WO1994007342A1

WO1994007342A1 - Bone conduction accelerometer microphone

Info

Publication number: WO1994007342A1
Application number: PCT/US1993/008660
Authority: WO
Inventors: Timothy K. Wickstrom; Elmer V. Carlson
Original assignee: Knowles Electronics, Inc.
Priority date: 1992-09-17
Filing date: 1993-09-14
Publication date: 1994-03-31
Also published as: AU4920793A

Abstract

A bone-conduction microphone of the acceleration type for emplacement in a user's ear employs a cantilever mounted piezoelectric bar having a pair of metallizations on opposite faces. An elongated weight is affixed to a free end of the bar and transversely disposed with respect thereto, resulting in a T-shaped structure having its principal mass concentrated at the extreme end of the piezoelectric bar. A closely fitting housing allows this microphone assembly to occupy a volume of approximately 4 mm, by 6 mm, by 3 mm. Associated electronics are mounted within the housing, and have contacting terminals extending outward therefrom.

Description

BONECONDUCTIONACCELEROMETER MICROPHONE

DESCRIPTION Technical Field of the Invention

The technical field of the invention is elec- troacoustical transducers, and in particular, acceleration-sensitive transducers. Background of the Invention Transducer assemblies are known which can be plugged into a user's ear, and which contain both an audible pick-up microphone, for detecting voice sounds generated by the user, and an earphone (or receiver) for generating sounds for the user to hear. One reason for incorporating the microphone into the earpiece is to confer immunity from am- bient noise sources, as well as to eliminate the need for a microphone disposed at the user's mouth. Such microphones are not of the conventional pres¬ sure-sensitive type, but are acceleration-sensitive units which pick up the vibration of the user's voice as transmitted through the bones of the user's skull.

Suitable circuitry and interconnect cabling connect the earphone and receiver transducers to a local communication unit, e.g. a two-way radio transmitter/receiver. A representative transducer assembly of this type is described in U.S. Patent No. 4,588,867 issued to Konomi on May 13, 1986. A problem with such units is that since the icro- phone is acceleration-sensitive, the microphone responds to accelerations caused not only by sound waves from the skull, but also to accelerations caused simply by movement of the cabling. It is thus desirable to eliminate cabling entirely for this reason. It has been proposed that a miniature transmitter-receiver be incorporated into the transducer assembly to provide short-range communi¬ cation to the local transmitter-receiver. This in turn has put a premium on available space in the transducer assembly, since sufficient volume must be provided to accommodate necessary electrical components.

To this end, if a significant volume of space could be made available, a satisfactory microphone could be made substantially shorter in dimension than current state-of-the-art microphones such as the microphones incorporated into earpiece assem¬ blies manufactured by Temco Japan Co. , Ltd. of Tokyo for their "Voice-Ducer" units. These micro- phones are elongated structures occupying a volume of approximately 12 millimeters by 4 millimeters by 4 millimeters. These units are configured as an extended piezoelectric bar cantilever mounted at one end, and having appropriate metallizations on opposite sides of the bar across which a signal is developed when the bar mounting system is accele¬ rated by vibratory excitation. The present inven¬ tion is oriented toward the solution of these and other problems. Summary of the Invention It is an object of the invention to provide an acceleration-type bone-conduction microphone suit¬ able for emplacement in a user's ear. In accor¬ dance with the invention, the microphone comprises a housing having an elongated interior chamber extending along a longitudinal axis. An elongated piezoelectric bar is disposed within the chamber to extend along this longitudinal axis. The bar has conducting electrode surfaces on opposing side faces. Mounting means are provided for mounting one end of the bar to the housing so that the bar is supported as a cantilever beam which will be deflected by acceleration of the housing. A weight, configured as a block elongated along an axis of elongation, is affixed to the other end of the bar with its axis of elongation disposed trans¬ verse to the chamber axis. The bar and block thus form a T-shaped structure with the mass of the block disposed at the free end of the cantilevered bar. The chamber is generally rectangular, and the block is configured to extend substantially the entire width of the chamber so as to place maximum possible mass at the extreme end of the cantile¬ vered bar.

The secured end of the piezoelectric bar is secured to a circuit board affixed to the housing and has a portion extending through an aperture carrying a pair of electrical terminals disposed for external access. Elements of an impedance shifting circuit are mounted on an interior portion of the circuit board and are connected between the electrode surfaces and the terminals. The imped¬ ance shifting elements preferably include a resis¬ tor connected between the electrode surfaces, a field effect transistor having its gate electrode connected to one of electrode surfaces, its source electrode connected to one of the other electrode surface and one of the terminals, the drain elec¬ trode being connected to the other of the termi¬ nals.

