Note: Descriptions are shown in the official language in which they were submitted.
CA 02283381 1999-09-03
WO 98139947 PCT/GB98/00621
1
TITLE . ACOUSTIC DEVICE
DESCRIPTION
FIELD OF THE INVENTION
This invention relates to acoustic devices capable of
acoustic action by bending waves and typically (but not
exclusively) for use in or as loudspeakers.
BACKGROUND TO THE INVENTION
Our co-pending PCT application no. GB96/02145 includes
general teaching as to nature, structure and configuration
of acoustic panel members having capability to sustain and
propagate input vibrational energy through bending waves in
acoustically operative areas) extending transversely of
thickness usually (if not necessarily) to edges of the
member(s). Specific teaching includes analyses of various
specific panel configurations with or without directional
anisotropy of bending stiffness through/across said areas)
so as to have resonant mode vibration components distrib-
uted over said areas) beneficially for acoustic coupling
with ambient air; and as to having determinable prefer-
ential locations) within said areas) for acoustic traps-
ducer means, particularly operationally active or moving
parts) thereof effective in relation to acoustic vibra-
tional activity in said areas) and related signals,
usually electrical, corresponding to acoustic content of
such vibrational activity. Uses are also envisaged in that
CA 02283381 1999-09-03
WO 98/39947 PCT/GB98/00621
2
PCT application for such members as or in "passive"
acoustic devices, i.e. without transducer means, such as
for reverberation or for acoustic filtering or for acoust-
ically "voicing" a space or room; and as or in "active"
acoustic devices with bending wave transducer means,
including in a remarkably wide range of loudspeakers as
sources of sound when supplied with input signals to be
converted to said sound, and also in such as microphones
when exposed to sound to be converted into other signals.
Our co-pending UK patent application no (P.5840)
concerns using features of mechanical impedance in
achieving refinements to geometry and/or locations) of
bending wave transducer means for such panel members as or
in acoustic devices. Contents of that U.K. patent applic-
ation and of the above PCT application are hereby incorp-
orated herein to any extent that may be useful in or to
explaining, understanding or defining the present
invention.
This invention arises particularly in relation to
active acoustic devices in the form of loudspeakers using
panel members to perform generally as above (and as may be
called distributed mode acoustic radiators/resonant panels
later herein), but further particularly achieve satisfact
ory combination of pistonic action with bending wave
action. However, more general or wider aspects of
invention arise, as will become apparent.
SUMMARY OF THE INVENTION
From a first viewpoint, this invention concerns active
acoustic devices relying on bending wave action in panel
members, particularly providing effective placements) for
bending wave transducer means different from specific
teachings of the above PCT and UK patent applications, i.e.
other than at locations) arising from analysis and prefer-
ence in that PCT application, even including at centres)
CA 02283381 1999-09-03
WO 98/3994'7 PCT/GB98/00621
3
of mass and/or geometry rather than off-set therefrom.
. From a second viewpoint, this invention concerns
acoustic devices relying on bending wave action in panel
members, particularly providing effective distributions of
resonant mode vibration that may be different from what
results from specific teachings and preferences of the
above PCT and UK patent applications even for the same
configurations or geometries.
From a third viewpoint, this invention concerns
acoustic devices relying on bending wave action in panel
members, particularly providing effective distributions of
resonant mode vibration in panel members of different
configurations or geometries from what are regarded as
inherently favourable in specific teachings and preferences
of the above PCT and UK patent applications.
It is considered useful to note that effective
specific embodiments of this invention utilise panel
members) intrinsically affording areal distribution of
resonant mode vibration components effective for acoustic
performance generally comparable or akin to the above PCT
and UK applications, essentially, relying on simple
excitement of such intrinsically areally distributed
acoustic bending wave action for successful acoustic
operation; rather than in any way resembling merely piece-
meal provisions for altering intendedly other acoustic
action in panel members) for which such intrinsic
distributed resonant mode action is not even a design
requirement indeed, usually where other particular
structural etc provisions are made to serve different
frequency ranges and/or selectively suppress or specific-
ally produce/superpose vibrations in a panel member that is
not intrinsically effective as in above PCT and UK patent
applications or herein, typically being inherently unsuit-
able as a matter of geometry and/or location of transducer
means.
CA 02283381 1999-09-03
WO 98!39947 PCTIGB98J00621
4
Effective inventive method and means hereof involve
area! distribution of variation in stiffness over at least
areas) of such panel members) that are acoustically
active in relation to bending wave action and desired
acoustic operation. As will become clear herein, such
variation can usefully be directly related effectively to
displacement of transducer means from locations as
specifically taught in the above PCT and UK patent
applications to different locations of this invention,
and/or, relative to such patent applications, to rendering
unfavourable configurations or geometries of panel members
more akin to favourable configurations or geometries for
acoustic operation involving area! distribution of resonant
modes of vibration consequential to bending wave action,
and/or with actual resonant mode distribution that may be
at least somewhat different, whether due simply to
different area! distribution of bending stiffness hereof or
to consequential different locations) for transducer
means, or both.
Specific teaching of the above PCT application extends
to panel members) having different bending stiffness(es)
in different directions across intendedly acoustically
active areas) that may be all or less than all of areas)
of the panel member(s), typically in or resolvable to two
coordinate related directions, and substantially constant
therealong. In contrast, advantageous panel members) of
embodiments) hereof have variation of bending stiff
ness(es) along some directions) across said areas) that
is/are irresolvable to constancy in normal coordinate or
any direction(s).
