Electroacoustic transducer assembly
United States Patent 3890423

An electroacoustic transducer assembly including a ceramic sonic transducer mounted in a cavity of a housing formed of material having low acoustic attenuation and distortion. The cavity is partially filled with a non-conductive liquid such as oil or distilled water and is evacuated to remove the dissolved and entrained air. The transducer is locked in place to thereby ensure good energy transfer between the tranducer and the medium in which the assembly is placed and to avoid changing the sound path length which might occur if the transducer were allowed to move in the cavity. The transducer assembly may be used in various structural embodiments for a plurality of applications in which acoustic information must be transitted, received and measured.

Application Number:
Publication Date:
Filing Date:
Primary Class:
Other Classes:
73/644, 310/327, 310/336, 367/165
International Classes:
B06B1/06; G01F1/66; (IPC1-7): H04R17/00
Field of Search:
View Patent Images:
US Patent References:
3771117TRANSDUCER INSTALLATION1973-11-06Shaffer et al.
3543065PROBE FOR BOND TESTER1970-11-24Phelan
3202845Ultra-sonic electrode applicator1965-08-24Kozinski
3187300Pressure-compensated transducer1965-06-01Brate
2626992Signal delay device1953-01-27Holman

Primary Examiner:
Budd, Mark O.
Attorney, Agent or Firm:
Samuelson & Jacob
The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows

1. An electroacoustic transducer assembly comprising:

2. The invention of claim 1 wherein the restraining means comprises:

3. The invention of claim 1 wherein the restraining means comprises:

4. An electroacoustic transducer system comprising:

5. An electroacoustic transducer system comprising:

6. An electroacoustic transducer system for mounting in the wall of a flow chamber comprising:

The invention relates to electroacoustic ceramic transducer assemblies. In particular, the invention is directed toward providing such transducer assemblies with superior energy transfer between the transducer and the housing to thereby eliminate losses due to attenuation and distortion.

Most prior art electroacoustic transducer assemblies lose efficiency because of poor coupling between the transducer and the acoustic window of the housing or require elaborate construction in order to maximize the energy transfer. Furthermore, it is not usually possible to use the same prior art, transducer assembly in many different applications. As a consequence it is often necessary to design a special assembly for each particular electroacoustic application.

Accordingly, it is an important object of the invention to provide an electroacoustic ceramic transducer assembly which may be used in a wide variety of applications and which is economical to fabricate and assemble.

It is another object of the invention to provide such an assembly wherein the thickness of the housing adjacent the face of the transducer is of such value that it provides strength necessary to withstand the pressure in the medium and efficient transfer of energy from the transducer to the medium.

It is a further object of the invention to provide such an assembly wherein the efficiency of energy transfer remains high over a relatively wide range of operating temperatures.

It is a still further object of the invention to provide such an assembly wherein no air pockets and/or voids are present at the interface between the active face of the transducer and the housing.

These and other objects, advantages, features and uses will be apparent as this description proceeds.

Broadly, the assembly of the invention comprises a housing having a cavity to receive the transducer. A few drops of liquid such as distilled water or oil are placed in the cavity, the transducer is put in place and the cavity is evacuated. This removes the dissolved and entrained air. Following return to atmospheric pressure, the transducer is locked in place in intimate contact with the housing. The assembly may now be mounted in a variety of configurations as will be evident in the following description.

For low temperature applications (below 100°C), water of the proper purity may be used whereas, for high temperature applications, a high temperature oil such as DOWTHERM G is preferred. It is most important that the liquid remain liquid or become solid but it must not become a gas. As the liquid expands due to a temperature rise, the interface between the transducer and the housing remains filled with liquid and no voids or air pockets are developed in the interface. If the cavity has not been evacuated, voids and air pockets could develop with a resulting loss of efficiency. When the temperature drops, the liquid which is trapped by an O-ring, contracts and remains at the interface between the transducer and the housing.

In the accompanying drawing, forming a part of this application, and in which like numerals designate like parts throughout the same:

FIG. 1 is a horizontal plan view of an embodiment of the invention wherein the transducer is undamped with cutaway portions to show the construction of the assembly;

FIG. 2 is a sectional view taken on line 2--2 of FIG. 1, viewed in the direction of the arrows;

FIG. 3 is a view similar to FIG. 1 of an embodiment of the invention wherein the transducer is damped;

FIG. 4 is a sectional view taken on line 4--4 of FIG. 3, viewed in the direction of the arrows;

FIG. 5 is a top plan view of a laboratory probe using an assembly of the invention;

FIG. 6 is an elevational view, partly in section of the embodiment of FIG. 5;

FIG. 7 is a bottom plan view of the embodiment of FIG. 5;

FIG. 8 is a cutaway elevational view of an embodiment of the invention utilizing two transducers and a pair of reflectors, one of the transducers being a transmitter and the other a receiver; and

FIG. 9 is a cutaway elevational view of a transducer assembly for use in measuring fluid flow in a pipe or other flow chamber.

