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United States Patent   Patent Number: 4,737,939
Ricketts  Date of Patent: Apr. 12,1988
US. Patent Apr. 12,1988 Sheet 2 of 2 4,737,939
 Appl. No.: 38,430
 Filed: Apr. 13,1987
Related U.S. Application Data
 Continuation of Ser. No. 818,992, Jan. 13, 1986, abandoned, which is a continuation of Ser. No. 693,228, Jan. 22, 1985, abandoned, which is a continuation of Ser. No. 497,365, May 23, 1983, abandoned.
 Int. CI.* H04R 1/00
 U.S. CI 367/158; 367/165;
 Field of Search 310/337, 800; 367/141,
367/153, 155, 157, 158, 165
 References Cited
U.S. PATENT DOCUMENTS
3,425,031 1/1969 Klee 367/903 X
3,716,828 2/1973 Massa 367/157
4,191,193 3/1980 Leo 310/800
4,354,132 10/1982 Borburgh et al 310/800
4,356,422 10/1982 Van Maanen 310/800
This application is a continuation of application Ser. No. 818,992 filed 1/13/86 (now abandoned), which is a 5 continuation of application Ser. No. 693,228, filed 1/22/85 (now abandoned), which is a continuation of application Ser. No. 497,365, filed 5/23/83 (now abandoned).
BACKGROUND OF THE INVENTION 10
This invention relates to transducers, and more particularly to a composite transducer for sonar applications which has separate, non-interfering transmit and receive transducers. 15
The longitudinal vibrator-type transducer 10 of FIG. 1 is widely used in the prior art as a transmitter and receiver in sonar applications. The transducer consists essentially of an electromechanically active element 11 (typically a piezoelectric ceramic), a head mass 12, a 20 rear mass 13, a bias rod 14, a pressure release system 15 and a waterproof housing 16,' as shown in FIG. 1. The bias rod 14 provides a bias compressive stress on both the active element 11 and the pressure release system 15. Acoustic decoupling of the assembly of these com- 25 ponents and the housing 16 is provided by the pressure release system 15. There are many variations of the transducer drawn in FIG. 1, but transducers of this general type have two characteristic frequencies that adversely affect receiving response. The two frequen- 30 cies are the head and tail mount resonances.
Because of the phase shifts associated with resonances and the deterioration of a beam produced by an array of transducers as a consequence of phase shift differences between the transducers, a relatively flat 35 receiving response is desired over a wide bandwidth. However, a typical transducer receiving response has uncontrolled head and tail mount resonances 20 and 21, respectively, as shown in FIG. 2. FIG. 2 shows a plot of receiving sensitivity versus normalized frequency 40 n=f/fr, where fr is the open-circuit (constant-current) resonant frequency 21. The peak 22 in the response below resonance is due to the head mass-tie rod resonance. Similarly, the response minimum 23 is caused by the resonance of the spring-mass formed by the pressure 45 release pad 15 and the rear mass 13.
In order to achieve a uniform or flat receiving response, the head and tail mount resonance frequencies must be equal, as well as the amplitudes of their resonances. Because of the difficulty of obtaining this bal- 50 ance in high volume production, damping is generally employed to compensate for any unbalance. The damping is often obtained by rubber bumpers 170 that are attached to the rear mass 13 and make frictional contact to the housing 16. A closely balanced transducer re- 55 quires tight tolerances on both the material parameters and physical dimensions of the transducer. This adds significantly to the cost of the transducer, particularly in high volume production.
In addition to uniformity of receiving sensitivity, 60 transducer self-noise is an extremely important performance parameter. Noisy transducers in a sonar array can cause a degradation of sonar system performance, as well as reveal the presence of the sonar platform. Longitudinal vibrator-type transducers such as that of 65 FIG. 1, in particular have been found to generate extraneous noise when exposed to a changing hydrostatic pressure head. Typically, the extraneous noise is deter
mined by measuring the open-circuit transducer voltage developed during pressure cycling. Polished contacting surfaces of the head mass 12, ceramic 11 and rear mass 13, very close tolerances on machine parts, and wellcontrolled alignment procedures have been found to be necessary to produce quiet transducers of the longitudinal vibrator type. These noise-quieting features have also added significantly to the cost of the transducer.
Also known in the prior art is a piezoelectric polymer which has low mass density and is mechanically flexible. These properties make the polymer more shock resistant than the prior art piezoceramics. Additionally, the characteristic impedance of the polymer more nearly matches that of water. Piezoelectric polymer film is presently made of polyvinylidene fluoride and is often referred to as PVF2. A polarization procedure must be used to render the polymer usefully piezoelectric. In one method of polarizing, both surfaces of the film are metallized to provide electrodes and a high d-c voltage is applied to the electrodes and held for about one hour at 100° C. Subsequent cooling to room temperature under the applied field results in permanent polarization with the strongest piezoelectric effect in the direction transverse to the metallized surfaces of the film.
The polymer PVF2 has been used previously as a transducer for the transmission and reception of ultrasound signals. Since the acoustic power which may be transmitted with this material is limited, its use has been confined to low power applications such as in medical ultrasound.
SUMMARY OF THE INVENTION
It is therefore an object of this invention to provide a composite transducer which utilizes the prior art longitudinal vibrator-type transducer for the transmitting function, but not for the receiving function, employing instead a separate transducer of piezoelectric polymer type for receiving signals. More particularly, the composite transducer of this invention comprises a prior art longitudinal vibrator-type "transducer 10 with a piezoelectric polymer 601 attached to its radiating face 121 for the receiving function as shown in FIG. 1. During transmission the receiver transducer is short-circuited, and during reception the transmitter transducer 10 is terminated in an electrical impedance that optimizes receiver response and minimizes transducer self-noise. The associated switching circuitry for switching between reception and transmission can be internal to the transducer.
It is a feature of the invention that the composite transducer occupies substantially the same space as required by the longitudinal vibrator-type transducer of the prior art, and can be retrofitted into a sonar system without modification of the transducer mounting arrangement.
BRIEF DESCRIPTION OF THE DRAWINGS
The objects and features of this invention are explained in the following description taken in conjuction with the accompanying drawings, in which:
FIG. 1 is an isometric view of the composite transducer of the invention;
FIG. 2 is a frequency response curve for a longitudinal vibrator-type transducer of the prior art;
FIG. 3 is an isometric exploded view of the PVF2 film; and