Area! variation of bending stiffness is, of course,
readily achieved by variation of thickness of acoustic
panel members, but other possibilities arise, say
concerning thickness and/or density and/or tensile strength
of skins of sandwich-type structures and/or reinforcements
CA 02283381 1999-09-03
WO 98139947 PCT/GB98/00621
of monolithic structures usually of composite materials)
type.
Whilst available practical analysis may not always
allow such investigation as precisely and fully to identify
5 and quantify changes in actual areal distribution of
acoustically effective resonant mode vibration for panel
members) hereof - even where having substantially similar
geometry and/or average stiffnesses in relevant directions
as for specific isotropic or anisotropic embodiments the
above PCT application - practical resulting performance
indicates little if any significant diminishing or
degradation in achieved successful acoustic performance
involving bending wave action, indeed encourages belief in
potential even for improving same. Beneficial effects (on
areal distribution of resonant mode vibration), of
basically favourable configuration/geometry of the above
PCT and UK patent applications can, however, be
substantially retained to very useful extent and effect in
two groups or strands of inventive aspects implementing
above one viewpoint.
One group/strand is as already foreshadowed,
specifically providing more convenient locations) for
transducer means in acoustically active panel members or
areas thereof having configurations or geometries known to
be favourable in isotropic or anisotropic implementations
of teachings of above PCT and UK patent applications,
effectively by displacing what are now called "natural"
locations for transducer means (in accordance with these
patent applications), to different locations hereof,
specifically by either or both of relatively greater and
lesser bending stiffnesses to one side and to the other
side, respectively, of such natural location(s). Region(sj
of greater bending stiffness serves) effectively to shift
such natural locations) away from such region(s),
typically from said one side towards said other side and
CA 02283381 1999-09-03
WO 98139947 PCT/GB98/0062I
6
regions) of lesser bending stiffness; regions) of lesser
bending stiffness serving to shift towards own region(s).
The other group/strand can be viewed as involving
capability only partially to so define same at least
notional sub-geometry of larger overall panel member
geometry not specifically favourable to good distributed
mode acoustic operation as in the above PCT and UK patent
applications; such sub-geometry being incompletely
circumscribed and not necessarily specifically so favour-
able of itself but the partial definition thereof having
significant improving effect on distributed mode acoustic
operation, say tending towards a type of configuration or
geometry known to include specific favourable ones if not
at least approaching such favourable ones; such improving
effect being particularly for distributing resonant modes
therefor at lower frequencies, but not necessarily (indeed
preferentially not) limiting higher frequency bending wave
action and resonant mode distribution to such sub-geometry,
i.e. allowing such higher frequency resonant mode distrib-
ution of vibration past and beyond the partial sub-geometry
definition.
As to readily achieving required or desired areal
variation of bending stiffness panel members) can have at
least core layers) first made as substantially uniformly
isotropic or anisotropic structure(s), say as used for
above PCT and UK applications, including sandwich
structures) having skin layers over core layer(s).
Variations) of thickness can then be readily imposed to
achieve desired areal distribution of stiffness(es). For
deformable material(s), such as foam(s), such variation of
thickness is achievable by selective compression or -
crushing to achieve desired contouring, say by controlled
heating and application of pressure, typically to any
desired profile and feasibly done even after application of
any skin layers (depending on stretch capability of such
CA 02283381 1999-09-03
WO 98/39947 PCT/GB98/00621
7
skin layer material). Another possibility is for the
member to have localised stiffening or weakening, perhaps
preferably graded series thereof. For through-cell or
honeycomb materials, e.g. of some suitable reticulated
section of its cells extending from skin to skin of an
ultimate sandwich structure, or rigidly form-sustaining
uncrushable composites, variation of thickness is readily
achievable by selective skimming to desired thickness
contouring/profiling. None of these possibilities involves
necessary change of geometrical centre, but skimming rather
than crushing inevitably results in change of centre of
mass. Further alternatives for desired thickness/stiffness
variation of as-made cores) will be discussed, including
without change of centre of mass as can be important for
transducer means combining pistonic and bending wave
actions, where pistonic action is manifestly best if
centred at coincidence of centre of mass and geometric
centre to avoid differential moments due to mass
distribution relative to transducer locations) and/or to
unbalanced air pressure effects.
Centre of mass is, of course, readily relocated,
typically to geometric centre, by selective addition of
masses) to panel members) concerned, preferably without
unacceptable effects on desired areal distribution of
stiffness, e.g. masses also small enough not unacceptably
to affect lower frequency bending wave action and
effectively decoupled from higher frequency acoustic
action(s), say small weights) suitably semi-compliantly
mounted in holes) in the panel also small enough not
unacceptably to affect acoustic action(s).
Increasing stiffness in one direction away from or to
one side of the 'natural' locations) for transducer
location means locations) of the above PCT and UR
applications, or decreasing stiffness in a generally
opposite direction or to other side, will result in
CA 02283381 1999-09-03
WO 98/39947
PCT/GB98/00621
8
transducer means locations) hereof generally in said one
direction to said one side, which can advantageously be
towards geometric centre. Such relative increasing/
decreasing of stiffness can be complex as to resulting
contouring of the panel member concerning, including
tapering down increased thickness/stiffness to edge of the
panel member and or sloping up decreased thickness/
stiffness, say to have a substantially uniform edge
thickness of the panel member.
