Hydrophone Response Research Articles

Sensitivity measurements of piezoelectric polymer hydrophones from 0.2–2 MHz using a broadband-pulse technique

It is widely recognized that the sensitivity of hydrophones used to measure medical diagnostic ultrasound fields should be uniform over several octaves above the center frequency (i.e., above the mean of the upper and lower −3 dB frequencies in the transmitted acoustic-pressure spectrum). However, a bandwidth extending to at least ten times below the diagnostic pulse-center frequency is needed for accurate (error ≈5%) measurement of the peak rarefactional pressure. Since at present it is not common for manufacturers of medical-use hydrophones to provide sensitivity information below 1–2 MHz, a study was undertaken to determine these low-frequency sensitivities. The technique uses broadband, plane-wave pressure pulses generated by electrical short-pulse excitation of a thick piezoelectric ceramic disk. The hydrophone response is calculated from measurements of the source transducer and hydrophone-voltage waveforms. The frequency responses of both needle-type and spot-poled membrane polymer hydrophones were measured using this technique. The spot-poled membrane hydrophones had −3-dB bandwidths extending below 0.2 MHz, the lower limit for the calibration technique. The needle-type hydrophones studied, however, all exhibited a response roll-off of greater than 3 dB in the frequency range studied. Therefore, given the above bandwidth criterion as a function of diagnostic pulse-center frequency, the sensitivity to at least 0.2 MHz should be established for diagnostic-use hydrophones, because a uniform response below 1 MHz cannot be assumed.

Development of a PVDF membrane hydrophone for use in air-coupled ultrasonic transducer calibration

This work describes the use of a polyvinylidene fluoride (PVDF) membrane hydrophone for application in air-coupled transducer calibration. A one-dimensional theoretical analysis is used to demonstrate the potential and performance of PVDF as a hydrophone material over the frequency range 100 kHz to 5 MHz included in the evaluation is the influence of deposited metallic electrode layers on the sensitivity of the material. Experimental validation over the restricted range 400 kHz to 1 MHz is provided by a coplanar 0.028 mm thick membrane hydrophone in conjunction with a custom built 1-3 piezocomposite transmitter. Calibration of the membrane hydrophone is performed by employing a standard hydrophone that has been calibrated to a primary standard in a water medium. Justification for such an approach is presented within the theoretical analysis which provides a close correlation with experimental data. The generation of Lamb waves at critical angles in the PVDF and their subsequent influence on the directional response of membrane hydrophones operating in air is also addressed. A method for partial suppression of the Lamb waves, based around perforation of the membrane (either in whole or in part), is evaluated experimentally with reasonable results.

A new method of ultrasonic hydrophone calibration using KZK wave modeling

Most commercially available ultrasonic transducers exhibit finite amplitude distortion in water during hydrophone measurements needed to comply with regulatory requirements. The frequencies observed due to finite amplitude distortion can easily exceed ten times the transducer center frequency and 100 MHz. Typically, hydrophone calibrations are supplied only up to 15 or 20 MHz and do not exhibit a flat response. The frequency response above 15 MHz should be known to accurately represent the acoustic information, especially for high-frequency transducers ranging between 7.5 and 15 MHz. A new hydrophone calibration technique has successfully predicted the frequency response of hydrophones up to 100 MHz. A circular source transducer was first characterized and then modeled using the KZK wave propagation model. This model accounts for diffraction, absorption, and nonlinearity. The transducer frequency response was measured with a hydrophone and compared to the simulation. This difference characterized the frequency response of the hydrophone and was used to estimate the hydrophone calibration. The estimated calibration at 20 MHz was checked and provided good agreement with the manufacturer calibration supplied. Acoustic measurement accuracy will be improved if the hydrophone frequency response is deconvolved from the actual acoustic transducer response.

Comments on “Effect of multilayer baffles and domes on hydrophone response” [J. Acoust. Soc. Am. 99, 1883–1893 (1996)

In the subject paper Ebenezer and Abraham used the transfer matrix method to investigate the response of a receiver located between a layered dome and a layered baffle. They considered this set as one multilayer system and obtained complicated matrix equations. The purpose here is to show that the sound field and the response of the receiver may be expressed as simple expressions using reflection and transmission coefficients having clear physical meaning.

