Free-field Voltage Sensitivity Research Articles

An end-capped lead zirconate titanate broadband hydrophone theoretical calculation and electroacoustic measurement for towed array applications

Hydrophone transducers for towed array applications require low self-noise, small form factor and a linear frequency response below 50 kHz for general purpose long-range ocean sensing. The design specification for this hydrophone application has several aspects including acoustic sensitivity, frequency response, dissipation factor, and physical size. A comprehensive noise model informs filter design for pre-amplifiers after the piezoelectric element. Theoretical calculation of pressure to voltage conversion is made here by estimating the Free-Field Voltage Sensitivity (FFVS) using methods from R.A. Langevin and G.W. McMahon. Using these methods to achieve the desired design specification, we use two cylindrical ceramics with a split electrode poled in the radial direction with series stacked ceramics using end caps with air backing to increase sensitivity and reduce overall size. An experiment is carried out to measure the resonant modes using a simple impedance measuring circuit which then can be used to estimate FFVS linearity. These results are compared with theoretical calculation for hydrophone response of resonant modes and results from measurement circuit.

Low cost acoustic transducer calibration system

An acoustic calibration system was developed primarily for underwater acoustic transducers, although applicable for a variety of acoustic applications. Historically, such systems have been large, highly customized, and very expensive. With the advent of compact cost-effective digital PC-based oscilloscopes and data acquisitions systems (e.g., devices from TiePie, Cleverscope, and Digilent), a development that began as a senior capstone-design project has recently been demonstrated. Our system is based on the TiePie Digital Oscilloscope, a PC based signal generation and acquisition systems, a power amplifier, optional preamplifier and stepper motor with controller for radiation patterns. More specifically, the receive sensor signals are acquired with a TiePie Handyscope HS6 DIFF (1000MS/s, USB 3.0 oscilloscope, with four differential analogue channels) and the transmit signal (signal generation, transmit voltage and current acquisition) utilizes a TiePie HS5-540XM 5 (500 MHz, 1-channel generator, 2-channel acquisition) measuring system. The calibration and data display is controlled with MATLAB and controlled through a graphical user interface (GUI). Examples of Transmit Pressure Response per Volt (TR/V or TVR), Free Field Voltage Sensitivity (FFVS), and Beam Pattern measurements will be provided. [Work supported by BTech Acoustics LLC.]

Reciprocity Calibration of Hydrophones in High Intensity Focused Ultrasound Field

The primary problem of safety and efficiency for the high intensity therapeutic ultrasound (HITU) is the acoustic measure and dose control. The key technique is the pressure and intensity in the acoustic field especial in the focal region using the small calibrated hydrophone. The calibration accuracy of the used hydrophone is very important for HITU. Although the small hydrophone calibration has realized but there was no report of the hydrophone calibration in high pressure field. In this paper, our objective is to develop an absolute calibration method for the measurement of free field voltage sensitivity of hydrophone for high intensity focused ultrasound. First the acoustic pressure at the focal point by the self-reciprocity method of spherically curved auxiliary transducer is calibrated, then the free field voltage sensitivity of hydrophone at the geometric focal point of the calibrated pressure is obtained. The spatial average effect of acoustic pressure on hydrophone surface at the focal point is theoretically modified, and the expression and value table of correction coefficient of spatial average effect of hydrophone are given. The maximum acoustic pressure measured at the focal point was up to 5.58MPa (1.02kW/cm2) and used to calibrate a hydrophone from 0.95 MHz to 1.10 MHz with maximum local distortion parameter 0.72. The results show the rationality and feasibility of the measurement principle and method.

Rapid measurement method of hydrophone in reverberation pool

In this paper, a new method based on spatial averaging is proposed to measure hydrophone rapidly in reverberation pool. The method is to overcome the influence of acoustic normal mode by spatial averaging technique, get the reverberation sound RMS pressure in reverberation control area. It is according to the comparison principle to calibration of hydrophone rapidly. The tank is 15m×9m×6m, in the frequency range of 400–10 kHz. The free field voltage sensitivity of the four hydrophones is measured. The measurement results are compared with the free field comparison method of measurement results, the deviation is less than 1 dB, to verify the effectiveness of the method. The method can realize the simultaneous measurement of multiple hydrophones, improve the calibration efficiency, and the lower limit of measurement frequency is much lower than the lower limit frequency of the conventional pulse comparison technique, which improves the measurement capability of the anechoic tank.