Additionally or alternatively, an inventive aspect of
at least the one group/strand is seen in a panel member
capable of acoustic bending wave action with a distribution
of bending stiffness(es) over its acoustically active area
that is in no sense centred coincidentally with centre of
mass and/or geometrical centre of that panel member, though
locations) of acoustic transducer means, whether for
bending wave action or for pistonic action or for both, may
be substantially so coincident, often and beneficially so.
It is noted at this point that there are two ways in
which areal distributions of stiffness(es) over a panel
member can be considered or treated as centred, one
analogous to how centre of mass is usually determined, i.e.
as putting first moment of stiffness to zero, thus in a
sense corresponding to high stiffness (so herein called
"high centre" of stiffness); the other in an inverse
manner, putting first moment of the reciprocal of stiffness
to zero, thus in another sense corresponding to weakness or
low stiffness (so herein called "low centre" of stiffness).
In panel members with isotropy or anisotropy as
specifically analysed in said PCT application, these
notional "high" and "low" centres of stiffness (so far as
meaningful in that context) are actually coincident,
further normally also coinciding with centre of mass and
with geometrical centre; but, for a panel member with
stiffness distribution as herein, these notional "high" and
CA 02283381 1999-09-03
WO 98139947 PCT/GB98/00621
9
"low" centres of stiffness are characteristically spaced
apart and typically further also from centre of mass and/or
geometric centre.
Reverting to effective or notional shifting (by
beneficial distributions of stiffness(es) hereof) of
practically effective locations) for bending wave action
transducer means (from locations) afforded by preferred
teachings/analyses of said PCT and UR patent applications
to different locations) hereof), such shifting can
usefully be viewed as towards said "low centre" of
stiffness which should thus be along same direction as
desired notional shifting, and/or away from said "high
centre" of stiffness that may usefully afford at least a
structural design reference position for providing
variations of bending stiffness(es) in the desired/
required corresponding distribution thereof. Variation of
bending stiffness outwards from such "low centre(s)" to
edges) of panel members) concerned, typically with
stiffness(es) increasing to different amounts and/or at
different rates in plural directions at least towards "high
centres)".
Feasible structures of honeycomb cellular cored
sandwich type can have desired stiffness distribution by
reason of contributions of as-made variant individual cell
geometries, and without necessarily substantial effects)
on distribution and centre of mass. Thus, desired areal
distributions of stiffness(es) are achievable by variations
of cells as to any or all of cell sectional area (if not
also shape), cell height (effectively core thickness) and
. cell wall thickness, including with such degree of
progressiveness applied to increase/decrease as may be
desired/required. Varying bending stiffness(es) without
. disturbing distribution of mass is achievable in such
context, say by varying cell wall thickness and cell height
for nominally same cell area, and/or by varying cell area
CA 02283381 1999-09-03
WO 98/39947 PCT/GB98/00621
and/or cell height for same thickness of cell walls, and
could, of course, be augmented or otherwise affected by
skin variations, including varying number and/or nature of
ply layers.
5 Also, it is seen as inventive for panel members hereof
to have at least "low" centres of stiffness(es) and pract-
ically most effective drive locations) that are identified
and typified oppositely in terms of minimum and maximum
diversity of transit times to panel edges) for notional or
10 actual bending waves considered as started from "low
centre" of stiffness and from transducer location(s),
respectively.
Reverting to above second general view, panel members
with distributions) of stiffness(es) as herein (as might
perhaps be called "eccentric") can have capability
applicable to securing that a said panel of some particular
given or desired shape (i.e. configuration or geometry) may
exhibit practically effective acoustic bending wave action
that was not considered achievable hitherto for that
particular shape, at least not according to any prior
helpful proposition; including not only for unfavourable
shapes related to known favourable shapes, but for shapes
not so related but treatable as herein to at least approach
what would hitherto be characteristic of some particular
favourable shape.
Indeed, this invention extends to capability of some
physically realisable areal distribution of bending
stiffness(es) of and for even irregularly shaped panel
members capable of bending wave acoustic action, to render
such action of satisfactorily distributed resonant mode
characteristic, and to afford practically effective -
location(s) for bending wave action transducer means
(including by finite element analysis), even irrespective
of and without reference to any envisaged or target shape
known to be favourable. Such procedures might proceed to
CA 02283381 1999-09-03
WO 98/39947 PCTIG898/OOb21
11
at least some extent pragmatically, by trial and error, as
to areal stiffness distributions, but can be helped by
analysing same using such as Finite Element Analysis at
least in terms of affording useful "low" and "high" centres
of stiffness shown herein to have positive (approaching/
attracting) and negative (distancing/repelling) location
effects on effective locations) for transducer means
within such areal stiffness distribution, whether itself
analysable or not.
In practice, useful benefits are seen by way of
seeking out constructs and/or transforms by which
derivations) can be made from what is known to be
effective for particular panel member geometries and
structures to what may, often will, be effective for a
different panel geometry/structure, particularly to
indicate structural specification for such different panel
geometry as to likely successful areal stiffness
distribution and as to transducer drive location(s).
