FIELD OF THE INVENTION
[0001] 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
[0002] 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
area(s) 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 area(s)
so as to have resonant mode vibration components distributed over said area(s) beneficially
for acoustic coupling with ambient air; and as to having determinable preferential
location(s) within said area(s) for acoustic transducer means, particularly operationally
active or moving part(s) thereof effective in relation to acoustic vibrational activity
in said area(s) and related signals, usually electrical, corresponding to acoustic
content of such vibrational activity. Uses are also envisaged in that 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 acoustically "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.
[0003] Our co-pending UK patent application no (P.5840) concerns using features of mechanical
impedance in achieving refinements to geometry and/or location(s) of bending wave
transducer means for such panel members as or in acoustic devices. Contents of that
U.K. patent application and of the above PCT application are hereby incorporated herein
to any extent that may be useful in or to explaining, understanding or defining the
present invention.
[0004] 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 satisfactory 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
[0005] From a first viewpoint, this invention concerns active acoustic devices relying on
bending wave action in panel members, particularly providing effective placement(s)
for bending wave transducer means different from specific teachings of the above PCT
and UK patent applications, i.e. other than at location(s) arising from analysis and
preference in that PCT application, even including at centre(s) of mass and/or geometry
rather than off-set therefrom.
[0006] 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.
[0007] 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.
[0008] It is considered useful to note that effective specific embodiments of this invention
utilise panel member(s) 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 piecemeal provisions
for altering intendedly other acoustic action in panel member(s) 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 specifically 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 unsuitable as a matter of geometry and/or location
of transducer means.
[0009] Effective inventive method and means hereof involve areal distribution of variation
in stiffness over at least area(s) of such panel member(s) 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 areal 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 areal distribution of bending stiffness
hereof or to consequential different location(s) for transducer means, or both.
[0010] Specific teaching of the above PCT application extends to panel member(s) having
different bending stiffness(es) in different directions across intendedly acoustically
active area(s) that may be all or less than all of area(s) of the panel member(s),
typically in or resolvable to two coordinate related directions, and substantially
constant therealong. In contrast, advantageous panel member(s) of embodiment(s) hereof
have variation of bending stiffness(es) along some direction(s) across said area(s)
that is/are irresolvable to constancy in normal coordinate or any direction(s).
[0011] Areal 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 of monolithic structures usually of composite material(s) type.
[0012] Whilst available practical analysis may not always allow such investigation as precisely
and fully to identify and quantify changes in actual areal distribution of acoustically
effective resonant mode vibration for panel member(s) 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.
[0013] One group/strand is as already foreshadowed, specifically providing more convenient
location(s) 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(s) of greater bending
stiffness serve(s) effectively to shift such natural location(s) away from such region(s),
typically from said one side towards said other side and region(s) of lesser bending
stiffness; region(s) 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 favourable 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 distribution of vibration past and beyond the partial
sub-geometry definition.
[0014] As to readily achieving required or desired areal variation of bending stiffness
panel member(s) can have at least core layer(s) first made as substantially uniformly
isotropic or anisotropic structure(s), say as used for above PCT and UK applications,
including sandwich structure(s) having skin layers over core layer(s). Variation(s)
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 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 core(s) 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 location(s) and/or to unbalanced air pressure effects.
[0015] Centre of mass is, of course, readily relocated, typically to geometric centre, by
selective addition of mass(es) to panel member(s) 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 weight(s) suitably semi-compliantly
mounted in hole(s) in the panel also small enough not unacceptably to affect acoustic
action(s).
[0016] Increasing stiffness in one direction away from or to one side of the 'natural' location(s)
for transducer location means location(s) of the above PCT and UK applications, or
decreasing stiffness in a generally opposite direction or to other side, will result
in transducer means location(s) 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.
[0017] 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 location(s) 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.
[0018] 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 "low" centres of stiffness are characteristically spaced apart and typically further
also from centre of mass and/or geometric centre.
[0019] Reverting to effective or notional shifting (by beneficial distributions of stiffness(es)
hereof) of practically effective location(s) for bending wave action transducer means
(from location(s) afforded by preferred teachings/analyses of said PCT and UK patent
applications to different location(s) 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 edge(s)
of panel member(s) concerned, typically with stiffness(es) increasing to different
amounts and/or at different rates in plural directions at least towards "high centre(s)".
[0020] 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 effect(s) 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 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.
[0021] Also, it is seen as inventive for panel members hereof to have at least "low" centres
of stiffness(es) and practically most effective drive location(s) that are identified
and typified oppositely in terms of minimum and maximum diversity of transit times
to panel edge(s) for notional or actual bending waves considered as started from "low
centre" of stiffness and from transducer location(s), respectively.
[0022] Reverting to above second general view, panel members with distribution(s) 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.
[0023] 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 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 location(s) for
transducer means within such areal stiffness distribution, whether itself analysable
or not.
[0024] In practice, useful benefits are seen by way of seeking out constructs and/or transforms
by which derivation(s) 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).
[0025] 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 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 (µ), i.e. B/µ, 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.
[0026] It is the case that panels of the above PCT application 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 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.
[0027] 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.
[0028] 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).
[0029] 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.
[0030] 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, variation(s) of bending stiffness and/or mass across at least
acoustically operative area(s) of panel member(s) 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 mass; and application is seen as in the above
PCT application.
[0031] 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).
[0032] 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.
[0033] 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.
[0034] 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 purpose.
[0035] 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.
[0036] 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 its geometric centre and a centre of stiffness which is
offset from its centre of mass.
BRIEF DESCRIPTION OF THE DRAWINGS
[0037] 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 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
[0038] Referring first to Figure 1A, 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 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 15 and its straight-line extensions
15G and 15L, respectively.
[0039] Figure 1B 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 distributed mode panel member 10B,
see projection lines 12P, 13P. Figure 1B gives no details for the actual structure
of the panel member 10B; 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.
[0040] 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 1C. Such crushing is indicated by thickness of the core 18C
diminishing 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 is
good for distributed mode acoustic action if compressively 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.
[0041] 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.
[0042] 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 1C. 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 material(s) 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; 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 10D 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 10D 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.
[0043] 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.
[0044] 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, 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 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.
[0045] Turning to Figures 2B, C and 3B, C the centre positions of the panels 10B 20B, 30B
are now labelled 25, 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 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 achieved by progressive
changes to cells 29, 39 particularly 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).
[0046] 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, including progressively
by spaced series of such provisions, say along the line extensions 15G, L of Figure
1A (not shown, but computable using such as Finite Element Analysis).
[0047] 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.
[0048] Figures 5A, B indicate constructs and transforms much 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.
[0049] Inspection of the B/µ quotient or the B and/or µ 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.
[0050] 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) 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 parameters, specifically thickness (h) and Young's modulus (E).
[0051] 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.
[0052] 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 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) .
[0053] 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.
[0054] 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 pistonically. 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.
[0055] The arrangements shown in Figures 8A and 8B are 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.
[0056] 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 which is attached to the
rear face of the lightweight rigid panel diaphragm 90 at the geometric centre thereof.
[0057] Figures 10A,B show a diaphragm coupler/actuator 100, conveniently a coil former of
a drive motor (not shown), 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 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-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.
[0058] 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 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.
[0059] Such differential frequency coupler (105) can be used 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 part(s) 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
part(s) 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 part(s) 108, or by tapering as indicated
at 108A.
[0060] 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 part(s) 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 sector(s)
of the coupler.
[0061] Fig 11B further shows displacement of effective variation of pistonic drive and distributed
mode excitation 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 satisfactory
favourable distribution of vibration modes.
[0062] 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 location(s)
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.
[0063] 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.