Patent Publication Number: US-11388523-B2

Title: Inertial exciters, drive units and loudspeakers

Description:
Cross Reference to Related Applications 
     This application is a U.S. National Stage Application of International Patent Application No. PCT/EP2019/084950 entitled “INERTIAL EXCITERS, DRIVE UNITS AND LOUDSPEAKERS” filed on 12 Dec. 2019, which claims priority from GB1820557.5 entitled “INERTIAL EXCITER” filed 17 Dec. 2018 and GB1908461.5 entitled “INERTIAL EXCITERS, DRIVE UNITS AND LOUDSPEAKERS” filed 13 Jun. 2019, the contents and elements of which are herein incorporated by reference for all purposes. 
     This application claims priority from GB1820557.5 filed 17 Dec. 2018 and GB1908461.5 filed 13 Jun. 2019, the contents and elements of which are herein incorporated by reference for all purposes. 
     FIELD OF THE INVENTION 
     The present invention relates to an inertial exciter, to a loudspeaker including an inertial exciter, to a drive unit, and also to a loudspeaker including a drive unit. Corresponding methods are also disclosed. 
     BACKGROUND 
     Traditional loudspeakers, an example of which is shown in  FIG. 1( a ) , typically include an acoustic radiator, typically referred to as a diaphragm, suspended from a frame mounted in a baffle or loudspeaker enclosure. Sound is produced as a result of movement of the diaphragm, actuated by a voice coil attached to the diaphragm, which interacts with a magnet system attached to the frame. The baffle or loudspeaker enclosure acts to inhibit cancellation between sound produced by the front and rear faces of the diaphragm. 
     Inertial exciters, an example of which is shown in  FIG. 1( b ) , typically are devices which are configured to attach to an acoustic radiator such as a panel or soundboard, and which are configured to apply inertial force to the acoustic radiator so as to cause the acoustic radiator to vibrate to produce sound. Inertial exciters are typically used in automotive, aviation and consumer products. 
     Loudspeakers incorporating inertial exciters are well known, with examples being disclosed in, for example [1]-[12]. 
     Acoustical exciters are capable of transmitting a wide bandwidth of mechanical vibration energy into acoustic radiators, typically panels or walls that are configured to sustain that vibration energy across their surface to produce acoustic output. For a loudspeaker incorporating an inertial exciter, the frequency spectrum of interest (the frequency spectrum across which the loudspeaker is able to produce sound) may be the audible range (20 Hz-20 kHz). 
     In order to produce sound over a wide bandwidth, inertial exciters typically need to have a coil assembly (the part of the inertial exciter that includes the voice coil) that has a low mass and is very stiff so as to maximize the efficiency across the audio bandwidth. Whereas the magnet assembly (the part of the inertial exciter that includes the magnet system) can have a much higher mass (and generally will have a higher mass in practice). 
     The mechanical fixation of the exciter to the acoustic panel requires special attention: when one wants to make use of moving coil (MC) excitation combined with moving magnet (MM) excitation (these types of excitation are discussed in more detail below), ideally the exciter is mounted to the acoustic radiator only via the coupler, i.e. with the magnet assembly being suspended from the acoustic radiator via the coil assembly, thereby leaving the magnet system freely suspended. 
       FIG. 2( a )  shows a loudspeaker  1  incorporating a wide bandwidth inertial exciter implementing principles derived from the prior art.  FIG. 2( b )  is a graph showing force level vs frequency for the loudspeaker shown in  FIG. 2( a ) . 
     In this example, a magnet assembly  2  including a magnet unit  10  and a frame  12  is suspended from an acoustic radiator  90  via a coil assembly including a voice coil  30  and a voice coil former  32 . The voice coil  30  sits in an air gap  16  of the magnet unit  10  when the exciter  1  is at rest 
     The voice coil will generate a force F according to:
 
F=BLI
 
where B is magnetic field, L is wire length and I is electric current (standard units).
 
     The inertia of the magnet assembly  2  (which is typically significantly heavier than the voice coil assembly  4 ) allows the voice coil assembly  4  to transmit vibrational energy to the acoustic radiator  90 . Excitation of the acoustic radiator  90  caused by movement of the voice coil assembly is referred to herein as “moving coil” or “MC” excitation. 
     Where the magnet assembly  2  is suspended from the acoustic radiator  90  via the coil assembly  4  (as in the example shown in  FIG. 2( a ) ), resonance of the magnet assembly  2  is able to give additional vibrational energy to the acoustic radiator  90  around the resonant frequency of the magnet assembly  2 . The resonant frequency of the magnet assembly  2  is defined by the mass of the magnet assembly  2  and the compliance of the suspension  60  from which the magnet assembly  2  is suspended. Excitation of the acoustic radiator  90  caused by resonance of the magnet assembly  2  is referred to herein as “moving magnet” or “MM” excitation. 
     As shown in  FIG. 2( b ) , MM excitation provides a force boost at low frequencies (labelled “MM” in  FIG. 2( b ) ), which is an advantage of systems in which the exciter is mounted to the acoustic radiator only via the coupler, as in the example of  FIG. 2( a ) . 
     The force level provided by MC excitation (labelled “MC” in  FIG. 2( b ) ) is boosted by the voice coil having a low weight and being very stiff. 
     The present inventor has observed a problem with the loudspeaker illustrated in  FIG. 2( a ) . This problem is illustrated by  FIG. 2( c ) . 
     In detail, when mounting the acoustic panel  90  (to which the wide bandwidth inertial exciter  1  is attached) vertically, e.g. in an interior door panel of a car, the gravitational force on the magnet assembly  2  tends to rotate its position relative to the voice coil  4  assembly over time. This is due to the compliance of the single suspension  60  (in this case a spider) that is configured to position the voice coil  30  in the air gap  16  (and does this job very well), but is not configured to inhibit rotation of the magnet assembly  2  relative to the voice coil assembly  4  when the acoustic radiator  90  is vertically mounted, e.g. as may be the case in a car door. 
     The prior art teaches some possible solutions to this problem, some of which are summarized as follows:
         Solution A as shown in  FIG. 3( a )( i )  and  FIG. 3( a ) ( ii ) (“Free magnet system”)
           Good MC &amp; MM operation; Minimal additional mass for MC; Similar to [1];   Problem: Motor mass on single suspension makes it unstable regarding buckling as depicted in  FIG. 2( c )     
           Solution B as shown in  FIG. 3( b )( i )  and  FIG. 3( b ) ( ii ) (“Grounded magnet system”)
           Stable magnet system; Similar to [ 13 ] and classic loudspeaker;   Problem: Large bracket for large panels, No MM excitation benefit, Not an inertial exciter design   
           Solution C as shown in  FIG. 3( c )( i )  and  FIG. 3( c ) ( ii ) (“Bracket to panel”)
           Stable magnet system; Similar to [6], [7], [11], [12]   Problem: Influence of panel acoustics, No MM benefit   
           Solution D as shown in  FIG. 3( d )( i )  and  FIG. 3( d ) ( ii ) (“Centrally suspended motor”)
           MC and MM excitation; Reasonably stable; Similar to [4]   Problem: Additional mass for MC operation, Breakup of large coupler causes a step in force profile   
           Solution E as shown in  FIG. 3( e )( i )  and  FIG. 3( e ) ( ii ) (“Double suspended motor”)
           MC and MM excitation; Stable motor suspension; Similar to [8], [9], [10]   Problem: Additional mass for MC operation, Breakup of large coupler causes a step in force profile   
           Solution F as shown in  FIG. 3( f )( i )  and  FIG. 3( f ) ( ii ) (“Shaker”)
           Only MM operation (for use as a shaker); Stable motor suspension   Problem: Not a wide bandwidth   
               

     Solution F uses an inertial exciter as a shaker to transmit a small bandwidth of mechanical vibration energy into structures such as a seat in a car or in a cinema to augment the experience via tactile stimulus. Generally, the frequency spectrum in which this seems enjoyable is very limited, e.g. 30 Hz-80 Hz. The design of shakers is less complicated as compared to acoustic exciters because they rely solely on the inertial vibration energy of the moving magnet system (MM) since their scope is to transfer only low frequency vibration. The fixation of such shaker to the panel is also less critical and may involve heavier constructions without compromising performance. Of course a wide bandwidth inertial exciter (with a freely suspended magnet system as in solutions A, D, E) can also be used solely as a shaker. 
     The inventor has observed that it is difficult to make an inertial exciter that successfully inhibits rotation of the magnet assembly relative to the voice coil assembly whilst allowing MM excitation and without adding significant weight to the voice coil assembly. Thus, it is difficult to produce an inertial exciter having good sound reproduction over a wideband bandwidth, without encountering rotation issues when the acoustic radiator is mounted vertically, e.g. as might be the case in a car door. 
     The present invention has been devised in light of the above considerations. 
     SUMMARY OF THE INVENTION 
     A first aspect of the invention provides: 
     An inertial exciter for use with an acoustic radiator, the inertial exciter comprising:
         a magnet assembly including a magnet unit configured to provide a magnetic field in an air gap, wherein the air gap extends around a movement axis of the inertial exciter;   a coil assembly including:
           an attachment portion configured to provide an attachment between the coil assembly and the acoustic radiator;   a voice coil;   a voice coil former which extends from the attachment portion into the air gap, wherein the voice coil is mounted to the voice coil former so that the voice coil sits in the air gap when the inertial exciter is at rest;   a tubular member, which is positioned radially outwardly of the voice coil former with respect to the movement axis, and which overlaps the voice coil former along at least a portion of the movement axis;   
           at least one suspension attached to the tubular member and a part of the magnet assembly positioned radially outwardly of the tubular member so that, when the coil assembly is attached to the acoustic radiator via the attachment portion, the magnet assembly is suspended from the acoustic radiator via the coil assembly by the at least one suspension.       

