Note: Descriptions are shown in the official language in which they were submitted.
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W O 97/09861 PCT/GB96/02160
INERTIAL VIBRATION TRANSDUCERS
D~SCRIPTIQN
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 (~) 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.
FR-A-2,569,931 of SAWAFUJI discloses a piezo-electric
vibrator and loudspeaker comprising a piezo plate loaded
with a mass located near to its centre of gravity and
coupled to a loudspeaker diaphragm to be vibrated via the
periphery of the piezo plate.
DISCLQSURE 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 dateherewith. 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
AMENDED SHEET
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envisaged in co-pending International publication No.
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 exposea to sound to
be converted into other signals.
This invention is particularly concerned with active
acoustic devices in the form o~ 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 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 the 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,
wherein the transducer has a plate-like piezo-electric
AMENDED SH~EI
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bender, means disposed centrally of the plate-like bender
adapted to mount the bender on the member to be vibrated,
the arrangement being such that a substantial part of the
bender is spaced from the member for movement relative
thereto, and a mass secured to the periphery of the bender.
The mounting means may be a lightweight rigid member. The
piezo-electric bender may be of crystalline form. From
another aspect the invention is a loudspeaker characterised
by a member having capabillty 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 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 DESCRIPTIQN 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
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and showing two alternative constructions;
Figure 3 is a diagram of a first embodiment of
transducer;
5Figure 4 is a diagram of a second embodiment of
transducer, and
Figure 5 is a diagram of a third 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
frame (1) carrying a resilient suspension (3)
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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 y, 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
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 2a and 2b 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.
'.- ~~~
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W O 97/09861 PCT/GB96/02160
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 2k 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, 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, Revlar (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 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
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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 Kevlar (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
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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 fre~uency 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 e~ually 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
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W O 97/09861 PCT/GB96/02160
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, thouyh
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
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
~ 25 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
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W O 97/09861 PCT/GB96/02160
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
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
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11
quality and requiring only one transducer and no crossover
for a full range sound from each panel diaphragm.
Figure 3 illustrates an embodiment of piezo-electric
transducer ~9) in which a crystalline disc-like piezo
bender (27) is mounted at its centre on one end of a
lightweight rigid cylindrical block (93) of rigid foam
plastics which is rigidly fixed in an aperture (20) in a
distributed mode radiator panel (2) e.g. by means of an
adhesive, the said one end of the block (28) projecting
from the face of the panel (2) so that the periphery (31)
of the bender (27) is freely suspended adjacent to a face
of the panel (2). An annular ring (25) of plastics, e.g.
mineral loaded polyvinylchloride is rigidly fixed to the
periphery of the piezo bender (27) to add mass to the free
periphery of the piezo bender. Thus when the transducer is
energized with an acoustic signal, the piezo bender (27)
vibrates and due to its mass launches bending waves into
the panel (2) to cause the panel to resonate and produce
and radiate an acoustic output. The transducer (9) may be
covered by a domed housing (26) which is fixed to the panel
(2) to protect the transducer.
The piezo-electric transducer (9) of Figure 4
comprises a disc-like piezo bender (27) fixedly mounted by
its periphery (31) on the surface of a panel (2) e.g. with
the aid of an adhesive, with the central portion of the
bender (27) freely suspended over a cavity (29) in the
panel (2) such that only the periphery (31) of the bender
(27) is in contact with the panel. A mass (25) e.g. of
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12
plastics material is attached to the centre of the bender
(27) with the interposition of a damping pad (30) of
resilient material, e.g. of an elastic polymer.
Thus an acoustic signal applied to the piezo bender
will cause the bender to vibrate and thus to launch bending
waves into the panel. The drive effect of the transducer
is enhanced by loading the driver (27) with the mass (25)
to increase its inertia.
The transducer arrangement (9) of Figure 5 is similar
to that of Figure 4 except that in this embodiment a pair
of piezo benders (27) are attached on opposite sides of a
cavity (29) through a panel (2) to operate in push/pull
mode. In this arrangement, the centres of both benders
(27) are connected together by a common mass (25~ with
resilient damping pads (30) positioned between each bender
(27) and the mass (25).
INDUSTRIAL APPLICABILITY
The transducers of the invention are relatively simply
in construction and are ef~ective in use.