Electroacoustic piezoelectric transducer having a broad operating range

An electroacoustic piezoelectric transducer comprises a concave diaphragm and at least two planar bimorph elements having different respective resonance frequency characteristics coupled to the diaphragm. The bimorph elements may comprise larger and smaller bimorph elements providing frequency responses in lower and higher frequency ranges respectively, and are preferably directly connected to the inner concave surface of the diaphragm.

BACKGROUND OF THE INVENTION 
The present invention relates generally to electroacoustic transducers and, 
more particularly, to an electroacoustic transducer of the type driven by 
or driving a piezoelectric element that bends in response to electrical 
energy applied across it, or which produces electrical energy in response 
to bending or warping. 
Reference is made to the applicant's U.S. Pat. No. 4,845,776 issued July 4, 
1989, the disclosure of which is hereby incorporated by reference, with 
respect to the description therein of such electroacoustic piezoelectric 
transducers. 
A piezoelectric element of the type conventionally used in such 
electroacoustic transducers is referred to herein as a bimorph element and 
comprises two piezoelectric wafers having outer electrodes and which are 
sandwiched over a conductive metallic interlayer which forms an electrode 
intermediate the two wafers. The bimorph element has a substantially 
planar configuration and is relatively thin between its outer electrodes 
with respect to its lateral dimension thereby permitting the bimorph to 
flex along its long dimensions or diameters. When an electrical signal is 
positive on the outer electrodes with respect to the intermediate 
electrode, the center of the bimorph element flexes in one direction. 
Reversing the electric field polarity causes the bimorph to flex in the 
opposite direction. Impressing an alternating voltage across the 
electrodes causes the bimorph element to oscillate. 
Electroacoustic piezoelectric transducers are known which convert 
alternating electric voltage to sound, thereby acting as loudspeakers. In 
a loudspeaker of this type, a bimorph element is mounted to a concave 
diaphragm and is electrically coupled to an alternating audio voltage 
source as described above. As the bimorph element vibrates or oscillates, 
the diaphragm is caused to vibrate resulting in compressional airwaves 
transmitting audio vibrations in the usual manner. 
The operating range of electroacoustic piezoelectric transducers varies as 
a function of several variables, including among other things, the 
physical characteristics of the diaphragm, the arrangement by which the 
bimorph element is coupled to the diaphragm, and the vibrational 
characteristics of the particular bimorph element that drives the system 
(in the case of a loudspeaker). For a particular transducer construction, 
the frequency response of the transducer depends on the resonance 
vibration characteristics of the bimorph element. The resonance 
frequencies of the bimorph element provides an indication of the operating 
range of a transducer using such an element. Since the bimorph element 
generally has a high resonance frequency, it is possible to use a large 
bimorph element for a range of frequencies. Although the frequency 
response of a transducer can be improved by inserting a coil or inductance 
in series with the bimorph element and tuning the reactances to the center 
frequency of the desired bandwidth, the operating range of an 
electroacoustic piezoelectric transducer is generally limited by the 
vibrational characteristics of the bimorph element which is coupled to the 
diaphragm. 
SUMMARY OF THE INVENTION 
It is therefore an object of the present invention to provide new and 
improved electroacoustic piezoelectric transducers in which piezoelectric 
elements which bend in response to electrical energy, or which produce 
electrical energy in response to bending, are coupled to diaphragms. 
Another object of the present invention is to provide new and improved 
electroacoustic piezoelectric transducers having broad operating ranges 
and improved frequency response. 
In accordance with the present invention, these and other objects are 
attained by providing an electroacoustic piezoelectric transducer 
including a concave diaphragm having a central apex and an outer 
peripheral region about which the diaphragm can be mounted on a housing, 
and at least two bimorph elements having different respective resonance 
frequency characteristics coupled to the diaphragm, and electric circuit 
means for coupling the electrodes of the bimorph elements to a voltage 
source. 
The bimorph elements are preferably mounted directly to the inner concave 
surface of the diaphragm and extend substantially transversely to an axis 
that passes through the apex of the diaphragm along which vibrations occur 
when sound waves are produced or received. 
The bimorph elements preferably are polygonal and are directly secured to 
the inner concave surface of the diaphragm at their corners, with one of 
the bimorph elements being situated closer to the apex of the diaphragm 
than another one of the bimorph elements. The corners of each of the 
bimorph elements may be advantageously situated in substantially the same 
orientation as, and in substantial alignment with, the corners of other 
ones of the bimorph elements with respect to the diaphragm axis. 
When the electroacoustic piezoelectric transducer of the invention is used 
as a loudspeaker, the electric circuit means couple the outer and inner 
electrodes of the plurality of bimorph elements to a common voltage 
source. 
In this manner, the band of frequencies over which the transducer operates 
is expanded relative to the operating range of conventional transducers 
which utilize only a single bimorph element. For example, in a case where 
two bimorph elements having different respective frequency characteristics 
are used, one of the bimorph elements will provide good frequency response 
over a range of lower frequencies, while the other bimorph element 
provides good frequency response over a range of higher frequencies. 