     The tubular member, by being positioned radially outwardly of the voice coil former (preferably also of the air gap) with respect to the movement axis, the tubular member facilitates the attachment of the at least one suspension, preferably two suspensions, to the part of the magnet assembly positioned radially outwardly of the tubular member. 
     The movement axis may be defined as an axis along which the voice coil assembly is configured to move relative to the magnet assembly when the inertial exciter is activated by supplying electrical current carrying an audio signal to the voice coil. 
     The inertial exciter may be considered to be at rest when electrical current is not supplied to the voice coil. 
     Note that in order for the magnet assembly to be suspended from the acoustic radiator via the coil assembly, the magnet assembly should only be attached to the acoustic radiator via the coil assembly, i.e. with no rigid attachment between the magnet assembly and the acoustic radiator. 
     Preferably, the inertial exciter comprises:
         a first suspension attached to the tubular member and the part of the magnet assembly positioned radially outwardly of the tubular member; and   a second suspension, separated from the first suspension in a direction extending parallel to the movement axis, wherein the second suspension is either: attached to the tubular member and the part of the magnet assembly positioned radially outwardly of the tubular member or is attached to the voice coil former and a part of the magnet assembly positioned radially inwardly of the voice coil former.       

     The use of two suspensions, separated in the direction of the movement axis, helps to significantly reduce the rotation described above with respect to  FIG. 2( c )  and maintain good performance without substantially increasing the weight of the coil assembly, noting that the tubular member allows a large separation of the first and second suspensions, and also noting that the tubular member has an inherently stiff shape and so can be formed from lightweight material. 
     For a typical application, the distance between locations at which the two suspensions attach to the part of the magnet assembly positioned radially outwardly of the tubular member may be at least 3 mm, more preferably at least 5 mm, more preferably at least 6 mm as measured in a direction extending parallel to the movement axis. A skilled person would appreciate that actual distances will vary in practice depending on various factors including weight of the magnet assembly (larger weight requires larger distance) and design limitations (e.g. space in aperture in which loudspeaker is to be installed). 
     The magnet assembly may include a frame to which the magnet unit is attached, wherein the part of the magnet assembly positioned radially outwardly of the tubular member (to which the at least one suspension is attached) is a part of the frame. 
     The part of the magnet assembly positioned radially outwardly of the tubular member (to which the at least one suspension is attached) could, for example, be a rim of the frame. 
     The part of the magnet assembly positioned radially outwardly of the tubular member (to which the at least one suspension is attached) may include a respective ledge for the/each suspension attached to the part of the magnet assembly positioned radially outwardly of the tubular member, thereby facilitating attachment of the suspension element(s) to the part of the magnet assembly positioned radially outwardly of the tubular member. 
     The frame (included in the magnet assembly) may include apertures configured to allow a jig to be inserted to centre the tubular member during assembly. 
     Some optional features of the inertial exciter described herein are described with reference to:
         a first plane perpendicular to the movement axis which extends through the attachment portion;   a second plane perpendicular to the movement axis which extends through the air gap.       

     Features described with reference to the first and second planes are preferably described with respect to the inertial exciter when the inertial exciter is at rest. As noted above, the inertial exciter may be considered to be at rest when electrical current is not supplied to the voice coil. 
     The part of the magnet assembly positioned radially outwardly of the tubular member (to which the at least one suspension is attached) may include:
         a proximal portion, wherein the proximal portion of the part of the magnet assembly positioned radially outwardly of the tubular member is located between the first plane and the second plane; and   a distal portion, wherein the distal portion of the part of the magnet unit positioned radially outwardly of the tubular member is located is on an opposite side of the second plane from the proximal portion (of the part of the magnet assembly positioned radially outwardly of the tubular member).       

     The magnet assembly may include a part of the magnet assembly positioned radially inwardly of the voice coil former, wherein the part of the magnet assembly position radially inwardly of the voice coil former includes:
         a proximal portion, wherein the proximal portion of the part of the magnet assembly positioned radially inwardly of the voice coil former is located between the first plane and the second plane; and   a distal portion, wherein the distal portion of the part of the magnet unit positioned radially inwardly of the voice coil former is located is on an opposite side of the second plane from the proximal portion (of the part of the magnet assembly positioned radially inwardly of the voice coil former).       

     The part of the magnet assembly positioned radially inwardly of the voice coil former may include part of the magnet unit. The proximal portion of the part of the magnet assembly positioned radially inwardly of the voice coil former may for example include part of the magnet unit, e.g. an extra magnet  114   a  as shown in  FIG. 4( a )( i ) . The distal portion of the part of the magnet assembly positioned radially inwardly of the voice coil former may for example include part of the magnet unit, e.g. a main magnet  112   a  as shown in  FIG. 4( a )( i ) . 
     The tubular member may include:
         a proximal portion, wherein the proximal portion of the tubular member is located between the first plane and the second plane;   a distal portion, wherein the distal portion of the tubular member is located is on an opposite side of the second plane from the proximal portion (of the tubular member).       

     The voice coil former may include:
         a proximal portion, wherein the proximal portion of the voice coil former is located between the first plane and the second plane;   a distal portion, wherein the distal portion of the voice coil former is located on an opposite side of the second plane from the proximal portion (of the voice coil former).       

     Note that if the tubular member has the distal portion (as described above), this allows the tubular member to reach past the air gap on the outside of the magnet unit, and allows the first and second suspensions to be separated by a larger distance, compared with an arrangement in which the two suspensions are attached to the voice coil former. 
     Preferably, the first suspension is attached to the distal portion of the tubular member and the distal portion of the part of the magnet assembly positioned radially outwardly of the tubular member. 
     Preferably, the second suspension is attached to the proximal portion of the tubular member and the proximal portion of the part of the magnet assembly positioned radially outwardly of the tubular member. 
     However, the second suspension could potentially instead attach to the proximal portion of the voice coil former and the proximal portion of the part of the magnet assembly positioned radially inwardly of the voice coil former, whilst still allowing a wide separation between the first and second suspensions, thereby still helping to reduce the rotation discussed above with reference to  FIG. 2( c ) . 
     Preferably, the inertial exciter comprises both:
         a first suspension that is attached to the distal portion of the tubular member and the distal portion of the part of the magnet assembly positioned radially outwardly of the tubular member; and   a second suspension that is attached to the proximal portion of the tubular member and the proximal portion of the part of the magnet assembly positioned radially outwardly of the tubular member.       

