Classifications

• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04R—LOUDSPEAKERS, MICROPHONES, GRAMOPHONE PICK-UPS OR LIKE ACOUSTIC ELECTROMECHANICAL TRANSDUCERS; DEAF-AID SETS; PUBLIC ADDRESS SYSTEMS
  • H04R7/00—Diaphragms for electromechanical transducers; Cones
  • H04R7/02—Diaphragms for electromechanical transducers; Cones characterised by the construction
  • H04R7/04—Plane diaphragms
  • H04R7/045—Plane diaphragms using the distributed mode principle, i.e. whereby the acoustic radiation is emanated from uniformly distributed free bending wave vibration induced in a stiff panel and not from pistonic motion
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04R—LOUDSPEAKERS, MICROPHONES, GRAMOPHONE PICK-UPS OR LIKE ACOUSTIC ELECTROMECHANICAL TRANSDUCERS; DEAF-AID SETS; PUBLIC ADDRESS SYSTEMS
  • H04R9/00—Transducers of moving-coil, moving-strip, or moving-wire type
  • H04R9/02—Details
  • H04R9/04—Construction, mounting, or centering of coil
  • H04R9/045—Mounting

Definitions

• the invention relates to bending wave panel loudspeakers , e.g. resonant bending wave panel speakers of the kind exemplified by W097/ 09842, and to drive motors for such speakers .
• a loudspeaker comprising a panel-form acoustic member intended for operation as a bending wave radiator and an electrodynamic moving coil transducer having a voice coil mounted to the acoustic member to excite bending wave vibration in the acoustic member, wherein the junction between the voice coil and the acoustic member is of sufficient length in relation to the size of the acoustic member to represent a line drive such that the acoustic member has a mechanical impedance which on average rises with bending wave frequency.
• the junction of the voice coil and the diaphragm may be circular and the junction may be substantially continuous.
• a sufficient length voice coil junction in the present context is one in which the length, or its diameter in the case of a circular junction, is equal to at least the length of a bending wave in the portion of the acoustic member defined by the junction, or circumscribed by the voice coil, at the highest operating frequency of the loudspeaker.
• the mechanical impedance of a panel is equal to the ratio of force applied at a single point to the resultant velocity at this point. Where the panel is driven by force acting over a line, the effective mechanical impedance is the ratio of total force applied over the line to the resultant velocity averaged over the length of the line. In the present description and claims the use of the term mechanical impedance is used to describe this ratio for both drive arrangements.
• the portion of the acoustic member circumscribed by the said voice coil may be of different stiffness to a portion of the acoustic member outside the voice coil .
• the transducer may be arranged both to move the acoustic member in whole body mode and to apply bending wave energy to the acoustic member.
• the size, shape and position of the junction between the voice coil and the acoustic member may be adjusted in relation to the modal distribution of the diaphragm or acoustic member in order to achieve a smooth transition from whole body motion at low frequencies to resonant bending wave behaviour at higher frequencies.
• the second resonance may give rise to an irregularity in the output.
• the effective perimeter of the driveline may be chosen in location and size to lie on or near to the nodal circle of the second resonance.
• the first resonance is the whole body or piston equivalent resonance.
• Mass loading may be applied to the acoustic member within the diameter of the voice coil.
• the acoustic member may be non-circular in shape.
• the transducer voice coil may be concentric with the geometric centre of the acoustic member.
• a second transducer may be coupled to the acoustic member within the portion thereof circumscribed by the said voice coil and adapted to cause high frequency bending wave activity of the said portion.
• the second transducer may be offset from the axis of the said voice coil .
• Coupling means may be provided to attach the said voice coil to the acoustic member, the coupling means having a footprint of non-circular shape.
• the portion of the acoustic member circumscribed by the voice coil may be stiffer than a portion of the acoustic member outside the voice coil .
• the bending stiffness of the acoustic member may be anisotropic.
