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The present invention relates to a panel type loudspeaker using a resonant multi-mode radiator
suitable for applications requiring a thin loudspeaker cross-section as in the case of a
loudspeaker loudspeaker. Concerned. The loudspeaker exhibits a conversion efficiency close to 1
and is thus suitable for applications requiring high acoustic power output from the loudspeaker.
BACKGROUND OF THE INVENTION Conventional speakers use a substantially universally
reciprocating diaphragm or similar element to generate an acoustic output. The motion of the
diaphragm must be in phase across the entire surface so that the diaphragm moves forward and
backward in response to the drive of the diaphragm drive, in particular, this means that the
loudspeaker It is realized by the nature and size of the diaphragm, which relates to the frequency
band required to operate over that band. Such a speaker can operate at a frequency higher than
the first resonant frequency of the diaphragm by appropriately attenuating the resonant mode of
the diaphragm, but in such a speaker, the diaphragm is mainly The diaphragm operates at a
lower frequency than that at which it exhibits a resonant mode, which adds undesirable spatial
and / or frequency limitations to the loudspeaker. Although small diaphragms are used to raise
the resonant frequency threshold, such small diaphragms are not sufficient radiators at low
There are two main types of speakers currently in use, both of which use a reciprocatingly driven
diaphragm. The first type of loudspeaker is an electrostatic loudspeaker, in which the diaphragm
is the charge generated between the diaphragm and a fixed backplate which is arranged slightly
behind the diaphragm. Driven by difference. Electrostatic loudspeakers are capable of producing
high fidelity output over a wide frequency band and are relatively flat in shape suitable for
loudspeaker applications. However, such electrostatic speakers are expensive and have very low
conversion efficiency which reduces the advantages of such speakers. The other established type
of piston diaphragm loudspeaker is the conventional dynamic loudspeaker which includes an
edge mounted diaphragm driven by an electromechanical drive. Such dynamic speakers have a
relatively narrow band and are more efficient emitters than electrostatic speakers, but still have
low conversion efficiency. In this type of loudspeaker it is necessary to prevent harmful
interference between the forward and reverse power of the diaphragm. This generally requires
that the diaphragm be mounted on the front face of a rigid box-like housing, thus making flat
panel configurations impossible.
SUMMARY OF THE INVENTION It is an object of the invention to provide a high conversion
efficiency flat panel loudspeaker having a frequency band at least suitable for loudspeaker
applications. This object is achieved by using the possibilities offered by some modern composite
panels in order to produce a loudspeaker operating in a novel way. For example, composite
panels comprising thin structural skins, between which light weight spacing cores are
sandwiched, are commonly used for aerospace structures, some of such composite panels being
claimed It can be used in the speakers described in the range. Several sandwich-like panel
materials have been used previously in diaphragm structures in conventional dynamic speakers,
as disclosed, for example, in West German patent specifications GB 2010 637 A, GB 2031691 A,
GB 2023 375 A, but this application To the best of human knowledge, such panel materials have
never been used as resonant multi-mode radiators in the method of the present invention.
SUMMARY OF THE INVENTION The claimed invention is directed to a single sandwich formed of
two material skins having a spacing core of transverse cellular construction. A resonant multimode radiation, which is a panel, and wherein said panel has a ratio of at least 10 "bending
stiffness (B)" to "third panel power per unit surface area (μ)" ratio in all directions. Internal
elements, mounting means for supporting the panel in a free state without damping or securing
the panel to the support body, and in the radiator panel in response to electrical input within the
operating frequency band for the speaker And electromechanical drive means coupled to the
radiator panel to serve to excite multimode resonances.
The term "transverse cell-like structure" used in the above definition and elsewhere in this
specification refers to a honeycomb core shape having a core cell extending through the
thickness of the panel material, and And other cellular based core constructions having
hexagonal core cross sections.
In the above definition of the invention, and throughout the specification and claims, all of the
units used are MKS units, in particular Nm and kg / m2 in the above paragraph.
Applicants refer to the above ratio value as "T" and the T value specified above is necessary to
enable the radiator panel to function properly in the manner required.