In one approach considered inventive herein, useful
attention has been concentrated on transducer location(s),
including by way of notionally superposing as a target
geometry a desired or given configuration of panel member
and a subject geometry of a panel member that is known to
be effective and for which detailed analysis is readily
done or available, so that desired target transducer
location coincides with actual preferentially effective
transducer location of the subject geometry. Then, a
bending stiffness mapping can be made so that, for any or
each of selected constructs relative to now-coincident
transducer locations of the target and subject geometries,
and over such geometries, so that the known/readily
analysed bending stiffness of the subject panel structure
can be subject to transformation relative to the target
geometry to give substantially the same or similar or
scaled comparable stiffness distribution as in the subject
CA 02283381 1999-09-03
WO 98/39947 PCT/GB98/00621
12
geometry and acoustically successful bending wave action in
the target geometry. Promising such constructs include
lines going from coincident transducer locations to/through
edges of the target and subject geometries (say as though
representing bending wave transits/traverses). Envisaged
related transforms depend on relative lengths of the same
construct lines in the target and subject geometries, and
a suitable relationship, typically involving the quotient
of bending stiffness (B) and mass per unit area (~), i.e.
B/~, for proportionality transforms involving the third
and/or fourth powers of such line lengths to edges of
target and subject geometries. It is preferred, at least
as feeling more natural, for a target geometry to be
smaller than a related subject geometry, further preferable
for superposition to seek to minimise excess of the latter
over the former, including to minimise transform
processing. Whilst generally similar types of target and
subject shapes may thus be preferred, or favourable subject
geometry closest to unfavourable target geometry, it is
seen as feasible for the target geometry to differ quite
substantially from any recognisable type of known
favourable configuration/structure.
It is the case that panels of the above PCT applic
ation that are isometric as to areal bending stiffness, and
well studied/analysed, are good starting points for subject
geometries/structures. Indeed, another construct/transform
approach seen as having potential involves seeking to match
in the target geometry/structure according to the way that
the (now common) transducer location splits bending
stiffnesses to each side thereof in the subject geometry/
structure. Moreover, similar or related mapping schemes
could be used not only as between differing geometry types,
but also in the event of wishing or requiring to give to a
target geometry of one type such a bending stiffness
distribution as to resemble or mimic another type of
CA 02283381 1999-09-03
WO 98/39947 PCT/GB98/00621
13
geometry/configuration, so far as practicable given type of
geometry/configuration (e. g. rectangular, elliptical) does
have profound influence on actual areal distribution of
resonant mode vibration that can be difficult to disturb
greatly.
For loudspeaker members capable of both pistonic and
bending wave types of action, coincidence of location of
bending wave transducer means with centre of mass and
geometric centre is particularly effective in allowing a
single transducer device at one location to combine and
perform both pistonic drive and bending wave excitation.
It is, however, feasible to use separate transducers
one for pistonic-only action at coincident centre of
mass/geometric centre, and another for spaced location
conveniently located as herein for bending wave-only
action, though mass balancing may then be required by added
masses (if not afforded conjointly with requisite
distribution of bending stiffness).
A particularly interesting aspect of invention,
concerning a single transducer that affords both of
pistonic action and spaced bending wave action but at
spaced positions, can be used whether spacing is achieved
by bending wave transducer location as herein (say to suit
convenient transducer configuration) or left as arises
without application of above aspects of invention.
Generally, of course, application of this invention
may involve distributions of mass with centre of mass
displaced from geometric centre and/or any transducer
location, or whatever. Indeed, variations) of bending
stiffness and/or mass across at least acoustically
operative areas) of panel members) can be in many
prescribed ways and/or distributions, usually progressively
in any particular direction to desired ends different from
hitherto, and same will generally represent anisotropy that
is asymmetric at least relative to geometric centre of
CA 02283381 1999-09-03
WO 98139947 PCT/GB98/00621
14
mass; and application is seen as in the above PCT
application.
Practical aspects of invention include a loudspeaker
drive unit comprising a chassis, a transducer supported on
the chassis, a stiff lightweight panel diaphragm drivingly
coupled to the transducer, and a resilient edge suspension
surrounding the diaphragm and mounting the diaphragm in the
chassis, wherein the transducer is arranged to drive the
diaphragm pistonically at relatively low audio frequencies
to produce an audio output and to vibrate the diaphragm in
bending wave action at higher audio frequencies to cause
the diaphragm to resonate to produce an audio output, the
arrangement being such that the transducer is coupled to
the centre of mass and/or geometric centre of the diaphragm
and the diaphragm has a distribution of bending stiffness
including variation such that acoustically effective
resonant behaviour of the diaphragm results (at least
preferably being. centred offset from the centre of mass) .
The diaphragm may be circular, or elliptical in shape
and the transducer may be coupled to the geometric centre
of the diaphragm. The diaphragm may comprise a lightweight
cellular core sandwiched between opposed skins, and one of
the skins may be extended beyond an edge of the diaphragm,
with a marginal portion of the extended skin being attached
to the resilient suspension.
The transducer may be electromagnetic and may comprise
a moving coil mounted on a coil former, the coil former
being drivingly connected to the diaphragm. A second
resilient suspension may be connected between the coil
former and the chassis. One end of the coil former may be
connected to the diaphragm, and the said second resilient
suspension may be disposed adjacent to the said one end of
the coil former, and a third resilient suspension may be
connected between the other end of the coil former and the
chassis.
CA 02283381 1999-09-03
WO 98139947 PCT/GB98/00621
The end of the coil former adjacent to the panel
. diaphragm may be coupled to drive the panel diaphragm
substantially at one point. Conical means may be connected
. between the coil former and the panel diaphragm for this
5 purpose.
