Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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VIBRATION TRANSDUCERS FOR RESONANT PANEL-FORM LOUDSPEAKER
AND LOUDSPEAKER WITH THE SAME
DESCRIPTION
TECHNICAL FIELD
The invention relates to loudspeakers. More
particularly, but not exclusively, the invention relates to
electrodynamic vibration exciters for resonant panel-form
loudspeakers of the kind in which bending wave energy is
imparted to the resonant panel to produce an acoustic
output. Such loudspeakers are described in our
International application W097/09842.
A vibration exciter for exciting a resonant
loudspeaker panel needs a form of fixture onto the panel
surface allowing the best possible conversion of driver
power into bending waves. An inertial reaction exciter
applies a push/pull force to the panel by reacting against
the inertia of the exciter mass. Only the motor element is
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rigidly attached to the panel and the ideal fixture where
the panel has a cellular core and skin layers is a small
single position contact that is larger than the cell size
- of the panel core and with a stronger attachment than the
-5 core-to-skin bond strength of the panel.
BACKGROUND ART
An already proven system of construction bonds the
tubular voice coil assembly directly to the panel skin to
make a strong circular contact with the panel. The
resultant large contact area under the voice coil is,
however, effectively reactive at higher frequencies which
limits the power input to the panel as the modal dimensions
approach the size of the circular contact area.
U.S. 4,514,599 describes an exciter for driving a
vehicle trim panel for such as a rear parcel shelf. In one
embodiment an elastic member is provided between the trim
panel and a dish-shaped plate coupled to the voice coil.
The elastic member has three legs facing the dish-shaped
plate.
DE 2 434 005 describes a piezoelectric transducer in
which the passive element has a plurality of projection for
evenly supporting the active member.
It is an object of the invention to mitigate such
problems.
DISCLOSURE OF INVENTION
According to the invention there is provided a
vibration exciter for imparting bending wave energy to a
resonant panel comprising a tubular voice coil assembly
AIUEWDED SHEET
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having a circular end shape for coupling directly to the
panel, characterised in that the tubular voice coil
assembly comprises an array at least three castellations
spaced around the circumference of the voice coil assembly
-5 at its end for coupling to the panel at a plurality of
discrete coupling positions.
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area is reduced and the originally enclosed circumference
contact is opened to allow distributed mode bending waves
in the panel to pass under the exciter area. A significant
consequence of this is that the exciter 'sees' a resistive
mechanical load extending to higher frequencies.
The most mechanically stable configuration is three
equally spaced castellations and this is confirmed by
mathematical modelling. Changing the contact patch shape
from segments of circumference to straight lines further
increases the modal input to the panel. For convenience,
generally circular contact patches may be employed.
The above description teaches the principle of
discontinuous contact using a moving coil vibration exciter
as the example. The same principle might, however, be
applied to any form of vibration exciter, with an
interrupted or discontinuous form of physical connection
between the drive element and the panel.
Additional contact between the exciter and the panel
may be provided by a centrally disposed diametrically
extending member, e.g. of cruciform shape, positioned
within the basic triangular array.
BRIEF DESCRIPTION OF DRAWINGS
The invention is diagrammatically illustrated, by way
of example, in the accompanying drawings, in which:-
Figure 1 is a partial view of a prior art resonant
panel loudspeaker and showing the area of the mounting or
connection between a vibration exciter and the resonant
panel;
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Figure 2 is a partial view generally similar to that
of Figures 1 and illustrating a first embodiment according
to the invention;
Figure 3 is a cross-sectional side elevation of a
vibration exciter attached to a resonant panel and
corresponding to Figure 2;
Figure 4 is a view generally similar to that of Figure
1 and illustrating a second embodiment according to the
invention;
Figure 5 is a view generally similar to that of Figure
1 illustrating a third embodiment according to the
invention, and
Figure 6 is a cross-sectional side elevation of a
vibration exciter attached to a resonant panel and
corresponding to Figure 5.
In Figure 1 of the drawings, there is shown an
acoustic device (1) of the kind described in International
patent application W097/09842, comprising an acoustic
member (2) in the form of a resonant panel to which bending
wave energy is applied by a vibration exciter (not shown)
to cause the panel (2) to resonate to produce an acoustic
output. As described in W097/09842, the vibration exciter
may comprise a moving coil electrodynamic motor having a
voice coil directly attached to a face of the panel (2).
Annulus or ring ( 3 ) is intended to represent the contact
area or 'footprint' of the voice coil, or at least the
tubular former on which the coil is wound, on the face of
the panel (2). We have found that this method of
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attachment, which is intended to maintain the inertia of
the voice coil at a minimum in the interests of efficiently
transmitting bending wave energy to the panel, introduces
a secondary effect, which we know as the "aperture effect"
5 in which maximum frequency response of the panel is related
to the diameter of the ring or annulus ( 3 ) formed by the
joint between the exciter and the panel (2). We have
identified another, at least potential, effect in which the
portion of the panel within the ring or annulus (3) can
exhibit a secondary resonance, which we know as the "cap"
resonance.
