US 3736533 A
An acoustic signal propagated in an acoustic medium having an interface with a piezoelectric transucer is efficiently converted by the transducer into electrical signal energy by the establishment of an evanescent electrical field within the piezoelectric transducer.
Description (OCR text may contain errors)
iliiited St; Alphonse APPARATUS FOR EFFICIENTLY CONVERTING ACOUSTIC ENERGY INTO ELECTRICAL ENERGY Gerard Argant Highstown, NJ.
Assignee: RCA Corporation, New York, NY. Filed: Dec. 15, 1971 Appl. No.: 208,344
US. Cl. ..333/30, 310/83, 333/73 Int. Cl ..I-I03h 7/30 Field of Search ..333/30, 73; BIO/8.3
References Cited UNITED STATESPATENTS 12/1962 Ranliin ..333/30 R 1 1 3,736,533 1 1 May 29, 1973 3,534,300 10/1970 Jouffroy et al ..333/30 R 3,464,033 8/1969 Tournois ..333/30 R 3,409,848 11/1968 Meitzler ct al ..333/30 R 2,527,986 Carlin ..333/30 R Primary ExaminerRudolph V. Rolinec Assistant ExaminerSaxfield Chatmon, Jr. AttorneyEdward J. Norton  ABSTRACT An acoustic signal propagated in an acoustic medium having an interface with a piezoelectric transucer is efficiently converted by the transducer into electrical signal energy by the establishment of an evanescent electrical field within the piezoelectric transducer.
7 Claims, 3 Drawing Figures OUTPUT ELECTRICAL SIGNAL INPUT ACOUSTIC SIGNAL Patented May 29, 1973 3,736,533
I ELECTRICAL 0 INPUT 2 9- ACOUSTIC SIGNAL l l l APPARATUS FOR EFFICIENTLY CONVERTING ACOUSTIC ENERGY INTO ELECTRICAL ENERGY DESCRIPTION OF THE PRIOR ART An electrical signal can be generated from an acoustic signal by a piezoelectric transducer. One method of converting acoustic energy to electrical energy is to allow the mechanical vibrations of the acoustic signal to induce an electric field between a pair of parallel electrodes located on opposite surfaces of a piezoelectric transducer. The conversion efficiency of this method is dependent on the separation between transducer electrodes. An electrode separation of several acoustic wavelengths causes the induced electric field between electrodes to change polarity many times, resulting in a decrease in conversion efficiency. Optimum conversion efficiency is obtained when there is a standing acoustic wave in the transducer, i.e., when the transducer thickness is one-half of an acoustic wavelength. Under these conditions, the induced electric field undergoes only one polarity change per acoustic cycle. However, an extremely thin transducer having a thickness or an electrode separation equivalent to a fraction of an acoustic wavelength at microwave frequencies is an impractical device. The conversion from acoustic energy to electrical energy has been accomplished by other techniques and systems that are relatively inefficient and frequency limited.
SUMMARY OF THE INVENTION An apparatus for efficiently converting acoustic energy to electrical energy is described. The apparatus comprises piezoelectric material capable of supporting an acoustic signal at a given velocity and having a surface interfaced with an acoustic signal propagating means capable of propagating an acoustic signal at a velocity less than the given velocity. The piezoelectric material and acoustic signal propagating means are interfaced in a manner that results in the acoustic signal establishing an evanescent electric field within the piezoelectric transducer. The piezoelectrics internal evanescent electric field provides an efficient conversion of the acoustic energy to electric energy.
DESCRIPTION OF THE DRAWINGS FIG. 1 is a diagram illustratig operating principles useful in an understanding of the invention.
FIG. 2 is a perspective view, partially sectioned, of an embodiment of the disclosedinvention.
FIG. 3 is an equivalent electrical circuit of the piezoelectric transducer of FIG. 2 when it is coupled to a waveguide transmission line.
DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to FIG. 1, there is shown a diagram illustraing the operating principles of an apparatus that efficiently converts acoustic energy into electromagnetic energy. An acoustic signal is transmitted in an acoustic medium toward a piezoelectric transducer 11. The direction of acoustictransmission is represented by the propagation vector, K,. The vector K, is incident at the interface 12 between the acoustic medium 10 and the piezoelectric transducer 11 at an incident angle, 0,. The incident angle, 0,, is measured from an axis nonnal to the interface 12. Under certain boundary conditions, the acoustic signal coupled to the piezoelectric transducer 11 is evanescent within the transducer 11. The evanescent acoustic signal induces a nonpropagating electric field within the piezoelectric transducer 11. The equation of the electric field, E, is obtained from the theory of piezoelectricity as:
E=gT e""' where T, is the acoustic pressure at the interface, a is a variable attenuation constant, x is the internal piezoelectric depth and g is the piezoelectric stress constant. A first boundary condition requires that the velocity of sound v, in the acoustic medium 10 be less than the velocity of sound v; of the piezoelectric transducer 11. A second boundary condition requires that the incident angle, 0,, exceed a critical angle of incidence 0,, sin 11 /1 where v, and v, are the respective velocities of sound in the acoustic medium ,10 and the piezoelectric transducer 1.1. The critical angle of incidence, 0, is also measured from an axis normal to the interface 12. The variable attenuation constant, a, of the transducers evanescent electric field is given by the equation:
where f is the frequency of the acoustic signal, v, and v, are the velocities of the acoustic medium 10 and piezoelectric transducer 11, respectively, and 0, is the intric thickness on conversion efficiency. The piezoelectric thickness need only be equal to or greater than (I /2, where 0,, is the acoustic wavelength at the lowest frequency of operation. Also, it can be shown by equations (1) and (3) that the difference in potential between the interface 12 and the internal piezoelectric depth, d, is independent of frequency.
Referring now to FIG. 2, a bulk acoustic wave, from a source not shown, propagates through an acoustic medium 20 bonded to a surface of a piezoelectric transducer 21. Vacuum indium bonding may be used to interface the piezoelectric transducer 21 to the acoustic medium 20. The magnitude of the velocity of sound, 11,, in the acoustic medium 20 is less than the magnitude of the velocity of sound, v,, in the transducer 21. An example of a suitable acoustic medium 20 is fused silica with a velocity of sound substantially equal to 5.96 Km/sec. A suitable piezoelectric transducer 21 is lithium niobate with a velocity of sound substantially equal to 7.5 Km/sec. Examples of acoustic energy sources are described in the November, 1969 issue of the IEEE Transactions of Microwave Theory and Techniques.
The acoustic propagation vector K represents the direction of acoustic transmission. The acoustic signal is incident at the interface 22 between the acoustic medium 20 and the piezoelectric transducer 21 at an angle 0,. The angle of incidence, 0,, is measured from an axis normal to the interface 22. The angle of incidence, 0,, exceeds the critical angle, 0, determined by equation (1). Therefore, the necessary boundary conditions are present in order to establish a nonpropagating electric field within the piezoelectric transducer 21. The exponential decay of the piezoelectrics electric field described by equation (I) and the resulting potential difference between the interface 22 and an internal piezoelectric point d (FIG. 1) permits the conversion from acoustic energy to electrical energy by the piezoelectric transducer 21. The nonpropagating electric field induced in the transducer 21 establishes a voltage source, V, defined by the equation:
13 v=-L EdX=% esinh at z V gT /a when the piezoelectric thickness t is in the order of several millimeters and the acoustic frequency is in the UHF range and above. The piezoelectric transducer 21 is coupled to a suitable electrical transmission line. In this embodiment, the piezoelectric transducer 21 is inside a section of waveguide transmission line 23 properly dimensioned for the propagation of electromagnetic energy at the frequency of the acoustic signal. The waveguide short circuit 24 acts as a tuning mechanism for the impedance of the transducer 21. The indium bond 22 between the acoustic medium 20 and the piezoelectric transducer 21 prevents unwanted radiation of electromagnetic energy outside of the waveguide transmission line 23.
