Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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5DISPLAY SCREENS INCORPORATING LOUDSPEAKERS
10DESCRIPTION
15TECHNICAL FIELD
The invention relates to display screens and more
particularly, but not exclusively, to projection screens.
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 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
to it a supporting body, in a free undamped manner;
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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-5,025,474 of MATSUSHITA discloses a projection
screen/loudspeaker combination in which the loudspeaker
comprises a box-like enclosure formed with ports so that
the loudspeaker operates as a bass-reflex speaker to
enhance its low frequency performance.
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 transducer
mounted on the chassis.
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 operational~ly active or moving part(s) thereof
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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
"active" acoustic devices with transducer means, such as in
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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 display
screens incorporating acoustic 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
10 in the above PCT application and/or otherwise as
specifically provided herein.
The invention is a display screen comprising a panel
having a light reflective or light emitti~g surface,
characterised in that the screen comprises a member having
15 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
20 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 acoustic output when resonating. The radiator
25 may comprise a stiff lightweight panel having a cellular
core sandwiched between a pair of high modulus skins. The
cellular core may be of honeycomb aluminium foil. The
Jskins may be of fibre reinforced plastics. The display
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screen may comprise a frame surrounding the panel. A
resiLient suspension may mount the panel in the frame.
Panel-form loudspeakers may be attached to opposite sides
of the frame to provide left and right hand channel
information. The left and right hand loudspeakers may be
hinged on the frame to be foldable against the radiator (2)
for storage. The left and right hand loudspeakers may each
comprise 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
acoustic output when resonating. The screen may be a
projection screen.
From another apsect the invention is audio visual
apparatus characterised by a projection screen. The audio
visual apparatus may comprise at least one rear channel
loudspeaker comprising 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
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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
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 perspective diagram of an embodiment of
projection screen according to the present invention;
Figure 4 is a partial view of a detail of the screen
of Figure 3, and
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Figure S is a plan view o~ a room incorporating the
projection screen of Figure 3.
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) e.g as described in
detail with reference to our co-pending International
publication Nos. W097/09859, WO97/09861, W097/09858 o~ even
date herewith, is mounted wholly and exclusivély 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
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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)
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
~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
synthetic plastics films of high modulus, such as Mylar
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(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
liguid 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
if of suitably low density. Some suitable core layer
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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
used for the edge mounting.
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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
area.
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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 st~n~ing
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
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
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stereo effect co~rAred 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 a multi-media audio-visual system
comprising a moving picture projector (31) arranged to
project an image onto a projection screen formed by a
loudspeaker panel (32) of the kind shown in Figures 1 and
2.
The loudspeaker/projection screen (32) comprises a
panel (2) having aluminium or carbon fibre reinforced skins
(21) sandwiching a honeycomb core (22) of aluminium foil.
The composite may be secured together using any epoxy
20adhesive. For a screen panel size of 1.22 x 1.38m, the
thickness of the aluminium skins may be 300 microns. The
core thickness may be llmm and the cell size of honeycomb
may be 9.5mm. Such a panel is stiff, of low density, high
modulus and is isotropic.
25A pair of smaller subsidiary loudspeakers (114) of the
kind described in Figures 1 and 2 are hinged on opposite
sides of the centre channel loudspeaker panel (32) by means
of hinges (34) whereby the subsidiary panels can be hinged
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against the primary panel (32) when not in use and can be
moved into the position as illustrated for use. The
subsidiary panels (114) are arranged to receive and radiate
respective left and right hand channel information, e.g.
S for stereo operation.
The subsidiary loudspeakers (114) may comprise panels
(2) having skins (21) of aluminium foil, or carbon fibre or
glass fibre reinforced plastics. A decorative film, e.g.
of polyester may be applied over one or both of the skin~.
The core (22) of the panels (114) may be of aluminium foil,
e.g. in a honeycomb cell arrangement, or may be of paper
cells. Where paper is employed it may be impregnated with
a plastics material such as a phenolic compound to improve
the stiffness of the paper. The cell size may be in the
lS range 3 to 6mm and the core thickness may be of the order
of 3 to lOmm. Where the skins are of aluminium foil they
may be 25 to 100 microns in thickness. An epoxy adhesive
may be used to assemble the panel.
Stereo, i.e. two channel sound reproduction, involves
the creation of sound stage illusion containing the
properties of source location, perspective and the ambience
of the original recording. Stereo with conventional
speakers is strong on aspects of phantom source location
and in some cases perspective, but is weaker in respect of
the expression of natural space and ambience. This is
because the near point source nature of conventional
pistonic speakers makes it easy aurally to identify their
physical location, which in conflict with the desire for
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overall stereo image localisation.
