Note: Descriptions are shown in the official language in which they were submitted.
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TITLE:
LOUDSPEAKERS COMPRISING PANEL-FORM ACOUSTIC RADIATING ELEMENTS
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
working frequency band for the loudspeaker.
There is a wide application for sound distribution
using speakers in stAn~Ard module form, interchangeable
with commercial ceiling tiles, generally on a 600 x 600mm
format, the objective being the even distribution of
articulate speech and music over a large area. Some
conventional moving coil drivers and panel derivatives are
presently made for this application.
Existing technology uses cone type moving coil
speakers fitted into frames and acoustic baffles. While
commonly used due to moderate cost and ready availability,
these suffer from serious hot spot (excessive sound
intensity) and directional effects and consequently poorer
intelligibility off axis. Many units are required to give
a uniform coverage over larger area.
Another known development uses a cone type speaker
where the 'cone' is a polystyrene structure with a flat
front surface, which may be painted. Here a combination of
additional moving coil drivers fitted to their diaphragm,
the latter structured to shrink acoustically with
increasing frequency, may give a wider radiation pattern
than a conventional cone speaker. These polystyrene foam
speaker units require chassis and acoustic baffles for
mounting them in position.
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US--A--4,506,117 of MULTIPHONIE discloses a suspended
ceiling panel provided with an electroacoustic transducer
having a metal base plate by which it is made solid with
the panel.
AMENDED SHFET
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DISCLOSURE QF 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 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 exposed to sound to be converted into
other signals.
CND~D SHFET
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'' .'' .,. .', ,.'
This invention is particularly concerned with active
acoustic devices in the form of loudspeakers the purpose of
use in a suspended ceiling tile.
Members as above are herein called distributed mode
radiators and are intended to be characterised as in the
said PCT application and/or otherwise as specifically
provided herein.
The invention is a suspended ceiling tile
incorporating a loudspeaker, characterised in that the tile
comprises 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 suspended ceiling
tile may be characterised in that the member comprises a
cellular core, e.g. of foamed plastics, sandwiched by high
modulus skins. A resilient suspension may be disposed at
the periphery of the radiator to support the radiator in a
suspended ceiling frame. The transducer may be an inertial
vibration transducer.
BRIEF DESCRIPTION OF DRAWINGS
The invention is diagrammatically illustrated, by way
~F~C~ T
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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 2k is an enlarged cross-section through a
distributed mode radiator of the kind shown in Figure 2a
~ ~ r ~ r ~
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and showing two alternative constructions;
Figure 3a is a perspective diagram of a room
incorporating a suspended ceiling, and
Figure 3k is a cross-sectioned side view of an
embodiment of distributed-mode loudspeaker according to the
present invention in the form of a ceiling tile.
BEST MODES FOR CARRYING OUT T~E 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 application No.
(our case P.5711) 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 applications Nos. (our cases P.5683/4/5) 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
application No. (our case P.5711) 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
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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
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
E~ se to increase their modulus.
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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), Raptan (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
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
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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
S 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
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
rsdiation or stray magnet fields. Conventional speakers
have a large magnetic field, up to 1 metre distant unless
specific compensation counter measures are taken.
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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
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
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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
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
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11
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.
Figure 3 illustrates a ceiling tile (36) of the kind
adapted to be supported in a grid-like suspended frame (99)
to form a suspended ceiling, and which is formed as a
loudspeaker (81) of the kind shown in Figures 1 and 2, that
is to say comprising a stiff, lightweight multi-mode
resonating panel (2) having a core (22) enclosed by skins
(21) on both sides. The panel (2) is mounted at its
periphery on a resilient suspension (3) of foam rubber
which is supported on the frame (99)~ The suspension (3)
may be attached to either the panel (2) or to the frame
(99) by means of an adhesive, but the connection may be by
gravity alone. The panel (2) carries a transducer (9),
e.g. of the kind shown in Figures 7 to 12, to launch
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bending waves into the panel to cause it to resonate to
produce an acoustic output. The transducer (9) may be
positioned on the panel as described in our co-pending
International publication No. W097/09842.
S In a preferred example of good quality the acoustic
panel is made as an expanded polystyrene foam core of
typically lOOg/m3 density, 8mm thick, skinned with hardened
aluminium alloy skins of O.lmm. A soft foam or felt strip,
some 3mm thick is fixed to the perimeter to provide a
partially compliant mounting when placed in the ceiling
frames and also helps to suppress any possible vibration in
the ceiling framing sections.
A preferred form of excitations is a unitary moving
coil inertial transducer with a 25mm or 38mm voice coil, 6
ohms impedance, 40 watt power handling, with the coil
bonded directly to the panel surface. A compact cup type
magnet system enclosed and self sealing may also be bonded
directly to the panel via a resilient decoupling ring
chosen for its vibro-mechanical properties and dimensional
stability.
Depending on application, a low cost form ceiling tile
can be made with a plastics foam cored paper faced board
material, which may have a light alloy foil layer for fire
retardancy, driven by low cost piezo vibration exciters.
Reduced maximum sound levels are obtained, still more than
sufficient for personnel announcements, voice overs and
background music distribution. The wide area coverage is
maintained.
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13
When metallic or carbon conductive skins or cores are
employed the speaker may be earth bonded or grounded to
maintain EMC screening of an installed structure.
T~DUSTRI~T APPLI~ABILITY
A ceiling tile loudspeaker according to the present
invention does not require a frame, chassis, or acoustic
baffle. The entire speaker panel is unitary and may be
placed in position just like a passive decorative ceiling
tile. The acoustic panel is relatively lightweight,
reducing ceiling loadings and aiding installation. It may
readily be made fire resistant. It can be decorated,
painted or papered to render it invisible in a ceiling
installation without significant acoustic impairment.
Minor damage does not impair the performance as
compared with the diaphragms of cone type speakers which
are very fragile. Also important is the great advantage in
sound distribution given by the acoustic panel speaker.
Its combination of high intelligibility and wide angle
coverage means that in a typical large area installation
superior acoustic performance may be achieved with around
half the number of conventional installed loudspeakers,
with a great saving in installed cost.
