Note: Descriptions are shown in the official language in which they were submitted.
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This invention relates generally to loudspeaker
systems, and more particularly to systems in which the audio
frequency signal is divided into upper and lower ranges for
higher fidelity reproduction from transducers particularly
designed for that purpose. It is well known that the size,
configuration, and even the operating principles of high
frequency acoustic transducers may differ substantially prom
those of low-frequency transducers. Separate and
independently operable transducers have been available for a
long time, which can faithfully reproduce sound within given
frequency bands. Efforts to reproduce high fidelity sound
for the human ears have targeted questions such as where the
frequency division should be made, how a transducer should
function within its assigned frequency range, how many
frequency divisions and transducers should be used, how the
transducers should be physically arranged and associated with
one another, and perhaps many other considerations of both
broad and narrow scope.
It has been a practice for some time to provide
speaker systems wherein the audio signal is divided into
upper and lower frequencies and distributed to transducers
particularly designed to best reproduce low or high frequency
sound. It has also been common, for various reasons, to
construct within a single assembly a combination of two or
more transducers in which the high frequency transducer is
coccal mounted with respect to the low frequency
transducer.
Coaxial loudspeakers have, in the past, employed
entirely independent transducers, their interrelationship
being almost entirely a matter of mechanical placement with
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some regard for the acoustical effects which result
therefrom. Typically "coaxial" speaker systems employ one or
more high frequency drivers mounted above the lower frequency
system by a post or bridge-like support, and, as a result,
often have irregular frequency response characteristics due
to phase cancellation between the drivers and diffraction
effects caused by the support apparatus.
Typical of the above features of the prior art, but
by no meat all-inclusive, are US. Patents Nos. 4,146,110
(Maloney); 3,796,839 (Torn); 3,158,697 (Gorky); and
2,259,907 (Olney). These patents all incorporate to varying
degrees the features mentioned above.
It is also well known that in acoustic transducers,
there are at least two types of drive mechanisms: the
permanent-magnet, moving-coil type and the piezoelectirc
type. US. Patent No 4,246,447 (Voyeur) is an example of the
piezoelectric mechanism.
The speaker system of the present invention
comprises a low frequency dynamic radiator type transducer or
woofer and one or more higher frequency transducer(s) or
tweeter(s) mounted in a single assembly, but not requiring
the elaborate and costly mounting techniques of the prior art
devices. The woofer unit typically is of the permanent-magnet,
moving-coil configuration, its dynamic radiator being a
diaphragm. The tweeter is mounted in the space defined by
the aforesaid diaphragm, and comprises a smaller diameter
diaphragm having situated at its apex a driver mechanism
comprising a piezoelectric element, or other driving element.
In this configuration, the entire mechanism
which constitutes the tweeter moves in unison with the
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low frequency diaphragm in the piston range and forms a part
of the total moving mass of the low frequency driver. This
configuration eliminates the customarily used mounting post or
brackets which support the high frequency unit(s) and also
improves the overall frequency response, dispersion, time, and
phase characteristics of the loudspeaker system.
Accordingly, it is an object of the present
invention to provide an improved multi-driver loudspeaker con-
struction having improved overall frequency response,
dispersion, and time and phase characteristics.
It is also an object of the present invention to
provide an improved multl-driver loudspeaker construction
which eliminates the need for a separate mounting apparatus
for the mid or upper frequency driving units.
The invention may be summarized according to one
broad aspect as a multi-driver loudspeaker combination
comprising: a first transducer of the dynamic radiator type
designed to reproduce sound in the lower portion of the audio
frequency range, said radiator including a diaphragm; a
piezoelectric transducer designed to reproduce sound in the
upper portion of the audio frequency range, intermediate
mounting means mounted on the diaphragm for supporting the
piezoelectric transducer, said piezoelec-tric transducer being
positioned within the periphery of the said diaphragm, said
piezoelectric transducer being mounted through the intermediate
mounting means upon said diaphragm and freely movable therewith
in an unrestrained manner.
These and other objects and advantages of the
present invention will be more readily apparent to those
skilled in the art upon reading the following detailed desk
Lowe
Croatian in conjunction with the accompanying drawing in which:
Figure 1 is a cross-sectional view of a multi-
driver loudspeaker system constructed according to the present
invention;
Figure 2 is a front elevation Al view of a multi-
driver loudspeaker system constructed according to the present
invention;
Figure 3 is a sectional view of the system of
Figure 2 r taken generally along section lines 3-3 thereof;
Figure 4 is a front elevation Al view of a multi-
driver loudspeaker system constructed according to the present
invention; and
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Figs. 5-7 are frequency response characteristics of
a prior art speaker and two speakers constructed according to
the present invention.