Hydrophone calibration using ambient noise

Hydrophone calibration typically requires a good signal-to-noise (SNR) ratio in order to calculate the free-field voltage sensitivity (FFVS). However, the SNR requirements can limit the calibration of hydrophones with low sensitivity, particularly in the low frequency range. Calibration methods using ambient noise in lieu of a generated signal will be explored at the Underwater Sound Reference Division (USRD) Leesburg Facility in Okahumpka, FL. The USRD Leesburg Facility is at a natural spring in rural central Florida and is one of the Navy’s quietest open water facilities with no boating noise, limited biological noise, an isolated location, low reverberation and an isothermal water temperature profile below 5 meters. Comparison calibrations will be made with two similar hydrophones using the ambient noise in the natural spring and the results will be compared to calibrations made with the same hydrophones using a generated signal.

Simulation Analysis on Cymbal Transducer

The resonance frequency of the cymbal transducer ranges from 2kHz to 40kHz and its effective electromechanical coupling factor is around 20%. Finite element analysis has been performed to ascertain how the transducer’s makeup affect the transducer’s performance parameters. Two-dimensional axisymmetric model of the cymbal transducer was founded by finite element software-ANSYS, the application of the element type was discussed and the FEM models were built up under the far field condition. Eight groups of cymbal transducers of resonance frequency around 3kHz with different structural dimensions were designed. It was better for choosing the cymbal transducer of the 8mm cavity coping diameter, 20.8mm cavity bottom diameter and 26.8mm piezoelectric ceramic wafer diameter than others for reducing distortion degree of the signal and improving communication turnover in the researched cymbal transducers. It was appropriate for choosing the cymbal transducer of the 8mm cavity coping diameter, 22.4mm cavity bottom diameter and 26.4mm piezoelectric ceramic wafer diameter in order to improve the free-field voltage sensitivity and transmission efficient.

Design of a Novel Encapsulation Based on Hair Cell Vector Hydrophone

According to sensitive structure of vector hydrophone which imitates fish lateral line cells, a novel encapsulation of polyurethane is proposed, that is, using polyurethane, the same material as sound-transparent cap, instead of castor oil to package. It is desirable that the novel encapsulation may effectively solve the problem of vector hydrophone’s low sensitivity. On the basis of analysis theoretical model of sound-transparent cap’s three layer mediums, this paper simulated the whole encapsulation structure by ANSYS. It can be concluded that the sensitivity is 0.267mV/Pa, resonance frequency is 1151.37 Hz. Finally, the prototype of the vector hydrophone was fabricated and preliminary characterization tests of the hydrophone were performed. Results show that the novel encapsulation hydrophone possesses satisfactory directional pattern and the free-field voltage sensitivity is -188.9 dB, which is 8.8dB higher than castor oil encapsulation.

Modeling and characterization of a micromachined artificial hair cell vector hydrophone

In the paper, a micromachined artificial vector hydrophone arises from a biological inspiration, the fish hair cell is presented. It is desirable that the application of piezoresistive effects combined with ingenious bionic structure and MEMS technology may improve the low-frequency sensitivity of the vector hydrophone as well as its miniaturization. Modeling processes for realizing the artificial hair cell hydrophone, along with preliminary characterization results in terms of sensitivity, frequency response and directivity patterns are also introduced. The microstructure of the sensor consists of four vertical cantilever beams with attached rigid plastic cylinder in the center of the structure. By locating eight piezoresistors logically formed the Wheatstone bridges; they can detect two components of underwater acoustic signal simultaneously. The prepared vector hydrophone has been measured in the standing wave field finally. The experiment results show that the vector hydrophone has good low-frequency characteristic, the free-field voltage sensitivity is −197.2 dB (0 dB = 1 V/μPa) at 400 Hz with a about 2 dB one-third octave positive slope over the 40–400 Hz bandwidth. The depth of pits of the directivity pattern is about 34.6 dB.