In the drawing, wherein, for the purpose of illustration, there are shown various embodiments of the invention, the numeral 20 designates a transducer assembly of the invention, generally, wherein the transducer is undamped. Transducer 20 (FIGS. 1 and 2) is seen to comprise a housing 22 having a cavity 24 into which a disk shaped, transducer 26 is placed. Transducer 26 is of the usual ceramic composition being largely formed of barium titanate, lead titanate-lead zirconate, lead metaniobate or similar compounds. The usual metallic electrodes 28 and 30 are applied to the opposite faces of transducer 26 in a manner well-known in the art.

The housing 22 is formed of a metal such as aluminum, titanium or other suitable material which have low acoustic attenuation and distortion. The thickness of the housing forward of the transducer 26, which is designated as A in FIG. 2, should be of such value as to present a relatively transparent window to the acoustic wave generated by the transducer or impinged on the housing from the medium. If the assembly is used in a corrosive environment, alumimum housings may be plated with a noncorrosive material such as gold.

Transducer assembly 20 is assembled in the following manner. An electrical lead 32 is affixed, preferably by soldering to electrode 30. Several drops of liquid such as distilled water or high temperature oil (depending on the use to which the assembly is to be put) are placed in cavity 24 until the cavity is, preferably, approximately one-half full. Transducer 26 is placed loosely in cavity 24 and tilted slightly to allow air to escape during evacuation. An elastomeric O-ring 34 is placed loosely on the back surface of transducer 26 and, if required, a sleeve or spacer 36 of metal such as aluminum is placed over O-ring 34. A retaining ring 38 such as a C-ring is placed loosely over sleeve 36. There should be sufficient liquid in the cavity 24 to cover the O-ring 34 and the C-ring 38. Now, the cavity is evacuated to remove the dissolved and entrained air from the liquid. Next, the combination is pushed toward the front of the housing and C-ring 38 is engaged in a slot 40 in the housing. This retains the combination in place and ensures that the interface between electrode 28 and the housing will be a liquid which is free of voids and/or air. In effect, the transducer is seated firmly against the face of the cavity.

When the temperature in the assembly rises and the liquid expands, the liquid is prevented from escaping by the O-ring 34. When the temperature drops and the liquid contracts, the liquid goes into the space between electrode 28 and the housing. To provide strain relief for the cable and restrain it from vibration at the solder junction, a plastic mastic 42 formed of material such as Dow-Corning Silastic is placed in the cavity 24 as shown in FIG. 2. A flange 44 is provided as a part of housing 22 to simplify installation in electroacoustic systems.

Transducer assembly 50 (FIGS. 3 and 4) is similar to assembly 20 except that the vibration of the transducer 56, which is contained in cavity 54 of housing 52, is damped. Assembly 50 comprises a housing 52 having a cavity 54 into which transducer 56 is placed. Transducer 56 is disk shaped and its opposite surfaces are provided with electrodes 58 and 60. An electrical lead or connection 62 is soldered to electrode 60 and a potting compound 72 of material such as Isochem Resins Co Type 1251Z Epoxy Resin is placed on the back surface of transducer 56. This will, when it hardens, serve to restrain the lead 62 from vibrating and will damp the motion of transducer 56. Other materials may be used as long as they are rigid and maintain their rigidity over the range of temperatures to which the assembly is to be subjected.

Transducer assembly 50 is assembled in the following manner. Transducer 56 with lead 62 affixed to electrode 60 and damping block 72 affixed to the back is placed loosely into cavity 54 after several drops of liquid have been dropped into the cavity preferably filling it halfway. The liquid is the same as that used in assembly 20. An elastomeric O-ring 64 is placed loosely at the back of block 72 and, if required, a sleeve or spacer 66 is placed loosely over O-ring 64. A retaining ring 68 such as a C-ring is placed loosely over sleeve 66, the transducer is tilted slightly and the cavity is evacuated to remove the dissolved and entrained air. Now, the C-ring 68 is forced into slot 70 in housing 52 thereby pushing electrode 58 into contact with the housing through an interface of liquid. A flange 74 is provided to facilitate mounting of the assembly in a probe or similar fixture. Operation of transducer assemblies 20 or 50 is accomplished by applying excitation signal between equipment ground and lead 32 or 62, respectively, when the transducer is in the transmit mode and by using the signal which appears between the equipment ground and the hot lead when the transducer is in the receive mode. Transducer assembly 20 is used when undamped operation is desired and transducer assembly 50 is used when damped operation is desired.

