Note: Descriptions are shown in the official language in which they were submitted.
CA 02349861 2001-05-04
WO 00/Z8781 PCTIGB99103656
ACOUSTIC DEVICE ACCORDING TO BENDING WAVE PRINCIPLE
DESCRIPTION
FIELD OF THE INVENTION
This invention relates to acoustic devices of the type
that use members that support bending wave action over the
surface of the member, the bending waves in turn coupling to
the ambient. Such devices may be used, for example, as
loudspeakers or microphones.
BACKGROUND TO THE INVENTION
The International patent application W097/09842 and
related applications describe speakers and other acoustic
devices having an acoustic member and a transducer coupled
to the acoustic member. In these devices, the various
parameters of the member may be adjusted so that resonant
bending wave modes in the member are distributed evenly in
frequency. The resonant bending wave modes may also be
distributed over the surface of the member. Preferential
locations for mounting the transducer on the member are
also disclosed. A typical preferential mounting location
CA 02349861 2001-05-04
WO OO/Z8781 ~ PCT/GB99f03656
2
is at a near-centre location, but not at the centre.
However, other preferential locations may also be available
depending on the shape of member.
However, it is not always easy to provide sufficient
modal density, especially at lower frequencies.
Accordingly, it would be advantageous if improved modal
density or other enhancement could be provided, especially
to the lower or mid frequency response.
SUMMARY OF THE INVENTION
According to a first aspect of the invention, there is
provided an acoustic device, comprising a substantially
continuous outer shell supporting bending waves, the shell
being bent to at least partially enclose an air volume so
that the bending waves couple to the volume to provide
coupled resonant modes, and a transducer coupled to the
shell for coupling an electrical signal in the transducer
with the coupled modes and hence in turn to ambient sound..
In the device according to the invention, in addition
to the resonant bending wave modes available in
conventional distributed mode devices, additional modes are
present. Calculations will be presented later showing the
increased number of modes with air coupled to the outer
shell. Accordingly, the device according to the invention
may increase the number of modes present in a predetermined
frequency range.
Preferably, the coupled resonant modes are evenly
distributed in frequency over a predetermined frequency
CA 02349861 2001-05-04
CVO 00/28781 PCT/GB99f93656
3
range. Usefully, this frequency range is around 1 to 2 or 3
octaves above the fundamental resonant frequency. It is in
this range that the resonant bending wave modes are most
sparse and in which distributing the bending and in-plane
modes gives the greatest benefit.
The outer shell may be fully closed to totally surround
the volume. Alternatively, ports or vents may be provided
in the outer shell. The ports or vents may be designed to
provide specific resonance effects, and in particular
enhance or control the output in the lower acoustic
frequency range.
Although it is not necessary that the outer shell fully
surrounds the enclosed volume the outer shell must be
substantially continuous so that it may demonstrate
effective acoustic action. In other words, the outer shell
must not have too many perforations or windows. Highly
perforate members may not be suitable as acoustic radiators,
since radiation from the front of the member will
destructively interfere with radiation from the rear that is
emitted in antiphase to radiation from the front. Moreover,
the shell should be sufficiently continuous for coupling of
the shell to the enclosed volume to be significant.
Tests on panels have shown very low coupling of
perforate members with ambient air. Accordingly, the outer
shell may define holes in its surface of total area no
greater than 20% of the surface area, preferably no greater
than 10% and further preferably no greater than 5%.
Air inside the volume may also exhibit cavity
CA 02349861 2001-05-04
-WO 00/Z8781 PCT/GB99f03656
4
resonances.
The acoustic device according to the invention supports
resonant bending wave modes over the surface of a three
dimensional shell and coupling to a volume at least
partially enclosed by the shell. In contrast, conventional
distributed mode loudspeakers have resonant bending wave
modes distributed over a single panel.
In the aforementioned W097/09842 it is suggested to
mount a distributed mode panel to the front of a frame. In
such prior art devices any resonant bending wave modes are
substantially restricted to the panel area, not the frame.
Accordingly, such devices do not provide the improved modal
density and hence acoustic performance offered by devices
according to the invention.