Referring to FIG. 3, there is shown an equivalent electrical circuit of the transducer 21 coupled to the transmission line 23. The generator of electromagnetic energy is the voltage source V. The voltage source, V, is a result of the time varying or oscillating electric field induced in the transducer 21 by the acoustic signal and is described by equation (5). The time varying oscillating electric field of a negative resistance semiconductive device responsive to an applied D.C. signal is an analogous source of electromagnetic energy. The impedance Z is the load impedance including the impedance of the transmission line 23. The matching impedance, Z is the complex conjugate of the electromagnetic impedance of the combination of transducer 21 and waveguide 23. The matching impedance, 2,, is substantially determined by the location of the variable waveguide short circuit 24 with respect to the transducer 21. It can be shown that the conversion efficiency, n, of the apparatus is defined by the equation:
where g is the piezoelectric stress constant, p is the density of the acoustic medium 20 having a velocity of sound 1/, and a cross-sectional area A, a is the attenuation constant given by equation (3), 2,, and Z are the respective impedances of the respective impedances of the load and matching structure. The conversion efficiency 1; can be as high as 50 percent.
An acoustic wavelength is much shorter than an electrical wavelength. This effect is advantageously used'in the construction of delay lines having relative long delay times. In the embodiment of FIG. 2, an input electrical signal is converted into a bulk acoustic wave by any suitable method described in the literature or in the November, 1969 issue of the IEEE Transactions On Microwave Theory and Techniques. The acoustic wave is propagated through the acoustic medium 20 and efficiently converted back to an output electrical signal by the apparatus of FIG. 2. The time delay between the input and output electrical signals is substantially determined by the length of the acoustic medium 20.
The application of the disclosed apparatus has been illustrated in an efficient delay line. Numerous and varied other arrangements can readily be devised in accordance with the disclosed principles.
What is claimed is:
1. An apparatus comprising:
a piezoelectric material capable of propagating an acoustic signal at a velocity v, and establishing an evanescent electric field within said material in response to an applied acoustic signal incident on a surface of said material at a given angle measured from an axis normal to said surface, and
an acoustic signal propagating means capable of propagating said applied acoustic signal at a velocity 1 which is less than said velocity v, and interfaced with said piezoelectric material at said surface to cause said applied acoustic signal to be incident on said piezoelectric surface at said given angle exceeding a predetermined critical angle, 0, measured from an axis normal to said surface and defined by the equation:
he i/ 1) where v, and v, are the respective velocities of an acoustic signal propagated through said acoustic propagating means and said piezoelectric material, whereby said acoustic signal is converted to electrical energy by said piezoelectric material.
2. An apparatus according to claim 1, wherein said given angle substantially determines the magnitude of the attenuation constant, a, of said piezoelectrics evanescent electric field, said attenuation constant, a, defined by the equation:
where v, and v, are the respective velocities of an acoustic signal propagated through said acoustic signal propagating means and said piezoelectric material, f is the frequency of said applied acoustic signal and 0, is said given angle.
3. An apparatus according to claim 1, wherein said evanescent electric field establishes a difference in electric potential between said interface and an internal piezoelectric depth located substantially at 0/2 from said interface, where .Q is the acoustic wavelength at the frequency of said applied acoustic signal, said difference in electric potential having a magnitude determined by said given angle.
4. An apparatus according to claim 1, including means for coupling said electrical energy converted from said acoustic signal by said piezoelectric material to an electrical transmission line.
5. An apparatus according to claim 4, wherein said electrical transmission line comprises a waveguide transmission line.
6. An apparatus according to claim 5, including means for varying the impedance of said waveguide transmission line. I
7. An acoustic delay line comprising:
an acoustic medium capable of propagating an acoustic signal at a given velocity 11,,
a piezoelectric transducer capable of propagating said acoustic signal at a velocity v greater than said given velocity v -and establishing an evanescent electric field within said transducer in response to said acoustic signal incident on a conducting surir: sin-l i/ 2) where 11 and v are the respective velocities of an acoustic signal propagated through said acoustic propagating means and said piezoelectric material, whereby said acoustic signal is converted into electrical energy,
an electrical transmission line, and said piezoelectric transducer being coupled with said transmission line tocause said electrical energy to propagate along said line.