Free Field Reciprocity Calibration in a ConvergentSpherical Acoustic Wave of a Focusing Transducer

Based on the reciprocity theorem of the acoustic field, we derivethe formula of the reciprocity coefficient of a convergent sphericalacoustic wave and we calculate a series of diffraction correctivefactor curves of the reciprocity coefficient of transducers. Using theseformulae and corrective factors, we calibrate the free fieldtransmitting current response and the free field voltage sensitivity ofa focusing transducer using the self-reciprocity method. The experimentalresults of the reciprocity calibration of the focusing transducer in thefrequency range of 2 MHz to 5.4 MHz are presented.

Considerations for a new high-accuracy transfer-coupler reciprocity system for absolute electro-acoustic calibration

A new, closed-chamber, reciprocity technique for determining the free-field voltage sensitivity of hydrophones to a very high accuracy has been described in a previous publication by the author. The subject of the current paper is a detailed uncertainty analysis for a proposed system based on this new technique. This system, which is under development, will have an overall uncertainty of 0.1 dB in the measurement of the free-field voltage sensitivity of hydrophones. All significant uncertainties are analysed and a realistic choice of individual uncertainties is made to achieve the target uncertainty. Design implications as a result of the uncertainty choices are indicated.

High pressure, high frequency reciprocal transducers

A high frequency transducer capable of transmitting and receiving having an operational range from 100 kHz to 2 MHz, a free field voltage sensitivity (FFVS) greater than -218 dB re 1 V/μPa (300 kHz), and a maximum hydrostatic pressure rating of 20.68 Mpa (3000 psi). The sensor element is made of lead titanate with a radial mode of 150 kHz being suppressed by the use of a rubber grommet surrounding the lead titanate element. The sensor element is retained within a sensor housing by a plurality of annealed iron wires attached to an annealed iron ring, both of the faces of the sensor housing are then covered with an acoustically transparent window of natural gum rubber. The sensor housing is attached to a non-metallic preamplifier housing made of a rigid polyurethane containing a preamplifier circuit, a transmit protection circuit, and a plurality of relays for switching the preamplifier circuit during the transmit and receive modes. A remotely control unit is connected to the transducer through a cable attached to a connector located on the preamplifier housing. The sensor housing and preamplifier housing are joined together electrically by a slip fit coaxial coupler for ease of maintenance and repair.

The effects of multilayering piezoelectric plates with implication to 1–3 piezocomposite materials

This paper discusses the considerations of stacking piezoelectric plates mechanically in series (on top of each other) and electrically in parallel or series. Benefits include increased displacement for projectors and increased signal‐to‐noise ratio (SNR) for hydrophones (because of self‐shielding and decreased electrical impedance). The presentation will show why these enhancements are realized by using a mathematical solution of the one‐dimensional (1‐D) wave equation to describe how each layer is related to that in adjacent layers via continuity of particle displacement and force. The resultant set of simultaneous equations is solved using matrix manipulation algorithms over a specified frequency range in terms of the free‐field voltage sensitivity (FFVS), transmitting voltage response (TVR), and electrical immitance characteristics. A temporal transient response is obtained through the inverse fast Fourier transform (IFFT). This computational efficient model is capable of piezoceramic or 1–3 piezocomposite transducer layers, as well as inactive layers. The 1–3 piezocomposites are input by using the Smith expressions for representing the piezocomposite as a homogeneous material. Application of 1–3 piezocomposites for multiple layering will be introduced and discussed. [Work sponsored by the Office of Naval Research.]

