Mark B. Moffett

Model for parametric acoustic sources

A theoretical model for the difference‐frequency radiation from a parametric acoustic array is developed. The (two‐frequency) primary wave is assumed to be radiated by a piston source and may be effectively limited in either the nearfield or the farfield of the piston. Either small‐signal absorption or saturation of the primary beam may serve as the limiting mechanism. The parametric gain is defined as a complex number whose (i) magnitude is the ratio of secondary and primary source pressures and (ii) phase is an indication of how much generation occurs within the nearfield and how much within the farfield of the primary beam. The parametric gain is evaluated as a function of the primary‐to‐secondary frequency (downshift) ratio, the amount of small‐signal primary absorption within the nearfield, and a ’’scaled’’ primary source level. Then, the parametric gain may be used to determine the beam pattern characteristics via a simple closed‐form expression. The theoretical results are found to be in good agree...

On fiber‐optic hydrophone sensitivity

Previous treatments of fiber‐optic hydrophone sensitivity have neglected the effect of dimensional changes of the fiber in response to acoustic pressure. It is shown here that the changes in length actually contribute more to hydrophone sensitivity than do the refractive index changes; the two contributions are of opposite sign. An experiment verifying the sensitivity calculation was performed using 60‐kHz pulses from a spherical source.

Comparison of Terfenol‐D and PZT‐4 power limitations

A previous article [Moffett et al., J. Acoust. Soc. Am. 89, 1448–1455 (1991)], comparing the radiated‐power limitations of Terfenol‐D and PZT‐4, is revisited. When the power at resonance is stress‐limited (as is the case for high mechanical quality factor Q), the maximum allowable stress amplitude Tmax is the compressive prestress value, so that tension in the Terfenol‐D is avoided. When this value is chosen for Tmax, rather than the smaller Tmax’s of the previous article, Terfenol‐D is shown to be superior to PZT‐4 in power capability at all values of the quality factor Q.

Broadband, acoustically transparent, nonresonant PVDF hydrophone

An acoustically transparent voided, polyvinylidene fluoride (PVDF) hydrope made of material whose impedance matches the characteristic acoustic impedance (ρc) of sea water, having drastically reduced diffraction and resonance effects. The frequency response is thus flat at frequencies less than one-half elastic wavelength in the PVDF material. An array of such hydrophones in front of a projector saves space without affecting projector performance.

Single-crystal lead magnesium niobate-lead titanate (PMN/PT) as a broadband high power transduction material

Two experimental underwater acoustic projectors, a tonpilz array, and a cylindrical line array, were built with single crystal, lead magnesium niobate/lead titanate, a piezoelectric transduction material possessing a large electromechanical coupling factor (k33 = 0.9). The mechanical quality factor, Q(m), and the effective coupling factor, k(eff), determine the frequency band over which high power can be transmitted; k(eff) cannot be greater than the piezoelectric material value, and so a high material coupling factor is a requisite for broadband operation. Stansfields bandwidth criteria are used to calculate the optimum Q(m) value, Q(opt) approximately 1.2 (1-k(eff)2 1/2/k(eff). The results for the tonpilz projector exhibited k(eff) = 0.730, Q(m) = 1.17 (very near optimal), and a fractional bandwidth of 0.93. For the cylindrical transducer array, k(eff) = 0.867, Q(m) = 0.91 (larger than the optimum value, 0.7), and the bandwidth was 1.16. Although the measured bandwidths were less than optimal, they were accurately predicted by the theory, despite the highly simplified nature of the Van Dyke equivalent circuit, on which the theory is based.

A ρc hydrophone

Because traditional piezoelectric materials have much higher acoustic impedances than water, most hydrophones are ‘‘hard’’ compared to the surrounding water medium. This hardness causes internal and external reflections that manifest themselves as resonance and diffraction effects, respectively. Even nonvoided polyvinylidene fluoride (PVDF), a piezoelectric polymer, has a characteristic acoustic impedance, ρoco, about 2.7 times that of water. With the advent of voided PVDF, however, it is possible to match water’s impedance as closely as desired. Using ρc window material and ρc PVDF, we have constructed a ρc hydrophone whose response is essentially free of resonance and diffraction effects.

