM>) configured to guide magnetic flux across an air gap (<NUM>);
a voice coil (<NUM>) rigidly connected to the diaphragm (<NUM>);
wherein the loudspeaker (<NUM>) is operable to energise the voice coil (<NUM>) to cause the voice coil (<NUM>) to move relative to the magnet unit (<NUM>) along a movement axis (<NUM>), thereby moving the diaphragm along the movement axis (<NUM>) to produce sound;
wherein the voice coil (<NUM>) is configured to sit in the air gap, with a centre of mass of the voice coil (<NUM>) having a position along the movement axis (<NUM>) that is between the first landing surface (<NUM>) and the second landing surface (<NUM>), when the diaphragm (<NUM>) is at rest; and
wherein a magnetic flux density at an outer perimeter of the voice coil (<NUM>) is <NUM>% or less of a magnetic flux density at an inner perimeter of the voice coil (<NUM>);
characterized in that
the magnet unit (<NUM>) and the air gap form a magnetic circuit (<NUM>) which has a magnetic reluctance of at least <NUM> x <NUM>^<NUM> [<NUM>/H] and wherein the air gap has a magnetic reluctance of at least <NUM> x <NUM>^<NUM> [<NUM>/H].