Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
CA 02230234 1998-02-23
W O 97/09848 PCT/GB96/02165
TTTT~
NOTICEBOARDS INCORPORATING LOUDSPEAKERS
DESCRIPTION
TECHNICAL FIELD
The invention relates to noticeboards and the like
visual display apparatus.
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
the panel is suc~ 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
to it a supporting body, in a free undamped manner;
~ CA 02230234 1998-02-23
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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-3,247,925 of WARNAKA discloses what purports to
be a low frequency resonant panel loudspeaker mounted in a
chassis and excited by an electromechanical transducers
mounted on the chassis.
DISC~O~URE OF INVENTIQN
- 10 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.
W097/09842 of even date herewith for such members as or in
AMENDED SHEET
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"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 visual
, . .
-10 display apparatus incorporating acoustics devices e.g. 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 visual display apparatus comprising
a notice or the like board to which notices or the like can
be fixed e.g. with pins, characterised in that the notice
--or the like board is 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
wholly and exclusively 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
AMENDcD S~{E' I
j ~ CA 02230234 1998-02-23
acoustic output when resonating. The radiator may have a
cellular core sandwiched between skin layers. The skins
may comprise paper. The core may comprise paper honeycomb.
The visual display apparatus may comprise a frame
surrounding the radiator. A resilient suspension may be
employed to mount the radiator in the frame. The frame may
have a return lip concealing the suspension. The
transducer may comprise a piezo electric bender.
BRIEF DESCRIPTION OF DRAWINGS
The invention is diagrammatically illustrated, by way
of example, in the accompanying drawings, in which:-
AMEi~D.D SHEErT '~
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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
~ 5 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 perspective diagram of a first
embodiment of noticeboard according to the present
invention;
Figure 4 is a perspective view of a second embodiment
of noticeboard according to the present invention;
Figure 5 is a partial cross-sectional view of the
noticeboard shown in Figures 3 and 4, and
Figure 6 illustrates one form of transducer.
BEST MODES FOR i-ARRYING 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) 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
A'~ S~
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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 beïng 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
sklns 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.
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)
r~
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W O 97/09848 PCT/GB96/02165
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 _ 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.
Z5 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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W O 97/09848 PCT/GB96/02165
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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W O 97/09848 PCT/GB96/02165
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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W O 97109848 PCT/GB96102165
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
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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W O 97/09848 PCT/GB96/02165
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
S 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.
The acoustic panel technology of Figures 1 and 2 can
be applied to the design of normal board material commonly
used for notices and display advertising, the normal
processing of these boards e.g. lamination, screen printing
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11
or spray painting, being unhindered.
The board itself may thus be specified, dimensioned
and electrodynamically driven to operate as a wide coverage
acoustic radiating panel a flat loudspeaker. As such, a
moderate audience of up to 10 persons for a small O.S6m
square metre panel or 30-50 persons for a 0.7 to 1.2 square
metre size may be served at natural volume levels with
highly articulate reproduction of speech and also
background music or relevant sound effects, as required.
Due to the naturally dispersed acoustic radiation
properties of the panel, the distorting effects of local
boundary reflections are minimised. A substantial physical
obstruction of a proportion of the acoustic panel does not
significantly impair the sound distribution.
Lower sound levels and lower levels of electrical
input power are required for a given level of
articulation/intelligibility.
In a preferred form the acoustic panel would comprise
low cost structure of paper honeycomb or foam core 3 to 6mm
thick bonded to reinforcing paper, plastic film or plastic
film coated paper skins of 0.08 to 0.3mm thickness. The
preferred drive for an inexpensive design using a flat
surface or even embedded transducer, is piezo electric,
operated in bending or mass loaded, vibration coupled
forms.
- These transducers have a naturally rising acoustic
output with frequency which is predictably and optimally
adjusted to a flat response by a combination of series
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W O 97/09848 PCT/GB96/02165 12
resistive drive to the capacitance load of the transducer,
and also by mass loading of the piezo element (for a given
mass the acceleration is inversely proportional to
freguency) and via the detailed specification of the
mechanical properties of the board. By control of the
fibrous loss factor in the skins, the visco elastic
properties of the adhesive bonding the skin to core, and
the piezo element to the skin by applied surface laminates,
including the backing for the displayed image or text, and
the bulk properties of the core in shear and compression,
the desired frequency response may be achieved.
Where higher sound levels and larger panels are
concerned, alloy or part alloy skins will provide good
energy coverage over the panel, with a proportionally lower
loss factor which appropriately matched to a higher power
moving coil type of transducer. The latter has a flat
region of frequency response. Where a flat, uninterrupted
surface is required on both sides of the panel, the
transducer may be of the embedded type concealed within.
If magnetic screening is required for the latter, thin
0.5mm mild steel foils may be included under the skin
surface over the transducer area. A small gain in
efficiency will also result due to improved flux linkage.
Small, low cost example of the notice board will not
require specific framing or specified damping. If of the
desk or counter top form, sufficient damping is provided by
the simple contact of the lower edge of the acoustic panel
with the surface it is placed on. The device would have a
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W O 97/09848 PCT/GB96/02165 13
photo frame type of back ~est, effective even when
fabricated in light card.
Certain classes of stiff foamed plastic, e.g.
unplasticised PVC, have appropriate bulk properties either
self skinning or unskinned, to operate within the acoustic
panel theory range. These can be used directly as acoustic
panels of this type without additional stiffening skins.
Figures 3 to 5 illustrate a noticeboard, advertising
display board or the like (48) incorporating the
loudspeaker technology of Figures 1 and 2. Thus a
loudspeaker (81) incorporating a rigid lightweight
distributed mode acoustic radiator panel (2) of the kind
shown in Figures 1 and 2 has graphic information, e.g. text
and/or pictures of any desired kind printed on the panel
(2). The graphic information can then be supplemented by
an audio message v a the loudspeaker to reinforce the
message.
As shown in Figure 3 the loudspeaker/display board
(48) is mounted on a stand (23) in the form of an easel.
Alternatively the loudspeaker/display board may be
suspended on wire (33) as shown in Figure 4. Alternatively
the noticeboard may be supported in any other desired
manner.
As shown in Figure 5 the frame (1) may incorporate a
return lip (41) to conceal the resilient suspension (3)
- which is attached between the rear face thereof and the
peripheral margin of the panel (2).
Figure 6 shows a transducer (9) for a distributed mode
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W O 97/09848 PCT/GR96/02165 14
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
S 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.