Other advantages and aspects of the invention will become apparent upon making reference to the specification, claims, and drawings to follow. Brief Description of Drawings

Figure 1 is a plan view of a microphone assem¬ bly of the present invention with a portion of its housing removed;

Figure 2 is a cutaway view taken along section lines 2-2 as shown in Figure 1; and

Figure 3 illustrates elements of an impedance shifting circuit shown in Figure 1 connected to a test circuit.

Detailed Description of the Invention

While this invention is susceptible of embodi¬ ment in many different forms, there is shown in the drawings and will herein be described in detail a preferred embodiment of the invention with the understanding that the present disclosure is to be considered as an exemplification of the principles of the invention and is not intended to limit the broad aspect of the invention to the embodiment illustrated. Figures l and 2 show cutaway plan and side elevational views of the microphone assembly 10 of the invention, the assembly 10 has a housing 12 made of upper and lower housing shells 14, 16. The housing 12 is rectangular and generally elongated. A passage 18 is disposed at one end of the housing, and a circuit board 20 is affixed to the lower housing shell 16 by epoxy cement 22 to secure the circuit board 20 in place extending partially into the interior of the housing 12. A piezoelectric bar 24 of square cross-section is secured at one end to the circuit board 20 by conducting epoxy cement 23. As will be discussed, this conducting epoxy cement 23 not only firmly supports this end of the bar 24, but also makes contact to an appro¬ priate metallization interconnecting certain cir¬ cuit elements on the circuit board 20.

The bar 24 extends generally parallel to the axis of elongation of the housing 12. At the other end of the bar 24, a generally rectangular weight 26 is affixed. The weight 26 is configured as an elongated block of length much greater than the transverse dimensions of the piezoelectric bar 24, with its axis of elongation perpendicular to that of the piezoelectric bar 24 and the planes of the respective housing confronting sidewalls 28, 30. The piezoelectric bar 24 and the weight 26 thus form a T-shaped structure placing maximum mass at the extreme end of the bar 24 to increase the sensitivity of the bar 24 to acceleration. Suit¬ able upper and lower metallizations disposed upon the piezoelectric bar 24 form upper and lower electrode surfaces 32, 34. As is well known, acceleration of the housing 12 will thus cause acceleration-dependent voltages to be developed between electrode surfaces 32 and 34. The circuitry on the circuit board 20 shown in Figure 1 is best understood with reference to Figure 3, which shows the circuitry on the circuit board 20 connected to a test circuit via intercon- nection leads 36, 38. A transducer 40, here sym¬ bolically representing the piezoelectric bar 24 and its electrode surfaces 32, 34 provides a signal voltage difference applied to a ground lead 42 and a gate lead 46 accessing the gate of a field effect transistor 48. A source lead 50 returns the source of the transistor 48 to the ground lead 42 and to an output terminal 52. A resistor 54 of from 6 to 10 gigaohms is connected between the gate lead 46 and the ground lead 42. The drain of the transis- tor 48 is connected via a drain lead 56 to an output terminal 58. The remaining portions of the circuit external to the circuit board 20 necessary to achieve an amplifying and impedance-shifting function are connected to the terminals 52, 58 by means of the interconnection leads 36, 38 intercon¬ necting these terminals to terminals 60, 62. Terminal 60 is connected to an output terminal 64 via a capacitor 66 and to one end of a resistor 68. The capacitor 66 is given a nominal value of 10 microfarads and the resistor a value of 2.2 kilohms. The other end of the resistor is connec¬ ted via a battery 70 of nominal voltage of 3 volts to the terminal 62, and to an output terminal 72. Referring to Figure 1 for the physical imple- mentation of these connections, the lower electrode surface 34 of the piezoelectric bar 24 contacts and is supported by a gate metallization 74 connected to the gate electrode of the transistor 48, and thus serves the function of the gate lead 46 shown in Figure 3. A ribbon-shaped foil 76 is secured by conducting epoxy cement to the upper electrode surface 34 of the piezoelectric bar 34, and is similarly secured to a ground metallization 78 con¬ nected to terminal 52. A portion of the foil lead 76 not shown in the drawings is connected to the housing 12 to minimize circuit noise. A source lead 80 is stitch-bonded from the transistor 48 to an extension of the ground metallization 78, these elements serving the function of ground lead 42 in Figure 3. Finally, the drain lead 82 of transistor 48 is stitch-bonded to metallization 86 to which terminal 58 is connected, thereby serving the function of the drain lead 56 shown in Figure 3. The resistor 54 of Figure 3 as shown in Figure 1 is a planar resistance element extending from metallizations 74 to 78.