The coil former may comprise a compliant section
radially offset from a rigid section to drive the diaphragm
pistonically and to provide offcentre resonant drive to the
diaphragm.
10 In other aspects the invention provides a loudspeaker
comprising a drive unit as described above; and/or is a
stiff lightweight panel loudspeaker drive unit diaphragm
adapted to be driven pistonically and to be vibrated to
resonate, the diaphragm having a centre of mass located at
15 its geometric centre and a centre of stiffness which is
offset from its centre of mass.
BRIEF DESCRIPTION OF THE DRAWINGS
Exemplary specific implementation is now illustrated/
described in/with reference to accompanying diagrammatic
drawings, in which .
Figures 1A-D are plan and three outline sectional
views indicating desired positioning of bending wave
transducer location of an acoustic panel member,
including and achievement by compressing deformable
core material or by profiling core or composite
material;
Figures 2A,B,C are outline overall plan view and core
sectional views for an elliptical acoustic panel
member hereof;
Figures 3A,B,C are similar views of another elliptical
panel member hereof;
Figures 4A,B,C indicate a acoustic panel member of
unfavourable circular shape rendered more favourable
by part-elliptical grooving/slotting, and model
CA 02283381 1999-09-03
WO 98139947 PCT/GB98/00621
16
distribution graphs without and with such grooving/
slotting;
Figures 5A,B,C are diagrams useful in explaining
possible mappings/constructs/transforms for deriving
stiffness distribution for desired or target geometry
for a rectangular panel member and a sectional/profile
representation of results;
Figures 6A,B,C are outline graphs of interest relative
to useful methodology including of Figure 5;
Figures 7A,B are sectional side and plan views of one
embodiment of loudspeaker drive unit of the present
invention;
Figures 8A,B are sectional side views of another
loudspeaker drive unit and a modification;
Figures 9A,B are sectional side view of a further
loudspeaker drive unit and modification;
Figures 10A,B are a perspective view of a loudspeaker
drive coupling or actuator for spaced application of
pistonic and bending wave action, and detail of
mounting to a diaphragm/panel member; and
Figures 11A,B show relationships for such actions and
crossover.
SPECIFIC DESCRIPTION OF EMBODIMENTS
Referring first to Figure lA, a substantially
rectangular acoustic distributed mode panel member 10A is
indicated as though resulting directly from teachings of
the above PCT and UK patent applications, thus having its
"natural" location 13 for bending wave transducer means
spaced from its geometrical centre 12 and off true diagonal
shown dashed at 11. In application of the present
invention, however, the transducer location 13 is to be at
the geometric centre 12 of the panel member 10A, i.e.
effectively to appear shifted along the solid line 15,
which is achieved by appropriate areal distribution of
CA 02283381 1999-09-03
WO 98139947 PCT/GB98/00621
17
bending stiffness of the panel member. To this end, the
bending stiffness is made relatively greater and lesser to
one side (right in Fig. 1A) and to the opposite side (left
. in Fig. 1C) of the geometric centre 12 and the "natural"
transducer location 13, specifically in opposite directions
along the line I5 and its straight-line extensions 15G and
15L, respectively.
Figure iB is an outline section along the line 15
including extensions 15G and 15L, and indicates the same
situation as Figure 1A, i.e. "natural" transducer location
13B likewise spaced from geometric centre 12B of distrib-
uted mode panel member 10B, see projection lines 12P, 13P.
Figure 1B gives no details for the actual structure of the
panel member 10H; but does indicate the alternatives of
being monolithic, see solid outer face lines 16X,Y, or
being of sandwich type, see dashed inner face lines 17X,Y
indicating skins bonded to an inner core 18, typically
(though not necessarily) of cellular foam type or of honey-
comb through-cell type.
Figure 1C indicates use of a core 18C of material that
is deformable, specifically compressible in being capable
of crushing to a lesser thickness, as is typically of many
foamed cellular materials suitable for distributed mode
acoustic panel members and assumed in Figure iC. Such
crushing is indicated by thickness of the core 18C dimin-
ishing from right to left in Figure 1C, and its cells going
from roundedly fully open (19X) to flattened (19Y). It is
not, of course, essential for those cells to be of the same
or similar size, or of regular arrangement, or be roundedly
fully open at maximum thickness (suitable foam materials
often being of partially compressed foamed type). The core
18C is further shown with facing skins 17A,B. It is
feasible, even normal, for the core material 18C to be
deformed to the desired profile before bonding-on the skins
17A,B - but not essential so long as the panel member 10C
CA 02283381 1999-09-03
WU-98/39947 PCT/GB98/006Z 1
18
is good for distributed mode acoustic action if compress-
ively deformed with the skins 17A,B attached. Resulting
greater and lesser thickness of the core 18C and the panel
member 10C will correspond with greater and lesser bending
stiffness; and the indicated profile of progressive
thickness, thus stiffness, variation is such as to cause
coincidence of the transducer location 13C with the
geometric centre 12C, see arrow 13S and circled combined
reference 12C,13C. Crushing deformation will normally be
done with thermal assistance and using a suitably profiled
pressure plate. There will be no change to the centre of
mass of the panel member 10C, i.e. centre of mass will
remain coincident with the geometric centre 12C, now also
coincident with the transducer location 13C.