• the acoustic member may be curved or dished or otherwise formed to increase its bending stiffness.
• the loudspeaker may comprise a chassis having a portion surrounding the acoustic member, and a further portion supporting the electrodynamic transducer, and may further comprise means connected between the acoustic member and the said chassis portion and resiliently suspending the acoustic member on the chassis.
• the suspension means may be connected between the chassis and the margin of the acoustic member.
• the resilient suspension may be adapted to mass load the acoustic member.
• the resilient suspension may be adapted to damp the acoustic member.
• the resilient suspension may be at least partly formed by a skin of the acoustic radiator.
• the acoustic member may have a front side from which acoustic energy is radiated, and may comprise acoustic masking means positioned over the portion of the acoustic member circumscribed by the said voice coil, the masking means defining an acoustic aperture.
• the electrodynamic moving coil transducer may be offset from the geometric centre of the acoustic member, and a counter balance mass may be provided on the acoustic member.
• the action of the large area voice coil on the diaphragm can produce a distribution of excited modes that results in significant beaming of the radiation on-axis, at least over some of the frequency range . In some applications, such as zoning of the output sound, this may be advantageous, but in many applications off-axis power is desirable.
• One approach to improving off-axis power is to excite the panel in bending wave vibrations at frequencies near to or greater than the coincidence frequency.
• the coincidence frequency is the frequency at which the bending wave velocity in the plate equals the velocity of sound in air. Above this frequency the velocity in the plate exceeds the velocity in air, and this supersonic vibration can give rise to strongly directional radiation off-axis. In fact at the coincidence frequency, radiation is beamed directly off-axis, with the angle of beaming moving closer to the on-axis direction with increasing frequency.
• the coincidence frequency of a plate is determined by its bending stiffness (B) and mass density
• the loudspeaker of the present invention may be adapted to operate as a full range device.
• Figure 1 is a front view of a loudspeaker driver motor
• Figure 2 is a cross-sectional side view of the drive motor of Figure 1 ;
• Figure 3 is a front view of a loudspeaker enclosure;
• Figure 4 is a side view of the loudspeaker enclosure;
• Figure 5 is a graph of frequency response;
• Figure 6 is a graph of near field bass frequency response;
• Figures 7 to 9 are front views of embodiments of diaphragm each having a supplementary vibration exciter;
• Figures 10 to 13 are front views of further embodiments of diaphragm;
• Figure 14 is a perspective diagram of a further embodiment of diaphragm
• Figures 19 to 21 show cross-sections of embodiments of diaphragm surrounds or suspensions
• Figure 22 is a cross-sectional view of an embodiment of speaker driver motor
• Figure 23 is front view of an embodiment of diaphragm
• Figure 24 is a cross-sectional view of an embodiment of diaphragm
• Figure 25 is a polar diagram comparing the response of a conventional pistonic speaker with that of the present invention.
• Figures 26 and 27 are front views of further embodiments of voice coil/diaphragm line drive junctions.
• BEST MODES FOR CARRYING OUT THE INVENTION In Figures 1 and 2 there is shown a loudspeaker driver motor (1) adapted to be mounted to a baffle, e.g. in an enclosure, see Figures 3 and 4 below, comprising a circular flat diaphragm of stiff lightweight material, comprising, for example, a core sandwiched between skins of high tensile sheet material, which forms an acoustic member or radiator intended to operate both pistonically and by flexure as a bending wave resonant device at higher frequencies.
• a loudspeaker driver motor (1) adapted to be mounted to a baffle, e.g. in an enclosure, see Figures 3 and 4 below, comprising a circular flat diaphragm of stiff lightweight material, comprising, for example, a core sandwiched between skins of high tensile sheet material, which forms an acoustic member
• the driver motor of the present invention is able to operate as a full range device covering substantially the whole of the audio spectrum with wide acoustic dispersion, unlike a conventional pistonic driver, whose frequency band or at least its dispersion angle is limited at high frequencies by the diameter of the diaphragm, see Figure 25 below, and a bending wave driver, which tends to roll-off at frequencies below about 200Hz, unless of very large diaphragm size.