     This arrangement allows the first and second suspensions to have a particularly large space between them, which helps to reduce the rotation discussed above with reference to  FIG. 2( c ) . 
     In this arrangement, the inertial exciter may optionally include a third suspension that is attached to the proximal portion of the voice coil former and the proximal portion of the part of the magnet assembly positioned radially inwardly of the voice coil former (e.g. as shown in  FIG. 5( b ) ). 
     The tubular member preferably extends around the magnet unit. 
     The tubular member preferably overlaps the magnet unit along at least a portion of the movement axis. 
     The tubular member may be shaped to include the attachment portion, e.g. so as to facilitate direct gluing (or some other attachment) of the tubular member to the acoustic radiator. 
     The tubular member may be shaped to include the attachment portion and the voice coil former. 
     The tubular member may include or be attached to a surface extending outwardly in a radial direction (with respect to the movement axis) from the distal portion of the tubular member to provide a surface for attaching the tubular member to the first suspension. The surface may be flat. The surface may be provided by a ring, e.g. made of plastic/cardboard. 
     The tubular member may include or be attached to a surface extending outwardly in a radial direction (with respect to the movement axis) from the proximal portion of the tubular member to provide a surface for attaching the tubular member to the second suspension. The surface may be flat. The surface may be provided by a ring, e.g. made of plastic/cardboard. 
     The wall of the tubular member could form an angle with respect to the movement axis, e.g. so that the distal portion of the tubular member is further from the movement axis than the proximal portion of the tubular member, thereby forming a frusto-conical tubular member. In this case, the angle is preferably no more than 15°. 
     The tubular member could have one or more extensions in radially outward direction (with respect to the movement axis) to provide a respective attachment surface for the/each suspension attached to the tubular member, thereby facilitating attachment of the/each suspension to the tubular member. 
     The width of the inertial exciter in the radial direction (perpendicular to the movement axis) will generally depend on design requirements. 
     The inertial exciter may include one or more wires configured to provide an electrical path for supplying an electrical current carrying an audio signal (representative of sound) to the voice coil. 
     The electrical path provided by the one or more wires may extend from a connector formed on the magnet assembly (e.g. on a frame of the magnet assembly) to the voice coil. 
     The one or more wires may include wire from the voice coil winding and/or a lead wire which connects to the voice coil winding. 
     The one or more wires may include a wire that passes through or around the tubular member. A coupling element (if present—see below) may be configured to guide said wire through or around the tubular member. 
     The one or more wires may include a wire that passes through or around (preferably through) a frame included in the magnet assembly. 
     The one or more wires may include two wires that meet at an electrical junction formed on an outwardly facing surface of the tubular member, e.g. at a solderpad or glue dot on the outwardly facing surface of the tubular member. 
     The magnet unit is preferably configured to provide a magnetic field in an air gap. The voice coil former and/or the tubular member may be cylindrical. But other shapes of air gap, voice coil former and tubular member are possible, e.g. oval, square. 
     Preferably the voice coil former is arranged around the movement axis. 
     The voice coil former preferably extends from the attachment portion in a direction which extends along the movement axis into the air gap. 
     The tubular member and voice coil former are each preferably made from lightweight materials such as paper, cardboard, Kapton, aluminium, kevlar, PE, ABS etc. 
     The tubular member and voice coil former are preferably made of the same material as each other, but could be made of different materials. 
     The tubular member and voice coil former may be formed integrally with each other (preferably also the attachment portion). 
     Preferably the attachment portion is arranged around the movement axis. 
     The attachment portion may be configured to provide an attachment between the coil assembly and the acoustic radiator by including a gluing surface configured to be glued to the acoustic radiator. 
     The attachment portion may be configured to provide an attachment between the coil assembly and the acoustic radiator by including bayonet features (e.g. projections) configured to engage with corresponding bayonet features (e.g. slots) on the acoustic radiator to provide a bayonet attachment between the attachment portion and the acoustic radiator. 
     The attachment portion may be a coupling element which is separately attached to the voice coil former and/or tubular member, e.g. by glue. 
     The coupling element could be a ring-shaped element, e.g. a cardboard or plastic ring. 
     The coupling element is not an essential element of the invention, since the attachment portion could be formed integrally with the voice coil former and/or the tubular member. Or the voice coil and tubular member could be configured to attach independently (e.g. by glue) to the acoustic radiator, in which case the attachment portion could include the glue and part of the acoustic radiator. 
     The/each suspension could take various forms. 
     Preferably, the/each suspension includes one or more corrugations. A suspension including one corrugation is preferred in some examples. 
     The at least one suspension may include a spider. The/each suspension may be a spider. 
     The at least one suspension may include a roll suspension. The/each suspension may be a roll suspension. 
     The at least one suspension may include a piece of sheet material having a geometry configured to allow deflection in a direction parallel to the movement axis, whilst inhibiting movement in a direction perpendicular to the movement axis. The/each suspension may be a piece of sheet material having a geometry configured to allow deflection in a direction parallel to the movement axis, whilst inhibiting movement in a direction perpendicular to the movement axis. 
     A potential advantage of a sheet material suspension could be a reduced height (in the movement axis direction) compared with classic suspensions which typically require a corrugation to facilitate deflection in the movement axis direction. 
     If there are two suspensions, each suspension including one or more corrugations, then the one or more corrugations in one suspension may mirror the one or more corrugations in the other spider, e.g. with respect to a plane perpendicular to the movement axis, e.g. to help cancel asymmetries in stiffness. 
     The magnet unit may include a central main magnet and a U-yoke. 
     In use, electrical current carrying an audio signal is supplied to the voice coil which energises the voice coil and causes a magnetic field to be produced by the current in the voice coil, which interacts with the magnetic field produced in the air gap by the magnet unit, and causes the voice coil assembly to move relative to the magnet assembly. This relative movement is accommodated by the at least one suspension. 
     We note for completeness that [13] teaches a loudspeaker that incorporates a tubular member similar to that shown in the loudspeaker according to the first aspect of the invention, but crucially in [13] the magnet assembly is attached to the frame (and is not suspended from the panel via the coil assembly), and therefore does not incorporate an inertial exciter. There is only one suspension connected between the tubular member and the frame in [13]. 
     A second aspect of the invention provides: 
     An inertial exciter for use with an acoustic radiator, the inertial exciter comprising:
         a magnet assembly including a magnet unit configured to provide a magnetic field in an air gap, wherein the air gap extends around a movement axis of the exciter;   a coil assembly including:
           an attachment portion configured to provide an attachment between the coil assembly and the acoustic radiator;   a voice coil;   a voice coil former which extends from the attachment portion into the air gap, wherein the voice coil is mounted to the voice coil former so that the voice coil sits in the air gap when the inertial exciter is at rest;   a tubular member, which is positioned radially inwardly of the voice coil former with respect to the movement axis, and which overlaps the voice coil former along at least a portion of the movement axis;   
           at least one suspension attached to the tubular member and a part of the magnet assembly positioned radially inwardly of the tubular member so that, when the coil assembly is attached to the acoustic radiator via the attachment portion, the magnet assembly is suspended from the acoustic radiator via the coil assembly by the at least one suspension.       

     The inertial exciter provided by the second aspect of the invention is similar to that provided by the first aspect of the invention, and provides essentially the same benefits as the inertial exciter provided by the first aspect of the invention, but with the components arranged in a different order in the radial direction with respect to the movement axis. 
     The inertial exciter provided by the second aspect of the invention permits use of a ring-shaped magnet, allow more magnet material to be used compared with the inner magnet type examples, and therefore enable more powerful inertial exciters, as may be desirable in some cases. 
     An inertial exciter according to the second aspect of the invention may thus incorporate any one or more features described in connection with an inertial exciter according to the first aspect of the invention, but with the ordering and direction of certain elements being altered in the radial direction (with respect to the movement axis) in order to provide equivalent benefits. Similarly, definitions described above with respect to the first aspect of the invention may be used in connection with the first aspect of the invention. 
     Some example features of an inertial exciter according to the second aspect of the invention will now be described. 
     The movement axis may be defined as an axis along which the voice coil assembly is configured to move relative to the magnet assembly when the inertial exciter is activated by supplying electrical current carrying an audio signal to the voice coil. 
     The inertial exciter may be considered to be at rest when electrical current is not supplied to the voice coil. 
     Note that in order for the magnet assembly to be suspended from the acoustic radiator via the coil assembly, the magnet assembly should only be attached to the acoustic radiator via the coil assembly, i.e. with no rigid attachment between the magnet assembly and the acoustic radiator. 
     Preferably, the inertial exciter comprises:
         a first suspension attached to the tubular member and the part of the magnet assembly positioned radially inwardly of the tubular member; and   a second suspension, separated from the first suspension in a direction extending parallel to the movement axis, wherein the second suspension is either: attached to the tubular member and the part of the magnet assembly positioned radially inwardly of the tubular member or is attached to the voice coil former and a part of the magnet assembly positioned radially outwardly of the voice coil former.       

     For a typical application, the distance between locations at which the two suspensions attach to the part of the magnet assembly positioned radially inwardly of the tubular member may be at least 3 mm, more preferably at least 5 mm, more preferably at least 6 mm as measured in a direction extending parallel to the movement axis. A skilled person would appreciate that actual distances will vary in practice depending on various factors including weight of the magnet assembly (larger weight requires larger distance) and design limitations (e.g. space in aperture in which loudspeaker is to be installed). 
     The magnet assembly may include a frame to which the magnet unit is attached, wherein the part of the magnet assembly positioned radially inwardly of the tubular member (to which the at least one suspension is attached) is a part of the frame. 
     The part of the magnet assembly positioned radially inwardly of the tubular member (to which the at least one suspension is attached) could, for example, be a hub of the frame. 
     The part of the magnet assembly positioned radially inwardly of the tubular member (to which the at least one suspension is attached) may include a respective ledge for the/each suspension attached to the part of the magnet assembly positioned radially inwardly of the tubular member, thereby facilitating attachment of the suspension element(s) to the part of the magnet assembly positioned radially inwardly of the tubular member. 
     The frame (included in the magnet assembly) may include apertures configured to allow a jig to be inserted to centre the tubular member during assembly. 
     Some optional features of the inertial exciter described herein are described with reference to:
         a first plane perpendicular to the movement axis which extends through the attachment portion;   a second plane perpendicular to the movement axis which extends through the air gap.       

     Features described with reference to the first and second planes are preferably described with respect to the inertial exciter when the inertial exciter is at rest. As noted above, the inertial exciter may be considered to be at rest when electrical current is not supplied to the voice coil. 
     The part of the magnet assembly positioned radially inwardly of the tubular member (to which the at least one suspension is attached) may include:
         a proximal portion, wherein the proximal portion of the part of the magnet assembly positioned radially inwardly of the tubular member is located between the first plane and the second plane; and   a distal portion, wherein the distal portion of the part of the magnet unit positioned radially inwardly of the tubular member is located is on an opposite side of the second plane from the proximal portion (of the part of the magnet assembly positioned radially inwardly of the tubular member).       

     The magnet assembly may include a part of the magnet assembly positioned radially outwardly of the voice coil former, wherein the part of the magnet assembly position radially outwardly of the voice coil former includes:
         a proximal portion, wherein the proximal portion of the part of the magnet assembly positioned radially outwardly of the voice coil former is located between the first plane and the second plane; and   a distal portion, wherein the distal portion of the part of the magnet unit positioned radially outwardly of the voice coil former is located is on an opposite side of the second plane from the proximal portion (of the part of the magnet assembly positioned radially outwardly of the voice coil former).       

     The part of the magnet assembly positioned radially outwardly of the voice coil former may include part of the magnet unit. The proximal portion of the part of the magnet assembly positioned radially outwardly of the voice coil former may for example include part of the magnet unit, e.g. a washer  213   a  as shown in  FIG. 5( a ) . The distal portion of the part of the magnet assembly positioned radially outwardly of the voice coil former may for example include part of the magnet unit, e.g. a main magnet  212   a  as shown in  FIG. 5( a ) . 
     The tubular member may include:
         a proximal portion, wherein the proximal portion of the tubular member is located between the first plane and the second plane;   a distal portion, wherein the distal portion of the tubular member is located is on an opposite side of the second plane from the proximal portion (of the tubular member).       

