Note: Descriptions are shown in the official language in which they were submitted.
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TITr.~ PANEL-FORM LOUDSPEAKERS
DESCRIPTION
TECHNICAL FIELD
The invention relates to loudspeakers and more
particularly to loudspeakers comprising panel-form acoustic
radiating elements.
BACRGROUND 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
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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
worXing frequency band for the loudspeaker.
DI5CLOSURE 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 i~
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"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 panel-~orm loudspeaker having 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, and wherein the
loudspeaker is also a microphone and comprises a second
transducer mounted on the member at one of the locations or
sites to produce an output 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 suspension. The member may comprise
Al'fi~N~D SHEET
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a stiff lightweight panel having a cellular core sandwiched
between skins. The suspension may be attached to the edge
of the panel. The first and second transducers may be
mounted wholly and exclusively on the radiator. At least
two said second transducers may be provided at
predetermined preferential locations or sites on the
radiator. A further transducer may be provided on the
radiator at a predetermined location or site to produce a
signal in response to resonance of the radiator due to
incident acoustic energy, and means may be provided for
comparing the signal generated by the said further
transducer with that of those gene,rated by the;said second
transducer(s). The comparison means may comprise a signal
receiver and conditioner and signal output means.
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
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 loudspeaker microphone according to the present
invention, and
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4a ,.
Figure 4 is a perspective view of a piezo-electric
transducer.
BEST MODES FOR CARRYING O~T 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
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International publication Nos. W097/09859, WO97/09861,
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 t~ 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 fra~me 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
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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
~QE 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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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
contributions from core and skin layers and transitions
between them.
Several of the preferred formulations for the panel
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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 freguency 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
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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
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
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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
conseguently 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 a distributed mode panel (2)
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according to the present invention e.g. of the kind shown
in Figures 1 and 2, intended for use both as a loudspeaker
and as a sound receiver or microphone, e.g. for use in an
interactive environment. Although not shown in Figure 3,
the panel (2) is mounted in a surrounding frame (1) and is
attached to the frame via 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 is driven to resonate and produce an
acoustic output by a transducer (9) of the kind described
above with reference to our co-pending International
publication Nos. W097/09859, WO97/09861, W097/09.858 which
in turn is connected to and driven by an amplifier (10).
The panel also carries a pair of vibration transducers
(63) which may be piezo-electric transducers of the kind
shown in Figure 4 which are coupled in parallel to drive a
signal receiver and conditioner (6S) connected to an output
(66). Another vibration transducer (63) on the panel (2),
e.g. of the kind shown in Figure 4, is coupled to drive a
filter/correlator the output from which is fed to the
signal receiver and conditioner (65), to provide signal
correction.
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 (Z0). In operation an acoustic signal applied to the
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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.
INDUSTRIAL APPLICABILITY
The invention thus provides a simple
loudspeaker/microphone e.g. for use in an interactive
environment.