Another prior publication, WO 98/31188, describes a
flat panel mounted in a tray. The tray is highly perforate,
with more window area than solid area, and accordingly not
substantially continuous. The tray will therefore not
couple effectively to the ambient nor to the air within the
tray.
The outer shell may be of constant thickness.
Alternatively, the thickness of the outer shell may be
varied, either slowly or continuously or more rapidly.
Ribs or other extensions may be provided on the outer
shell.
The shell may be a single integral shell.
Alternatively, the outer shell may comprise a combination of
members mechanically coupled to create the desired
CA 02349861 2001-05-04
WO 00/28781 PCT/GB99/03656
_..
acoustical radiator structure. The boundary condition at
the joins between the members may usefully be specified.
The outer shell may be in the form of a polyhedron or
part thereof having a plurality of individual faces.
5 Each face may have a natural resonant frequency and
the natural resonance frequencies may be selected to have
different values. This can increase the modal density of
the outer shell as a whole. Further, the different natural
resonant frequencies may be selected so that the resonant
modes on different faces are at interleaved frequencies.
This approach may be particularly useful to increase the
modal density for the lower 10 to 20 resonant modes.
The individual faces or separate panel members need not
have uniform mechanical properties and they may vary in
stiffness, isotropy of stiffness, damping, or thickness.
The acoustic device may be a loudspeaker, the
transducer being an exciter.
The acoustic device may comprise front and rear faces
in the form of panels together with at least one further
panel providing a path for resonant modes from front to rear
and so coupling the front and rear panels. The front and
rear panels may be substantially planar. The front and rear
panels may be driven by separate discrete exciters, or a
single exciter may be coupled to both the front and the rear
panel.
In embodiments, the voice coil of an exciter may be
coupled to one of the panels and the magnet assembly of the
exciter to the other of the panels. Since the magnet
CA 02349861 2001-05-04
- W0 00/28781 PCT/GB99/03656
6
assembly is heavy, the coupling of it to the panel will
result in high frequency roll-off. This may enhance the
bass response of the acoustic device. A plurality of
exciters may be provided. The exciters may be driven in-
s phase, out of phase or in any suitable phase relationship to
one another.
In a conventional flat panel loudspeaker any in-plane
compression waves produce little or no acoustic output.
This is because in-plane compression and expansion of a flat
panel does not couple to ambient air. In contrast, in a
device according to the invention the outer shell is bent,
so in-plane compression and expansion causes shrinking and
expansion of the shell, locally or globally, which acts as a
mechanism to couple the compression waves to the enclosed
volume and to the ambient. Accordingly, in-plane
compression waves may usefully. contribute to the coupled
modes. In fact, the resonant modes may in some embodiments
couple bending waves, in-plane modes and the enclosed
volume. The modal density may accordingly be improved.
According to a second aspect of the invention, there
is a provided an acoustic device, comprising a
substantially continuous shell enclosing a volume,
supporting a plurality of resonant modes coupling the shell
and the enclosed volume, the resonant modes spanning the
shell, and a transducer coupled to the shell for coupling
an electrical signal in the transducer with the resonant
modes and hence in turn to ambient sound.
In the device according to the second aspect of the
CA 02349861 2001-05-04
-WO 00/28781 PCT/GB99/03656
7
invention, the resonant modes span the surface, from front
to back, side to side and top to bottom. This allows good
coupling of the modes over the whole surface with the
enclosed volume. It is not necessary that the modes cover
the whole surface; the shell may for example have ports or
areas that do not resonate.
According to a further aspect of the invention there is
provided a method of driving an acoustic device comprising
a substantially continuous outer shell supporting bending
waves, the shell being bent to at least partially enclose
an air volume so that the bending waves couple to the
volume to provide coupled resonant modes, and two
transducers coupled to the shell, including driving the two
transducers in phase with a common electrical signal, so
that the transducers drive the coupled modes of the shell
and volume in a monopole configuration, and radiating sound
energy from the coupled modes into the ambient air.