     The voice coil former may include:
         a proximal portion, wherein the proximal portion of the voice coil former is located between the first plane and the second plane;   a distal portion, wherein the distal portion of the voice coil former is located is on an opposite side of the second plane from the proximal portion (of the voice coil former).       

     Note that if the tubular member has the distal portion (as described above), this allows the tubular member to reach past the air gap on the inside of the magnet unit, and allows the first and second suspensions to be separated by a larger distance, compared with an arrangement in which the two suspensions are attached to the voice coil former. 
     Preferably, the first suspension is attached to the distal portion of the tubular member and the distal portion of the part of the magnet assembly positioned radially inwardly of the tubular member. 
     Preferably, the second suspension is attached to the proximal portion of the tubular member and the proximal portion of the part of the magnet assembly positioned radially inwardly of the tubular member. 
     However, the second suspension could potentially instead attach to the proximal portion of the voice coil former and the proximal portion of the part of the magnet assembly positioned radially outwardly of the voice coil former (e.g. as shown in  FIG. 5( c ) ), whilst still allowing a wide separation between the first and second suspensions, thereby still helping to reduce the rotation discussed above with reference to  FIG. 2( c ) . 
     Preferably, the inertial exciter comprises both:
         a first suspension that is attached to the distal portion of the tubular member and the distal portion of the part of the magnet assembly positioned radially inwardly of the tubular member; and   a second suspension that is attached to the proximal portion of the tubular member and the proximal portion of the part of the magnet assembly positioned radially inwardly of the tubular member.       

     This arrangement allows the first and second suspensions to have a particularly large space between them, which helps to reduce the rotation discussed above with reference to  FIG. 2( c ) . 
     In this arrangement, the inertial exciter may optionally include a third suspension that is attached to the proximal portion of the voice coil former and the proximal portion of the part of the magnet assembly positioned radially outwardly of the voice coil former (e.g. as shown in  FIG. 5( b ) ). 
     The magnet unit preferably extends around the tubular member. 
     The tubular member preferably overlaps the magnet unit along at least a portion of the movement axis. 
     The tubular member may be shaped to include the attachment portion, e.g. so as to facilitate direct gluing (or some other attachment) of the tubular member to the acoustic radiator. 
     The tubular member may be shaped to include the attachment portion and the voice coil former. 
     The tubular member may include or be attached to a surface extending inwardly in a radial direction (with respect to the movement axis) from the distal portion of the tubular member to provide a surface for attaching the tubular member to the first suspension. The surface may be flat. The surface may be provided by a ring, e.g. made of plastic/cardboard. 
     The tubular member may include or be attached to a surface extending inwardly in a radial direction (with respect to the movement axis) from the proximal portion of the tubular member to provide a surface for attaching the tubular member to the second suspension. The surface may be flat. The surface may be provided by a ring, e.g. made of plastic/cardboard. 
     The wall of the tubular member could form an angle with respect to the movement axis, e.g. so that the distal portion of the tubular member is closer to the movement axis that the proximal portion of the tubular member, thereby forming a frusto-conical tubular member. In this case, the angle is preferably no more than 15°. 
     The tubular member could have one or more extensions in radially inward direction (with respect to the movement axis) to provide a respective attachment surface for the/each suspension attached to the tubular member, thereby facilitating attachment of the/each suspension to the tubular member. 
     The width of the inertial exciter in the radial direction (perpendicular to the movement axis) will generally depend on design requirements. 
     The inertial exciter may include one or more wires configured to provide an electrical path for supplying an electrical current carrying an audio signal (representative of sound) to the voice coil. 
     The electrical path provided by the one or more wires may extend from a connector formed on the magnet assembly (e.g. on a frame of the magnet assembly) to the voice coil. 
     The one or more wires may include wire from the voice coil winding and/or a lead wire which connects to the voice coil winding. 
     The one or more wires may include a wire that passes through or around the tubular member. A coupling element (if present—see below) may be configured to guide said wire through or around the tubular member. 
     The one or more wires may include a wire that passes through or around (preferably through) a frame included in the magnet assembly. 
     The one or more wires may include two wires that meet at an electrical junction formed on an inwardly facing surface of the tubular member, e.g. at a solderpad or glue dot on the inwardly facing surface of the tubular member. 
     The magnet unit is preferably configured to provide a magnetic field in an air gap. The voice coil former and/or the tubular member may be cylindrical. But other shapes of air gap, voice coil former and tubular member are possible, e.g. oval, square. 
     Preferably the voice coil former is arranged around the movement axis. 
     The voice coil former preferably extends from the attachment portion in a direction which extends along the movement axis into the air gap. 
     The tubular member and voice coil former are each preferably made from lightweight materials such as paper, cardboard, Kapton, aluminium, kevlar, PE, ABS etc. 
     The tubular member and voice coil former are preferably made of the same material as each other, but could be made of different materials. 
     The tubular member and voice coil former may be formed integrally with each other (preferably also the attachment portion). 
     Preferably the attachment portion is arranged around the movement axis. 
     The attachment portion may be configured to provide an attachment between the coil assembly and the acoustic radiator by including a gluing surface configured to be glued to the acoustic radiator. 
     The attachment portion may be configured to provide an attachment between the coil assembly and the acoustic radiator by including bayonet features configured to engage with corresponding bayonet features on the acoustic radiator to provide a bayonet attachment between the attachment portion and the acoustic radiator. 
     The attachment portion may be a coupling element which is separately attached to the voice coil former and/or tubular member, e.g. by glue. 
     The coupling element could be a ring-shaped element, e.g. a cardboard or plastic ring. 
     The coupling element is not an essential element of the invention, since the attachment portion could be formed integrally with the voice coil former and/or the tubular member. Or the voice coil and tubular member could be configured to attach independently (e.g. by glue) to the acoustic radiator, in which case the attachment portion could include the glue and part of the acoustic radiator. 
     The/each suspension could take various forms. 
     Preferably, the/each suspension includes one or more corrugations. A suspension including one corrugation, e.g. a roll suspension, is preferred in some examples. 
     The at least one suspension may include a spider. The/each suspension may be a spider. 
     The at least one suspension may include a roll suspension. The/each suspension may be a roll suspension. 
     The at least one suspension may include a piece of sheet material having a geometry configured to allow deflection in a direction parallel to the movement axis, whilst inhibiting movement in a direction perpendicular to the movement axis. The/each suspension may be a piece of sheet material having a geometry configured to allow deflection in a direction parallel to the movement axis, whilst inhibiting movement in a direction perpendicular to the movement axis. 
     A potential advantage of a sheet material suspension could be a reduced height (in the movement axis direction) compared with classic suspensions which typically require a corrugation to facilitate deflection in the movement axis direction. 
     If there are two suspensions, each suspension including one or more corrugations, then the one or more corrugations in one suspension may mirror the one or more corrugations in the other spider, e.g. with respect to a plane perpendicular to the movement axis, e.g. to help cancel asymmetries in stiffness. 
     The magnet unit may include a ring-shaped main magnet and a T-yoke. 
     In use, electrical current carrying an audio signal is supplied to the voice coil which energises the voice coil and causes a magnetic field to be produced by the current in the voice coil, which interacts with the magnetic field produced in the air gap by the magnet unit, and causes the voice coil assembly to move relative to the magnet assembly. This relative movement is accommodated by the at least one suspension. 
     A third aspect of the invention provides:
         A loudspeaker including:   an acoustic radiator;   an inertial exciter according to the first or second aspect of the invention, wherein the coil assembly of the inertial exciter is attached to the acoustic radiator via the attachment portion so that the magnet assembly is suspended from the acoustic radiator via the coil assembly via the at least one suspension.       

     The acoustic radiator could have various shapes (e.g. flat, curved, small, large, geometric, free-form). 
     The acoustic radiator may be suspended from a frame. 
     The loudspeaker is preferably a dipole loudspeaker, wherein the acoustic radiator is suspended from a frame (of the dipole loudspeaker) via one or more suspension elements, wherein the frame is configured to allow sound produced by a first radiating surface of the acoustic radiator to propagate out from a first side of the dipole loudspeaker and to allow sound produced by a second radiating surface of the acoustic radiator to propagate out from a second side of the dipole loudspeaker. 
     Here, the first radiating surface and the second radiating surface should be located on opposite faces of the acoustic radiator. 
     The coil assembly of the inertial exciter may be attached to the second radiating surface of the acoustic radiator (via the attachment portion). 
     An inertial exciter according to the first or second aspect of the invention is particularly well suited for use in a dipole loudspeaker because its construction is such that it can obstruct a smaller area of the radiating surface of the acoustic radiator to which it is attached compared with some of the prior art examples discussed above (see e.g. [13], which requires a frame). 
     A fourth aspect of the invention provides:
         A method of manufacturing a loudspeaker according to the third aspect of the invention.       