• the diaphragm (2) is supported in a chassis or basket (3) , e.g.
• a corrugated suspension (6) e.g. of rubber-like material is fixed to the diaphragm round its periphery by means of an adhesive and the suspension is clamped to the annular flange (4) with the aid of a clamping ring (7) , whereby the diaphragm can move pistonically relative to the chassis.
• the chassis supports an electrodynamic moving coil transducer (8) for moving the diaphragm pistonically and for applying bending wave energy to the diaphragm to cause it to resonate, e.g. in the manner generally described in WO97/09842.
• the transducer comprises a magnet assembly (9) fixed to the chassis and defining an annular gap (10) concentric with the diaphragm and a voice coil and former assembly (11) mounted for axial movement in the annular gap and which is fixed to the diaphragm concentrically therewith by a coupler ring (12) .
• a corrugated suspension spider (13) is fixed between the voice coil assembly and the chassis to ensure the proper axial movement of the voice coil in the annular gap.
• the voice coil diameter is large in relation to the bending wave length and the effect of this is that of a line drive to the diaphragm instead of a point drive as is normal for bending wave radiators using electrodynamic exciters having small diameter voice coils.
• This line drive provides a significant increase in the mechanical drive impedance presented to the voice coil, and this higher mechanical impedance enables the system to tolerate relatively high mass voice coils without premature roll off of high frequencies.
• An inner portion (16) of the diaphragm is circumscribed by the voice coil as seen in Figure 1, while an outer portion (17) of the diaphragm extends radially outside the voice coil.
• small masses are attached to the diaphragm inside the voice coil diameter to tune and/or smooth the frequency response of the acoustic radiator.
• Such masses are not always essential but may usually be desirable.
• These masses are shown as discrete masses but need not necessarily be discrete. They may have masses in the range 0.5 g to 100 g, and typically in the range 2 to 20 g. One or more such mass may be provided.
• the loudspeaker driver embodiment of Figures 1 and 2 has been optimised for use in a hi-fi loudspeaker, when coupled to an amplifier which has a flat voltage transfer function throughout the audio band.
• the transducer has a large 75mm diameter voice coil mounted in a low inductance motor system having a vent 0 (18) , having a copper eddy current shield (19) over the pole piece or front plate (20) .
• Figure 2 shows a cross section of a magnetic ring (21) of neodymium, centrally mounted in a steel magnetic circuit comprising a magnet cup (22) and the front plate (20) resulting in an average 5 B field of 0.8T.
• the voice coil (11) over-hangs the magnet front plate (20) to give an over-hung configuration.
• the voice coil consists of a winding height of 14.5mm of aluminium turns on a 0.1mm thick aluminium former.
• the voice coil parameters are given below:
• the coupler ring (12) is required to provide a secure interface between the voice coil and the diaphragm. This nests inside of the voice coil. A 2.5mm overlap is provided to allow for a good bond area between the coupler and the voice coil former.
• the coupler ring extends the effective length of the voice coil by 1.7mm, giving a ring width of 3.5mm to couple to the diaphragm. This is show in Figure 2.
• the material of the coupler ring is commercial grade thermoplastic or thermoset resin e.g. ABS which gives a mass of 3.4g.
• a thermal resistant cyanoacrylate is used
• the diaphragm material used along with its parameters are given below:
• the wavelength of the panel may be calculated at the highest frequency of operation, i.e. 20kHz. This calculation gives a wavelength of 28mm, based on an average bending stiffness of 2.1 Nm.
• the voice coil diameter is therefore 2.7 times the wavelength at the highest frequency of operation.
• the first aperture resonance corresponds to a half wavelength within the voice coil.