In the embodiment of the invention illustrated in
Fig. 1, the low frequency transducer or woofer is of the
permanent-magnet, moving-coil type and comprises a
permanent-magnet assembly 10 to which is secured a frame 12
having a generally somewhat conical configuration. The frame
12 defines an aperture 13 which defines generally the frontal
shape and area of the transducer. The shape of the aperture
13 formed by the frame can be other than circular, for
example, oval. The woofer diaphragm I extends or flares
generally conically outwardly and has its outer edge secured
to the periphery of the frame 12 by means of a compliant
suspension 16. The inner portion of the diaphragm 14 is
secured to a voice coil form 18 upon the lower portion of
which is the voice coil 20 which surrounds the center pole 22
of the permanent-magnet assembly 10 with the voice coil
positioned in the magnetic air gap 24 in the customary
fashion. Up to this point in the description, the
construction of the transducer is entirely conventional.
The high frequency transducer or tweeter comprises
the tweeter cone 30, the central axis of which is typically
aligned with the central axis of the woofer cone 14. The
tweeter cone 30 has a somewhat greater flare rate and is of
substantially smaller dimension than the woofer cone 14. At
the outer periphery of cone 30, a foam compliance ring 34 may
be positioned between the edge of cone 30 and the surface of
diaphragm 14. Behind the diaphragm 30 and extending along a
portion of the surface thereof, dampening or stiffening
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material 32 and 36 can be provided to smooth response and isolate the lead
wires if desired. The driver element 38 is positioned at the apex of cone
30. This driver element 38 comprises a piezoelectric crystal commonly
known in the trade as a bimorph or multi morph. The electrical leads 40 are
coupled to the crystal 38, and extend out through the woofer cone 14 in con-
ventional manner to input terminals 44 mounted upon a portion of the frame
12. The leads 40 from the crystal 38 join leads 43 which couple terminals
44 to the voice coil 20. The crystal 38 and voice coil 20 are thus elect
tribally coupled in parallel.
The connection of the single pair of input leads to both drivers
38 and 20 without utilization of a crossover network is made possible be-
cause the crystal driver 38 functions as a high-pass filter as well as a
tweeter driver, and depending upon the thickness, coupling coefficient and
diameter of the crystal 38 and the diameter of cone 30 and its shape, etc.,
provides an effective crossover frequency in the range anywhere from one to
ten kilohertz. An external filter network can be used if desired.
The damping rings 32 and 36, which illustratively can be formed of
fiberglass insulating material, are to suppress undesired vibrational modes
while the foam compliance ring 34 provides a means to control the mechanical
coupling between the woofer and tweeter cones 14, 30 in the crossover region
of response. A desirable acoustic response can thus be achieved by appear-
private selection of the material, the dimensions, the symmetry, and the post-
lion of the tweeter mechanism as well as variations in the decoupling ring
34 and damping rings 32 and 36.
hen operating in response to low frequency electrical signals,
the transducer assembly appears much as if it were a single piston. The
operation in response to high frequency signals above the crossover ire-
quench adds to the translational motion of the high frequency cone 30
essentially as if it were acting alone except that it is, in effect, mounted
upon a support which exhibits little or no movement at these high ire-
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quenches. The decoupling arrangement disposed between the woofer cone Andy tweeter cone 30 provides a method to control the degree of motion and
phase between the two cones in the midland and upper band response regions,
thus providing a means to control the electromechanical feedback to the
tweeter driving element, as described by the reciprocity principle. This
provides a smooth frequency response characteristic in the mid- and upper
band response regions. This mounting arrangement between the diaphragms 14,
30 leads to improved frequency response and dispersion for the overall
system and to improved time phase coherence throughout the desired frequency
lo range. From a mechanical point of view, the arrangement of the present
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invention also eliminates the need for the supplemental
mounting brackets customarily used in other coaxial systems
to support the higher frequency drivers.
In another embodiment of the invention illustrated
in Figs. 2 and 3, a permanent-magnet assembly 110 is secured
to a frame 112 having a generally elliptical or oval frontal
opening, illustratively 6 inches by 3 inches ~15.24 cm by
22.86 cm). The woofer diaphragm 114 extends generally
conically outwardly. The outer rim of diaphragm 114 is
secured to the oval frontal opening of the frame 112 by means
of a compliant suspension 116. The inward portion of the
diaphragm 114 is secured to a voice coil form 118 to which is
attached a woofer voice coil 120 positioned in the magnetic
air gap 124 in the customary fashion.
The tweeter of this embodiment comprises a tweeter
cone 130, the central axis of which is about 45 off the axis
of the woofer cone 114, as best illustrated in Fig. 3. A
junction area 131 is provided at the outer perimeter of
cone 130. This junction area 131 is glued or otherwise
attached, with or without a compliant member, to the
peremptorily edge 135 of an opening 133 provided in the woofer
cone 114. piezoelectric bimorph crystal driver element 138
is positioned at the apex of cone 130. Electrical leads 140
are coupled to the crystal 138 and extend to terminals 145
provided on the outside surface of woofer cone 114. The
leads 140 from the crystal 138 are coupled by leads 142 to
the input terminals 144 provided on the supporting frame 112.