Where core density contribution is small, ie bending
stiffness is dominant, the linear factor of core mass
contribution may be neglected and the desired areal
thickness distribution may be achieved by shaping the
thickness of an isotropic core of polymer foam or
fabricated honeycomb sandwich or monolithic without skin
and a core; and any such structure can be fabricated,
machined or moulded as desired herein.
Figure 1D shows distributed mode acoustic panel member
10D with progressive relief of its lower surface so that
its thickness reduces with similar profile to that of
Figure iC. Such profile might be somewhat different for
the same intended effect, i.e. achieving coincidence of
transducer location 13D with geometric centre 12D, say
depending on materials) used for the panel member 10D.
Such materials may be monolithic reinforced composites or
any kind of cellular, typically then as a skinned core,
including of honey-comb type with through-cells extending
from skin-to-skin. The foamed-cell-like.indication 19Z of
Figure 1D could correspond with use of foamed material that
is by choice not crushed or is not suitable for crushing;
CA 02283381 1999-09-03
WO 98/39947 PCT/GB98/00621
19
but is intended to do no more than indicate that there is
no significant change of density. There must, of course,
then be a change in the distribution of mass and the centre
of mass of the panel member 10D as such will be spaced from
the geometric centre, generally in the direction of arrow
CM. In order to achieve coincidence of overall centre of
mass with geometric centre 12D, the panel member lOD is
shown with at least one additional balancing mass 22
indicated mounted in preferably blind receiving hole 23,
further preferably by semi-compliant means 24, say in a
suitable mechanically or adhesively secured bush or sleeve,
so that its inertial compress is progressively decoupled
from the panel member lOD at higher frequencies of desired
vibration distribution. There may be more than one
balancing mass (22), say in a less than 180° locus through
the notional extension line 15L, or some other array
disposition, and need not all be of the same mass, say
diminishing in mass progressively away from the line 15L.
At simplest, the thickness may be simply tapered along
through the section of Figure 1B, though a more complex
taper is normal, including to a common equal edge thickness
and/or progressively less away from the line 15 - 15G, L.
Geometric relations of bending frequency to size are used
need to be taken into account. For any given shape,
increasing its size lowers the fundamental frequencies of
vibration, and vice versa. Effective shift of preferential
transducer location can be seen as equivalent to shortening
the effective panel size in relation bending along the
direction of such shift.
Turning to Figures 2A - C and 3A - C, all panel
members are shown as being of generally elliptical shape,
those referenced 20A, 30A being isotropic, thus showing
coincidence at 25, 35 of geometrical centre and centre of
mass. To the extent meaningful for isometric panel
geometries and structures, distributions of stiffness will,
CA 02283381 1999-09-03
WO 98139947 PCT/GB98/00621
of course, also be centred at 25, 35 - whether as to "high
centre" (stiffness as such) or as to "low centre" (softness
or compliance). In addition, Figures 2A, 3A show at 26, 36
one preferentially good or best location (as in the above
5 PCT application) for a bending wave action transducer and
operative for desired resonant mode acoustic performance of
the panel member 20A, 30A, say as or in a loudspeaker.
Turning to Figures 2B, C and 3B, C the centre
positions of the panels lOB 20B, 30B are now labelled 25,
10 26 and 35, 36 and still correspond to both of geometric
centre and centre of mass (25, 35), but now also further to
acoustically effective bending wave transducer location
(26, 36). Compared with Figures 2A, 3A the transducer
locations 26, 36 have effectively been displaced by a
15 distribution of bending stiffness(es), hereof, and
accompanying displacements of "high and "low" centres of
stiffness, are indicated 27, 28 and 37, 38 as generally
oppositely relative to the geometric centres 25, 35. This
different asymmetric stiffness distribution is shown
20 achieved by progressive changes to cells 29, 39 partic-
ularly as to their heights, thus thickness of the panel
members 20A, 30A; but also as to their areas and
population density (see Figures 2B, C), or as to their
areas and wall thicknesses but not their population density
(see Figures 3B, C) thereby achieving desired distribution
of stiffness without at least operatively significant
disturbance to distribution of mass, thus centre of mass is
now coincident with both geometric centre and transducer
location (25, 26; 35, 36).
There are further feasible approaches to varying
stiffness(es), thus areal distribution; say by introducing
out-of-planar formations, such as bends, curves etc
affecting stiffness in generally understood Ways; or such
as grooves, slots or scorings in surfaces to reduce
stiffness or rib formations to increase stiffness,
CA 02283381 1999-09-03
WO 98139947 PCT/GB98/00621
21
including progressively by spaced series of such
provisions, say along the line extensions 15G, L of Figure
lA (not shown, but computable using such as Finite Element
Analysis).
Figure 4A shows another application of into-surface
grooving, slotting or scoring, specifically to improving
distributed mode bending wave action for an acoustic panel
member 40 that is actually of a configuration or geometry,
namely circular, that is known to be unfavourable as a
distributed mode acoustic panel member, especially with
central location of exciting transducer means. This known
unsatisfactory performance capability is indicated by the
modal frequency distribution indicated in Figure 4B as will
be readily recognised and understood by those skilled in
the art, specifically corresponding to concentric vibration
patterning. Profound improvement on what is shown in
Figure 4C has been achieved by grooving, slotting or
scoring as indicated at 45 in the form of part of an
ellipse, i.e. in a class of configurations/geometries known
to include some highly favourable as distributed mode
acoustic panel members (as in Figures 2, 3 above), though
not actually according to such a known favourable
particular ellipse. However, effect on lower frequency
modal action is markedly better distributed than the
symmetry of simple centrally excited circular shapes, and
higher frequency modal action is able to extend past and
beyond the open ends of the groove 45. The shape of the
groove 45 was developed using Finite Element Analysis, see
indicated complex element patterning, such techniques being
of general value to detail implementation of teachings
hereof. Lesser arcuate formations asymmetrically spaced
relative to centre of a circular panel member have also
shown promise, and should be readily refined by further
Finite Element Analysis.