Preferably, the value of T should be at least 100. The T value is a measure of the acoustic
conversion efficiency of the radiator panel when the speaker is operating as intended at a
frequency higher than its coincidence frequency (see below). High T values are best obtained by
the use of honeycomb cored panels with thin metal skins. The presently preferred panel type is a
panel having a honeycomb core structure and a thin skin, wherein both the core and the skin are
made of aluminum or an aluminum alloy. For such panels, T values of 200 or more can be
obtained. In the case of a solid plate material, it is probably impossible to give the required
minimum value of T for any material. A full steel panel will have a T value of about 0.5, which is
much lower than the required T value, whatever the thickness. A solid carbon fiber reinforced
plastic sheet with equiaxed reinforcement would have a T value of about 0.85, but this T value is
also well below the required minimum. The operating mode of the claimed loudspeaker is
fundamentally different from prior art diaphragm loudspeakers having substantially "reciprocal"
diaphragm motion. As mentioned above, such conventional speakers are intended to cause the
reciprocating and in-phase motion of the diaphragm, and furthermore, the diaphragm is designed
to exclude the mode resonant motion of the diaphragm from the speaker frequency band And /
or by incorporating appropriate damping to suppress the modal resonant motion of the
diaphragm, it is intended to avoid modal resonant motion of the diaphragm. In contrast, the
present invention does not incorporate any conventional diaphragm, but rather multimode
radiation that functions by exciting the resonant modes in the panel, rather than by moving the
panel in a reciprocal, non-resonant manner Use a panel that meets the above criteria. The
differences in this mode of operation are from the "stiffness" versus "mass" criteria, from the
avoidance of edge attenuation and the absence of internal damping layers etc within the radiator
panel, and also from the matched and fundamental frequencies of the composite panel. Resulting
from the operation of the radiator at frequencies above both.
The "coincident frequency" is the frequency at which the bending wave velocity of the radiator
panel coincides with the speed of sound in air. This coincidence frequency is a kind of threshold
for efficient operation of the loudspeaker, since many of the modern composite sandwich panels
efficiently radiate at frequencies above the loudspeaker coincidence frequency. Using the
information provided herein to fabricate a radiator panel suitable for a given frequency band
where the coincident frequency of the radiator panel matches or falls below the required band It
is possible, as a result, the loudspeaker will convert almost all of the mechanical input from the
electromechanical drive means into an acoustic output. This is more than just a requirement, as
this feature of high conversion efficiency overcomes the potential problems in systems based on
resonant multimode radiators. The high conversion efficiency (which can be achieved by the
choice of appropriate materials according to the design rules presented here) is rather due to
internal structure damping within the panel material or rather than damping imposed due to
panel movement It is obtained when the panel movement is suppressed by sound attenuation.
When this is obtained, the acoustic distortion will be small.
The value of "B" in the "T" criteria given above is the static bending stiffness of the panel rather
than the stiffness of the panel when subjected to a rapid bending action. However, this bending
stiffness decreases as the frequency increases due to the increasing influence of the shear
movement in the core. It is important that the action of this shear movement be minimized, which
can be obtained by the use of panels having a sufficiently high shear modulus. This requirement
leads to a second criterion, which means that the shear modulus (G) of the core is related to the
relationship “μc 2 / d” (“c” in the above equation is the velocity of sound in air and , “D”
is the depth of the panel core) must be equal to or greater than the value given by This
expression is represented by another expression “μ.c2 / d. It is convenient to rearrange to G>
Two representative forms of the present invention will now be described by way of example with
reference to the accompanying drawings, in which:
The loudspeaker illustrated in FIG. 1 comprises a resonant multi-mode radiator 1, a simple
support frame 2 from which the radiator is suspended by a suspension loop 3 and an
electromechanical exciter 4.
