NASA CASE NO. 1 AR 1 5 138-1
PRINT FIG L
The invention disclosed in this document resulted from research in aeronautical and
space activities performed under programs of the National Aeronautics and Space
Administration. The invention is owned by NASA and is, therefore, available for
licensing in accordance with the NASA Patent Licensing Regulation (14 Code of
Federal Regulations 1245.2).
To encourage commercial utilization of NASA-owned inventions, it is NASA policy to
grant licenses to commercial concerns. Although NASA encourages nonexclusive
licensing to promote competition and achieve the widest possible utilization, NASA
will provide the necessary incentive to the licensee to achieve early practical
application of the invention.
Address inquiries and all applications for license for this invention to NASA Patent
Counsel, Langley Research Center, Code 212, Hampton, Virginia 23681-0001.
Serial No.: 08/326,804
PIEZOELECTRIC LOUDSPEAKER Patent
Application (NASA. Langley
Research Center) 16 p Unclas
LAR- 15138-1 AWARDS ABSTRACT
The present invention relates in general to loudspeakers for
sound reproduction, and specifically to loudspeakers using
piezoelectric actuators to drive a speaker membrane.
According to the present invention, it is possible to make
a piezoelectric loudspeaker suitable as a midrange driver by
using a dome shaped piezoelectric actuator to move a speaker
membrane. The dome shaped actuator is made from a reduced and
internally biased oxygen wafer ("RAINBOW") of piezoelectric
material . The height of the dome apex varies with a voltage
applied between the convex and the concave surfaces of the dome
shaped actuator. Excursions in the order of 0.02 - 0.05 inches
and load capacity over 10 lbs. can be obtained at a rated drive
voltage of 350 V rms. The domed actuator is mounted between the
speaker membrane and a speaker frame so the dome height
determines an axial distance between the speaker membrane and the
frame. The rim of the domed wafer must be free to rock on its
edge when the dome height varies to ensure low distortion in the
loudspeaker. This is achieved by mounting the rim of the domed
wafer on a support surface by prestress only. An exceptionally
simple and inexpensive midrange driver uses a planar disc
supported on the rim of a domed piezoelectric actuator as a
speaker membrane. The center part of one side of the disc is
pressed against the rim of the domed actuator by prestress from
a latex surround stretched between the speaker membrane and the
speaker frame. Previous piezoelectric drivers could only be used
as tweeters, and even then horn loading was required to get
reasonable coupling to the listening area.
Inventor: Curtis Randall Regan
Home address: 138 E. Leicester Avenue
Norfolk, VA 23503
Employer: NASA Lang ley Research Center, Hampton, VA
Inventor: Antony Jalink ,~Jr^
Home address: 5 Madison Lane South
Newport News, VA 23606
Employer: NASA Langley Research Center, Hampton, VA
Inventor: Richard F. Hellbaum
Home address: 74 Banister Drive
Hampton, VA 23666
Employer: NASA Langley Research Center, Hampton, VA
Inventor : Wayne /Rohrbach
Home address: 204 Harris Grove Lane
Yorktown, VA 23692
employer: NASA Langley Research Center, Hampton, VA
Initial Evaluator: Glenn R. Taylor
Serial No.: 08/326,804
LAR 15138-1 -1- PATENT APPLICATION
Origin of the Invention
The invention described herein was made in the performance of work
done by employees of the U.S. Government and may be manufactured and
5 used by or for the government for governmental purposes without the
payment of any royalties thereon or therefore.
Backg round of the Invention
Field of the Invention
The present invention relates to loudspeakers for sound reproduction,
10 and more particularly to loudspeakers utilizing piezoelectric actuators to
drive a speaker membrane.
Descri ption of the Related Art
A loudspeaker system for sound reproduction typically consists of a
cabinet with one or more loudspeakers ("drivers") covering separate parts
15 of the desired frequency range. Typically there will be a high frequency
driver ("tweeter"), a midrange driver, and a bass driver ("woofer"). The
drivers are usually direct drivers, which have a speaker membrane coupled
directly to the air for radiation to the listening area. Horn drivers, which
have acoustic horns connected between the driven membrane and the free
20 air to improve the coupling efficiency, are used mostly for high power public
address applications. In either type of driver, the speaker membrane is
moved back and forth in response to an electric voltage from an amplifier
LAR 15138-1 -2- PATENT APPLICATION
by means of an actuator, which can be either electromagnetic, electrostatic,
An electromagnetic loudspeaker uses a cylindrical voice coil of
metal wire suspended in a radial magnetic field as an actuator. The voice
5 coil is connected electrically to the amplifier output and mechanically to the
speaker membrane, which moves in response to the axial force generated
by the current flowing in the voice coil wire. The speaker membrane is
usually a cone or small dome of thin walled material. Electromagnetic
loudspeakers are today the dominant type of drivers.
1 An electrostatic loudspeaker uses a thin metallized film suspended in
an electrostatic field as both actuator and speaker membrane. The
metallized film is suspended between two acoustically open wire mesh
screens. A high voltage electrostatic field is set up between the two mesh
screens, and an alternating voltage derived from the amplifier output is
15 impressed on the metallized film, which makes the film/membrane move
back and forth in the electrostatic field to generate sound waves. The force
per unit area of the membrane is small, so the membrane must be large to
provide substantial sound pressure levels. Electrostatic loudspeakers are
20 A piezoelectric loudspeaker uses a piezoelectric actuator to drive the
speaker membrane. A conventional piezoelectric actuator has very small
maximum excursions, so piezoelectric drivers have been limited to use in
earphones and high frequency horn speakers.
U.S. Patent No. 3,900,748 to Adler describes a coiled element of
25 ferroelectric material for use as a piezoelectric actuator for driving a speaker
membrane. Large axial excursions of the coil ends are possible by arranging
electrode pairs to set up shear stresses in the material so the element will
twist along its center line when an electric potential difference is imposed
between the electrode pairs. The element may be coiled either helically or
LAR 15138-1 -3- PATENT APPLICATION
spirally. In either case, the moving end of the material is coupled
mechanically to a cone shaped membrane. Adler states that the described
piezoelectric actuator has high compliance. This means that the force
exerted on the speaker cone will be low, and that the moving end of the coil
5 will require centering and guiding. The coiled piezoelectric elements are
complicated and expensive to manufacture.
Summary of the Invention
It is an object of the present invention to provide a piezoelectric
loudspeaker suitable for use as a direct coupled midrange driver.
10 It is a further object of the invention to provide a midrange driver of
simple and rugged design using a dome shaped actuator of piezoelectric
It is a still further object of the present invention to provide a direct
coupled loudspeaker utilizing a dome shaped piezoelectric actuator that has
15 low distortion.
These and other objects are accomplished by a loudspeaker
comprising a speaker membrane; a speaker frame; a dome shaped actuator
made from a reduced and internally biased oxide wafer of piezoelectric
ceramic material, and which has a dome height that varies with a voltage
20 applied between the outside and inside surfaces of the dome shaped
actuator; and means for mounting the actuator between the speaker
membrane and the speaker frame so an axial distance between the speaker
membrane and the frame is determined by the dome height of the actuator.
A preferred embodiment allows the edge of the rim of the dome shaped
25 actuator to rock on a support surface when the dome height changes.
LAR 15138-1 -4- PATENT APPLICATION
Brief Description of the Drawing s
The present invention and the objects achieved by it will be
understood from the description herein, with reference to the accompanying
drawings, in which:
5 FIG. 1 is an axial sectional view through a piezoelectric actuator for
a loudspeaker according to a preferred embodiment of the invention.
FIG. 2 is an axial sectional view through a pair of piezoelectric
actuators as shown in FIG. 1 stacked rim against rim in clamshell fashion
according to a preferred embodiment of the invention.
10 FIG 3 is an axial sectional view through a midrange driver according
to a preferred embodiment of the invention.
FIG. 4 is an axial sectional view through a midrange driver of planar
design along line 4-4 as shown in FIG. 5 according to a preferred
embodiment of the invention.
15 FIG. 5 is a view from the rear of the midrange driver shown in FIG.
4 taken along line 5-5 therein.
Detailed Descri ption of the Preferred Embodiments
FIG. 1 is an axial sectional view through a piezoelectric actuator 10
made from a reduced and internally biased oxide wafer 12. The actuator
20 10 is dome shaped and is made from a flat wafer of a piezoelectric ceramic
material, such as lead-lanthanide-zirconium-titanate (PLZT), by reducing one
surface 15 while the other surface 14 is protected from the reducing
medium. The reduced surface shrinks, so internal strains are set up in the
wafer 1 2, and the wafer 1 2 takes on a shallow dome shape as illustrated
25 in FIG. 1. The curvature (r) and the height (h) from rim 17 to apex 16 of
the domed actuator 10 are exaggerated in FIG. 1 to be readily visible in the
LAR 15138-1 -5- PATENT APPLICATION
drawing. Actuators 10 are available with a diameter (d) from 0.5" to 4" and
wafer thickness from 0.006" to 0.060".
The concave inner surface 15 of the actuator 10 is reduced to a
conductive form of lead oxide, so it can directly serve as an electrode in the
5 actuator 10. A conducting film 14 is applied to the convex outer surface
of the actuator 10 to serve as a second electrode. The conducting film can
be a metallic film deposited by sputtering, a conductive paint, or any other
conductive film known in the art.
When an electric voltage is applied between the electrodes 14 and
10 15, a piezoelectric strain is generated in the wafer 12. This causes the
radius of curvature (r) of the actuator 10 and the corresponding height (h)
from rim 17 to apex 16 to change. The change in height (h) is typically
about ± 0.02" in a 1.5" diameter actuator 10 for a voltage variation of
± 500 V.
15 The excursion provided by this type of dome shaped actuator 10 is
about 100 times larger than the maximum excursions generated by
conventional direct extending piezoelectric actuators, and about 10 times
the excursion of bimorph piezoelectric actuators. The typical load capacity
of the dome shaped actuator 10 is about 10 lbs., which is the about the
20 same as the load capacity of direct extender piezoelectric actuators, but
more than 100 times the load capacity of bimorphs. Large excursion
combined with large load capacity makes the domed piezoelectric actuator
10 suitable for driving speaker membranes in loudspeakers for midrange
25 Twice as large excursions can be obtained from a pair of dome
shaped actuators 10, 10' stacked rim against rim in clamshell fashion, as
shown in FIG. 2. A strip of metal foil 25 inserted between the rims of the
two actuators 10, 10' contacts the inner surface electrodes 15 of both
actuators 10 and 10', and another strip of metal foil 26 interconnects the
LAR 15138-1 -6- PATENT APPLICATION
two external electrodes 14. When a voltage is applied between the metal
foil strips 25 and 26, both actuators 10 and 10' change their heights (h) in
the same direction. Several such clamshell assemblies can be cascaded if
still larger excursions are needed.
5 A first preferred embodiment of the invention is illustrated in FIG. 3,
which is an axial sectional view through a loudspeaker 30 using a dome
shaped piezoelectric actuator 10 to directly drive a speaker membrane in the
form of a conventional speaker cone 32. The speaker cone 32 is mounted,
as is common in the art, to a mounting flange 36 via a surround member 34
10 of rubber. The mounting flange 36 is part of a conventional speaker basket
40 with a flange 42 for support of the actuator 1 driving the speaker cone
32. The surround member 34 is weak axially, but sufficiently rigid in the
lateral plane to keep the speaker cone 32 centered. When the loudspeaker
30 is mounted on the wall of a loudspeaker cabinet, the surround member
1 5 34 also seals the cabinet so out of phase sound pressure from the rear of
the loudspeaker cone 32 does not interfere with the sound waves radiated
from the front of the speaker cone 32.
The narrow end of the speaker cone 32 is closed by a semi-spherical
bottom end. The apex of a dome shaped piezoelectric actuator 10 as
20 shown in FIG. 1 is mechanically connected to the bottom end of the speaker
cone 32 by a screw or a rivet 28 passing through holes in the actuator 10
and the bottom end of the speaker cone 32. Insulation must be provided
to avoid short circuiting the outer electrode 1 4 and inner electrode 1 5 of the
dome shaped actuator 10, e.g., by using a plastic fastener for connecting
25 the apex of the actuator 10 to the speaker cone 32.
The rim 17 of the actuator 10 is mounted to the flange 42 via a
mounting ring 46 of insulating material and an O-ring 48 of soft elastomeric
material. The mounting ring 46 is fastened to the flange 42 by screws 49,
and a spacer ring 44 is inserted between the mounting ring 46 and the
LAR 15138-1 -7- PATENT APPLICATION
flange 42 to maintain a predetermined pressure by the O-ring 48 on the rim
of the actuator 10. The pressure from the O-ring 48 provides a prestress
force of 4 to 8 oz between the rim 1 7 of the actuator 1 and the mounting
plate 46. Strips 24 and 25 of metal foil are applied to the outside and inside
5 electrodes 14, 1 5 of the actuator 10 to serve as leads for the drive voltage.
An alternating voltage applied between metal strips 24 and 25, will cause
the height (h) from the rim 1 7 to the apex 1 6 of the actuator 1 to alternate
with the voltage.
When the polarity of the drive voltage is such that the height (h)
10 increases, the apex 16 of the actuator 10 will push the speaker cone 32
outward, away from the mounting plate 46, so the sound pressure in front
of the speaker cone 32 increases. The force exerted by the apex 1 6 of the
actuator 10 will cause a reaction force between the rim 17 and the fixed
mounting ring 46, which adds to the prestress force from the O-ring 48.
1 5 When the drive voltage has the opposite polarity, the apex 1 6 of the
actuator 10 will pull the speaker cone inward, thereby reducing the sound
pressure in front of the speaker cone 32. At the same time, the rim 1 7 of
the actuator 10 will be pulled away from the mounting plate 46. As long
as the prestress force exerted by the O-ring 48 is larger than the maximum
20 pulling force on the actuator 10, the rim 17 of the actuator 10 will remain
pressed against the mounting ring 48, and the actuator 10 will behave as
if it were firmly attached to the mounting plate 46. The limited pressure
from the O-ring 48, however, does allow the edge of the rim 17 on the
dome shaped actuator 10 to rock on the mounting ring 46 when the radius
25 of curvature (r) of the actuator 10 changes in response to the drive voltage.
One way to mount the rim 17 of the actuator 10 on the mounting ring
46 would be by soldering or gluing. This would allow for much larger
negative forces on the apex 16 of the actuator 10, but the rim 17 would
then be locked in place, so it could not rock on its edge when the radius of
LAR 15138-1 -8- PATENT APPLICATION
curvature (r) of the actuator 10 changes in response to the drive voltage.
The actuator 10, accordingly, would not be able to maintain a true spherical
curvature when its height apex (h) varies. This introduces spurious strains
in the actuator 10 and causes nonlinearities in the apex excursions (h).
5 Accordingly, as embodied herein, a main source of nonlinearity in the
loudspeaker 30 is eliminated by mounting the rim 1 7 of the actuator on its
contact surface by prestress only, so the edge of the rim 1 7 is free to rock
when the apex height (h) changes.
The apex 16 of the dome shaped actuator 10 is laterally stable, so it
10 can center the narrow end of the speaker cone 32 without need for a
separate centering spider, which is required in electromagnetic
A second preferred embodiment of the invention is a planar midrange
driver illustrated in FIGS. 4 and 5. FIG. 4 is an axial sectional view through
15 a loudspeaker 60, and FIG. 5 is a rear view of the moving parts of the
loudspeaker 60 taken along section line 5-5 in FIG. 4. The radiating element
of the loudspeaker 60 is a 3" diameter planar disc membrane 62 made from
0.064" thick styrofoam. The styrofoam disc membrane 62 is supported by
the rim 1 7 of a dome shaped actuator 1 0. The apex 1 6 of the actuator 1
20 is supported by a frame member 78 in the form of a 0.092" thick steel wire
via a rubber disc 77. The speaker membrane 62 is prestressed against the
rim 17 of the actuator 10 and the frame member 78 by means of a 0.0005 -
0.001" thick latex film 64 serving as a surround.
The loudspeaker 60 is assembled by first stretching the latex film 64
25 flat on a mounting ring 72, and then clamping the rim of the film 64
between the mounting ring 72 and a mounting flange 68 by screws 71.
The membrane 62 and the actuator 10 with contact strips 24, 25 attached
are next centered on the inside of the flat latex film 64, the rubber disc 77
is placed on the apex 16 of the actuator 10, and the frame wire 78 is
LAR 15138-1 -9- PATENT APPLICATION
pressed against the actuator 10 until its ends fit in cut-outs in the mounting
ring 72. The thickness of the mounting ring 72 is designed to provide
sufficient stretching of the latex film 64 to provide a prestress force of 4 to
8 oz between the rim 17 of the actuator 10 and the membrane 62 on one
5 side and apex 16 of the actuator 10 and the frame wire 78 on the other
The mounting ring 72 is finally mounted on a closed driver box 70 by
means of screws 71, so the frame wire 78 is clamped in place in its cutout.
The driver box 70 is lightly filled with acoustic damping material 73, such
10 as glass fiber insulation or acoustic foam, as is common in the art.
Connectors 66, 67 for the drive voltage are provided in the bottom of the
An increase in apex height (h) of the actuator 10 caused by a drive
voltage between terminals 66, 67 forces the membrane 62 outward against
1 5 tension in the latex film 64. A decrease in the apex height (h) makes the
latex film 64 pull the membrane 62 inward to remain in contact with the
retreating rim 17 of the actuator 10. In either case, the movement of the
membrane 62 is determined by the apex height (h) of the actuator 10. The
inward movement of the membrane 62 follows a decrease in apex height (h)
20 in the actuator 10 only as long as the axial pull from the latex film 64 is
larger than the outward force on the membrane 62 from the reduced sound
pressure and acceleration forces. For a midrange driver 60, the sum of
such forces are lower than the initial 4 oz prestress force, so there is no risk
that the actuator 10 will lose its mechanical contact with the speaker
25 membrane 62 or the frame wire 78.
The loudspeaker 60 functions the same way as the loudspeaker 30
described earlier with reference to FIG. 3. In both cases, the rim 17 of the
dome shaped actuator 10 is free to rock and expand on its support surface,
so nonlinearities are minimized. In the planar loudspeaker 60 (FIGS. 4-5),
LAR 15138-1 -10- PATENT APPLICATION
the prestress force holding the rim 17 of the actuator 10 in place on its
contact surface, however, is applied across the actuator 10, which is
sandwiched between the speaker membrane 62 and the speaker frame
comprising box 70 and frame wire 78. The source of the prestress force in
5 this case, therefore, should be able to accommodate the full excursions of
the apex 16 of the dome shaped actuator 10 without excessive changes in
the prestress force. This is accomplished by the relatively wide and thin
latex film used as the surround member in loudspeaker 60. The source of
the prestress in loudspeaker 30 (FIG. 3) needs only accommodate the slight
1 rocking motion of the edge of the rim 1 7 of the actuator 1 0, so a relatively
rigid O-ring 48 is a suitable means for prestressing the rim 17 of the
actuator 10 against its support surface in that loudspeaker 30.
The planar loudspeaker 60 illustrated in FIGS. 4 and 5 is extremely
simple in design and can be manufactured at very low cost. The rim 1 7 of
15 the actuator 10 provides support for the planar speaker membrane 62
between the center and the periphery of the membrane 62, so a very thin
and light membrane can be used.
Twice as large excursions as those obtained from the single dome
shaped actuator 10 can be obtained by supporting the planar speaker
20 membrane 62 by a pair of actuators connected at their apexes by a rivet or
screw and providing a flat support surface for the rim of the second
actuator on the frame wire 78.
The rated drive voltage for either of the piezoelectric speakers 30, 60
described above is about 350 V rms, while the rated output voltage from
25 commercially available audio amplifiers is only about 20 V rms. The output
voltage from an audio amplifier, however, can easily be converted to a
higher voltage by a small transformer, which would be included as part of
a crossover network regularly included in a loudspeaker system for deriving
separate signals for tweeters, midrange drivers, and woofers. A
LAR 15138-1 -11- PATENT APPLICATION
loudspeaker according to the preferred embodiments of the invention thus
can easily be incorporated in a loudspeaker system powered by a
conventional audio amplifier.
A loudspeaker 30 or 60 as described above and illustrated in FIGS.
5 3-5 with excursion in the order of 0.020" and a speaker membrane with
diameter 3" - 3.5" generates sound pressures sufficient for a hi-fi system
down to frequencies below 1 ,000 Hz. The described loudspeakers 30, 60
according to the above-described preferred embodiments of the invention
thus can be used as midrange drivers. This was not possible with
1 previously known piezoelectric loudspeakers, which had so small excursions
that they could only generate sufficient sound pressure as tweeters.
The dome shaped actuator 10 has low compliance and large load
capacity so it can drive a loudspeaker 30, 60 up to the highest audible
frequencies. A loudspeaker 30, 60 could thus theoretically be used as a
1 5 combined tweeter/midrange driver. The limiting factor would in practice be
"beaming" at high frequencies, because the diameter of the speaker
membrane is large compared to the sound wavelength at frequencies in the
mid to upper kHz range. Beaming can to some extent be controlled by
diffusers or acoustic lenses. The cost of a loudspeaker 30, 60 according
20 to the embodiments of the invention, however, is so low that it may in most
cases be more economical to build separate tweeters similar to the
described midrange drivers 30, 60, but with small diameter domed speaker
membranes for better dispersion at the highest frequencies.
Numerous modifications and adaptations of the present invention will
25 be apparent to those skilled in the art. Thus, the following claims and their
equivalents are intended to cover all such modifications and adaptations
which fall within the true spirit and scope of the present invention.
What is claimed is:
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Ahstrant of thm Disclosure
A piezoelectric loudspeaker suitable for midrange frequencies uses a
dome shaped piezoelectric actuator to drive a speaker membrane directly.
The dome shaped actuator is made from a reduced and internally biased
5 oxygen wafer, and generates excursion of the apex of the dome in the order
of 0.02 - 0.05 inches when a rated drive voltage of 350 V rms is applied
between the convex and the concave surfaces of the dome shaped
actuator. The load capacity exceeds 10 lbs. The edge of the rim of the
dome shaped actuator must be free to rock when the dome height varies to
10 ensure low distortion in the loudspeaker. This is achieved by mounting the
rim of the dome shaped actuator on a support surface by prestress only.
An exceptionally simple design uses a planar speaker membrane with the
center part of one side pressed against the rim of a dome shaped actuator
by prestress from a stretched latex surround member.
77 16 78 66"