BRIEF DESCRIPTION OF THE DRAWINGS
Specific embodiments of the invention will now be
described, purely by way of example, with reference to the
accompanying drawings in which:-
Figure 1 shows sections through a loudspeaker according
to a first embodiment of the invention having an ellipsoidal
shell;
Figure 2 shows a section through a loudspeaker
according to a second embodiment;
Figure 3 shows a section through a loudspeaker
CA 02349861 2001-05-04
WO 00/28?81 PCT/GB99f03656
8
according to a third embodiment having a port;
Figure 4 shows a section through a modification of the
port;
Figure 5 shows a view of a loudspeaker according a
fourth embodiment to the invention in the form of an open
box;
Figure 6 shows a view of a fifth embodiment of the
invention having a closed box;
Figure 7 illustrates various excitation techniques that
can be used with the loudspeaker shown in Figure 6;
Figure 8 illustrates the response to excitation in an
open box without air;
Figure 9 illustrates the response to excitation in the
open box shown in Figure 8, including the effects of air
coupling;
Figure l0A shows the velocity response at the exciter
as a function of frequency for the box modelled in Figure 8,
without air;
Figure 20H shows the velocity response at the exciter
as a function of frequency for the box modelled in Figure 9,
including the effects of air;
Figure 11 shows the pressure inside the box of Figure
9B
Figure 12 shows the modes in a loudspeaker having a
closed box with two exciters driven in antiphase;
Figure 13 shows the modes in the loudspeaker modelled
in Figure 12 with two exciters driven in phase;
Figure 14 shows the velocity response of the device
CA 02349861 2001-05-04
WO 00/28781 ~ PCT/GB99l03656
9
modelled for Figures 12 and 13;
Figure 15 shows the velocity response of a six sided
closed box in which the bending stiffness of the front face
does not match that of the rear face;
Figure 16 shows views of a loudspeaker according to the
invention in the form of a truncated pyramid;
Figure 17 shows a view of a loudspeaker according to
the invention in the form of a tetrahedron;
Figure 18 shows a view of a loudspeaker according to
the invention in the form of a dodecahedron;
Figure 19 shows a view of a loudspeaker according to
the invention in the form of a cylinder;
Figure 20 shows a view of a loudspeaker according to
the invention in the form of a cone section;
Figure 21 shows the root mean square (rms) central
difference of mode frequency as a function of aspect ratio
of the front face of the device of Figure 5,
Figure 22 shows velocity profiles for three exciter
positions of the device of Figure 5 having a front face
aspect ratio of 2:1; and
Figure 23 shows a figure of merit for exciter location
as a function of exciter position on the device used in the
model for Figure 22.
DETAILED DESCRIPTION
Referring to Figure 1, a closed ellipsoid shell (1)
encloses a volume (2) and has transducers (3), (5) mounted
on the interior of the shell at opposed positions on the
CA 02349861 2001-05-04
-WO 00/28781 PCT/GB991fl3656
minor axis of the ellipsoid. The shell (1) supports
resonant modes formed from resonant bending wave components
coupled to the enclosed volume.
The transducers couple an electrical signal to the
5 coupled resonant modes of the shell and the volume . In the
present embodiment, the transducers are exciters than can,
in use, be driven to excite coupled modes to produce an
acoustic output. The transducer may be of conventional type
in which a voice coil moves relative to a grounded magnet
10 assembly when electrical current is passed through the voice
coil. The transducer may be inertial, in which case the
magnet assembly is free and the force of the voice coil acts
against the inertia of the magnet assembly. Alternatively,
a grounded transducer may be used in which case the magnet
assembly is supported. In the present embodiment,
commercially available exciters normally used to drive
distributed mode panels are used in an inertial
configuration.
By driving the transducers in a known polarity it is
possible to produce a desired polar behaviour in emitted
sound. For a monopole source, the transducers can be driven
in phase, whereas to produce a dipole the transducers can be
driven in antiphase. Alternatively, the exciters can be
driven at any suitable phase relation.
The invention allows a loudspeaker to be used
unbaffled. In a conventional pistonic or distributed mode
loudspeaker using a single diaphragm or panel the sound
radiated from the rear is in antiphase with the sound
CA 02349861 2001-05-04
WO 00/28781 PCT/GB99f03656
11
radiated from the front. Accordingly, to avoid interference
effects the sound radiated from the rear has to be prevented
from reaching the front, by enclosing the diaphragm in a box
or providing a baffle around the loudspeaker. By driving
the loudspeaker according to the present invention as a
monopole with two transducers operating in phase it is
possible to avoid the need for such baffles.