The elements shown in Figures 1 and 2 are illustrated approximately to scale. The housing 12 employed for test and evaluation purposes is that of a standard transducer housing. It clearly has not been optimized for minimum housing volume. The case width is 0.154 inches (3.90 mm.). The length of the housing 12 is 0.270 inches (6.93 mm.). This dimension may be reduced to 0.233 inches (5.91 mm.). The height of the housing 12 is 0.119 inches (3.03 mm.); a value which may be reduced to 0.107 inches (2.90 mm.). The piezoelectric bar 24 has an unsupported length of 0.172 inches (4.30 mm.). Its cross-section dimensions are 0.020 inches (0.51 mm.) and 0.019 inches (0.48 mm.). The piezoelec- trie bar 24 is made of a ceramic manufactured by Piezoelectric Products, Inc. of Metuchen, New Jersey under the trade name "PZT". The weight 26 is made of lead and has an overall length of 0.19 inches (3.03 mm.), a major transverse dimension of 0.055 inches (1.38 mm.), a minor transverse dimen- sion of 0.025 inches (0.62 mm.), and a weight of 29 milligrams.

The transverse dimensions of the microphone assembly 10 are comparable with those of the previ- ously mentioned Temco unit; however, the overall length is approximately half that of the Temco unit. Thus, a significant volume has been freed to accommodate additional electronics.

The performance of such units as measured by the circuit shown in Figure 3 under constant lg acceleration shows a generally flat response throughout the lower end of the audio band at -22 dB. referred to one volt. The output slowly rises in the kilohertz range, reaching a value of -15 dB. at 2 kilohertz and peaking at -4 dB. at 2.7 kilo¬ hertz, thereafter dropping off rapidly as the frequency increases.

While the invention has been described with reference to a preferred embodiment, it will be understood by those skilled in the art that various changes may be made and equivalents may be substi¬ tuted for elements thereof without departing from the broader aspects of the invention. Also, it is intended that broad claims not specifying details of a particular embodiment disclosed herein as the best mode contemplated for carrying out the inven¬ tion should not be limited to such details.

Claims

C L A I M S

1. A bone-conduction microphone for emplace¬ ment in a user's ear comprising: a housing having an elongated interior chamber extending along a longitudinal chamber axis and having a chamber width and chamber height perpen¬ dicular to said longitudinal chamber axis; an elongated piezoelectric bar disposed within said chamber and extending parallel to said longi- tudinal chamber axis, said bar having conducting electrode surfaces on a pair of opposing side faces of said bar; mounting means for mounting one end of said bar to said housing so that said bar is supported as a cantilever beam by said mounting means; and a weight configured as a block having a mass and elongated along an axis of elongation and affixed to the other end of said bar with said axis of elongation of said block disposed transverse to said chamber axis, so that said bar and said block form a T-shaped structure with the mass of said block disposed at said other end of said bar, said block having a length substantially greater than the transverse dimensions of said bar.

2. The microphone of claim 1 wherein said chamber is generally rectangular.

3. The microphone of claim 2 wherein said block axis of elongation is perpendicular to a pair of confronting walls of said chamber, and said bar is configured to extend substantially the entire width of said chamber between said confronting walls.

4. The microphone of claim 3 wherein said housing has an aperture communicating through an end wall of said chamber, said mounting means includes a circuit board affixed to said housing and extending through said aperture and to which said one end of said bar is affixed, a plurality of electrical terminals disposed on an externally accessible portion of said circuit board, and elements of an impedance shifting circuit mounted on a portion of said circuit board extending into said chamber and connected between said electrode surfaces and chosen ones of said plurality of terminals.

5. The microphone of claim 4 wherein said circuit elements include a resistor connected between said electrode surfaces, a field effect transistor having source, drain and gate elec¬ trodes, said gate electrode being connected to one of said electrode surfaces, said source electrode being connected to the other of said electrode surfaces and one of said terminals, said drain electrode being connected to another of said termi¬ nals.

6. The microphone of claim 3 wherein said housing is in the form of a generally rectangular shell and the length of said housing is approxi¬ mately 6 millimeters, the width of said housing is approximately 4 millimeters, and the height of said housing is approximately 3 millimeters. AMENDED CLAIMS

[received by the International Bureau on 17 February 1994 (17.02.94); original claim 1 replaced by amended claim 1; other claims unchanged (1 page)]

1. A bone-conduction microphone for emplace¬ ment in a user's ear comprising: a housing having an elongated interior chamber extending along a longitudinal chamber axis and having a chamber width and chamber height perpen¬ dicular to said longitudinal chamber axis; an elongated piezoelectric bar disposed within said chamber and extending parallel to said longi- tudinal chamber axis, said bar having conducting electrode surfaces on a pair of opposing side faces of said bar defining a direction of deflection; mounting means for mounting one end of said bar to said housing so that said bar is supported as a cantilever beam by said mounting means for movement along said direction of deflection; and a weight, separate and independent from said housing, said weight being configured as a block having a mass and elongated along an axis of elon- gation and affixed to the other end of said bar with said axis of elongation of said block disposed transverse to said chamber axis, so that said bar and said block form a T-shaped structure with the mass of said block disposed at said other end of said bar, said block having a length substantially greater than the transverse dimensions of said bar and extending transverse to said direction of deflection.

2. The microphone of claim 1 wherein said chamber is generally rectangular.