• the coincidence lobe of this panel gives strong acoustic output off axis close to or above coincidence frequency as given in Table 1 above .
• the thin line or trace (45) is a plot of a speaker according to the invention with a 300 mm diameter diaphragm
• the thick trace (44) is of a conventional pistonic diaphragm of 250 mm diameter.
• the chassis consists of an aluminium back plate (23) to support the transducer (8) and which is connected to the front flange (4) .
• Allen bolts (not shown) are used to secure the clamping ring (7) to the flange (4) .
• the pair of masses (14,15) fixed to the diaphragm are to smooth ' the first drum mode within the inner portion of the diaphragm, at approximately 2kHz.
• the motor drive unit parameters are given below:
• dD 14cm (Diameter of radiating area (centre to centre of the surround) )
• Figures 3 and 4 show a loudspeaker enclosure (24) for the drive unit of Figures 1 and 2 and having a sloping front (25) and sides (26) .
• An aperture (27) is provided in the front (25) to receive the drive unit or motor (1) .
• the enclosure has been designed to give a volume of 17 litres giving a maximally flat alignment.
• the enclosure form is chosen to smear internal enclosure standing waves, although this is not essential to the design and operation of the speaker.
• the enclosure is constructed from 18mm medium density fibreboard (MDF) .
• the joints are glued (using PVA wood glue) and screwed to give an air tight seal .
• a supplementary exciting device could be used to improve the high frequency level and/or extension and directivity performance of the loudspeaker.
• the supplementary exciter could be placed anywhere on the diaphragm to provide a smaller radiation area.
• Devices such as piezos of large area, small area or strip-like form or smaller moving coil devices could be used. This is illustrated in Figures 7 to 9.
• a circular piezo disc vibration exciter (28) has been mounted on the diaphragm (2) at its centre and inside the diameter of the voice coil (11) .
• a piezo strip vibration exciter (29) has been mounted on the diaphragm
• the voice coil moving mass has little effect on the high frequency extension of the speaker. Therefore the present invention is not restricted to lightweight voice coils. This implies scope for employing moving magnet motor systems and/or relatively high mass coupler rings between the voice coil assembly and the diaphragm which currently might be excluded from small drive area or point drive designs of bending wave speaker. This could allow complex coupler designs to transform the voice coil ring to other beneficial shapes so as to improve performance. Examples of triangular, square and oval shapes of coupler ring are shown in Figures 10 to 12 respectively under references (31) to (33) respectively. These shapes have implications on the distribution of modes excited and therefore directivity implications.
• a rectangular diaphragm (34) has been chosen this, together with a rectangular coupler ring (32) rotated by an angle relative to the diaphragm sides, could provide a more irregular modal pattern in the diaphragm. This could also further improve frequency response on and off axis.
• the voice coil diameter is 75mm. This can be increased or decreased depending on the design specification. If the design specification requires narrow directivity for zoning applications, a larger voice coil coupled to a low wave speed panel, i.e. having a very high Fc, could be used. Conversely, if wide directivity is required a smaller voice coil can be used, within the criteria of line drive. However this may need electrical high frequency boost to maintain constant pressure throughout the audio band.
• the invention is not limited to the circular panel shape shown in the embodiment of Figures 1 and 2.
• Other shapes can be beneficial in directivity and/or frequency response, because of the different mode shapes that result from the geometry of the panel . It is expected that the more complex the mode shapes in the panel, the less directivity there will be in the acoustic output . Examples include square, rectangular and hexagonal panels.
• the invention is not restricted to pure piston behaviour of the diaphragm at low frequency, and may be quasi-tympanic at low frequencies.
• the diaphragm (34) could be a large radiating panel. This would provide a means of self-baffling giving a dipole bass response as indicated by opposed arrows.
• the panel edges could be free or clamped.
• the invention is not restricted to a flat diaphragm or to a single material type.
• Profiling and shaping of the diaphragm can be used to alter the modal behaviour.
