Ultrasonic transducers

• US 6,775,388 B1
• Filed: 04/27/1999
• Issued: 08/10/2004
• Est. Priority Date: 07/16/1998
• Status: Expired due to Term

• Current Assignee: Massachusetts Institute of Technology
• Original Assignee: Massachusetts Institute of Technology
• Inventors: Pompei, F. Joseph
• Primary Examiner(s): Harvey, Minsun Oh
• Assistant Examiner(s): PENDLETON, BRIAN T
• Application Number:
  US09/300,200
• Time in Patent Office: 1,932 Days
• Field of Search: 381/190, 381/191, 381/173, 381/174, 367/140, 367/180, 367/181
• US Class Current: 381/191
• CPC Class Codes: B06B 1/0292 Electrostatic transducers, e.g. electret-type
• CPC Class Codes: B06B 1/0614 for generating several frequencies
• CPC Class Codes: B06B 1/0688 with foil-type piezoelectric elements, e.g. PVDF

Abstract

Sonic transducers utilize resonant cavities of varying depths to achieve wide operational bandwidth. The transducers may include a conductive membrane spaced apart from one or more backplate electrodes. In one approach, spacing is achieved using a dielectric spacer having a series of depressions arranged in a pattern, the depressions forming cavities each resonant at a predetermined frequency. In another approach, the conductive membrane is piezoelectrically active, and the transducer is simultaneously driven in both piezoelectric and electrostatic modes.

Claims

1. A sonic transducer comprising:

(a) a conductive membrane;

(b) a backplate comprising at least one electrode; and

(c) disposed between the membrane and the backplate, a dielectric spacer comprising a series of depressions arranged in a pattern, the depressions forming cavities each resonant at a predetermined frequency, wherein

(d) the backplate comprises a plurality of electrodes and the depressions vary in depth through the spacer, depressions of different depths forming cavities resonant at different frequencies, different ones of the electrodes being aligned with depressions having a consistent depth; and

(e) the depressions of varying depth facilitate wide operational bandwidth at least over ultrasound frequencies such that generation, by the membrane, of ultrasound modulated with an audio signal is demodulated as it passes through the atmosphere to thereby create a highly directional audible sound.

2. The transducer of claim 1 wherein at least some of the depressions extend completely through the spacer.

3. The transducer of claim 1 wherein the depressions are annular grooves arranged concentrically.

4. The transducer of claim 1 wherein the depressions have a circular or polygonal cross section.

5. The transducer of claim 1 further comprising means for urging the backplate, the spacer, and the conductive membrane into intimate contact.

6. The transducer of claim 1 wherein the conductive membrane is a polymer film metallized on at least one side thereof.

7. The transducer of claim 6 wherein the conductive membrane has first and second opposed surfaces, the first surface being metallized and in contact with the spacer, none of the depressions extending fully through the spacer.

8. The transducer of claim 1 wherein the conductive membrane comprises a nonconductive piezoelectric material sandwiched between first and second metallized surfaces.

9. The transducer of claim 8 wherein the first metallized surface is in contact with the spacer and further comprising:

(a) a DC source connected across the second metallized surface of the membrane and the at least one backplate electrode; and

(b) an AC source connected across the first and second metallized surfaces of the membrane for driving the membrane piezoelectrically.

10. The transducer of claim 9 further comprising an AC source connected across the second metallized surface of the membrane and the at least one backplate electrode for driving the membrane electrostatically in mutually reinforcing conjunction with the piezoelectric AC source.

11. A sonic transducer comprising:

(a) a conductive membrane;

(b) a backplate comprising at least one electrode; and

(c) disposed between the membrane and the backplate, a dielectric spacer comprising a series of depressions arranged in a pattern, the depressions forming cavities each resonant at a predetermined frequency, wherein

(d) the backplate comprises a plurality of electrodes and the depressions vary in depth through the spacer, depressions of different depths forming cavities resonant at different frequencies, different ones of the electrodes being aligned with depressions having a consistent depth; and

(e) the spacer comprises at least first and second contiguous layers, the depressions extending fully through the first layer, the second layer comprising a second series of depressions fewer in number than the the depressions of the first layer, the depressions of the second layer registering with depressions of the first layer to form a first series of resonant cavities, the depressions of the first layer not registered with second-layer depressions forming a second series of resonant cavities, the first and second series of cavities having different resonant frequencies.

12. A sonic transducer comprising:

(a) a conductive membrane;

(b) a backplate comprising at least one electrode; and

(c) disposed between the membrane and the backplate, a dielectric spacer comprising a series of depressions arranged in a pattern, the depressions forming cavities each resonant at a predetermined frequency, wherein

(d) the conductive membrane is a polymer film metallized on at least one side thereof; and

(e) the conductive membrane has first and second opposed surfaces, the first surface being unmetallized and in contact with the spacer, the second surface being metallized.

