Hydrophones

There is considerable practical interest in developing low-density, compliant, flexible piezoelectric transducers. A low-density piezoelectric would have better coupling to water and have a more easily adjusted buoyancy than the higher-density ceramics used for hydrophones. A complaint material would have a better resistance to mechanical shock than a conventional ceramic transducer, and a large compliance would also mean high damping, which is desirable in a passive device. A flexible material could also be formed to any desired profile. 

On the basis of these ideas, there has been intensive research attempting to use the unique properties of composites in many applications. The following sections will discuss some of the many areas in which composites are used. 

Ferroelectric composites can be used as underwater transducers [22] to detect (passive mode) and/or generate (active mode) sound. Transducers made from ceramics have the disadvantage that their density makes it difficult to obtain good impedance matching with water. One of the problems with PVDF (aside from the problems associated with poling) is its low permittivity. This will produce a low element capacitance, which will load the output. The equivalent circuit for a flank array transducer linked to an amplifier via a cable is shown in Fig. 6.15. The open-circuit voltage sensitivity Moc/ is loaded by stray capacitance Cg, and/or cable capacitance Q. The end-of-cable sensitivity Mgc is given by 

amplifier voltage noise, which is component-dependent  

The predicted noise spectra for a hydrophone are shown in Fig. 6.16. It can be seen that the sea-state zero noise dominates at low frequencies and the amplifier noise at high frequencies. So, for most practical applications, is the best figure of merit. 

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As any conventional probe, acoustic beam pattern of ultrasound array probes can be characterized either in water tank with reflector tip, hydrophone receiver, or using steel blocks with side-drilled holes or spherical holes, etc. Nevertheless, in case of longitudinal waves probes, we prefer acoustic beam evaluation in water tank because of the great versatility of equipment. Also, the use of an hydrophone receiver, when it is possible, yields a great sensitivity and a large signal to noise ratio. 

The encircling probe was characterised with its mirror in water. As we did not own very tiny hydrophone, we used a reflector with hemispherical tip with a radius of curvature of 2 mm (see figure 3c). As a result, it was possible to monitor the beam at the tube entrance and to measure the position of the beam at the desired angle relatively to the angular 0° position. A few acoustic apertures were verified. They were selected on an homogeneous criteria a good one with less than 2 dB of relative sensitivity variations, medium one would be 4 dB and a bad one with more than 6 dB. 

The development of active ceramic-polymer composites was undertaken for underwater hydrophones having hydrostatic piezoelectric coefficients larger than those of the commonly used lead zirconate titanate (PZT) ceramics (60—70). It has been demonstrated that certain composite hydrophone materials are two to three orders of magnitude more sensitive than PZT ceramics while satisfying such other requirements as pressure dependency of sensitivity. The idea of composite ferroelectrics has been extended to other appHcations such as ultrasonic transducers for acoustic imaging, thermistors having both negative and positive temperature coefficients of resistance, and active sound absorbers. 

Composite Devices. Composites made of active-phase PZT and polymer-matrix phase are used for the hydrophone and medical imaging devices (see Composite materials, polymer-matrix Imaging technology). A usehil figure of merit for hydrophone materials is the product of hydrostatic strain coefficient dj and hydrostatic voltage coefficient gj where gj is related to the dj coefficient by (74)... 

Ferroelectric—polymer composite devices have been developed for large-area transducers, active noise control, and medical imaging appHcations. North American Philips, Hewlett-Packard, and Toshiba make composite medical imaging probes for in-house use. Krautkramer Branson Co. produces the same purpose composite transducer for the open market. NTK Technical Ceramics and Mitsubishi Petrochemical market ferroelectric—polymer composite materials (108) for various device appHcations, such as a towed array hydrophone and robotic use. Whereas the composite market is growing with the invention of new devices, total unit volume and doUar amounts are small compared to the ferroelectric capacitor and ferroelectric—piezoelectric ceramic markets (see Medical imaging technology). 

R. Y. Ting, "Evaluation of New Piezoelectric Composite Materials for Hydrophone AppUcations," presented at the Bernard Jaffe Memorial Colloquium, American Ceramics Society, 86 Meeting, Pittsburgh, 1984. 

Lead zirconate [12060-01 -4] PbZrO, mol wt 346.41, has two colorless crystal stmctures a cubic perovskite form above 230°C (Curie point) and a pseudotetragonal or orthorhombic form below 230°C. It is insoluble in water and aqueous alkaUes, but soluble in strong mineral acids. Lead zirconate is usually prepared by heating together the oxides of lead and zirconium in the proper proportion. It readily forms soHd solutions with other compounds with the ABO stmcture, such as barium zirconate or lead titanate. Mixed lead titanate-zirconates have particularly high piezoelectric properties. They are used in high power acoustic-radiating transducers, hydrophones, and specialty instmments (146). 

Fig. 1.2 Numerically simulated frequency spectra of the hydrophone signal due to acoustic cavitation noise. The driving ultrasound is 515 kHz in frequency and 2.6 bar in pressure amplitude, (a) For stable cavitation bubbles of 1.5 pm in ambient radius, (b) For transient cavitation bubbles of 3 pm in ambient radius. Reprinted from Ultrasonics Sonochemistry, vol. 17, K. Yasui, T. Tuziuti, J. Lee, T. Kozuka, A. Towata, and Y. lida, Numerical simulations of acoustic cavitation noise with the temporal fluctuation in the number of bubbles, pp. 460-472, Copyright (2010), with permission from Elsevier...

Cavitation medium gets disturbed due to the presence of external instrument such as thermocouple, hydrophone, aluminum foil, test tube etc. and hence we may not get a realistic picture of the cavitational activity distribution... 

Illustration of acoustic/hydrophone system for leak and shock detection. 

For third-party damage, other sensors such as acoustic (hydrophone)-based technology is also being developed (Figure 10.20). 

Britain contributed technical innovation, industrial and financial power, and military manpower to the Allied victory. Hydrophones, tanks and aircraft are obvious examples of new weapons, and hardly suggest industrial backwardness or military conservatism. However, innovation with traditional weapons was no less important. New scientific artillery techniques made a bigger contribution to the defeat of the German army in 1918 than the more publicised tank. Even new weapons depended upon tactical innovation to be effective. The army s success was possible only when the different arms - artillery, infantry, tanks (when available) and aircraft - had learned to operate together. The navy s success over the U-boat required the adoption of the convoy system as well as the development of hydrophones. 

Given that the appropriate relative permittivities of the ceramic and the polymer are respectively 1500 and 3.5, and that the d33 coefficient for the ceramic is 375 pCN-1, calculate a value for the hydrophone figure of merit and show that the units are m2 N 1. For this estimate it may be assumed that the composite has been structurally modified so that the d33 contribution is negligible. Comment on the realism or otherwise of the calculated value. 

Position a calibrated hydrophone at focal distance from the transducer. The hydrophone is connected to a standard oscilloscope to measure its voltage. We have used a calibrated membrane hydrophone (spot diameter 0.5 mm, GEC-Marconi... 

Move the hydrophone across the focal plane, while the ultrasound is pulsed, until the maximum voltage generated in the hydrophone is found. 

Calculate the acoustic pressure amplitude values using the calibration coefficient of the hydrophone. 

In the panel test, a sample of the material is submerged in water and a sound projector is placed distant from the sample. A second hydrophone is placed close to the sample so that it can receive both the transmitted and reflected tone burst. These tone... 

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