Note: Descriptions are shown in the official language in which they were submitted.
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INERTIAL VIBRATION TRANSDUCERS
DESCRIPTION
TECHNICAL FIELD
The invention relates to transducers and more
particularly to vibration transducers for loudspeakers
comprising panel-form acoustic radiating elements.
BACKGROUND ART
It is known from GB-A-2262861 to suggest a panel-form
loudspeaker comprising:-
a resonant multi-mode radiator element being a unitary
sandwich panel formed of two skins of material with a
spacing core of transverse cellular construction, wherein
the panel is such as to have ratio of bending stiffness
(B), in all orientations, to the cube power of panel mass
per unit surface area (~c) of at least 10;
a mounting means which supports the panel or attaches
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to it a supporting body, in a free undamped manner;
and an electro-mechanical drive means coupled to the
' panel which serves to excite a multi-modal resonance in the
radiator panel in response to an electrical input within a
working frequency band for the loudspeaker.
US-A-4,506,117 of MULTIPHONIE discloses an
electroacoustic transducer comprising an inertial mass
adapted to be attached rigidly by its base plate to a panel
to be vibrated.
DISCLOSURE OF INVENTION
Embodiments of the present invention use members of
nature, structure and configuration achievable generally
and/or specifically by implementing teachings of our co-
pending PCT publication No. W097/09842 of even date
herewith. Such members thus have capability to sustain and
propagate input vibrational energy by bending waves in
operative areas) extending transversely of thickness often
but not necessarily to edges of the member(s); are
configured with or without anisotropy of bending stiffness
to have resonant mode vibration components distributed over
said areas) beneficially for acoustic coupling with
ambient air; and have predetermined preferential locations
or sites within said area for transducer means,
particularly operationally active or moving parts) thereof
effective in relation to acoustic vibrational activity~in
said areas) and signals, usually electrical, corresponding
to acoustic content of such vibrational activity. Uses are
envisaged in co-pending International publication No.
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W097/09842 of even date herewith for such members as or in
"passive" acoustic devices without transducer means, such
as for reverberation or fo'r acoustic filtering or for
acoustically "voicing" a space or room; and as or in
"active" acoustic devices with transducer means, such as in
a remarkably wide range of sources of sound or loudspeakers
when supplied with input signals to be converted to said
sound, or in such as microphones when exposed to sound to
be converted into other signals.
This invention is particularly concerned with active
acoustic devices in the form of loudspeakers.
Members as above are herein called distributed mode
radiators and are intended to be characterised as in the
above PCT application and/or otherwise as specifically
provided herein.
The invention is an inertial vibration transducer for
exciting a member having capability to sustain and
propagate input vibrational energy by bending waves in at
least one operative area extending transversely of
thickness to have resonant mode vibration components
distributed over said at least one area and have
predetermined preferential locations or sites within said
area for transducer means and having a transducer mounted
on said member at one of said locations or sites to vibrate
the member to cause it to resonate forming an acoustic
radiator which provides an acoustic output when resonating,
comprising a motor coil assembly having a coil rigidly
fixed to a tubular member, a magnet assembly disposed
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concentrically with respect to the motor coil assembly, and
resilient means supporting the magnet assembly for axial
movement relative to the motor coil 'assembly, and wherein
the tubular member is adapted to be rigidly mounted
directly to the member to be vibrated by adhesive ,means.
The resilient means may comprise mutually opposed
elastomeric members disposed on opposite sides of the
magnet assembly. Caps may be provided for closing the
axial ends of the tubular member, and the resilient means
may be mounted on the caps. The caps may comprise the
resilient means. Each cap may comprise an annular
compliant roll surround. Magnetic shields may be provided
over the caps. The coil may be mounted on the inner face
of the tubular member_ The motor assembly may be adapted
for reception in a correspondingly shaped cavity in the
radiator. The motor coil assembly may be adapted to be
rigidly fixed to a face of the radiator. The magnet
assembly may comprise opposed generally disc-like pole
pieces, the periphery of one of which is disposed within
and adjacent to the motor coil assembly, and the periphery
of the other of which pole pieces is formed with a flange
arranged to lie adjacent to and to surround the motor coil
assembly. A resilient member may be sandwiched between one
of the pole pieces and a face of the radiator. The magnet
assembly may comprise an opposed pair of magnets
sandwiching a pole piece. Complementary magnet assemblies
and motor coil assemblies may be arranged on opposite faces
of the radiator, and means may be provided tying the magnet
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a CA 02229858 1998-02-18
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assemblies together for push-pull operation.