A planar-omnidirectional sensor for broadband and high-frequency underwater acoustic applications

A prototype hydrophone with planar-omnidirectional radiation characteristics has been developed for broadband, high-frequency, underwater acoustic applications. A 9-μm sheet of KYNAR-type polyvinylidene fluoride (PVDF) was wrapped about a 2.5-mm solid Rho-c rod with a height of 86 mm such that the resulting cylindrical shape had an omnidirectional radiation behavior in the X-Y plane. The PVDF was oriented with the 3-1 direction radially wrapped and the 3-2 axis positioned along the mounting rod for improved vibration strum rejection from the mounting rod with improved radial sensitivity because of the orthotropic nature of the KYNAR material. Further discussions will detail the selection of an impedance matching mounting rod and waterproofing elastomer and a cable with the low capacitance of 0.018 pF/m (8.5 pF/ft). Measured data to be shown includes the free-field voltage sensitivity (FFVS) and X-Y plane receiving radiation patterns from 100 kHz to over 1.5 MHz. The presentation will conclude with a conceptual discussion on an advanced version of this same sensor design for a broadband and high-frequency, three-dimensional omnidirectional sensor.

U.S. Navy 0–3 piezocomposite transducers

Interest in 0–3 piezoelectric composite materials for use in hydrophone applications has grown considerably in recent years. Designs which require operation in the hydrostatic mode with a large hydrostatic voltage coefficient gh, are desired to increase free-field voltage sensitivity and meet the ever increasing demands placed on hydrophone performance. U.S. Navy scientists at the Naval Research Laboratory, Underwater Sound Reference Detachment, have investigated the utilization of 0–3 piezocomposite for sonar applications and have developed various hydrophone configurations based on specific program requirements. The requirements of the programs, such as a lightweight, low-profile, and high receive sensitivity, demanded unique designs and the need for utilizing the unique characteristics of 0–3 composite materials. The hydrophones presented in this discussion used a 0–3 composite known at NRL simply as piezorubber or PZR. This material consists of lead titanate particles embedded in a neoprene elastomeric matrix. This discussion will cover the development, including problems and solutions, fabrication, and testing of two unique 0–3 composite hydrophones.

Construction and evaluation of a noise-suppressing hydrophone

The construction and evaluation of an acoustic sensor is described which consists of two small pieces of piezoceramic bonded together with their poling axes collinear, one piece containing ordered macrovoids, the other solid. Electrical leads attached to each piece allow their outputs to be separately processed. The solid piece has a small hydrostatic voltage coefficient while the voided has a significantly larger voltage coefficient. Both have appreciable responses to vibration excitation along their poling axes. Hence, if the two electrical outputs are appropriately weighted, phase reversed, and then combined, the net response to vibration excitation should be negligible while the net response to an incident pressure wave should be essentially that of the voided piece alone. Hence, the assembly should function simultaneously as a poor accelerometer but a good hydrophone. The data show that at least 10 dB of vibration response cancellation and a large, nearly uniform, free-field voltage sensitivity are achieved over the frequency range from 5 to 20 kHz.

Finite element modeling of active periodic structures: Application to 1–3 piezocompositesa)

The finite-element approach has been previously used, with the help of the ATILA code, to model the scattering of acoustic waves by single periodic passive structures, such as compliant tube gratings [A.-C. Hennion et al., J. Acoust. Soc. Am. 87, 1861–1870 (1990)], or by doubly periodic passive structures, such as Alberich anechoic coatings [A.-C. Hladky-Hennion et al., J. Acoust. Soc. Am. 90, 3356–3367 (1991)]. This paper presents an extension of this technique to active periodic structures, and describes with particular emphasis its application to the modeling of 1–3 piezocomposites made of parallel piezoelectric connecting strips in a passive matrix. The method can also be applied to many other piezocomposites or to large high-frequency arrays. In the proposed approach, only the unit cell of the active structure and a small part of the surrounding fluid domain have to be meshed, while the acoustic field on both sides of this mesh is described by a plane-wave expansion including progressive and evanescent contributions. Internal losses and anisotropy of the materials as well as normal or oblique incidence of the impinging wave, if this wave exists, can be taken into account in the model. In this paper, the general method is first described, and particularly the aspects related to the piezoelectric elements. Then, one test example is given, for which analytical results exist. This example is followed by a detailed presentation of finite-element results, which are compared with the corresponding measurements (free-field voltage sensitivity or transmitting voltage response), in the case of a given 1–3 composite. The accuracy of the whole approach is thus clearly demonstrated. Finally, the influence of the geometric parameters of a 1–3 composite is studied, while at the same time the limitations of previously published simple analytical models are discussed.