     The method may include pre-assembling the coil assembly, before suspending the magnet assembly from the coil assembly by the at least one suspension. 
     A fifth aspect of the invention provides:
         A drive unit for use with an acoustic radiator, the drive unit comprising:   a magnet assembly including a magnet unit configured to provide a magnetic field in an air gap, wherein the air gap extends around a movement axis of the inertial exciter;   a coil assembly including:
           an attachment portion configured to provide an attachment between the coil assembly and the acoustic radiator;   a voice coil;   a voice coil former which extends from the attachment portion into the air gap, wherein the voice coil is mounted to the voice coil former so that the voice coil sits in the air gap when the drive unit is at rest;   a tubular member, which is positioned radially outwardly of the voice coil former with respect to the movement axis, and which overlaps the voice coil former along at least a portion of the movement axis;   
           at least one suspension attached to the tubular member and a part of the magnet assembly positioned radially outwardly of the tubular member so that, when the coil assembly is attached to the acoustic radiator via the attachment portion, the acoustic radiator is suspended from the magnet assembly via the coil assembly by the at least one suspension.       

     The drive unit according to the fifth aspect of the invention therefore has essentially the same construction as the inertial exciter according to the first aspect of the invention, except that in the drive unit according to the fifth aspect of the invention, the acoustic radiator is suspended from the magnet assembly, rather than magnet assembly being suspended from the acoustic radiator. 
     This is advantageous because it helps to provide stable pistonic movement of the acoustic radiator and reduces rocking of the acoustic radiator when the magnet assembly is rigidly attached to an external body (e.g. frame), i.e. when the drive assembly is “grounded”. 
     Preferably, the magnet assembly is rigidly attached to a frame, which may be rigidly attached to an external body. 
     Preferably, the magnet assembly is rigidly attached to a frame from which the acoustic radiator is suspended. 
     Any feature described in connection with the inertial exciter according to the first aspect of the invention may be incorporated in the drive unit according to the fifth aspect of the invention, except where such a combination is clearly impermissible or expressly avoided. 
     A sixth aspect of the invention provides:
         A drive unit for use with an acoustic radiator, the drive unit comprising:   a magnet assembly including a magnet unit configured to provide a magnetic field in an air gap, wherein the air gap extends around a movement axis of the exciter;   a coil assembly including:
           an attachment portion configured to provide an attachment between the coil assembly and the acoustic radiator;   a voice coil;   a voice coil former which extends from the attachment portion into the air gap, wherein the voice coil is mounted to the voice coil former so that the voice coil sits in the air gap when the drive unit is at rest;   a tubular member, which is positioned radially inwardly of the voice coil former with respect to the movement axis, and which overlaps the voice coil former along at least a portion of the movement axis;   
           at least one suspension attached to the tubular member and a part of the magnet assembly positioned radially inwardly of the tubular member so that, when the coil assembly is attached to the acoustic radiator via the attachment portion, the acoustic radiator is suspended from the magnet assembly via the coil assembly by the at least one suspension.       

     The drive unit according to the sixth aspect of the invention therefore has essentially the same construction as the inertial exciter according to the second aspect of the invention, except that in the drive unit according to the sixth aspect of the invention, the acoustic radiator is suspended from the magnet assembly, rather than magnet assembly being suspended from the acoustic radiator. 
     This is advantageous because it helps to provide stable pistonic movement of the acoustic radiator and reduces rocking of the acoustic radiator when the magnet assembly is rigidly attached to an external body (e.g. frame), i.e. when the drive assembly is “grounded”. 
     Preferably, the magnet assembly is rigidly attached to a frame, which may be rigidly attached to an external body. 
     Preferably, the magnet assembly is rigidly attached to a frame from which the acoustic radiator is suspended. 
     Any feature described in connection with the inertial exciter according to the first aspect of the invention may be incorporated in the drive unit according to the fifth aspect of the invention, except where such a combination is clearly impermissible or expressly avoided. 
     A seventh aspect of the invention provides:
         A loudspeaker including:   an acoustic radiator;   a drive unit according to the fifth or sixth aspect of the invention, wherein the coil assembly of the drive unit is attached to the acoustic radiator via the attachment portion so that the acoustic radiator is suspended from the magnet assembly via the coil assembly by the at least one suspension.       

     The acoustic radiator could have various shapes (e.g. flat, curved, small, large, geometric, free-form). 
     The acoustic radiator may be suspended from a frame. 
     The loudspeaker is preferably a dipole loudspeaker, wherein the acoustic radiator is suspended from a frame (of the dipole loudspeaker) via one or more suspension elements, wherein the frame is configured to allow sound produced by a first radiating surface of the acoustic radiator to propagate out from a first side of the dipole loudspeaker and to allow sound produced by a second radiating surface of the acoustic radiator to propagate out from a second side of the dipole loudspeaker. 
     Here, the first radiating surface and the second radiating surface should be located on opposite faces of the acoustic radiator. 
     The coil assembly of the inertial exciter may be attached to the second radiating surface of the acoustic radiator (via the attachment portion). 
     Preferably, the magnet assembly is rigidly attached to the frame from which the acoustic radiator is suspended. 
     A drive unit according to the fifth or sixth aspect of the invention is particularly well suited for use in a dipole loudspeaker because its construction is such that it can obstruct a smaller area of the radiating surface of the acoustic radiator to which it is attached compared with some of the prior art examples discussed above (see e.g. [13], which requires a frame). 
     An eighth aspect of the invention provides:
         A method of manufacturing a loudspeaker according to the third aspect of the invention.       

     The method may include pre-assembling the coil assembly, before suspending the magnet assembly from the coil assembly by the at least one suspension. 
     The invention includes the combination of the aspects and preferred features described except where such a combination is clearly impermissible or expressly avoided. 
     SUMMARY OF THE FIGURES 
     Embodiments and experiments illustrating the principles of the invention will now be discussed with reference to the accompanying figures in which: 
       FIG. 1( a )  shows an example traditional loudspeaker. 
       FIG. 1( b )  shows an example inertial exciter. 
       FIG. 2( a )  shows a loudspeaker incorporating a wide bandwidth inertial exciter implementing principles derived from the prior art. 
       FIG. 2( b )  is a graph showing force level vs frequency for the loudspeaker shown in  FIG. 2( a ) . 
       FIG. 2( c )  illustrates a problem with the inertial exciter shown in  FIG. 2( a ) . 
       FIG. 3( a )( i )  and (ii) illustrate “Solution A” as taught by the prior art. 
       FIG. 3( b )( i )  and (ii) illustrate “Solution B” as taught by the prior art. 
       FIG. 3( c )( i )  and (ii) illustrate “Solution C” as taught by the prior art. 
       FIG. 3( d )( i )  and (ii) illustrate “Solution D” as taught by the prior art. 
       FIG. 3( e )( i )  and (ii) illustrate “Solution E” as taught by the prior art. 
       FIG. 3( f )( i )  and (ii) illustrate “Solution F” as taught by the prior art. 
       FIG. 4( a )( i ) -(vi) show a first inertial exciter  101   a  that exemplifies an inertial exciter of the inner magnet type, and a loudspeaker  180   a  incorporating the first initial exciter  101   a.    
       FIG. 4( b )( i ) -(iv) show a second inertial exciter  101   b  that exemplifies an inertial exciter of the inner magnet type, and a loudspeaker  180   b  incorporating the first initial exciter  101   b.    
       FIG. 4( c )  shows a third inertial exciter  101   c  that exemplifies an inertial exciter of the inner magnet type. 
       FIG. 4( d )  shows a fourth inertial exciter  101   d  that exemplifies an inertial exciter of the inner magnet type. 
       FIG. 4( e )  shows a fifth inertial exciter  101   e  that exemplifies an inertial exciter of the inner magnet type. 
       FIG. 4( f )  shows a sixth inertial exciter  101   f  that exemplifies an inertial exciter of the inner magnet type. 
       FIG. 4( g )  shows a seventh inertial exciter  101   g  that exemplifies an inertial exciter of the inner magnet type. 
       FIG. 4( h )  shows an eighth inertial exciter  101   h  that exemplifies an inertial exciter of the inner magnet type. 
       FIG. 4( i )( i ) -(viii) shows a ninth inertial exciter  101   i  that exemplifies an inertial exciter of the inner magnet type. 
       FIG. 4( j )( i ) -(ii) show a tenth inertial exciter  101   j  that exemplifies an inertial exciter of the inner magnet type. 
       FIG. 4( k )  shows an eleventh inertial exciter  101   k  that exemplifies an inertial exciter of the inner magnet type. 
       FIG. 4( l )  shows a twelfth inertial exciter  101   l  that exemplifies an inertial exciter of the inner magnet type. 
       FIG. 5( a )  shows a first inertial exciter  201   a  that exemplifies an inertial exciter of the outer magnet type. 
       FIG. 5( b )  shows a second inertial exciter  201   b  that exemplifies an inertial exciter of the outer magnet type. 
       FIG. 5( c )  shows a third inertial exciter  201   c  that exemplifies an inertial exciter of the outer magnet type. 
       FIG. 5( d )  shows a fourth inertial exciter  201   d  that exemplifies an inertial exciter of the outer magnet type. 
       FIG. 6( a )  shows a drive unit that exemplifies a drive unit of the inner magnet type. 
       FIG. 6( b )  shows a drive unit that exemplifies a drive unit of the outer magnet type. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Aspects and embodiments of the present invention will now be discussed with reference to the accompanying figures. Further aspects and embodiments will be apparent to those skilled in the art. All documents mentioned in this text are incorporated herein by reference. 
     For the purpose of this description, example inertial exciters are divided into two types, referred to as “inner magnet” type according to the first aspect of the invention and “outer magnet” type according to the second aspect of the invention. Similarly, example drive units are divided into two types, referred to as “inner magnet” type according to the fifth aspect of the invention and “outer magnet” type according to the sixth aspect of the invention 
     Inertial Exciter—Inner Magnet Type Examples 
     A first inertial exciter  101   a  that exemplifies an inertial exciter of the inner magnet type is shown in  FIG. 4( a )( i ) . 
     The inertial exciter  101   a  of  FIG. 4( a )  comprises a magnet assembly  102   a  and a coil assembly  104   a.    
     The magnet assembly  102   a  includes a magnet unit  110   a  and a frame  120   a  to which the magnet unit  110   a  is attached. 
     In this example, the magnet unit  110   a  includes a main magnet  112   a,  a washer  113   a  an extra magnet  114   a  and a U-yoke  115   a.  The magnet unit  110   a  is configured to provide a magnetic field in an air gap  116   a.  The air gap  116   a  extends around a movement axis  106   a  of the inertial exciter  101   a.    
     The frame  120   a  includes a base portion  122   a  which extends radially outwardly with respect to the movement axis  106   a  (in this example from a base of the U-yoke  115   a ), and a rim  124   a  which extends axially with respect to the movement axis  106   a,  that is at least partly along the movement axis  106   a.  The rim  124   a  of the frame  120   a  is positioned at the periphery of the base portion  122   a,  and is positioned radially outwardly of the magnet unit  110   a.    
     The rim  124   a  of the frame  120   a  is positioned radially outwardly of a tubular member  140   a,  and thus serves as the “part of the magnet assembly positioned radially outwardly of the tubular member” referenced in the “Summary of the invention” section of this document, above. 
     In this example, the main magnet  112   a,  washer  113   a,  extra magnet  114   a,  U-yoke  115   a,  and air gap  116   a  are circular, though other forms are possible. 
     In this example, the washer  114   a  and U-yoke  116   a  may be made of steel, though other materials are possible. 
     In this example, the coil assembly  104   a  includes a voice coil  130   a,  a voice coil former  132   a,  a tubular member  140   a  and an attachment portion  150   a.    
     In this example, the attachment portion  150   a  is a coupling element which is separately attached to the voice coil former and tubular member, e.g. by glue. The coupling element  150   a  is configured to provide an attachment between the coil assembly  104   a  and an acoustic radiator (not shown) by including a gluing surface  151   a  configured to be glued to the acoustic radiator. The coupling element  150   a  could for example be a plastic or cardboard ring-shaped element. 
     The voice coil former  132   a  extends axially with respect to the movement axis  106   a  from the coupling element  150   a  into the air gap  116   a.  The voice coil  130   a  is mounted to the voice coil former  132   a  so that the voice coil  130   a  sits in the air gap  116   a  when the inertial exciter  101   a  is at rest. 
     The tubular member  140   a  is positioned radially outwardly of the voice coil former  132   a  with respect to the movement axis  106   a.  The tubular member  140   a  also overlaps the voice coil former  132   a  along a portion of the movement axis (this portion corresponding to the full length of the voice coil former  132   a ). 
     In this example, the voice coil former  132   a  and tubular member  140   a  are cylindrical, though other shapes are possible. 
     Two planes are depicted in  FIG. 4( a )( i ) . 
     A first plane  108   a  is perpendicular to the movement axis  106   a  and extends through the attachment portion which as noted above is the coupling element  150   a.    
     A second plane  109   a  is perpendicular to the movement axis  106   a  and extends through the air gap  116   a.    
     The rim  124   a  of the frame  120   a  includes:
         a proximal portion, wherein the proximal portion of the rim  124   a  is located between the first plane  108   a  and the second plane  109   a;  and   a distal portion, wherein the distal portion of the rim  124   a  is located is on an opposite side of the second plane  109   a  from the proximal portion of the rim  124   a.          