The coupled modes may be made up of two types of shell
vibrations coupled to the enclosed volume. One of these
l0 types is bending waves that bend the shell out of the local
plane of the shell. The other type is an expansion or
contraction in the plane of the shell.
A totally flat plate would not provide such coupled
expansion-contraction modes with the resonant bending wave
modes. Although a flat plate can have oscillation modes of
expansion and contraction, these simply move the plate in
its own plane and do not affect the motion of ambient air
molecules. Accordingly, such modes in a flat plate have
little or no acoustic effect. In contrast, if the plate is
sufficiently bent back in on itself, or even forms a
completely closed body, the in-plane compression wave modes
cause a global expansion and contraction of the body, which
can couple to air and hence have an acoustic effect.
The actual modes of vibration of the shell, need not be
pure bending wave modes, nor pure compression wave modes.
Rather, the modes may interfere and couple with one another
to provide coupled modes. These modes may still however
retain a predominant bending wave character. These waves in
CA 02349861 2001-05-04
WO 00/28781 PCT/GB99Ifl3656
12
the shell then couple to the included volume to produce
coupled resonant modes.
It is not essential that the transducers are mounted on
the minor axis. It may be convenient to mount them off
axis, as illustrated in Figure 2, or indeed at any suitable
location. It is preferred to mount the transducers at a
position that is selected for optimum or desired response.
The use of a regular geometry, such as an ellipsoid, makes
this easier. Alternatively, approaches such as finite
element analysis can be used to investigate suitable
transducer locations. In general, approaches similar to
those used for distributed mode loudspeakers may be
suitable; in particular asymmetric transducer locations may
prove suitable. An example of this will be discussed later
with reference to Figures 21 to 23.
A single transducer may be sufficient for some
applications; others may require several transducers spaced
over the shell. The placement of transducers may influence
the directivity of coupling of the outer shell to the
ambient.
The provision of an enclosed volume allows the use of
ports to control the resonance inside the volume. Figure 3
illustrates a port (7) in the form of a simple hole in one
end of the ellipsoid. Alternatively, a ducted port (9) may
be provided as illustrated in Figure 4.
The port may result in effects analogous to effects
that such ports produce in conventional pistonic-type
loudspeakers, or pipes. The ports may have an asymmetric
CA 02349861 2001-05-04
WO 00/28781 PCTIGB99I03656
13
cross-section.
As indicated above, it is not necessary that the volume
is wholly enclosed by the shell. Rather, all that is
required is that the shell is .sufficiently bent back on
5~itself that resonant modes in the shell couple to the air in
the enclosed volume so as to produce an acoustic effect.
Figure 5 illustrates an open box comprising a large front
face (11) surrounded by a frame (13) comprising four side
faces (15) at right angles to the front face (11). A single
transducer (3) is provided, connected to an amplifier via
audio connections (9). The side faces (15) are all
acoustically coupled to the front face (11). Resonant
bending wave modes in the front face (11) do not simply
remain in the front face but couple round to the side faces
(15) .
A box can also be implemented in the form of a sealed
enclosure (Figure 6), with front and rear faces (il), (17)
and four side f aces ( 13 ) j oining the front ( 11 ) to the rear
(17) face to form a sealed enclosure containing a volume.
Two transducers (3), (5) are provided, one on each face
(11) , (17) .
Also shown in Figure 6 is an electrical circuit (19)
that can switch between inverting and non-inverting drive of
the two transducers. A double pole double throw switch (21)
switches between a parallel drive of the transducers and an
anti-parallel drive.
The transducers or front and rear faces can be
uncoupled, as shown in Figure 7A. Alternatively, the magnet
CA 02349861 2001-05-04
WO 00/28781 PCT/GB99Ifl3656
14
assemblies of two conventional moving coil transducers can
be coupled together as shown in Figure 7B. As a further
alternative, a single transducer can have its voice coils
connected to the front face (11) and the magnet assembly
connected to the rear face, (17). The magnet assembly is
much heavier than the voice coil , and so this assembly will
preferentially couple low frequencies to the rear face.