The response of transiently excited thick transducers at low frequencies

The generation of acoustic transients via short-pulse excitation of thick piezoelectric disks has proven to be useful in a number of applications, including broadband measurements of attenuation, directivity, and frequency response. However, one aspect of this technique that has received little attention is the lower-frequency limit. In the present study the frequency response was analyzed theoretically using the Mason/Redwood model for a thickness-expansion drive transducer. Two factors were found that limit the useful range of the technique at low frequencies. The first is related to the negative capacitance that appears in the equivalent circuit of a thickness expander. This element can be ignored in some transducer applications, but its presence must be accounted for in the low-frequency case. The second factor is due to the finite measurement time available, which for small transmitter/receiver separation distances is equal to the acoustic transit time through the piezoelectric disk. For an air-backed transducer radiating into water, the two limitations restrict the usable bandwidth to frequencies above approximately (10h2εS/πzTxT)exp(2h2εS/πcTzT ) and cT/xT, respectively, where h is a piezoelectric constant, εS is the clamped dielectric permittivity, and zT, xT, and cT are the transducer characteristic impedance, thickness, and speed of sound. For a 2.5-cm-thick lead zirconate titanate transducer (Navy Type I), the lower limit considering both factors is approximately 200 kHz. This limit is acceptable for determining the frequency response of hydrophones used in diagnostic ultrasound field measurements, but it is insufficient for characterizing hydrophones used to measure extracorporeal shock-wave lithotripsy pressure pulses.

Coherent localization of an unknown cw source

Let D j =D j (f) be the data spectrum from thej th element of a hydrophone array of any shape, all of whose phones have the same unknown transfer function with unknown gains. ThenD j =B dG j ( m ¯ d ), where B d is the product of the unknown source spectrum and the unknown hydrophone response, G j is the Green’s function for thej th hydrophone location, and m ¯ d is the unknown true value of the vector m ¯ ̄ containing source location and environmental parameters. Similarly, let S j =B sG j ( m ¯ s ) be a candidate synthetic spectrum for comparison with D j ; here B s is a guess at B d and m ¯ s is a candidate parameter vector. Conventional coherent localization searches for an m ¯ s that maximizes the agreement of D j and S j , an agreement clearly dependent on the guess B s . To remove the problem of the unknown source define F ij =D iG j ( m ¯ s ) and search for the m ¯ s that maximizes agreement of F ij with F ji . To see why this works note that F ij =B dG i ( m ¯ d )G j ( m ¯ s ), whereasF ji =B dG j ( m ¯ d )G i ( m ¯ s ). Thus F ij and F ji have the same source function B d , and F ij equals F ji only if m̄ s =m̄ d . Note that knowledge of the source is not required; in particular it need not be compact in time, hence cw data can be used to localize with ≥ two hydrophones. The localizer for hydrophones with equal gains is φ1=|∑ i≠j ξ i ξ j ∑ f ζ fF ij F ji *|2/(∑ i≠j ξ i ξ j ∑ f ζ f F ij F ij *)2, in which optional real ξ i give array shading, and optional real ζ f give spectral shading. A localizer allowing hydrophones to have different unknown gains is φ2=|∑ i≠j ξ i ξ j ∑ f ζ f F ̂ ij F ̂ F̂ ji *|2/(∑ i≠j ξ i ξ j )2, where F̂ ij =F ij / ∑ f ζ f F i j F i j * .

Are current hydrophone low frequency response standards acceptable for measuring mechanical/cavitation indices?

The purpose of this paper is to determine the error introduced by ultrasonic hydrophones used to measure current or proposed Mechanical (MI) and Cavitation (CI) Indices, assuming that the hydrophones meet bandwidth specifications contained in US and IEC measurement standards. These indices are based on the peak rarefactional pressure, pr. Since the portion of the pressure waveform where pr occurs is dominated by low frequency components, attention was placed on the low frequency hydrophone response specifications. Both simulated and actual diagnostic pressure pulses (with center frequency fc) were subjected to single-pole high-pass filtering for a range of -3 dB cut-off frequencies (fa). The error in the indices introduced by this filtration was evaluated. At both fa = 0.5fc (the US requirement) and fa = 0.86fc (calculated from the IEC -6dB bandwidth specification at 0.5fc), results showed that errors exceeding -30% could be expected. Furthermore, to reduce errors to less than 5%, the low frequency hydrophone response should extend at least an order of magnitude below the center frequency of the pressure wave. For example, for a 3.5 MHz transducer, the hydrophone should have a lower cut-off frequency of less than 350 kHz, which at present constitutes a challenge because of the lack of commercial hydrophones calibrated below 1 MHz.