13. A sonic transducer comprising:

(a) a conductive membrane;

(b) a backplate comprising at least one electrode; and

(c) disposed between the membrane and the backplate, a dielectric spacer comprising a series of depressions arranged in a pattern, the depressions forming cavities each resonant at a predetermined frequency, wherein

(d) the backplate comprises a plurality of electrodes and the depressions vary in depth through the spacer, depressions of different depths forming cavities resonant at different frequencies, different ones of the electrodes being aligned with depressions having a consistent depth; and

(e) the depressions of different depths form cavities having different mechanical resonance frequencies, the transducer further comprising, for each different depression depth, a separate resonant drive circuit tuned to the corresponding mechanical resonant frequency.

14. The transducer of claim 13 wherein the transducer has a capacitance and each drive circuit includes an inductor coupled with the transducer capacitance to provide an electrical resonance corresponding to the mechanical resonance frequency.

15. A sonic transducer comprising:

(a) a dielectric spacer having a pair of opposed surfaces and a series of apertures extending therethrough, the apertures being arranged in a pattern;

(b) a backplate comprising at least one electrode conforming to the aperture pattern and means for coupling an AC signal to the at least one electrode, the backplate being disposed against a first surface of the spacer;

(c) a conductive membrane disposed against a second surface of the membrane; and

(d) means for urging the backplate and the conductive membrane into intimate contact with the first and second surfaces of the spacer, the apertures forming cavities each resonant at a predetermined frequency, wherein

(e) the backplate comprises a plurality of electrodes and the depressions vary in depth through the spacer depressions of different depths forming cavities resonant at different frequencies, different ones of the electrodes being aligned with depressions having a consistent depth; and

(f) the depressions of varying depth facilitate wide operational bandwidth at least over ultrasound frequencies such that generation, by the membrane in response to the AC signal, of ultrasound modulated with an audio signal is demodulated as it passes through the atmosphere to thereby create a highly directional audible sound.

16. A sonic transducer comprising:

(a) a substantially nonconductive piezoelectric membrane having a pair of opposed conductive surfaces;

(b) a backplate comprising at least one electrode;

(c) means for creating a plurality of resonant cavities between the membrane and the at least one electrode;

(d) means for urging the membrane into the resonant cavities; and

(e) an AC source connected across the membrane, wherein

(f) the backplate comprises a plurality of electrodes and the depressions vary in depth through the spacer, depressions of different depths forming cavities resonant at different frequencies, different ones of the electrodes being aligned with depressions having a consistent depth; and

(g) the depressions of varying depth facilitate wide operational bandwidth over ultrasound frequencies such that generation, by the membrane in response to the AC source, of ultrasound modulated with an audio signal is demodulated as it passes through the atmosphere to thereby create a highly directional audible sound.

17. The transducer of claim 16 wherein the means for creating a plurality of resonant cavities comprises a perforated plate spaced above the at least one electrode.

18. A method of driving a sonic transducer comprising (a) a conductive membrane, (b) a backplate comprising at least one electrode, and (c) disposed between the membrane and the backplate, a dielectric spacer comprising a series of depressions arranged in a pattern, the depressions forming cavities each resonant at a predetermined frequency, wherein the backplate comprises a plurality of electrodes and the depressions vary in depth through the spacer, depressions of different depths forming cavities resonant at different frequencies, different ones of the electrodes being aligned with depressions having a consistent depth, the method comprising the steps of:

(a) for each different depression depth, providing a separate resonant drive circuit tuned to the corresponding mechanical resonant frequency; and

(b) driving the cavities with the respective drive circuits tuned thereto.

19. The method of claim 18 wherein the transducer has a capacitance and each drive circuit includes an inductor coupled with the transducer capacitance to provide an electrical resonance corresponding to the mechanical resonance frequency.

20. A method of operating a sonic transducer comprising a sonic transducer comprising (a) a conductive membrane, (b) a backplate comprising at least one electrode, and (c) disposed between the membrane and the backplate, a dielectric spacer comprising a series of depressions arranged in a pattern, the depressions forming cavities each resonant at a predetermined frequency, wherein the backplate comprises a plurality of electrodes and the depressions vary in depth through the spacer, depressions of different depths forming cavities resonant at different frequencies, different ones of the electrodes being aligned with depressions having a consistent depth, the method comprising the steps of driving the transducer to generate ultrasound modulated with an audio signal such that the generated ultrasound is demodulated as it passes through the atmosphere to thereby create a highly directional audible sound.