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NASA CASE NO.

PRINT FIG. ___

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-tAR-15138-1)

PIEZOELECTRIC LOUDSPEAKER Patent

AppliCdtion (NASA. Langley

Research Center) 16 p

N95-14110

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 (llRAINBOW1l) 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 

23503 

Employer : Research Center, Hampton, VA 

Inventor : Antony Jalink) TT, 

Home address: 5 Madison Lane South , VA 23606 

oyer : NASA Langley Research Center, Hampton, VA 

Inventor : Richard F. Hellbaum 

Home address: 74 Banister Drive 

23666 

yer : NASA Langley Research Center, Hampton, VA 

Inventor : Wayne ?'. Rohrbach 

Home address: 204 Harris Grove Lane 

F 23692 

mployer : y 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

5

The invention described herein was made in the performance of work

done by employees of the U.S. Government and may be manufactured and

used by or for the government for governmental purposes without the

payment of any royalties thereon or therefore.

Background of the Invention

Field of the Invention

10

The present invention relates to loudspeakers for sound reproduction,

and more particularly to loudspeakers utilizing piezoelectric actuators to

drive a speaker membrane.

Description 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.

10 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
5

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

will require centering and guiding. The coiled piezoelectric elements are

complicated and expensive to manufacture.

Summary of the Inv_ntinn

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-

BriAf DA_cription of thA Drawing_

PATENT APPLICATION

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.

DetailAd Description of thA PrAfArrAd l=mhodimAnts

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 12, and the wafer 12 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 Ibs., 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 10 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

15 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 14 and inner electrode 15 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 17 of the actuator 10 and the mounting

plate 46. Strips 24 and 25 of metal foil are applied to the outside and inside

5 electrodes 14, 15 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 17 to the apex 16 of the actuator 10 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 16 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.

15 When the drive voltage has the opposite polarity, the apex 16 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 17 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 17 of the actuator on its

contact surface by prestress only, so the edge of the rim 17 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 17 of a dome shaped actuator 10. The apex 16 of the actuator 10

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

15 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 -1 O- 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

10 rocking motion of the edge of the rim 17 of the actuator 10, 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 17 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

10 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

15 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

Ab,qtrant of the Di_nlosure

PATENT APPLICATION

5

10

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

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 Ibs. The edge of the rim of the

dome shaped actuator must be free to rock when the dome height varies to

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.
I0 16 14

FIG. I

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FIG_ 3
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64 62 't' ( _ z4 zs

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77 16 78 66 --M

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FIG. 4

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