The present invention seeks at least to mitigate
occurrence of the aperture effect and/or cap resonance.
BEST MODES FOR CARRYING OUT THE INVENTION
Referring now to Figures 2 to 6 of the drawings, and
more particularly to Figures 2 and 3 of the drawings, the
present invention provides mounting arrangements for
vibration exciters (4) (see Figures 3 and 6) on resonant
panels (2) such that the resonant behaviour of the panel
(2) is not interrupted or is less so than is the case with
the mounting footprint of the arrangement of Figure 1.
Thus Figure 2 shows a contact area or footprint for an
exciter on the panel in the ~orm of a circular array (5) of
discrete contact patches (6). The basic requirement for
stability is met by three contact patches forming a
triangle, but in the present embodiment there are eight
contact patches for increased strength. From Figure 3 is
will be seen that the exciter (4) is an inertial device of
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the general kind shown in Figure 11b of International
patent application W097/09842 and has a voice coil (21)
comprising a coil wound on a cylindrical former, one axial
end of which voice coil (11) being drivingly mounted on the
panel (2) to impart bending wave energy into the panel, and
a magnet assembly (12,13,14) surrounding the voice coil
(11) and comprising a magnet (12) sandwiched between a pole
piece (13) which fits inside the voice coil (11) and a
second pole piece (14) in the form of a cap which surrounds
the voice coil, the magnet assembly and the voice coil
being connected by a resilient suspension (15) which also
serves the function of a dust excluder.
An axial end (16) of the voice coil (11) is supported
on the panel in a generally annular mounting member (9)
which may be a plastics moulding. The end (16) of the
voice coil may be attached to the mounting member (9) by
means of an adhesive or alternatively the mounting member
may be directly moulded onto the voice coil. A side (17)
of the mounting member facing away from the exciter (4) is
formed with a series of short castellations or projections
(10) extending away from the side (17) and disposed to form
an annular array, the end faces (18) of the castellations
(10) forming the array of discrete contact patches (6)
which together define the footprint of the exciter (4) on
the panel (2). The exciter may be secured to the panel by
means of adhesive applied to the end faces (18) of the
mounting member (9).
The embodiment of Figure 4 is generally similar to
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that of Figures 2 and 3 with the exceptions that the number
of discrete contact patches (6) in the array (5a) is
increased and in addition the area defined by the circular
array (5a) is occupied by a centrally disposed additional
contact (29) in the form of a cross member in the interests
of improving the coupling between the exciter and the
panel. The cross shape of the contact (19) is chosen since
it is radially discontinuous, that is to say it does not
form a closed figure and thus does not substantially
adversely affect the bending wave behaviour of the panel as
may be the case with the continuous ring footprint of
Figure 1.
Figures 5 and 6 show an alternative embodiment of the
invention which is very similar to that of Figures 2 and 3
as modified by the embodiment of Figure 4. In the present
case the contact array (5b) is modified in that the solid
cross-like additional contact (19) of Figure 4 is replaced
by a cross-like array (20) of discrete contact patches
(21), further to reduce any interference to the bending
wave behaviour of the resonant panel (2) due to the exciter
mounting.
INDUSTRIAL APPLICABILITY
Division of the driving contact is thus a means for
improving termination of the mechanical junction between
the exciter and the panel. It may be adjusted by design to
tune the upper frequency properties of the panel radiation
to match the characteristics of the panel chosen. Thus
simple castellation may be chosen to couple from as few as
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three points with a contact area sufficient to reliably
bond the exciter to the panel surface. Different
configurations of contact geometry provide further scope
for fine tuning this interface where in some cases the
several factors of contact area, glue bond, panel loss,
aperture effect and cap resonance may interact with each
other.
Although not shown in the drawings, it will be
appreciated that instead of mounting the voice coil on the
panel via a castellated mounting member (9), it might be
possible instead to mount the voice coil directly on the
panel while still achieving the benefits of this invention.
In this case it would be necessary to castellate or
otherwise adapt the end of the voice coil former to provide
a ring of discrete mounting positions. Possibly the ends
of the castellations could be folded to form flanges to
increase the area of contact as a strengthening aid when
adhesively fixing the voice coil to the panel.
The invention thus provides a simple method of
reducing aperture and/or cap effects in fixing vibration
exciters to resonant panels. While the invention has been
particularly described with reference to moving coil
exciters, it will be appreciated that the method can be
applied also to other (non-voice-coil) type exciters to
achieve the same benefits.