Lightweight, broadband Rayleigh wave transducer

A broadband angle-beam transducer system for producing and/or receiving Rayleigh waves on the surface of a metal medium comprising a thickness mode piezoelectric polymer transducer element embedded within and impedance matched to a plastic wedge. The transducer element is oriented at an angle determined to be the critical angle for Rayleigh waves in the metal structure to be tested. As a transmitter, the transducer element generates discrete dilatational wave pulses, one of which propagates within the plastic wedge to the plastic face in contact with the metal structure. At this face some of the wave energy is reflected as dilatational and shear waves within the wedge and the remainder of the wave energy passes through a coupling medium into the metal structure producing a Rayleigh wave traveling along the surface of the structure. When the system is used as a receiver, a Rayleigh wave propagating on the surface of the metal structure is received by the polymer element which then generates a voltage proportional to the Rayleigh waveform.

Characterization of PMN‐PT‐La for use in high‐power electrostrictive projectors

There is increasing interest in relaxor ferroelectrics such as PMN‐PT‐La,Ba (lead magnesium niobate‐lead titanate doped with lanthanum or barium) for use as high‐power transducer driver materials in lightweight projector arrays. These electrostrictive materials have been shown to have induced strains an order of magnitude greater than PZT at modest electric fields. The drawback, however, is that the response of the material is dependent on frequency, prestress, temperature, and ac drive and dc bias fields. To date no measurements have been made of all these relationships for 33‐mode vibration applications. In this study, using various mechanical prestresses, quasistatic room temperature measurements were made of the piezoelectric constant, d33, of the relative permittivity, e33T/e0, and of the short‐circuit Young’s modulus, Y33E for PMN‐PT‐La (TRS Ceramics, Inc., 0.90 / 0.10 /1%, 31 March 1995). Electric fields of up to 2 MV/m were applied to samples (2×2×10 mm that were prestressed from 0 to 12 ksi (83 M...

BIASED LEAD ZIRCONATE TITANATE AS A HIGH-POWER TRANSDUCTION MATERIAL

High‐power, underwater transducers using polarized piezoelectric ceramic material are usually limited in drive amplitude so that depolarization does not occur, but application of a dc bias field in the polarization direction allows the use of higher ac drive fields. To demonstrate the feasibility of a biased operation as a means of achieving higher power, a thin‐walled, spherical‐shell transducer was constructed of Channel 5800 and tested in NUWC’s Acoustic Pressure Tank Facility. The transducer was successfully driven to 33 V/mil (1.3 MV/m) rms with an accompanying bias field of 31 V/mil (1.2 MV/m) and a hydrostatic pressure of 1400 psig (9.65 MPa). The source level was 206 dB//mPa‐m at 61 kHz, corresponding to a 10‐dB improvement over the usual Navy standard unbiased drive limit of 10 V/mil (0.4 MV/m) rms. [Work supported by ONR.]

Nearfield characteristics of parametric acoustic sources

Because it is often impractical to make measurements in the farfield of a parametric acoustic source, it is desirable to be able to predict the farfield from nearfield measurements. A nearfield theory applicable to saturation‐limited as well as absorption‐limited sources, previously outlined [J. Acoustic. Soc. Am. 63, 1622–1624 (1978)], has been programmed for digital computation. Growth curves for the source level and beamwidth of absorption‐limited sources are presented in generalized form. The theoretical results are compared with experimental data from several sources. Sources involving substantial difference‐frequency generation in the primary farfield have broader nearfield beam patterns and lower apparent source levels than in the farfield. On the other hand, when most of the generation occurs in the primary nearfield, the pattern is narrower and the apparent source level higher than in the farfield.