• the part of the diaphragm circumscribed by the voice coil could be constructed from a different material or the same material but thicker or thinner.
• Exemplary embodiments are shown in Figures 15 to 18.
• Stiffness can be applied to the diaphragm by profiling. Stiffness variation can also be realised by using material isotropy.
• the diaphragm surround affects acoustic performance. Both the piston and modal region can be varied by changing the material properties of the surround. In particular, if mass is applied to the perimeter of the diaphragm as shown at (36) in Figure 19, high frequency performance can be improved. Edge damping of the diaphragm can be applied to control its modal behaviour. This can be in the form of surface treatment or edge damping can be by means of the surround footprint, as indicated at (37) in Figure 20.
• the panel skins, or one of them, could be used to form the surround as indicated in Figure 21.
• the diaphragm comprises a core (38) and skins (39,40) covering the core.
• the lower skin (40) is extended to form the surround or suspension (6) . This may give cost advantages. Advantages could also include low-loss termination of the diaphragm.
• isotropic diaphragms e.g. at approximately two times Fc, will give side lobes in the same position in both planes.
• coincidence can be set independently in alternate planes thus giving a smoother total power response .
• Mechanical components e.g. mass or voice coil coupling to the panel, can provide a means of mechanical filtering.
• the frequency response can be modified.
• Passive component electrical shelving or amplifier transfer function shelving/high frequency boost could also be employed to modify the acoustic output of the device.
• (11) may be positioned off-centre on the diaphragm (2) to improve the distribution of resonant modes excited m the diaphragm, with a counterbalancing mass (35) positioned on the diaphragm to prevent rocking.
• the diaphragm (2) need not be flat and can be dished or otherwise formed to increase its stiffness.
• This may be m the form of a curvature which varies across the diaphragm so that the stiffness is greater towards the edges of the diaphragm, as shown.
• This curvature or profiling of the diaphragm may assist in scaling the diaphragm while keeping the fc constant, and may also be beneficial in smoothing the piston to modal transition, especially for larger diaphragms.
• FIG 26 there is shown a circular diaphragm (2) which is driven by the voice coil of a transducer, not shown, having a rectilinear coupler (46) , equivalent to the coupler ring (12) of the embodiment of Figures 1 and 2, connected between the voice coil and the diaphragm to provide a straight line drive junction.
• the coupler (46) is arranged and disposed on a diameter of the diaphragm and with its ends equally spaced from the opposite edges of the diaphragm.
• the coupler (46) is positioned off centre of the diaphragm and angled with respect to the sides of the diaphragm.
• the present invention thus provides an effective way of increasing the frequency bandwidth of a bending wave speaker.

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• Engineering & Computer Science (AREA)
• Physics & Mathematics (AREA)
• Acoustics & Sound (AREA)
• Signal Processing (AREA)
• Multimedia (AREA)
• Audible-Bandwidth Dynamoelectric Transducers Other Than Pickups (AREA)
• Diaphragms For Electromechanical Transducers (AREA)

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• 2000
  • 2000-11-30 GB GBGB0029098.1A patent/GB0029098D0/en not_active Ceased
• 2001
  • 2001-11-14 DE DE60132357T patent/DE60132357T2/de not_active Expired - Lifetime
  • 2001-11-14 EP EP01999128A patent/EP1340407B1/de not_active Expired - Lifetime
  • 2001-11-14 WO PCT/GB2001/005014 patent/WO2002045460A2/en active IP Right Grant
  • 2001-11-14 CN CNB018173284A patent/CN100433937C/zh not_active Expired - Lifetime
  • 2001-11-14 CZ CZ20031501A patent/CZ20031501A3/cs unknown
  • 2001-11-14 AU AU2002223800A patent/AU2002223800A1/en not_active Abandoned
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  • 2001-11-14 JP JP2002546462A patent/JP2004515178A/ja active Pending
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• 2003
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