Accordingly, this arrangement can be used to increase the
bass response of a loudspeaker. Front and rear faces may be
reversed.
Finite element calculation of the response of an
acoustic device has been performed for a five-sided device
similar to that shown in Figure 5. For convenience, this
configuration will be referred to as an open box. Figure 8
shows the behaviour of the box in response to excitation, in
the absence of air, at 178Hz (Figure 8A), at 348Hz (Figure
8B) and at 1000Hz (Figure 8C). Figure 9 shows the behaviour
in the presence of air at the same frequencies, i.e. at
178Hz (Figure 9A), at 348Hz (Figure 9B) and at 1000Hz
(Figure 9C). As can be seen, the responses are not
restricted to any one of the planar surfaces, but instead
couple over the whole of the five surfaces of the box.
Moreover, the presence of air beneficially adds to the
complexity of the shapes.
The velocity response at the transducer as a function
of frequency is shown in Figure 10: Figure l0A shows the
results without air and Figure lOB shows the results with
air. A large value indicates a high velocity achieved by
CA 02349861 2001-05-04
-WO OO/Z8781 PCT/GB99/43656
excitation at that frequency. Particularly large velocities
occur at resonance. As can be seen, the response with no
air shows a smaller number of larger peaks. This is
characteristic of a smaller number of resonant modes. The
5 response when there is air enclosed shows a larger number of
peaks, each of which is smaller. This is characteristic of
a larger number of weaker modes. As can be seen, the
coupling of the modes in the panel to the enclosed volume
increases the number of resonant modes and so improves the
10 acoustic device. What is surprising is that this effect is
so marked even in an open box.
The air pressure inside the box at 348 Hz is shown in
Figure I1. The asymmetric air pressure pattern can clearly
be seen. It is this air pressure distribution that causes
15 the complex mode shapes of the air coupled resonant modes.
Similar calculations have been carried out for an
acoustic device similar to that shown in Figure 6, i.e. a
closed six-sided box having a front, a rear and four side
faces. Some of these results are shown in Figure 12 and 13.
All the calculations include air. Again, the coupled modes
couple over all of the surfaces of the box, front and back,
left and right sides together with the top and bottom.
Figure 12A illustrates the mode at 178 Hz caused by
driving the closed box with the velocity in phase. Since
the front and rear faces face the opposite directions, this
is achieved with the transducer on the front panel moves the
panel outwards when the transducer on the rear panel moves
the panel inwards. This may be achieved by electrically
CA 02349861 2001-05-04
WO OO/Z8781 PC'flGB99f03656
16
connecting the transducers out of phase, for example using
the switch shown in Figure 6. Figure 12B shows the
oscillation at 1000Hz.
Figures 13A and 13B show the same frequency modes with
the box acting as a monopole with the front and rear panel
moving in antiphase, i.e. with the electrical connections to
the transducers in phase. As can be seen, a complex
response is again obtained.'
Figures 12C and 13C shows the air pressure inside the
box driven at 1000Hz as a dipole and a monopole
respectively, corresponding to the response of the box shown
in Figures 12B and 13B. Figure 12C clearly shows an
asymmetric response, even though the drive and the box are
symmetric. Figure 13C shows the very different pressure
response just caused by driving the same box in a different
way.
Some of the transducer velocity against frequency
graphs obtained with the closed box are presented in Figure
14. Figure 14A and 14B show that the response on the front
and rear faces (driven as a monopole) of a symmetrical
closed box match, as might have been expected. Figure 14C
shows the significantly less even, and hence worse, results
obtained for a dipole drive of the same box.
Of course, all the above results are just calculations
but they do show improvement possible using a shell bent
back on itself to enclose a volume.
Figure 15 shows the velocity response graph for a box
in which the front face has a different stiffness from the
CA 02349861 2001-05-04
WO 00/28781 PC1'/GB99/03656
17
rear, driven by two transducers one on the front (shown in
Figure 15A) and one on the rear face (Figure 15B). As can
be seen, the response of the front face is beneficially
different from that of the rear face. Accordingly, the use
of asymmetry can beneficially increase the modal density in
frequency.