The radiator 1 comprises a rectangular panel of aluminum alloy honeycomb sandwich structure
with an aluminum alloy skin. The details of this panel and the dimensioning rules will be
described later. The electromagnetic exciter 4 has an axis 5 and this axis 5 pushes against the
back of the radiator panel 1 and excites the radiator panel 1 by the reciprocating movement of
the axis when the exciter 4 is supplied with an electrical signal. The electromagnetic exciter 4 is
mounted on the support frame 2 as follows. At the point of contact between the shaft 5 and the
panel 1, the panel is reinforced by the patch 6 to resist wear and tear. In order to prevent the
radiator panel 1 from being excited preferentially in the symmetric mode, the exciter 4 is not
located on the panel near the center of the panel but on the panel near one corner of the panel.
An exciter 4 is arranged to excite the radiator panel 1. The inertial mass of the exciter 4 and the
inertial mass of the radiator panel 1 are adapted to ensure an efficient inertial coupling between
the exciter 4 and the radiator panel 1 for efficient power transfer .
A second embodiment of the loudspeaker according to the invention, shown in FIG. 2, is an
analogue of the embodiment of the loudspeaker described above with reference to FIG. 1 except
for some minor details which will be described later. is there. Common reference numbers are
used for common parts in these two figures.
The second embodiment of the loudspeaker is suspended from the ceiling 7 rather than
suspended from the support frame. Instead of the two suspension loops in the first embodiment
described above, four suspension loops are used so that the radiator panel is disposed under the
ceiling rather than hanging from the ceiling. The exciter 4 is arranged above the radiator 1.
The two loudspeaker embodiments described above operate in exactly the same way and follow
the same design rules with regard to the choice of panel material and the structure and size of
the panel, which relate to the required frequency band of the loudspeaker. The "T" and "Shear
Modulus" criteria, both described above, relate to panel shape and panel material rather than
panel size and speaker frequency range. In order to make a loudspeaker optimized for a
particular frequency range, it is useful to mention some of the design rules given below.
Since the fundamental frequency of the panel must be lower than the lowest frequency of the
required frequency range of the speaker, the lower limit of the required frequency range of the
speaker limits the fundamental frequency of the panel. Furthermore, the matched frequency of
the panel must also be lower than the lowest frequency of the required frequency range of the
loudspeaker. The coincidence frequency (f1) is independent of the panel area and is given by:
fc2 = μ. c4 / 4. π2. The required bandwidth for B-specific speakers determines the value of fc,
thus giving rise to the relationship between μ and B. If the value of the fundamental frequency
(f1) is also determined, then the value of this fundamental frequency determines an
approximation for the panel area, since f1 is given by the following approximation:
f12 = B / μ. A2 Finally, the frequency at which the first air resonance occurs in the core of the
panel must be higher than the upper frequency limit of the loudspeaker. This frequency (fa) is
given by still another equation as follows, and fa = c / 2. d In the above formula, d is the depth of
the panel core. Thus, this equation determines the depth of the panel core according to the
frequency band of the speaker.
In the following, design considerations are described by way of example with reference to one
loudspeaker embodiment using a radiator panel comprising a 1 m × 1 m square aluminum
honeycomb with aluminum outer skin complex. For this panel, the core depth is 0.04 m and the
thickness of each skin is 0.0003 m. In the case of this panel, B is 18850 Nm, μ is 3.38 kg / m 2
and T is 488 Nm 7 / kg 2.
From the f1 equation, f1 is [18850 / 3.38 × 1] 1/2 = 75 Hz.
From the fc equation, fc is [3.38 × 3404/4 × 3.14162 × 18850] 1⁄2 = 246 Hz.
From the fa equation, fa is 340/2 × 0.04 = 4250 Hz.
The shear stiffness of the panel varies with the direction in the plane of the panel.
For the axis of minimum value of "G", the equation "μ.c2 / G.d" has a value of 0.056, and for the
axis of maximum value of "G", the above equation has a value of 0.122 Have.
These values are both much less than the limit value "1", thus indicating that the performance of
the loudspeaker will not be limited by the core shear movement over the intended frequency
band .
Based on these calculations, a claimed speaker using a 1 m by 1 m square shaped radiator panel
made of the material detailed above has high conversion efficiency and low distortion in its band
It will be inferred that it will have a frequency band of 250 Hz to 4 kHz with.
It is expected that such a band would be very suitable for loudspeaker speakers.
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