     The tubular member  140   a  similarly includes:
         a proximal portion, wherein the proximal portion of the tubular member  140   a  is located between the first plane  108   a  and the second plane  109   a;  and   a distal portion, wherein the distal portion of the tubular member  140   a  is located is on an opposite side of the second plane  109   a  from the proximal portion of the of the tubular member  140   a.          

     The inertial exciter  101   a  includes:
         a first suspension  160   a  that is attached to the distal portion of the tubular member  140   a  and the distal portion of the rim  124   a;  and   a second suspension  165   a  that is attached to the proximal portion of the tubular member  140   a  and the proximal portion of the rim  124   a.          

     Each suspension  160   a,    165   a  in this example is a spider including multiple corrugations. Such suspensions are well known in the art. 
     Thus, when the coil assembly  104   a  is attached to the acoustic radiator via the attachment portion/coupling element  150   a,  the magnet assembly  102   a  is suspended from the acoustic radiator via the coil assembly  104   a  by the first and second suspensions  160   a,    165   a.    
     As can be seen from  FIG. 4( a )( i ) , the rim  124   a  of the frame  120   a  includes a first ledge  125   a  to which the first suspension  160   a  is attached, and a second ledge  126   a  to which the second suspension  165   a  is attached. 
     In this example, the first and second suspensions  160   a,    165   a  are each shown as a respective spider having multiple corrugations. 
     The inertial exciter  101   a  includes wires  134   a,    135   a  configured to provide an electrical path for supplying an electrical current carrying an audio signal (representative of sound) to the voice coil  130   a.    
     The electrical path provided by the wires  134   a,    135   a  extend from a connector  138   a  formed on an outwardly facing surface of the rim  124   a  of the frame  120   a  to the voice coil  130   a.    
     In this example, the wires include part of the voice coil winding  134   a  as well as a lead wire  135   a.  In this example, the voice coil winding  134   a  extends around the tubular member  140   a  as guided by the coupling element  150   a.    
     The voice coil winding  134   a  and lead wire  135   a  meet at an electrical junction formed at a solderpad or glue dot  136   a  on an outwardly facing surface of the tubular member  140   a.    
       FIG. 4( a ) ( ii ) shows a loudspeaker  180   a  including the inertial exciter  101   a  of  FIG. 4( a )( i )  and an acoustic radiator  190   a  suspended from a frame  192 , wherein the coil assembly  104   a  of the inertial exciter  101   a  is attached to the acoustic radiator  190   a  via the attachment portion/coupling element  150   a  so that the magnet assembly  102   a  is suspended from the acoustic radiator  190   a  via the coil assembly by the first and second suspensions  160   a,    165   a.    
     In use, electrical current carrying an audio signal is supplied to the voice coil  130   a  via the connector  138   a  and wires  134   a,    135   a.  This energises the voice coil  130   a  and causes a magnetic field to be produced by the current in the voice coil  130   a,  which interacts with the magnetic field produced in the air gap  116   a  by the magnet unit  110   a,  and causes the voice coil assembly  104   a  to move relative to the magnet assembly  102   a.  This relative movement is accommodated by the first and second suspensions  160   a,    165   a.    
     Because the magnet assembly  102   a  is suspended from the acoustic radiator via the coil assembly  104   a  by the first and second suspensions  160   a,    165   a,  the loudspeaker is able to be moved by MC and MM excitation, as indicated by  FIG. 4( a ) ( iii ). 
     Because the voice coil former  132   a  and tubular member  140   a  are tubular, they provide good stiffness even when made of a lightweight material such as paper, cardboard, Kapton, aluminium, kevlar etc. Thus, the voice coil assembly  104   a  can have low weight and good stiffness, as is needed for good wide bandwidth performance from MC excitation. 
     Moreover, because the tubular member  140   a  has a distal portion which overlaps the voice coil former  132   a  so as to extend beyond the air gap  116   a,  i.e. to the opposite side of the second plane  109   a  from the proximal portion of the tubular member  140   a,  it is possible to have a large distance between the first and second suspensions  160   a,    165   a,  which helps inhibit rotation of the magnet assembly  102   a  relative to the voice coil assembly  104   a  when the acoustic radiator  190   a  is vertically mounted, e.g. as may be the case in a car door, as depicted in  FIG. 4( a ) ( iii ). 
     Note that this is achieved whilst providing one interface (the glue surface of the coupling element  150   a ) with the acoustic radiator  190   a,  and also whilst permitting MC excitation. The low mass of the voice coil assembly (see above) help to achieve acoustic sensitivity and balance in the upper frequency band, as depicted in  FIG. 4( a ) ( iv ). 
       FIG. 4( a )( v )  shows a method step involved in assembling the inertial exciter  101   a  which makes use of a conventional centering jig  195   a  to align the voice coil former  132   a  in the air gap  116   a  before the components of the voice coil assembly  104   a  are glued together. The coupling element  150   a  may be flush with an inwardly facing surface of the voice coil former  132   a  to facilitate use of the centering jig  195   a.    
       FIG. 4( a ) ( vi ) shows an alternative or additional method step involved in assembling the inertial exciter  101   a  in which apertures are incorporated into the frame  120   a  to allow a centering jig  196   a  to be inserted into the apertures during assembly, e.g. to help with aligning the voice coil former  132   a  in the air gap  116   a.    
     Preferably the voice coil assembly (including the coupling element  150   am  the voice coil  130   a,  voice coil former  132   a  and the tubular member  140   a ) could be pre-assembled on a separate jig (not shown) before being assembled in the magnet assembly  102   a.    
     Various alternative inner magnet examples will now be described. Alike features have been given alike reference numerals where appropriate and are not described in further detail, except where necessary. 
     A second inertial exciter  101   b  that exemplifies an inertial exciter of the inner magnet type is shown in  FIG. 4( b )( i ) . 
     The coupling element  150   b  of the inertial exciter  101   b  is shown in  FIG. 4( b ) ( ii ) and includes bayonet features in the form of radial extensions  151   b  configured to engage with corresponding bayonet features  191   b  on the acoustic radiator  190   b  shown in  FIG. 4( b ) ( iii ) to provide a bayonet attachment between the coupling element  150   b  and the acoustic radiator  190   b.  The bayonet features  191   b  on the acoustic radiator preferably form slots for accommodating the radial extensions  151   b.  The resulting loudspeaker  180   b  is shown in  FIG. 4( b ) ( iv ). 
     The above-described bayonet feature could facilitate assembly and replacement of the inertial exciter  101   b  to the acoustic radiator  190   b.    
     The above-described bayonet features could be combined with adhesives or filler (e.g. grease) to avoid rattling during operation. The adhesive or filler could have temperature dependent properties so that by applying heat the inertial exciter  101   b  can be replaced. 
     A third inertial exciter  101   c  that exemplifies an inertial exciter of the inner magnet type is shown in  FIG. 4( c ) . 
     In this example, the tubular member  140   c  includes a collar  141   c  that provides a flat face to facilitate gluing of the first suspension  160   c,  which in this example could be a fabric damper, a metal or plastic spiral spring, a rubber element, etc. 
     A fourth inertial exciter  101   d  that exemplifies an inertial exciter of the inner magnet type is shown in  FIG. 4( d ) . 
     In this example, the a ring  141   d,  e.g. made of cardboard or plastic, is attached to the distal portion of the tubular member  140   d  to provide a flat surface  141   d  to facilitate gluing of the first suspension  160   d.    
     A fifth inertial exciter  101   e  that exemplifies an inertial exciter of the inner magnet type is shown in  FIG. 4( e ) . 
     In this example, the tubular member  140   e  is integrally formed with the attachment portion  150   e  by appropriately shaping the tubular member  140   e  to include the attachment portion  150   e.  This allows the tubular member  140   e  to be glued directly to the voice coil former  132   e,  and avoids the use of a coupling element as described in previous examples. In this example, the attachment portion  150   e  is a flat face of the tubular member  140   e  that is configured to be glued to the acoustic radiator (not shown). 