Electroacoustic evaluations of 1–3 piezocomposite rings

Two 1–3 piezocomposite rings have been recently electroacoustically evaluated for sonar application. The two rings have outer diameters of 345 mm, 101.6-mm height and a wall thickness of 5 mm. Both have a 5% Navy type II piezoceramic volume concentration loading. The host epoxy of one ring is soft while the other is hard. Both rings had 17.78 by 25.4 mm rectangular elements etched into a 3 by 3 element pattern in their outer electrode for direct in-water performance comparison with a previous design utilizing individual elements of the same dimensions [Howarth etal., J. Acoust. Soc. Am. 91, 2325–2326 (A) (1992)]. The in-water measurements include wide-band free-field voltage sensitivity (FFVS), transmitting voltage response (TVR), and radiation patterns in both the XY and XZ planes. A laser Doppler vibrometry was used for in-air evaluation of the piezoelectric charge constant and surface displacement. Interelement coupling data will also be presented. [Work sponsored by the Office of Naval Research.]

Evaluation of the properties of 1-3 piezocomposites of a new lead titanate in epoxy resins

Abstract By using a new calcium-modified lead titanate ceramic with a near-zero planar coupling coefficient, a series of 1-3 piezocomposite samples was fabricated with a dice-and-fill technique. The ceramic rods were approximately 0.10 mm in size, and the percent of ceramic loading varied from 10 to 30%. Two epoxy resins with different glass transition temperatures and moduli were used. The dielectric properties and the piezoelectric dh and gh coefficients of the composites were measured as a function of pressure and temperature and were found to exhibit little variation, but 10-25% lower than theoretical predictions. A prototype hydrophone made from one of the piezocomposite samples was tested to show a constant free-field voltage sensitivity of -201 dB re V/μPa from 100 Hz to 6 kHz.

Finite element modeling of active periodic structures.

The finite element approach has been previously used, with the help of the ATILA code, to model the scattering of acoustic waves by single passive periodic structures, such as compliant tube gratings [A.-C. Hennion etal., J. Acoust. Soc. Am. 87, 1861–1870 (1990)], or by doubly passive periodic structures, such as Alberich anechoic coatings [A.-C. Hennion etal., J. Acoust. Soc. Am. Suppl. 1 88, S115 (1990)]. Within this approach, only the unit cell of the structure, including a small part of the surrounding fluid domain, has to be meshed, while the acoustic field on both sides of this mesh is described by a plane wave expansion, with progressive as well as evanescent contributions. Internal losses and anisotropy of the materials as well as normal or oblique incidence of the wave can be described in the model. This technique has been extended to active periodic structures, such as composite piezoelectric materials, parallel piezoelectric connecting strips in a passive matrix, for example, or large high frequency arrays. In this communication, the general method is first presented, particularly the aspects related to the piezoelectric elements. Then, several examples are given, for which analytical models exist. Finally, finite element results are compared with experiments in the case of the free-field voltage sensitivity or the transmitting voltage response of 1–3 piezocomposites and demonstrate the accuracy of the whole approach.

Piezoelectric properties of calcium-modified lead titanate and its application in underwater transducers

The dielectric and piezoelectric properties of a calcium-modified PbTiO3 ceramic were determined as a function of pressure from 0 to 70 MPa by using an acoustic reciprocity technique. The transverse piezoelectric coefficients (d31, g31) of this material are very small, which results in high values of hydrostatic gh (=g33+2g31) and dh (=d33+2d31) constant. Consequently, the material is considerably more sensitive than lead zirconate titanate (PZT). Measurements of the resonance properties of this ceramic indicated that it is suitable for use in a high-frequency underwater transducer. Prototype transducers were designed and constructed. The free-field voltage sensitivity was measured over the frequency ranges 1–100 kHz and from 200 kHz–2 MHz. The results of these measurements are compared with those obtained from existing high-frequency hydrophone standards. Since this material is a volume expander, it is very useful for electroacoustic transducer applications.