Figures 5A, B indicate constructs and transforms much
CA 02283381 1999-09-03
WO 98/39947 PCTIGB98/00621
22
as discussed above, specifically shown for rectangular
target (51A, B) and subject (52A, B) configurations/
geometrie. Construct lines 53A, B processed according to
different lengths and desired/required bending stiffnesses
show highly promising effectiveness of the approach at
least as applied to shapes of the same rectangular type.
The methodology of Figure 5B is particularly attractive in
that the subject configuration/geometry 52B is efficiently
constructed from the target configuration/geometry 51B
placed at one corner by extensions from that corner so that
a preferential transducer location 54B of a well-understood
and analysed isometric shape 52B simply coincides with
geometrical centre of the target shape 51B. Figure 5C
indicates a typical section through target member 50 of
target shape 51A resulting from methodology according to
Figure 5B.
Inspection of the B/~ quotient or the B and/or a
parameter values, specifically alone with the other held
constant, in the various radial directions 53B, and
mathematical mapping from panel of shape 52B to panel of
shape 51B, allows distribution of stiffness hereof to be
computed in those directions (53B) further using a power
relation including fourth power of length and second or
third powers of thickness depending on whether bending
stiffness required is of skinned core sandwich panel or an
unskinned monolithic solid composite structure.
Figure 6A shows ratiometric results of length mapping
for Figure 5B methodology, and Figure 6B shows how required
(target) bending behaviour is related to the ratiometric
results of Figure 6A and relative to material properties,
specifically stiffness alone involving fourth power of
length (solid line), thickness of a sandwich structure
involving a square power (dotted line), and thickness of a
monolith structure involving a 4/3 power (dashed line).
For a sandwich structure, skin stiffness (tensile strength)
CA 02283381 1999-09-03
WO 98/39947 PCT/GB98/00621
23
would also involve fourth power of length; and skin
thickness a 4/3 power. Figure 6C shows modal density
mapping with 3$ damping for a target square panel member,
without bending stiffness distribution hereof, a subject
1.134:1 aspect ration isometric panel member of the above
PCT application, ie involving adjustment relative to one
side difference only; and the square panel improved by
bending stiffness distribution according to skin param-
eters, specifically thickness (h) and Young's modulus (E).
Referring to Figures 7A and 7B, a loudspeaker drive
unit comprises a chassis 71 in the form of an open frame
shaped as a shallow circular basket or dish having an
outwardly projecting peripheral flange 71F pierced with
holes whereby the drive unit can be mounted on a baffle
(not shown), e.g. in a loudspeaker enclosure (not shown) in
generally conventional fashion. The chassis 71 supports a
transducer 72 in the form of an electrodynamic drive motor
comprising a magnet 73 sandwiched between pole pieces 74A,B
and affording an annular gap in which is mounted a tubular
coil 75 former carrying a coil 75C which forms the drive
coupling or actuating movable member of the motor.
The coil former is mounted on resilient suspensions
76A,B at its opposite ends to guide the coil former 75 for
axial movement in the gap of the magnet assembly. One end
of the coil former 75 is secured, e.g. by bonding 77, to
the rear face of a lightweight rigid panel 70 which forms
an acoustic radiator diaphragm of the loudspeaker drive
unit and which comprises a lightweight cellular core 70C,
e.g. of honeycomb material, sandwiched between opposed
front and rear skins 70F,R. The panel 70 is generally as
herein taught, specifically with distribution of bending
stiffness affording coincidence of centre of mass and
_ preferential bending wave exciter location at its geometric
centre. In the example shown, the front skin is
conveniently of conventional circular form integrating with
CA 02283381 1999-09-03
WO 98139947 PCT/GB98/00621
24
the contour and in some cases blending in effective
operation with the surround/suspension. The rear skin is
chosen to be rectangular to form a composite panel
compliant with distributed mode teaching (it may be driven
directly by the differential coupler of Figures 10A and
10B) .
For a simple central, or central equivalent drive the
distributed mode panel section will be designed with
preferential modal distribution as per the invention herein
generated for example by control of areal stiffness, so as
usefully to place the modal driving point or region at or
close to the geometric and mass centre. Thus good modal
drive at higher frequencies and pistonic operation at lower
frequencies is obtained for a conventional style of driver
build and geometry.
The front facing skin 70F of the panel 70 is extended
beyond the edge of the panel and its peripheral margin is
attached to a roll surround or suspension 77 supported by
the chassis 71 whereby the panel is free to move pistonic-
ally. The transducer 72 is arranged to move the panel 70
pistonically at low frequencies and to vibrate the panel 70
at high frequencies to impart bending waves to the panel
whereby it resonates as discussed at length above.