From another aspect the invention is a loudspeaker
' comprising an inertial transducer as described above.
BRIEF DESCRIPTION QF DRAWINGS
The invention is diagrammatically illustrated, by way
of example, in the accompanying drawings, in which:-
Figure 1 is a diagram showing a distributed-mode
loudspeaker as described and claimed in our co-pending
International publication No. W097/09842; '
Figure 2a is a partial section on the line A-A of
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Figure 1;
Figure 2b is an enlarged cross-section through a
dist.~ibuted mode radiator of the kind shown in Figure 2a
and showing two alternative constructions;
S Figure 3 is a sectional side view of a first
embodiment of transducer;
Figure 4 is a sectional side view of a second
embodiment of transducer;
Figure 5a is a sectional side view of a third
embodiment of transducer;
Figure 5b is a sectional side view of a fourth
embodiment of transducer;
Figure 5c is a sectional side view of a fifth
embodiment of transducer, and
1~ Figure 6 is a sectional side view of a sixth
embodiment of transducer.
BEST MODES FOR CARRYING OUT THE INVENTION
Referring to Figure 1 of the drawings, there is shown
a panel-form loudspeaker (81) of the kind described and
claimed in our co-pending International publication No.
W097/09842 of even date herewith comprising a rectangular
5 frame (1) carrying a resilient suspension (3) round its
inner periphery which supports a distributed mode sound
radiating panel (2). A transducer (9) e.g as described in
detail with reference to our co-pending International
publication Nos. W097/09859, W097/09861, W097/09858 of even
10 date herewith, is mounted wholly and exclusively on or in
the panel (2) at a predetermined location defined by
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dimensions x and y, the position of which location is
calculated as described in our co-pending International
publication No. TrJ097/09842 of even date herewith, to launch
bending waves into the panel to cause the panel to resonate
to radiate an acoustic output.
The transducer (9) is driven by a signal amplifier
(10), e.g. an audio amplifier, connected to the transducer
by conductors (28). Amplifier loading and power
requirements can be entirely normal, similar to
conventional cone type speakers, sensitivity being of the
order of 86 - 88dB/watt under room loaded conditions.
Amplifier load impedance is largely resistive at 6 ohms,
power handling 20-80 watts_ Where the panel core and/or
skins are of metal, they may be made to act as a heat sink
for the transducer to remove heat from the motor coil of
the transducer and thus improve power handling.
Figures 2~. and 2~ are partial typical cross-sections
through the loudspeaker (81) of Figure 1. Figure 2a shows
that the frame (1), surround (3) and panel (2) are
connected together by respective adhesive-bonded joints
(20). Suitable materials for the frame include lightweight
framing, e.g. picture framing of extruded metal e.g.
aluminium alloy or plastics. Suitable surround materials
include resilient materials such as foam rubber and foam
plastics. Suitable adhesives for the joints (20) include
epoxy, acrylic and cyano-acrylate etc. adhesives.
Figure 2b illustrates, to an enlarged scale, that the
panel (2) is a rigid lightweight panel having a core (22) ,
I~M~NDED ~hLET
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e.g. of a rigid plastics foam (97) e.g. cross linked
polyvinylchloride or a cellular matrix (98) i.e. a
honeycomb matrix of metal foil, plastics or the like, with
the cells extending transversely to the plane of the panel,
and enclosed by opposed skins (21) e.g. of paper, card,
plastics or metal foil or sheet. Where the skins are of
plastics, they may be reinforced with fibres e.g. of
carbon, glass, Kevlar (RTM) or the like in a manner known
per-se to increase their modulus.
Envisaged skin layer materials and reinforcements thus
include carbon, glass, Kevlar (RTM), Nomex (RTM) i.e.
aramid etc. fibres in various lays and weaves, as well as
paper, bonded paper laminates, melamine, and various
synthetic plastics films of high modulus, such as Mylar
(RTM), Kaptan (RTM), polycarbonate, phenolic, polyester o.r
related plastics, and fibre reinforced plastics, etc. and
metal sheet or foil. Investigation of the Vectra grade of
liquid crystal polymer thermoplastics shows that they may
be useful for the injection moulding of ultra thin skins or
shells of smaller size, say up to around 30cm diameter.