     The tubular member  140   e  could be made of paper, cardboard, Kapton, aluminium, kevlar, PE, ABS etc. 
     A sixth inertial exciter  101   f  that exemplifies an inertial exciter of the inner magnet type is shown in  FIG. 4( f ) . 
     The inertial exciter  101   f  is the same as the fifth inertial exciter  101   e  shown in  FIG. 4( e ) , except that holes are formed in the attachment portion  150   f  to enhance the glue attachment to the acoustic radiator (not shown). 
     A seventh inertial exciter  101   g  that exemplifies an inertial exciter of the inner magnet type is shown in  FIG. 4( g ) . 
     In this example, the coupling element  150   g  is attached only to the voice coil former  132   g,  with the tubular member  140   g  being attached to the voice coil former  132   g.    
     An eighth inertial exciter  101   h  that exemplifies an inertial exciter of the inner magnet type is shown in  FIG. 4( h ) . 
     In this example, the tubular member  140   h  forms an angle with respect to the movement axis, thereby forming a frusto-conical tubular member  140   h.  In this case, the angle is preferably no more than 15°. 
     A tubular member  140   h  shaped in this way could facilitate the making of the tubular member  140   h  from paper or from plastic in a deep draw process. 
     In this example, the tubular member  140   h  is again integrally formed with the attachment portion  150   h  by appropriately shaping the tubular member  140   h  to include the attachment portion  150   h.    
     A ninth inertial exciter  101   i  that exemplifies an inertial exciter of the inner magnet type is shown in  FIG. 4( i )( i ) . 
     This example is essentially the same as the first inertial exciter  101   a  shown in  FIG. 4( a )( i ) , except that in this case the first and second suspensions  160   i,    165   i  include only a single corrugation, and the single corrugations mirror each other (in a plane  108   i  perpendicular to the movement axis  106   i ) to help cancel asymmetries in stiffness between the two suspensions  160   i,    165   i.  The first and second suspensions  160   i ,  165   i  may in this case be roll suspensions, e.g. made of rubber, textile or foam. 
       FIG. 4( i ) ( ii ) show the attachment between the frame  120   i  and the suspensions  160   i,    165   i.  In this particular example, the rim of the frame  120   i  is provided in two parts,  124   i ( i ) and  124   i ( ii ). 
     Example dimensions are drawn on  FIG. 4( i )( i )  and  FIG. 4( i ) ( ii ), noting that the distance between locations at which the two suspensions  160   i,    165   i  attach to the rim of the magnet assembly is 6.3 mm in this example, which is large given the overall size of the inertial exciter  101   i.    
       FIGS. 4( i ) ( iii )-(viii) are 3D views showing the inertial exciter  101   i  from various angles. 
     A tenth inertial exciter  101   j  that exemplifies an inertial exciter of the inner magnet type is shown in  FIG. 4( j )( i ) . 
     The inertial exciter  101   j  shown in  FIG. 4( j )( i )  is the same as the inertial exciter  101   a  shown in  FIG. 4( a )( i )  except that the inertial exciter includes an alternative form of first and second suspensions  160   j,    165   j.    
     The alternative form of suspension used for the first and second suspensions  160   j,    165   j  is shown in more detail in  FIG. 4( j ) ( ii ). 
     As can be seen most clearly from  FIG. 4( j ) ( ii ), the alternative form of first and second suspensions  160   j ,  165   j  is a piece of sheet material having a geometry configured to allow deflection in a direction parallel to the movement axis  106   j,  whilst inhibiting movement in a direction perpendicular to the movement axis  106   j.    
     A suitable material for the alternative form of first and second suspensions  160   j,    165   j  could be a fiber-reinforced plastic, e.g. a polymer matrix reinforced with glass fibres or carbon fibres, or a metal, e.g. steel spring material. 
     An eleventh inertial exciter  101   k  that exemplifies an inertial exciter of the inner magnet type is shown in  FIG. 4( k ) . 
     The inertial exciter  101   k  shown in  FIG. 4( k )  is the same as the inertial exciter  101   a  shown in  FIG. 4( a )( i )  except that in this example the second suspension  165   k  is attached to a proximal portion of the voice coil former  132   k  and a proximal portion of a part of the magnet assembly positioned radially inwardly of the voice coil former (in this case the extra magnet  114   k ). 
     Note, that in this case the inertial exciter  101   k  has:
         a first suspension  160   k  that is attached to a distal portion of the tubular member  140   k  and the distal portion of the rim  124   k;  and   a second suspension  165   k  that is attached to a proximal portion of the voice coil former  132   k  and a proximal portion of a part of the magnet assembly positioned radially inwardly of the voice coil former  132   k  (in this case the extra magnet  114   k ).       

     Thus, this arrangement still allows for a wide separation between the first and second suspensions  160   k ,  165   k,  thereby helping to inhibit rotation of the magnet assembly  102   k  relative to the voice coil assembly  104   k.    
     Besides providing suspension, the second suspension  165   k  can also serve as a dust cover to prevent dust in the airgap  116   k  prior to mounting the inertial exciter  101   k  to an acoustic radiator. 
     In this example, the first suspension  165   k  is a roll suspension including only one corrugation. 
     A twelfth inertial exciter  101   l  that exemplifies an inertial exciter of the inner magnet type is shown in  FIG. 4( l ) . 
     The inertial exciter  101   l  shown in  FIG. 4( l )  is the same as the inertial exciter  101   a  shown in  FIG. 4( a )( i )  except that a third suspension  168   l  is attached to a proximal portion of the voice coil former  132   l  and a proximal portion of a part of the magnet assembly positioned radially inwardly of the voice coil former (in this case the extra magnet  114   l ). 
     Besides providing suspension, the third suspension  168   l  can also serve as a dust cover to prevent dust in the airgap  116   l  prior to mounting the inertial exciter  101   l  to an acoustic radiator. 
     Inertial Exciter—Outer Magnet Type Examples 
     A first inertial exciter  201   a  that exemplifies an inertial exciter of the outer magnet type is shown in  FIG. 5( a ) . 
     The inertial exciter  201   a  shown in  FIG. 5( a )  includes many features which are common to the inertial exciter  101   a  shown in  FIG. 4( a )( i ) . Alike features have been given alike reference numerals where appropriate and are not described in further detail, except where necessary. 
     The magnet assembly  202   a  includes a magnet unit  210   a  and a frame  220   a  to which the magnet unit  210   a  is attached. 
     In this example, the magnet unit  210   a  includes a (ring-shaped) main magnet  212   a,  a (ring-shaped) washer  213   a  and a T-yoke  215   a  (which looks like an upside down “T” as drawn). The magnet unit  210   a  is configured to provide a magnetic field in an air gap  216   a.  The air gap  216   a  extends around a movement axis  206   a  of the inertial exciter  201   a.    
     The outer magnet type examples can be useful as they allow more magnet material to be used compared with the inner magnet type examples, and therefore enable more powerful exciters, as may be desirable in some cases. 
     In this example, the frame  220   a  includes a base portion  222   a  which extends radially inwardly with respect to the movement axis  206   a  (in this example from a base of the T-yoke  215   a ). 
     In this example, the frame  220   a  also include a hub  224   a  which extends axially with respect to the movement axis  206   a,  that is at least partly along the movement axis  206   a.  The hub  224   a  of the frame  220   a  is positioned at the centre of the base portion  222   a,  and is positioned radially inwardly of the tubular member  240   a.    
     In this example, the tubular member  240   a  is positioned radially inwardly of the voice coil former  232   a  with respect to the movement axis  206   a,  and overlaps the voice coil former  232   a  along at least a portion of the movement axis  206   a.    
     The inertial exciter  201   a  includes:
         a first suspension  260   a  that is attached to a distal portion of the tubular member  240   a  and the distal portion of the hub  224   a;  and   a second suspension  265   a  that is attached to the proximal portion of the tubular member  240   a  and the proximal portion of the hub  224   a.          

