Note: Descriptions are shown in the official language in which they were submitted.
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W O 97/09857 PCT/GB96/02164
MUSICAL INSTRUMENTS INCORPORATING LOUDSPEAKERS
DESCRIPTION
TECHNICAL FIELD
The invention relates to musical instruments and more
particularly to electronic keyboard musical instruments
incorporating loudspeakers.
BACKGROUND 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
~ 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 022301~4 1998-02-20
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to it a supporting body, in a free lln~mped 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.
WO-A-90/03025 of SCHIMMEL discloses a device for
actuating the soundboard of a piano so that the piano can
reproduce recorded piano music.
DISCLOSURE OF INVENTION
Embodiments of the present invention use members of
nature, structure and co~figuration 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
electronic keyboard musical instruments incorporating
active acoustic devices e.g. in the form of loudspeakers.
Members as above are herein called distributed mode
radiators and are intended to be characterised as in the
above PCT application and/or otherwise as specifically
provided herein.
The invention is an electronic musical instrument
having a keyboard and a loudspeaker, characterised in that
the loudspeaker comprises 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 vibrate the member to
cause it to resonate forming an acoustic radiator which
provides an acoustic output when resonating. The radiator
~ENDEDSHFET
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-, - 3a
may have a cellular core sandwiched by a pair of high
modulus skins. A frame may surround the radiator. A
resilient suspension may support the radiator on the frame.
The electronic musical instrument may comprise a body
having a base and legs supporting the body above the
ground, the radiator being positioned at the base of the
body. The loudspeaker is positioned with the radiator
substantially vertical. The frame may form a ground
support for the instrument.
r~3 ~
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BRIEF DESCRIPTION OF DRAWINÇS
The invention is diagrammatically illustrated, by way
of example, in the accompanying drawings, in which:-
Figure 1 is a diagram showing a distributed-mode
S 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 a first
embodiment of keyboard electronic instrument according to
the present invention;
Figure 4 is a plan view of the underside of the
instrument of Figure 3;
Figure 5 is a partial cross-sectional view of the
instrument of Figures 3 and 4, and
Figure 6 is a perspective view of a second embodiment
~0 of musical instrument according to the present invention.
BFST 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
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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
dimensions x and v, the position of which location is
calculated as described in our co-pending International
application 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, si,milar 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
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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
~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.
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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 fre~uency 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
~ 25 for an overall lightness and stiffness suited to a
particular purpose, specifically including optimising
contributions from core and skin layers and transitions
between them.
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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
5 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
10 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
15 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)
20 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
25 bitumen-based material, as commonly used in conventional
loudspeaker enclosures or may be of a resilient or rigid r
polymeric sheet material. Some materials, notably paper
and card, and some cores may be self-damping. Where
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desired, the damping may be increased in the construction
of the panels by employing resiliently setting, rather than
rigid setting adhesives.
Ef~ective 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 notstrongly 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
-~ 25 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
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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
Ob~A i nAhle,
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.
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11
Figures 3,4 and 5 illustrate an electronic keyboard
musical instrument (137), e.g. a piano, comprising a body
(138) supported on ground engaging legs (139). The body
(138) is formed with a keyboard (140) by which the
instrument is played.
In conventional fashion the instrument is provided
with a signal generator connected to the keys and with a
signal amplifier which drives a loudspeaker. In the
present embodiment the loudspeaker comprises a distributed
mode acoustic radiator (81) of the kind shown in Figures 1
and 2, incorporating a stiff lightweight rectangular panel
(2) mounted by its periphery in a frame (1) with an
interposed resilient suspension (3) and driven by a
transducer (9), all as described above with reference to
Figures 1 and 2. As shown, the loudspeaker (81) forms the
base of the body (138).
Figure 6 illustrates an electronic keyboard musical
instrument, (137) very similar to that of Figures 3 to 5
having a body (138) having a keyboard (140). The body is
supported on front legs (141) and is provided with a panel-
form loudspeaker (81) to form the back support of the
instrument.
The loudspeaker (81) comprises a rigid lightweight
distributed mode acoustic radiator panel (2) mounted by its
periphery in a surrounding resilient suspension (3) e.g. of
foam rubber, the suspension being supported in a baffle-
like frame (6), e.g. of medium density fibreboard. A
transducer (9)~ is attached to the panel (2) to launch
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bending waves into the panel to cause the panel to resonate
to produce an acoustic output. The arrangement is thus of
the kind shown in Figures 1 and 2. The transducer (9) will
be driven by an amplifier (not shown) which is connected to
receive signals produced by depression of the keys of the
keyboard, in generally conventional fashion.