Leads 142 also couple terminals 144 to the woofer voice coil
120. The woofer voice coil 120 and tweeter driver 138 thus
are coupled in parallel.
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Again, the coupling of the single pair of input
leads 142 to both drivers 138 and 120 without a divider or
crossover network is made possible because the crystal driver
138 acts as a high pass filter.
In another embodiment of the invention illustrated
in Fig. 4, a pexmanent-magnet assembly (not shown) is secured
to a frame 212 having a generally circular frontal opening.
The tweeter cones 230 can be molded into the woofer cone body
214, making the surrounding portion of the woofer cone 214 an
extension of the tweeter cone body. A woofer diaphragm 214
flares generally conically outwardly. Its outer perimeter is
secured to a circular frontal opening provided in the frame
212 by means of compliant suspension 216. The inner portion
of the diaphragm 214 is secured to a voice coil form upon
which is provided a voice coil which surrounds the center
pole of the permanent-magnet assembly with the voice coil
positioned in the air gap, all in a manner previously
discussed.
Four high frequency transducers or tweeters 229 are
20 mounted in the woofer diaphragm ~14 in a manner similar to
the tweeter diaphragm mounting illustrated in Fig. 3. Each
tweeter 229 comprises a tweeter cone 230~ the central axis of
which is illustratively 45 off the central axis of the
woofer cone 214, as in the embodiment of Figs. and 3. The
tweeter cones' axes are also positioned at 90 intervals
about the woofer cone 214 axis. As before, the tweeter cones
230 have somewhat greater flares and are of substantially
smaller dimension than the woofer cone 214D A piezoelectric
driver element (nut shown) is positioned at the apex of each
cone 230. The electrical terminations (not shown) to the
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crystals which drive tweeter cones 230 are made as in the
preceding embodiments. Again, the crystal drivers function
as high pass filters, and the frequency responses of the
drivers are selectable in part by proper selection of the
physical parameters of the various drivers and tweeter
cones 230.
The advantages of the off-axis placement of the
tweeter axes from the woofer axis in the embodiments of
jigs. 1-4 can best be appreciated with reference to
Figs. 5-7.
Fig. 5 illustrates the frequency response of a
prior art 6" by 9" (15.24 cm by 22.86 cm) oval speaker with a
coaxial secondary cone called a "whizzer." The
three-frequency response curves correspond to the on-axis
(0) frequency response of the speaker, the 30 off-axis
frequency response of the speaker, and the 45 off-axis
frequency response of the speaker. It will be appreciated
that, even with the whizzer cone, the off-axis (30 and
~5 off-axis) response of the speaker is significantly below
I the on-axis response (1-3 dub) even at such low frequencies as
2 K~z. At about 4 KHz, the off-axis performance has degraded
even more seriously (30 off-axis down about 5 dB9
45 off-axis down 14 dub). AT 15 KHz, 30 off-axis is down
13 dub, and 45 off-axis is down about the same amount.
Fig. 6 illustrates the frequency responses of a
6" by 9" (15.24 cm by 22.86 cm) elliptical constructed in
accordance with Fig. 1. Although the off-axis response at
2 KHz remains down about 1 and 3 dub (at 30 off-axis and
I off-axis, respectively), at 5 K~z, the 30 off-axis
response is down only about 1-1.5 dub, a 3.5-4 dub improvement
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over Fig. 5, and the 45 off-axis response is only down
8-8.5 dub, a 5.5-6 dub improvement over Fig. 5. At 15 KHz, the
improvement is equally as significant, with the 30 off-axis
response being down only about 10.5 dub, a 2.5 dub improvement
over Fig. 51 and the 45 off-axis only being down 8.5 dub, a
5.5 dub improvement over Fig. 5.
The frequency response characteristics of the
Figs. 2 and 3 embodiment of the invention are illustrated in
Fig. 7. In the embodiment tested for Fig. 7, the apex of the
tweeter cone projected into the plane of the surrounding
woofer cone lay half-way from the woofer cone axis to the
compliance ring. In other words, the tweeter was mounted
half-way out the woofer cone from the axis to the compliance
ring. AT 2 KHz, the 30 off-axis response was down about
1.5 -2 dub and the 45 off-axis response was down 5 dub. At
4 KHz, the 30 off-axis performance was actually 1-1.5 dub
above the on-axis performance and the 45 off-axis
performance was only about 1.5-2 dub lower than on-axis, both
substantial improvements over the embodiment of Fig. 5. At
15 KHz, the 30 off-axis performance and 45 off-axis
performance were actually both substantially above the
on-axis performance with 30 being about 4-5 dub above and
45 being about 10 dub above the on-axis performance.