This material self forms an orientated crystal structure in
the direction of injection, a preferred orientation for the
good propagation of treble energy from the driving point to
the panel perimeter.
Additional such moulding for this and other
thermoplastics allows for the mould tooling to carry
location and registration features such as grooves or rings
for the accurate location of transducer parts e.g. the
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motor coil, and the magnet suspension. Additional with
some weaker core materials it is calculated that it would
be advantageous to increase the skin thickness locally e.g. '
in an area or annulus up to 150 of the transducer
diameter, to reinforce that area and beneficially couple
vibration energy into the panel. High frequency response
will be improved with the softer foam materials by this
means.
Envisaged core layer materials include fabricated
honeycombs or corrugations of aluminium alloy sheet or
foil, or Revlar (RTM), Nomex (RTM), plain or bonded papers,
and various synthetic plastics films, as well as expanded
or foamed plastics or pulp materials, even aerogel metals
if of suitably low density. Some suitable core layer
materials effectively exhibit usable self-skinning in their
manufacture and/or otherwise have enough inherent stiffness
for use without lamination between skin layers. A high
performance cellular core material is known under the trade
name 'Rohacell' which may be suitable as a radiator panel
and which is without skins. In practical terms, the aim is
for an overall lightness and stiffness suited to a
particular purpose, specifically including optimising
contributions from core and skin layers and transitions
between them.
Several of the preferred formulations for the panel
employ metal and metal alloy skins, or alternatively a '
carbon fibre reinforcement. Both of these, and also
designs with an alloy Aerogel or metal honeycomb core, will
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9
have substantial radio frequency screening properties which
should be important in several EMC applications.
Conventional panel or cone type speakers have no inherent
EMC screening capability.
In addition the preferred form of piezo and electro
dynamic transducers have negligible electromagnetic
radiation or stray magnet fields. Conventional speakers
have a large magnetic field, up to 1 metre distant unless
specific compensation counter measures are taken.
Where it is important to maintain the screening in an
application, electrical connection can be made to the
conductive parts of an appropriate DML panel or an
electrically conductive foam or similar interface may be
used for the edge mounting.
The suspension (3) may damp the edges of the panel (2)
to prevent excessive edge movement of the panel.
Additionally or alternatively, further damping may be
applied, e.g. as patches, bonded to the panel in selected
positions to damp excessive movement to distribute
resonance equally over the panel. The patches may be of
bitumen-based material, as commonly used in conventional
loudspeaker enclosures or may be of a resilient or rigid
polymeric sheet material. Some materials, notably paper
and card, and some cores may be self-damping. Where
desired, the damping may be increased in the construction
~ of the panels by employing resiliently setting, rather than
rigid setting adhesives.
Effective said selective damping includes specific
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WO 97/09859 PCT/GB96/02167
application to the panel including its sheet material of
means permanently associated therewith. Edges and corners
can be particularly significant for dominant and less
dispersed low frequency vibration modes of panels hereof.
5 Edge-wise fixing of damping means can usefully lead to a
panel with its said sheet material fully framed, though
their corners can often be relatively free, say for desired
extension to lower frequency operation. Attachment can be
by adhesive or self-adhesive materials. Other forms of
10 useful damping, particularly in terms of more subtle
effects and/or mid- and higher frequencies can be by way of
suitable mass or masses affixed to the sheet material at
predetermined effective medial localised positions of said
area.
An acoustic panel as described above is bi-
directional. The sound energy from the back is not
strongly phase related to that from the front.
Consequently there is the benefit of overall summation of
acoustic power in the room, sound energy of uniform
frequency distribution, reduced reflective and standing
wave effects and with the advantage of superior
reproduction of the natural space and ambience in the
reproduced sound recordings.
While the radiation from the acoustic panel is largely
non-directional, the percentage of phase related
information increases off axis. For improved focus for the
phantom stereo image, placement of the speakers, like
pictures, at the usual standing person height, confers the
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benefit of a moderate off-axis placement for the normally
seated listener optimising the stereo effect. Likewise the
triangular left/right geometry with respect to the listener
provides a further angular component. Good stereo is thus
obtainable.
There is a further advantage for a group of listeners
compared with conventional speaker reproduction. The
intrinsically dispersed nature of acoustic panel sound
radiation gives it a sound volume which does not obey the
inverse square law for distance for an equivalent point
source. Because the intensity fall-off with distance is
much less than predicted by inverse square law then
consequently for off-centre and poorly placed listeners the
intensity field for the panel speaker promotes a superior
stereo effect compared to conventional speakers. This is
because the off-centre placed listener does not suffer the
doubled problem due to proximity to the nearer speaker;
firstly the excessive increase in loudness from the nearer
speaker, and then the corresponding decrease in loudness
from the further loudspeaker.
There is also the advantage of a flat, lightweight
panel-form speaker, visually attractive, of good sound
quality and requiring only one transducer and no crossover
for a full range sound from each panel diaphragm.
Figure 3 illustrates an embodiment of moving coil
° transducer (9) arranged to be embedded entirely within the
interior of a stiff lightweight distributed mode panel (2)
of the kind comprising a core (22) faced on both sides with
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skins (21) to launch bending waves into the panel.
The transducer comprises a coil (13) embedded in a
fixing (16), e.g. of epoxy resin, in a cavity (29) in the
core (22) of the panel (2), and surrounding a cylindrical
coil former (18), the coil (13) and former (18) thus being
rigidly fixed in the panel (2).
Mounted in the portion of the cavity (29) defined by
the coil former (18) is a magnet assembly comprising an
opposed pair of magnets (15) separated by a pole-forming
member (14), the magnet assembly being mounted on the inner
faces of skins (21) of the panel (2) by means of opposed
compliant suspension members (19) of rubber-like material,
e.g. foam rubber, which are adhesively bonded to the magnet
assembly and to the interior surfaces of the respective
skins (21) of the panel. The magnet assembly (14,15) is
thus mounted concentrically of the coil (13) and is axially
movable on its suspension (19).
The transducer (9) operates to launch bending waves
into the panel (2) by vibrating to cause local resilient
deformation of the panel due to relative axial motion
between the magnet assembly and the coil. The drive effect
is enhanced by increasing the mass of the magnet assembly.
In operation, at least at high frequencies, since the
mass of the magnet assembly is relatively large in
comparison to that of the panel, the inertia of the magnet
assembly will tend to hold the magnet assembly stationary
and to vibrate the panel relatively thereto.
Figure 4 illustrates an embodiment of moving coil
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transducer (9) similar to that shown in Figure 3 and
arranged to be embedded entirely within the interior of a
' stiff lightweight distributed mode radiator panel (2) of
the kind comprising a core (22) faced with skins (21) to
launch bending waves into the panel. The transducer (9) is
formed as a modular assembly to facilitate its assembly
into a panel (2). As shown, the panel (2) is formed with
a suitable cavity (120) to receive the transducer (9).
The transducer comprises a coil (13) fixed to the
interior wall of a cylindrical coil former (18) e.g. by
means of a rigid adhesive potting (20), the former (18)
providing the outer casing of the transducer and being
closed at its opposite axial ends by lightweight end caps
(119) which are rigidly fixed to the coil former in any
desired fashion, e.g. by means of adhesive bonds (220).
The assembly is arranged to be located in the transducer
cavity (120) in a distributed mode panel (2), by movement
in direction of arrow 'A' as indicated in. The transducer
is fixed in the cavity by means of an adhesive.
Mounted in the cavity (29) defined by the coil former
(18) is a magnet assembly comprising an opposed pair of
magnets (15) separated by a pole-forming member (14), the
magnet assembly being mounted on the end caps (119) of the
coil former (18) by means of opposed compliant suspension
members (19) of rubber-like material, e.g. foam rubber,
which are adhesively bonded to the magnet assembly and to
the interior surfaces of the respective end caps.
The magnet assembly (14,15) is thus mounted
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concentrically of the coil (13) and is axially movable on
its suspension (19). The transducer (9) operates to launch
bending waves into the panel (2) by vibrating to cause
local resilient deformation of the panel in the same way as
described above with reference to the embodiment of Figure
3.