It is often said that as reproducing devices the
loudspeakers should disappear into the sound stage
illusion. Part of the problem lies in the relatively
narrow forward radiating directivity of conventional
speakers. In addition, the sound balance to the sides and
rear of the enclosure, sound which strongly drives the
reverberant sound field in the room, is coloured and
unbalanced with significant variations in frequency
response. This detracts from the sense of natural acoustic
space and ambience.
The embodiment of Figure 3 employs a pair of acoustic
panel speakers for left and right channels which are set in
complex vibration over the whole surface over a wide
frequency range typically lOOHz to 20kHz.
The primary loudspeaker panel (32) is shown suspended
on suspension means (33) but alternatively the panel may be
supported e.g. on a floor stand.
Figure 5 shows how the projection apparatus of the
present invention may be arranged in a room (14S) equipped
with seating (146). The apparatus has a projector (31)
projecting an image onto the screen (32) and also includes
a pair of subwoofers (35), which may be of conventional
construction, at the sides of the room to improve bass
audio extension and a pair of rear effect loudspeakers
(117) i.e. so-called ambience speakers, at the rear of the
room. Suitably the rear speakers (117) are also of the
kind shown in Figures 1 and 2 in view of their wide and
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even sound dispersion characteristics. The rear effect
loudspeakers may be of the same construction as the
subsidiary loudspeakers ( 114).
A panel loudspeaker according to Figures 1 and 2 has
remarkable non directional properties. For acoustic
reproduction of ambience channels of a sound system, the
energy must be widely distributed, ideally from non
directional sources. It is important that the sound source
is not well localised otherwise the perception of a large
ambient space, the simulated acoustic region behind the
listener, is unsatisfactory.
Hitherto conventional directional and/or small source
speakers, generally moving coil types, are used for
ambience reproduction. Due to the intensity phenomenon of
aural perception, audience members seated closer to a
nearby ambience speaker find their perception strongly
localised on that speaker greatly impairing the ambience
effect and their whole appreciation of the multichannel
sound field. The localisation may be so powerful that
aural attention is drawn away from the primary front stage
sound channels this working in conjunction with the Haas
effect which reinforces the localisation to proximate
sources.
An ambience reproducing system built with one or more
loudspeakers according to Figures 1 and 2 deliver a large
sound field or near uniform intensity which has
deliberately poor localisation. A large audience may be
handled, even with some persons in close proximity ~as near
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as 0.5m) to the panel loudspeakers without any significant
localisation of the ; m~e~ iate reproducing channel and with
r the vital property of an unimpaired aural perception of the
important front channels. Greatly improved realism is
5 achieved for the multi-channel sound reproducing system as
a whole as a result of the desirable radiating
characteristics of the acoustic panel sound reproducer.
The ambience loudspeakers may if desired be suspended
on wires and disguised, by the application of a suitable
10 image to the panel (2) to resemble pictures.
Figure 4 shows how the frames (1) of the
projection/loudspeaker panel may be formed with a return
lip (36) whereby the suspension (3) can be concealed. The
frames of the subsidiary loudspeakers (114) and the
15 ambience loudspeakers (117) may be similarly formed.
INDUSTRIAL APPLICABILITY
An acoustic panel build to sufficient size to serve as
a projection screen for still, film and video images, is
thus simultaneously a sound reproducer for example for the
20 centre or dialogue channel of home theatre. Uniquely,
acoustic panels according to the present invention of good
size, say over 0.6m wide, provide very good sound coverage
for audiences. Working demonstrations have shown high
intelligibility and sound clarity over the whole audience
25 region with a major advantage that persons nearest to the
screen do not suffer blasting from excessive proximate
sound levels, invariably a flaw of conventional direct
radiating cone based speakers.
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There is a second and unique aspect of a projection
screen of the invention. With conventional centre channel
speakers the ear is easily capable of locating the acoustic
centre of the speaker. All sounds appear to come from this
concentrated small source, detracting from the sense of
realism. With the acoustic panel, its uniquely non-
directional radiation property means that the sound appears
to come from the general acoustic region of the screen but
not from one isolated point. When the image is combined
with sound on the panel, there is a powerful synaesthetic
effect. Here the desirable lack of specific sound source
localisation allows the ear/brain sensing combination
freely to associate an imagined, virtual and approximate
location for the sound sources, synchronised with the
locations presented by the visual image on the acoustic
surface.
With well recorded dialogue sections, not only does
the virtual acoustic image appear to track the visual
image, it can also convey the information needed for the
perception to depth and perspective. The quality of
audience involvement in the cinematic experience is
substantially enhanced.
