Note: Descriptions are shown in the official language in which they were submitted.
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W O 97/09858 PCT/GB96/02148
VIBRATION TRANSDUCERS
DESCRIPTION
TECHNICAL FIF~T~n
The invention relates to transducers and more
particularly to vibration transducers for loudspeakers
comprising panel-form acoustic radiating elements.
BACRGROUND 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
~ 25 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 (~) 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 area(s) 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 area(s) 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 part(s) thereof
effective in relation to acoustic vibrational activity in
said area(s) and signals, usually electrical, corresponding
to acoustic content of such vibrational activity. Uses are
envisaged in co-pending International publication No.
A~FN~EI:) SH~E~
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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 for 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 acoustic radiators
and are intended to be characterised as in the above PCT
application and/or otherwise as specifically provided
herein.
The invention is a vibration transducer for exciting
a member having a face and 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, the motor coil assembly being
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adapted to be fixed to the said face of the member, and a
magnet assembly having opposed disc-like pole pieces, the
periphery of one of which pole pieces is arranged to be
disposed within and adjacent to the motor coil assembly,
and the periphery of the other of which pole pieces is
formed with a surrounding flange adapted to surround and to
be disposed adjacent to the motor coil assembly, and
wherein the magnet assembly is adapted to be secured at its
centre to the said member to be vibrated. Fixing means may
be provided to secure the magnet assembly to the member.
The fixing means may comprise a fastener adapted to engage
in a cavity in the member. The fastener may comprise a
spacer for spacing the peripheries of the polerpieces from
the said member. The vibration transducer may comprise
complementary motor coil assemblies and magnet assemblies
adapted for mounting on opposed faces of the said member,
and means tying the centres of the magnet assemblies
together for push/pull operation. Thus the fastener may
have heads at opposite ends and adapted to engage the
respective magnet assemblies, the fastener comprising a
pair of interengaging screw-threaded portions, and spacer
means adapted for disposition adjacent to the fastener and
adapted for sandwiching between the respective magnet
assemblies and the opposed faces of the said member.
From another aspect the invention is a loudspeaker
characterised by a member having capability to sustain and
propagate input vibrational energy by bending waves in at
least one operative area extending transversely of
E3 ~ CT
CA 022298~6 1998-02-18
' 4a . .. ..
tAickness 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 vibration transducer
as described above 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.
~BRIEF DESCRIPTION OF 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
Figure 1;
Figure 2b is an enlarged cross-section through a
distributed mode radiator of the kind shown in Figure 2a
and showing two alternative constructions;
Figure 3 is a diagram of a first embodiment of
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transducer according to the present invention, and
Figure 4 is a diagram a second embodiment of
transducer according to the presen~t invention;
BEST MQDES FOR CARRYING QUT 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
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
date herewith, is mounted wholly and exclusively on or in
the panel (2) at a predetermined location defined by
dimensions x and ~, the position of which location is
calculated as described in our co-pending International
publication No. W097/09842 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
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W O 97/09858 PCT/GB96/02148
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 2k 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)
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, Revlar (RTM) or the like in a manner known
E~E 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
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W O 97/09858 PCT/GB96/02148
synthetic plastics films of high modulus, such as Mylar
(RTM), Raptan (RTM), polycarbonate, phenolic, polyester or
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
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
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W O 97/09858 PCT/GB96/02148
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
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
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W O 97/09858 PCT/GB96/02148
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
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.
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
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
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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 stAn~;ng person height, confers the
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
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11
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
guality and requiring only one transducer and no crossover
for a full range sound from each panel diaphragm.
Figure 3 illustrates an embodiment of transducer (9)
for launching bending waves into a rigid lightweight
distributed mode radiator panel (2), e.g. of the kind shown
in Figures 1 and 2 comprising a core (22) enclosed by
opposed skins (21), to cause the panel to resonate.
The transducer comprises a coil (13) rigidly fixed,
e.g. by means of an adhesive, on the outside of a coil
former (18) which is rigidly bonded to a surface skin (21)
of the radiator panel (2), e.g. by means of an epoxy
adhesive bond (16). A magnet (15) is enclosed by a pair of
poles (14), one of which is disc-like and is disposed with
its periphery close to the interior of the coil former
(18), and the other of which has a peripheral flange (90)
arranged to surround the coil (13).
The magnet assembly including the magnet (15) and
poles (14) is mounted on the panel (2) by means of a fixing
(93), e.g. of metal or hard plastics, which passes through
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12
a cavity (29) extending through the panel (2). The fixing
(93) comprises a complementary pair of threaded members
(91,92) each having heads (95), one of which heads bears
against an outer face of the transducer (9) and the other
of which heads bear against a face of the panel (2) on the
side of the panel opposite to that on which the transducer
is mounted. A spacer (127) is trapped between the
transducer (9) and the panel (2) to space the transducer
from the panel.
The transducer (9) of Figure 3 operates by locally
resiliently bending the panel between the fixing (93) and
the ~ormer (18) when an acoustic signal is applied to the
transducer to launch bending waves into the panel to cause
it to resonate.
The transducer arrangement (9) of Figure 4 is similar
to that described in Figure 3, except that in this
embodiment the transducer comprises complementary push/pull
drivers of the kind shown in Figure 3 disposed on opposite
sides of the panel. A fixing member (93) is arranged to
pass through an aperture (29) in the panel (2) to tie the
two transducers together and to the panel. 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 together. The complementary parts of
the fixing member (93) are secured together by
complementary screw-threaded portions (94,96). The fixing
member may be of any suitable material e.g. plastics or
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W O 97/09858 PCT/GB96;02148 13
metal.
In this embodiment the transducer device (9) is
rigidly clamped to the panel ~2) by means of rigid pads
(19), e.g. of hard plastics, positioned between the panel
and the poles (14) adjacent to the aperture (29), whereby
the transducer works to launch bending waves into the panel
by local resilient bending of the panel between the pads
and the coil former (18).