The arrangements shown in Figures 8A and 8B are
..5 generally similar to that described above, except that in
these cases the chassis 81 is even shallower, the motor 72
is largely outside the chassis 81, and the coupler/
actuator coil former 85 extends into the chassis with
consequent modification of its suspension 86. Modification
of Figure 9B involves use of a smaller neodymium motor 82N
and sectional end reduction 85A of the coil former 85.
The arrangements shown in Figures 9A and 9B are very
similar to those shown in Figure 8A and 8B except that the
extended end 95A,B of the coil former 95 is formed with a
[single) of double conic section, the pointed end 95P of
CA 02283381 1999-09-03
WO 98/39947 PCT/GB98/00621
which is attached to the rear face of the lightweight rigid
panel diaphragm 90 at the geometric centre thereof.
Figures lOA,B show a diaphragm coupler/actuator 100,
conveniently a coil former of a drive motor (not shown),
5 having a major arcuate peripheral part 108 of its drive
end, which is adapted to be attached (107) to a rigid
lightweight panel 100 made of a semi-compliant material;
and with arcuate peripheral part 109 of the same end rigid.
The drive applied to the panel 100 will be pistonic at low
10 frequencies through both of the arcuate peripheral end
parts 108,109. At high frequencies the coupler/actuator
will excite bending wave action by the minor part 109, thus
vibrational energy in the panel 100 at a position offset
from the axis of the coupler/actuator 105. By its semi-
15 compliant nature, the major arcuate peripheral end part 108
will be substantially quiescent at high frequencies. Thus
the true actuation position of the drive is frequency
dependent even though applied in the same way and by the
same means 105.
20 The simple illustrated case of one direct coupling
section and one semi compliant section may be extended to
multiple firm contact points and more complex semi-
compliant arrangements, e.g. two or more preferential
distributed mode panel member transducer locations may be
25 involved. The semi compliant section may be tapered or
graded, or plurally stepped in thickness or bulk property,
to provide a gradation of coupled stiffness interactively
calculated with the panel acoustic performance criteria to
improve overall performance, whether with a distributed
mode acoustic panel with bending wave transducer location
spaced from geometric/mass centre to suit convenient
structure for the coupler/actuator 105, or with the latter
suited to such as transducer locations of above PCT and UK
patent applications.
Such differential frequency coupler (105) can be used
CA 02283381 1999-09-03
WO 98/39947 PCT/GB98/00621
26
with the usual motor coil employed in electrodynamic
exciters. While such coupler 105 may be a separate
component of predetermined size or diameter, it is
convenient to see its application as part of the attachment
plane of a motor coil of similar diameter, which may as
indicated above be chosen to encompass one or more of the
preferential drive transducer locations of a distributed
mode acoustic panel member, specifically at and excited by
rigid end parts) 108 as intended higher frequency response
is by bending mode vibration in a distributed mode acoustic
panel diaphragm member 100. At lower frequencies the
semi-resilient parts/inserts 108 become more contributory,
and progressively bring the whole circumference of the
actuator/coupler 105 into effect for balanced, centre of
mass action, thus satisfactory pistonic operation at low
frequencies. The fundamental bending frequency of the
panel member 100 and the resilience of the coupler/actuator
parts) 108 are chosen to allow for satisfactorily smooth
transition in acoustic power from the pistonic to the
bending vibration regions of the frequency range. Such
transition may be further aided by plural stepping of the
parts) 108, or by tapering as indicated at 108A.
Understanding operation of this coupler 108 is aided
by Figure 11A outlining intended variation of velocity
applied to the acoustic panel, including in the region of
crossover. At low frequencies the semi compliant parts)
108 contribute effective power to the panel member 100 in
a balanced pistonic manner. That piston like action decays
with increasing frequency as the mechanical impedance of
the vibrating panel member 100 becomes predominant and is
excited at preferential eccentric position(s). Thus the
active velocity contribution at higher frequencies arises'
from the rigid, offset sectors) of the coupler.
Fig 11B further shows displacement of effective
variation of pistonic drive and distributed mode excitation
CA 02283381 1999-09-03
WO 98/39947 PCT/GB98/00621
27
points with frequency. At low frequencies the pistonic
. drive point is predominantly at the centre and centre of
mass. With increasing frequency there is a transition to
a bending wave excitation point offset from the centre,
aligned by suitable choice of panel design and also complex
coupler actuator diameter and parts geometry to drive at or
close to the preferred distributed mode point for satis-
factory favourable distribution of vibration modes.
In above Figures 7A,B bending wave transducer means of
this type with an overall diameter in the range 150 to
200mm would operate "natural" transducer locations) of a
distributed mode panel member of satisfactory bending mode
performance commencing in the range 150Hz to 500Hz.
Pistonic operation will be effective from lower
frequencies, eg from 30Hz for a suitable acoustic mounting,
and would decline in its upper range as the panel member
enters the bending mode range.
The differential frequency capability of couplers of
this invention allows subtle refinements to use of
distributed mode acoustic panel members. For example, in
a given panel a change in the driving point with frequency
may be found desirable for purposes of frequency control
seen in particular applications, such as close to wall
mounting in small enclosures and related response modifying
environments. More than one grade and/or size /area of
semi-compliant parts or inserts may be used on suitable
geometries of coupler effectively to gradually or step-wise
move between more or most effective drive point of the
modal pattern with frequency, and advantageously modify the
radiated sound.
a. .~~ .:
