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Full text of "Piezoelectric loudspeaker"

NASA CASE NO. 1 AR 1 5 138-1 

PRINT FIG L 

NOTICE 
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 

10/11/94 LaRC 



(NASA-Case-LAR-15138-1) N95-14110 

PIEZOELECTRIC LOUDSPEAKER Patent 

Application (NASA. Langley 

Research Center) 16 p Unclas 



G3/71 0029855 



LAR- 15138-1 AWARDS ABSTRACT 

PIEZOELECTRIC LOUDSPEAKER 

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 



Emplc 




employer: NASA Langley Research Center, Hampton, VA 
Initial Evaluator: Glenn R. Taylor 

Serial No.: 08/326,804 
Filed: 10/11/94 



LAR 15138-1 -1- PATENT APPLICATION 

PIEZOELECTRIC LOUDSPEAKER 

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, 
or piezoelectric. 

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 
expensive. 

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 
material. 

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 
frequencies. 

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 
loudspeakers. 

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 
side. 

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 
box 70. 

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: 



LAR 15138-1 -44^ PATENT APPLICATION 

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. 




FIG. I 



25 



10 




zzzzzzzzszz 



10- 



26 




FIG. 2 




10 28 



FIG 3 



10 60 



25 68 




77 16 78 66" 

FIG. 4 



F 




1 

4 



FIG. 5