Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
CA 02230449 1998-02-2~
W O 97/09862 PCT/GB96/02155
PANEL-FORM M~C~OPHONES
DESCRIPTION
TECHNICAL FIELD
The invention relates to microphones and more
particularly to microphones comprising panel-form acoustic
elements.
~ACKGROUND ART
20It 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
-
CA 02230449 1998-02-2~
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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.
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 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
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
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CA 02230449 1998-02-2~
"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 microphones.
Members as above are herein called distributed mode
acoustic radiators and are intended to be characterised as
in the above PCT publication and/or otherwise as
specifically provided herein.
The invention is a panel-form microphone characterised
by a stiff lightweight 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
wholly and exclusively on said member at one of said
locations or sites to produce a signal in response to
resonance of the member due to incident acoustic energy.
The member may be mounted in a surrounding frame by means
of an interposed resilient support. Two or more of the
said transducers may be positioned at the locations or
sites on the member. The member may have a cellular core
sandwiched between skins. The or each transducer may be a
piezo-electric device.
AMENDE~ Sl tEET
' CA 02230449 1998-02-2~
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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 Za 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 an embodiment of distributed-
mode microphone according to the present inventi~on, and
Figure 4 is a perspective view of a vibration
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) round its
inner periphery which supports a distributed mode sound
radiating panel (2). A transducer (9) as described in
detail with reference to our co-pending International
~flEN~ED ~E~
CA 02230449 1998-02-2~
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 v, 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 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
A~A~ ,'~F~
.
CA 02230449 1998-02-2~
- W O 97109862 PCT/GB96/02155
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, Kevlar or the like in a manner known ~er 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 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
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- W O 97/09862 PCT/GB96/02155
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 wouid
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 stif~ness 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
CA 02230449 1998-02-2~
W O 97/09862 PCT/GB96/~2155
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.
Tn 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
CA 02230449 l998-02-2~
- W O 97/09862 PCT/GB96/02155
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
S 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 fre~uency 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.
A panel as described above is a good receiver of sound
which appears as acoustic vibration over the panel. A
preferably lightweight panel structure aids sensitivity and
the vibration may be sensed by one and preferably more
simple bending transducers e.g. of the piezo variety as
described in Figure 4 below. A plurality of transducers
and transducer placement positions optimises the ~uality of
coupling from the distributed panel vibrations to the
desired electrical output signal. Placement should be in
position(s) of high modal density, inboard of the panel,
while the panel itself should have the preferred actual or
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- W O 97/09862 PCT/GB96/0215S
equivalent geometry for good modal distribution.
Sound energy incident on the panel is converted into
free mode vibration. This vibration may be sensed by
optical or electrodynamic vibration transducers and the
result is a microphone. For non-critical applications a
single sensor is effective, placed at an equivalent,
optimised driving point.
For superior ~uality the non reciprocal nature of the
transduction principle must be considered. Two factors are
pertinent; firstly, some fre~uency dependent e~ualisation
to reach a flat frequency response, and secondly, the need
to capture a broader sampling of the complex vibrations of
the acoustic panel. A minimllm of three transducers is
indicated; they may be inexpensive piezo electric benders
with their outputs connected in parallel. Alternatively
larger area polymer piezo films may be applied, with
suitable geometric pickup patterning to define the
vibration integration areas for the required optimisation
of sensitivity versus fre~uency response.
For microphone applications it is advantageous that
the panel be light to provide the best match between the
radiation impedance of the air and the panel. Higher
sensitivity is achieved with lower mass panels. For a
single transducer the calculations for the theoretical
model indicates an optimal location at a panel corner since
all vibrational modes are 'voiced' at the corners.
Figure 3 illustrates a distributed mode panel (2)
according to the present invention e.g. of the kind shown
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W O 97/09862 PCT/GB96/02155
11 .
in Figures 1 and 2, intended for use as a sound receiver or
microphone. Although not shown in the drawing, the panel
(2) is mounted in a surrounding frame (1) and is attached
to the frame v a a resilient suspension (3) in the manner
shown in Figures 1 and 2. The frame is suspended on a pair
of wires (33), e.g. from a ceiling or on a floor standing
frame (not shown).
The panel carries an array of four vibration
transducers (63) spaced over the panel and which may be
piezo-electric transducers of the kind shown in Figure 4
below which are coupled in parallel to drive a signal
receiver and conditioner (65) connected to an output (66).
Figure 4 shows a transducer (9) for a distributed mode
panel (2) in the form of a crystalline disc-like piezo
bender (27) mounted on a disc (118), e.g. of brass, which
is bonded to a face of the panel (2), e.g. by an adhesive
bond (20). In operation an acoustic signal applied to the
transducer (9) via leads (28) will cause the piezo disc
(27) to bend and thus locally resiliently deform the panel
(2) to launch bending waves into the panel.