The transducer arrangement (9) of Figure 5~ comprises
complementary push/pull drivers disposed on opposite sides
of the panel (2) to launch bending waves into a rigid
lightweight distributed mode radiator (2) of the kind
comprising a core (22) enclosed by opposed skins (21), to
cause the panel to resonate.
In this embodiment, coils (13) are rigidly fixed, e.g.
by means of an adhesive, on the outside of a coil former
(18) to form a motor coil assembly which is rigidly bonded
to the opposed surface skin (21) of the radiator panel (2),
e.g. by means of an epoxy adhesive bond (16). Magnets (15)
are enclosed by pairs of poles (14), one of which is disc-
like and is disposed with its periphery close to the
interior of each coil former (18), and the other of which
has a peripheral flange (162) arranged to surround the coil
(13).
A fixing member (93) which is generally cylindrical in
shape is arranged to pass freely through an aperture (29)
in the panel (2). The fixing member (93) comprises opposed
generally complementary parts each formed with a head (95)
which are clamped against the axial extremities of the
respective pair of transducers (9) to couple the drivers
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together. The complementary parts of the fixing member
(93) are secured together by complementary screw-threaded
- portions (160, 161). The fixing member may be of any
suitable material e.g. plastics or metal.
5 The transducer arrangement (9) of Figure 5~ is not
rigidly clamped to the panel (2) adjacent to the aperture
(29) but is instead coupled to the panel via resilient pads
(17) e.g. of foam rubber positioned close to the panel
aperture (29) in much the same manner as is shown with
10 reference to Figures 3 and 3 whereby the transducer works
to launch bending waves into the panel by inertial effects
due to the combined mass of the respective drivers.
The transducer (9) of Figures 5~,is generally similar
to that of Figure 5a but is intended for attachment to only
15 one side of a panel (2). Thus the magnet assembly (14,15)
is secured to the surface of the panel ( 2 ) by means of a
resilient suspension (17) e.g. of rubber, which is attached
to the periphery of the flange (162) of the outer pole
pieces (14).
Figure 5~ shown a transducer (9) of the kind shown in
Figure 5~ and intended for easy application to a panel
surface. Thus the transducer (9) is mounted, by way of the
former (18) and resilient suspension (17) on a thin
substrate (147) formed with a self adhesive outer layer
whereby the transducer can be mounted in position.
The transducer ( 9 ) of Figure 6 is intended as a low
profile device which can be buried substantially within the
thickness of a distributed mode panel (2). The transducer
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comprises a cylindrical coil former (18) adapted to be
fixed, e.g. by means of an adhesive, in a corresponding
aperture (29) in the panel (2). A coil (13) is secured to '
the interior face of the former (18) e.g. with the aid of
an adhesive to form a motor coil assembly. The opposed
axial ends of the former (18) are closed by disc-like
compliant suspension members (59), e.g. of rubber or the
like, each of which is formed with an annular corrugation
(136) near to its periphery to form a roll surround similar
to that used on conventional pistonic cone loudspeaker
drive units. The peripheries of the members (59) are
secured to the axial ends of the coil former (18) e.g. by
clamping, with the aid of an adhesive or in any other
suitable fashion. The centre portions of the members (59),
which centre portions are defined by the annular
corrugations (136) carry between them a magnet assembly
comprising an opposed pair of magnets (15) sandwiching a
pole piece (14). The outer faces of the magnets (15) are
bonded or otherwise secured to the centre portions of the
members (59), whereby the magnet assembly (14,15) is
located concentrically with respect to the coil (13) and is
capable of limited axial movement relative thereto.
The magnet assembly is shielded by means of disc-like
screens (121) mounted on annular resilient members (17)
supported on the panel (2) to prevent or limit the stray
magnet field surrounding the panel adjacent to the
transducer.
Although in the above described embodiments of
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transducer, the motor coil assemblies have been shown to be
fixed to the panel (2) and the magnet assemblies have been
shown to be compliantly mounted with respect to the panel,
it will be understood that this arrangement could be
reversed so that the magnet assemblies are fixed to the
panel and the motor coil assemblies are compliantly
suspended. In such a case the magnet assemblies will be
made relatively light and the motor coil assemblies will be
made relatively heavy to increase the drive effect.