It should be noted that there is a difference between
devices such as that illustrated in Figure 5 with a
plurality of faces, and devices such as that of Figure 1 in
the form of a continuous curve. The joins (23) between the
f aces act as hinges and so resonant bending wave modes do
not simply travel from one facet to the next. Rather, a
more complex coupling of the modes occurs.
Other multi-faced structures are also possible. Figures
16 to 20 illustrate various such forms, namely a truncated
square pyramid, a tetrahedron, a dodecahedron, a cylinder
and a cone section. Each of these forms may be open or
closed; for example the cylinder may be either with or
without end faces, and the cone section may have a rear
face, or not. The individual faces may be formed
separately, and then joined, or groups of faces or even the
whole structure may be integrally formed.
As discussed in W097/09842, good aspect ratios for an
isotropic rectangular panel are 0.707:1 and 0.882 to 1. It
is also possible to optimise acoustic devices according to
the invention to maximise the distribution of resonant modes
in frequency by adjusting the properties of the shell and to
provide a good even coupling of a transducer to the modes by
CA 02349861 2001-05-04
- WO OO/Z8781 PCT/GB99f03656
18
correctly locating the transducer on the panel.
This may be done using the techniques discussed in
various distributed mode patent applications. In
particular, the use of an orderly approach to finding
optimum aspect ratios and transducer locations to provide
results that are as good as possible has been described in
W099/41939, published 19 August 1999, in the names of New
Transducers Ltd, etc.
In order to find suitable properties for an open box,
the first step that was carried out was to model the
variation of the aspect ratio of the central panel in the
open box of Figure 5. The aspect ratio was varied from 1 to
2.25, and the corresponding frequencies of the modes were
calculated by finite element analysis. The root mean square
central difference of mode frequencies plotted against
aspect ratio (see Figure 21). The central difference of
mode frequency is, for the nth mode, the frequency of the
(n+1) th mode plus that of the (n-1) th mode, minus twice the
frequency of the nth mode. If the modes were equally
spaced, this measure would equal zero. Accordingly, the
root mean square (rms) central difference provides a Figure
of merit for various aspect ratios. The smaller the rms
central difference the better.
From Figure 21 it may be seen that good results are
obtained for aspect ratios between 1.6 and 2.2, with
especially good results between 1.95 and 2.05. An aspect
ratio of 2 was taken as a convenient value for further
study.
CA 02349861 2001-05-04
WO 00/28781 PCT/GB99/03656
19
The next stage is to find the optimised drive point on
the face. The velocity response as a function of frequency
is calculated for several drive point positions. Figure 22
gives three examples, at the centre (22A), at the standard
drive point for a flat distributed mode panel (22B) , and at
an optimum drive point (22C). The standard deviation for
graphs such as these is plotted as a function of position on
Figure 23. The best results are those with the smallest
deviation, shown in black. Note that the edges of the panel
are not shown - these are poor drive points.
As can be seen by inspecting the Figure the optimum
drive point occurs in four regions, located around 30% of
the distance along the long side and 30% of the distance
along the short side, together with three other regions
found by reflecting the first region about the central
symmetry axis, to locations around 70% along the long axis
and 30% along the short axis, 30% and 70% along the
respective axes, and 70% and 70% along the respective axes.
These positions are different from the optimum drive point
for a simple rectangle, which occurs near centrally at
coordinates around (3/7,4/9) expressed as a ratio of the
distance along the sides.
The positions are quite tolerant to variation and good
results are obtained at positions from 14% to 42% along the
long side and from 22% to 34% along the short side, together
with reflections of these values.
Although the above calculations are carried out without
the influence of air being taken into account, they provide
CA 02349861 2001-05-04
WO 00/28781 PCT/GB99f03656
good indications of suitable aspect ratios and transducer
locations even for a real device. Of course, features such
as air coupling or slight anisotropy of the faces may move
the optimum aspect ratios and drive positions slightly.
5 The embodiments described above relate to loudspeakers,
i.e. devices that convert electrical energy into sound. The
methods of the present invention are equally applicable to
microphones, in which incident sound energy is converted by
a transducer to electrical energy.