     The proximal portions of the tubular member  240   a  and hub  224   a  are located between the first plane  208   a  and the second plane  209   a  as defined above. The proximal portions of the tubular member  240   a  and hub  224   a  are located on an opposite side of the second plane  209   a  from the proximal portions. 
     As can be seen from  FIG. 5( a ) , the hub  224   a  of the frame  220   a  includes a first ledge  225   a  to which the first suspension  260   a  is attached, and a second ledge  226   a  to which the second suspension  265   a  is attached. 
     In this example, the inertial exciter  201   a  includes a lead wire  234   a  configured to provide an electrical path for supplying an electrical current carrying an audio signal (representative of sound) to the voice coil  130   a.    
     In this example, the electrical path provided by the lead wire  234   a  extend from a connector  238   a  formed on an outwardly facing surface of the base portion  222  of the frame  220   a  (outward in the sense of facing away from the hub  224   a ) to the voice coil  230   a.    
     In this example, the lead wire  234   a  extends through the frame  220   a.    
     In this example, the coupling element  250   a  is similar to that shown in  FIG. 4( a )( i ) . 
     In use, electrical current carrying an audio signal is supplied to the voice coil  230   a  via the connector  238   a  and lead wire  234   a.  This energises the voice coil  230   a  and causes a magnetic field to be produced by the current in the voice coil  230   a,  which interacts with the magnetic field produced in the air gap  216   a  by the magnet unit  210   a,  and causes the voice coil assembly  204   a  to move relative to the magnet assembly  202   a.  This relative movement is accommodated by the first and second suspensions  260   a,    265   a.    
     Various alternative inner magnet examples will now be described. Alike features have been given alike reference numerals where appropriate and are not described in further detail, except where necessary. 
     A second inertial exciter  201   b  that exemplifies an inertial exciter of the outer magnet type is shown in  FIG. 5( b ) . 
     This example is that same as that shown in  FIG. 5( a ) , except that a third suspension  268   b  is attached to the voice coil former  232   b  and to a part of the magnet assembly  202   b  (in this case the washer  213   b ) positioned radially outwardly of the voice coil former  232   a.    
     Besides providing suspension, the third suspension  268   b  can also serve as a dust cover to prevent dust in the airgap  216   b  when the inertial exciter  201   b  is in use. 
     A third inertial exciter  201   c  that exemplifies an inertial exciter of the outer magnet type is shown in  FIG. 5( c ) . 
     This example is that same as that shown in  FIG. 5( a ) , except that in this example the second suspension  265   c  is attached to the voice coil former  232   b  and to a part of the magnet assembly  202   b  (in this case the washer  213   b ) positioned radially outwardly of the voice coil former  232   a.    
     Note, that in this case the inertial exciter  201   c  has:
         a first suspension  260   c  that is attached to a distal portion of the tubular member  240   c  and the distal portion of the hub  224   c;  and   an second suspension  265   c  that is attached to a proximal portion of the tubular member  240   c  and a proximal portion of a part of the magnet assembly positioned radially outwardly of the tubular member  240   c  (in this case the washer  213   b ).       

     Thus, this arrangement still allows for a wide separation between the first and second suspensions  160   k ,  165   k,  thereby helping to inhibit rotation of the magnet assembly  202   c  relative to the voice coil assembly  204   c.    
     Besides providing suspension, the second suspension  265   c  can also serve as a dust cover to prevent dust in the airgap  216   c  when the inertial exciter  201   b  is in use. 
     A fourth inertial exciter  201   d  that exemplifies an inertial exciter of the outer magnet type is shown in  FIG. 5( d ) . 
     This example is that same as that shown in  FIG. 5( b ) , except that:
         the tubular member  240   d  is integrally formed with the attachment portion  250   d  by appropriately shaping the tubular member  240   d  to include the attachment portion  250   d.      holes are formed in the attachment portion  250   d  to enhance the glue attachment to the acoustic radiator (not shown)       

     Drive Unit—Inner Magnet Type Example 
     A drive unit  301   a  that exemplifies a drive unit of the inner magnet type is shown in  FIG. 6( a ) . 
     The construction of the drive unit  301   a  is essentially the same as the inertial exciter  101   a  of the inner magnet type shown in  FIG. 4( a )( i ) , with alike features being given alike reference numerals that do not need to be described further here. 
     A key difference from the inertial exciter  101   a  shown in  FIG. 4( a )( i )  is that the magnet assembly  302   a  of the drive unit  301   a  shown here is grounded, i.e. with the magnet assembly  302   a  (preferably the frame  320   a  of the magnet assembly  302   a ) being rigidly attached to an external body, preferably a frame from which the acoustic radiator  390   a  is suspended. 
     Thus, the acoustic radiator  390   a  is suspended from the magnet assembly  302   a  via the coil assembly  304   a  by the first and second suspensions  360   a,    365   a,  rather than magnet assembly  302   a  being suspended from the acoustic radiator  390   a.    
     Of course, any of the inertial exciters of the inner magnet type as shown in any of the preceding drawings could be configured for use as a drive unit in this way. 
     As noted previously, this arrangement helps to provide stable pistonic movement of the acoustic radiator  390   a  and reduces rocking of the acoustic radiator  390   a,  and also takes up a small surface area on a radiating surface of the acoustic radiator  390   a  and thus is particularly useful in a dipole loudspeaker. 
     Drive Unit—Outer Magnet Type Example 
     A drive unit  401   a  that exemplifies a drive unit of the outer magnet type is shown in  FIG. 6( b ) . 
     The construction of the drive unit  401   a  is essentially the same as the inertial exciter  201   a  of the outer magnet type shown in  FIG. 5( a ) , with alike features being given alike reference numerals that do not need to be described further here. 
     A key difference from the inertial exciter  201  shown in  FIG. 5( a )  is that the magnet assembly  402   a  of the drive unit  401   a  shown here is grounded, i.e. with the magnet assembly  402   a  (preferably the frame  420   a  of the magnet assembly  402   a ) being rigidly attached to an external body, preferably a frame from which the acoustic radiator is suspended. 
     Thus, the acoustic radiator  490   a  is suspended from the magnet assembly  402   a  via the coil assembly  404   a  by the first and second suspensions  460   a,    465   a,  rather than magnet assembly  402   a  being suspended from the acoustic radiator  490   a.    
     Of course, any of the inertial exciters of the outer magnet type as shown in any of the preceding drawings could be configured for use as a drive unit in this way. 
     As noted previously, this arrangement helps to provide stable pistonic movement of the acoustic radiator  490   a  and reduces rocking of the acoustic radiator  490   a,  and also takes up a small surface area on a radiating surface of the acoustic radiator  490   a  and thus is particularly useful in a dipole loudspeaker. 
     The features disclosed in the foregoing description, or in the following claims, or in the accompanying drawings, expressed in their specific forms or in terms of a means for performing the disclosed function, or a method or process for obtaining the disclosed results, as appropriate, may, separately, or in any combination of such features, be utilised for realising the invention in diverse forms thereof. 
     While the invention has been described in conjunction with the exemplary embodiments described above, many equivalent modifications and variations will be apparent to those skilled in the art when given this disclosure. Accordingly, the exemplary embodiments of the invention set forth above are considered to be illustrative and not limiting. Various changes to the described embodiments may be made without departing from the spirit and scope of the invention. 
     For the avoidance of any doubt, any theoretical explanations provided herein are provided for the purposes of improving the understanding of a reader. The inventor does not wish to be bound by any of these theoretical explanations. 
     Any section headings used herein are for organizational purposes only and are not to be construed as limiting the subject matter described. 
     Throughout this specification, including the claims which follow, unless the context requires otherwise, the word “comprise” and “include”, and variations such as “comprises”, “comprising”, and “including” will be understood to imply the inclusion of a stated integer or step or group of integers or steps but not the exclusion of any other integer or step or group of integers or steps. 
     It must be noted that, as used in the specification and the appended claims, the singular forms “a,” “an,” and “the” include plural referents unless the context clearly dictates otherwise. Ranges may be expressed herein as from “about” one particular value, and/or to “about” another particular value. When such a range is expressed, another embodiment includes from the one particular value and/or to the other particular value. Similarly, when values are expressed as approximations, by the use of the antecedent “about,” it will be understood that the particular value forms another embodiment. The term “about” in relation to a numerical value is optional and means for example +/−10%. 
     REFERENCES 
     A number of publications are cited above in order to more fully describe and disclose the invention and the state of the art to which the invention pertains. Full citations for these references are provided below. 
     The entirety of each of these references is incorporated herein.
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