Transducer probe 80 (FIGS. 5-7) is seen to comprise a transducer assembly 82 of the type of either FIGS. 1 and 2 or 3 and 4 and a metal reflector 84. Probe 80 is of the type generally used in beakers or vessels for laboratory investigation of fluids in situ. Probe 80 utilizes 4 spacer rods 86 which are, for example, drilled and internally threaded at their lower ends. The lower ends are engaged with reflector 84 and are held in place by screws 88 and the upper ends are either integral with header 92 or are held in place in header 92 by screws 90 which join headers 92 and 100. The spacer rods 86 are of suitable length to make the acoustic path between the assembly 82 and the reflector 84 the correct length.

Flange 94 rests on a ledge 96 of header 92 and an elastomeric O-ring 98 which fits in a groove in upper header 100 is placed on the upper surface of flange 94. When the two headers are mounted together by means of screws 90, the O-ring 98 is placed in compression to thereby prevent entry of liquid into the rear of transducer assembly 82. Electrical connection is made to the transducer between equipment ground and electrical lead 102. Electrical lead 102 is carried to the external electronic circuitry through an insulated bushing 104 which is threaded into upper header 100. Lead 102 preferably terminates in a hook shaped connection 106.

Probe 80 is placed in the fluid to be studied and operation proceeds in a manner well-known in the art. Openings 108 are provided if it is desired to mount the probe to a plate, board or stem by means of bolts or screws for use, for example, as a process probe.

In FIG. 8, there is illustrated a probe 110 which employs two transducer assemblies of the invention and which may be used in either a laboratory or fluid flow (process) environment. Probe 110 is seen to comprise a pair of transducer assemblies 112A and 112B which are mounted in shells 114 and 118 by means of threaded insulated bushings 116A and 116B. A sealing gasket 115 is used to seal the junction between the shells. By way of example, assembly 112A is designated the transmitter and assembly 112B is designated the receiver. However, this situation can be reversed or both of them can alternately serve as transmitter and receiver. Liquid is placed in cavities 122A and 122B of housing 118 so they are about one half full of liquid and the transducers 120A abd 120B are placed loosely in cavities 122A and 122B. Transducers 120A and 120B are respectively provided with electrodes 124A and 126A and 124B and 126B.

Pins 128A and 128B of metal are inserted in metallic springs 130A and 130B, respectively, and the combinations are dropped loosely into the respective cavities so that the springs are preferably about one-half submerged in the liquid. The transducers are slightly tilted and the cavities are evacuated to remove the dissolved and entrained air. Now, the bushings 116A and 116B are threaded into place to seat the transducers in position in the cavities. At the same time shells 114 and 118 are coupled together by means of a plurality of bolts 117 and ring 119. Other bolts (not shown) join ring 119 to mounting stem 121 to complete the assembly. Reflector support rods 123 may be formed from the same piece of material as ring 119 or may be threaded into it. Electrical connections to the backs of the transducers are brought out through the associated pins 128A and 128B, springs 130A and 130B and leads 132A and 132B. The connections between the pins and the leads are held in firm contact by means of screws 134A and 134B.

In use, probe 110 is placed in the environment to be studied and the necessary connections are made between leads 132A and ground and 132B and ground and the associated electronic equipment. Excitation is applied to lead 132A causing transducer 120A to vibrate. The acoustic signal is transmitted across the medium to a pair of reflectors 136A and 136B and thence to the assembly 118 and transducer 120B. Transducer 120B produces an electrical signal from the acoustic excitation, which signal appears on lead 132B.

Reflectors 136A and 136B are perpendicular to each other and have their surfaces at 45° to the direction of the rays from and to the transducers. While this is a preferred reflector orientation, other orientations may also be used to obtain the desired path length and orientation.

In FIG. 9, there is illustrated a transducer assembly 140 of the invention to be affixed to a pipe or flow chamber for use as a flowmeter probe. Probe 140 is mounted on a pipe 142 to make the desired angle with the fluid flow. Transducer assembly 144 is normally mounted so that its face is in contact with the fluid but does not impede the flow. However, it may protrude into the flow if desired. It comprises a housing 146 and an electroded transducer 148 (details not shown).

Housing 146 is preferably filled halfway with liquid and transducer 148 is put loosely in place and the O-ring 149, a sleeve or spacer 153 (if needed) and C-ring 151, similar to those previously described (details not shown), are placed over the transducer and the cavity is evacuated to remove dissolved and entrained air. The C-ring 151 is pushed into position in a groove in the housing and the transducer is now in place. The foregoing assembly construction is similar to that shown and described in connection with the embodiments of FIGS. 1 and 2 or 3 and 4.

Housing 146 is threaded onto a pipe stem 150 and a bushing or nipple 152 is threaded to the other end of pipe stem 150 and to probe housing 154. The electrical lead 155 is carried through the pipe stem and the bushing to a terminal 156 from which it is connected to the associated equipment. The equipment ground connection is made to a terminal 158.

While particular embodiments of the invention have been shown and described, it is apparent to those skilled in the art that modifications are possible without departing from the spirit of the invention or the scope of the subjoined claims.