Other features and advantages of the invention will become apparent from 
the following description.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring now to the drawings wherein like reference characters designate 
identical or corresponding parts throughout the several views, and more 
particularly to FIGS. 1 and 2, an electroacoustic piezoelectric transducer 
10 in accordance with the invention is illustrated. Transducer 10 
comprises a substantially conical compliant diaphragm 12 mounted on a 
housing 14 (FIG. 1) and a pair of bimorph elements 16 and 18 coupled to 
the diaphragm 12. 
Diaphragm 12 has a rearward substantially central apex 20 and a forward 
outer peripheral region 22 along which the diaphragm 12 is mounted on 
housing 14. The diaphragm 12 has an axis 24 (FIG. 2) that extends through 
the apex 20 along which vibrations occur when the diaphragm is producing 
or receiving sound waves. 
In accordance with the invention, bimorph elements 16 and 18 which are 
connected to the diaphragm 12 in a manner described below have different 
respective resonance frequency characteristics. For example, the smaller 
bimorph 16 may have a fundamental resonance frequency of 20 kilohertz 
while the larger bimorph element 18 may have a fundamental resonance 
frequency of about 2 kilohertz. 
Each of the bimorph elements 16 and 18 has a square configuration and is 
directly connected to diaphragm 12 by securing each of its four corners to 
the inner concave surface 26 of diaphragm, such as by means of epoxy 
adhesive or enamel. The corners 28 of bimorph element 16 are secured to 
the inner surface 26 of diaphragm 12 at locations substantially 
symmetrical with respect to the axis 24. Similarly, the corners 30 of 
bimorph element 18 are secured to the inner diaphragm surface 26 
substantially symmetrically with respect to axis 24 so that the planar 
bimorph elements 16 and 18 are substantially parallel to each other. 
Moreover, the corners 28 and 30 of bimorph elements 16 and 18 are secured 
to the inner diaphragm surface 26 in substantially the same orientation 
with respect to the axis 24 so that the corners of the respective bimorph 
elements are in substantial alignment with each other. This arrangement 
permits the diaphragm to be driven (in the case of a loudspeaker) in a 
manner which accurately approaches a desirable piston-like movement. 
Referring to FIG. 3, each of the bimorph elements 16 and 18 comprises a 
respective pair of piezoelectric wafers 32a,32a; 32b,32b on which outer 
layers of conductive material, such as nickel, are deposited to function 
as outer electrodes. The piezoelectric wafers 32 are sandwiched over 
respective conductive metallic interlayers 34a, 34b of bimorph elements 16 
and 18 which form intermediate electrodes of the bimorph elements 16 and 
18. The bimorph elements 16 and 18 are of the "bender" type in which 
in-phase vibrational displacements of the corner regions of each bimorph 
element occur upon the electrodes of the bimorph element being coupled to 
an alternating voltage source. 
Electric circuit means are provided for coupling the outer and inner 
electrodes of the bimorph elements to a common voltage source. Still 
referring to FIG. 3, the circuit means include conductor 36 for coupling 
the outer electrodes of the piezoelectric wafers 32 of bimorph elements 16 
and 18 to a positive terminal of a voltage source and conductor 38 for 
coupling the intermediate electrodes 34 of bimorph elements 16 and 18 to 
the negative terminal of the voltage source. 
When the diaphragm 12 is driven by impressing an alternating voltage across 
conductors 36 and 38, an improved frequency response extending over an 
operating range that is significantly broader relative to an operating 
range of a conventional transducer incorporating a single piezoelectric 
bimorph element is obtained. For example, the electroacoustic 
piezoelectric transducer may operate over a broad band of frequencies from 
1.5 to 40 kilohertz. The smaller bimorph element 16 will have good 
frequency response in a high band of frequencies while the larger bimorph 
element 18 will provide good frequency response in a lower band of 
frequencies. When the electroacoustic piezoelectric transducer is used as 
a loudspeaker, good frequency response is thereby obtained over a broad 
range of frequencies not possible heretofore. Bimorph elements having 
different resonance frequency characteristics are simply designed by 
varying the dimensions of the bimorph elements, and generally, larger 
bimorph elements have good frequency response over lower frequency ranges 
while smaller bimorph elements provide good frequency response in higher 
frequency ranges. Such different-size bimorph elements are easily directly 
connected to the diaphragm of a transducer by securing their exterior 
edges, such as their corners in the case of polygonal-shaped bimorph 
elements, to the inner concave surface of the diaphragm as described 
above. In this case, the smaller bimorph element is located closer to the 
apex of the diaphragm as clearly seen in FIG. 2. This arrangement also 
enhances the piston-like drive mode for the transducer. 
It will be understood that transducers in accordance with the invention can 
vary from the one shown and described herein. For example, the planar 
bimorph elements need not be square or even polygonal, but may have a 
circular wafer-type configuration, in which case the entire external edge 
of each bimorph element may be secured to the inner surface of the 
diaphragm. More than two bimorph elements having different resonance 
frequency characteristics may be coupled to a diaphragm as desired. 
Obviously, numerous other modifications and variations of the present 
invention are possible in the light of the above teachings. It is 
therefore to be understood that within the scope of the claims appended 
hereto, the invention may be practiced otherwise than as specifically 
disclosed herein.