Effect of multilayer baffles and domes on hydrophone response

The effect of infinite multilayer planar baffles and domes on the response of point hydrophones mounted on the baffles with their centers at some distance from the face of the baffles is studied. The analysis is based on the use of transfer matrices and matched boundary conditions. A two-port transfer matrix for a baffle–dome system with several solid and liquid layers is first developed by using four- and two-port transfer matrices for solid and liquid layers, respectively. This is then used to determine the voltage generated by a hydrophone in response to an incident plane wave as well as a specified velocity distribution on the rear of the baffle. The voltage depends on the acoustic and acceleration sensitivities of the hydrophone and on the characteristics of the baffle–dome system. It is also shown that in most cases moderately low values of acceleration sensitivity are sufficient to ensure that the waterborne noise sensed by hydrophones is much greater than the structureborne noise. Elastic theory is used to model the solid layers. Numerical results are presented to illustrate the effect of single, double, and triple layer baffles, compliant layers, speed of shear waves, and the distance between the hydrophone and the baffle on the generated voltage. Experimental results are also presented.

Broadband hydrophone calibration below 1 MHz

Recent calibration efforts for miniature hydrophones used to measure medical diagnostic ultrasound fields have been devoted to increasing the upper frequency range of calibration (≳10–15 MHz). However, a bandwidth extending to at least 10 times below the diagnostic pulse center frequency is needed for accurate (error ≊5%) measurement of the peak rarefactional pressure and mechanical index, both important quantities. Since at present no commercial hydrophones for medical ultrasound use provide sensitivity information below 1 MHz, a study was undertaken to determine these low frequency sensitivities. The technique uses broadband, plane‐wave pressure pulses generated by electrical shock excitation of a thick piezoceramic disk. The hydrophone response is calculated from measurements of the source transducer and hydrophone voltage waveforms. The frequency responses of both needle and membrane polymer hydrophones were measured using this technique. The membrane hydrophones studied had bandwidths extending below...

Sonar baffles

Acoustic decoupling baffles are often used to minimize noise contamination at hydrophone and transducer arrays. To maintain sensitivity near the nominally pressure release surface of the baffle, hydrophones can be placed at an odd multiple of a quarter wavelength from the baffle or near a heavy signal conditioning plate inserted between the hydrophones and the baffle. In either case, coherent interference between the incident wave and the wave reflected from the baffle limits the bandwidth of high sensitivity. Wayne Reader reasoned that by inserting a broadband absorber between the hydrophones and the decoupling baffle, a smoother hydrophone response could be attained of potentially less weight with relatively small loss in sensitivity. The talk describes the use of gradual transition absorbers developed by Wayne for sonar applications and his interactions with the author to combine these materials with broadband decouplers such as arrays of compliant tubes. His careful and thoughtful approach to the theoretical and experimental aspects of acoustic and material research was inspirational and we miss his insight, advice, and encouragement.

Particle size dependence of piezoelectric and acoustical response of a composite hydrophone

Abstract Ceramic-polymer composites composed of highly anisotropic modified lead titanate ceramic and polymer have been prepared utilizing 0–3 connectivity pattern. The composites were formed using ceramic filler powders having five different particle sizes (in the range 5–200 microns). These were found to possess lower densities (2.8 to 4 gm/cm3) and lower relative dielectric constant ∼ 150. Under the condition of fixed density and ceramic/polymer volume ratio, the dielectric constant and the piezoelectric charge constant were found to increase with increasing particle size of the ceramic used. However, these properties were not altered when the particle size of the ceramic exceeded 150 microns. These results could be explained in terms of the existence of surface layer. Three tubular sections cast out of these composites were used for fabricating the hydrophones. The receiving sensitivity of these hydrophones was evaluated using comparison calibration by pulse calibration technique. The measured values of the receiving sensitivity was found to be inversely proportional to the ceramic particle size used for making the composite. The hydrophones studied were found to have flat frequency response suitable for broad band applications in underwater acoustics.

Frequency response of PVDF needle-type hydrophones

This paper examines the factors governing the frequency response of ultrasonic polyvinylidene flouride (PVDF) polymer needle-type hydrophones, in particular the sensitivity variations in the lower frequency range of 1–6 MHz. A theoretical model was used to analyze the influence of the hydrophone's diameter, the metal electrodes, thickness, PVDF material properties, the adhesive layer acoustical characteristics and the backing material, on the frequency response of the hydrophone. The results of the theoretical modelling differ by less than ± 0.5 dB from those obtained experimentally from the reciprocity calibration in the frequency range 1–20 MHz. It is shown that the needle hydrophone's diameter and backing material are the main reasons for the sensitivity variations observed in the frequency range below 6 MHz.

Study of surface elastic wave induced on backing material and diffracted field of a piezoelectric polymer film hydrophone

A distinctive irregularity in the frequency response of a piezoelectric polymer film hydrophone, which is not predicted by the use of conventional theory, has been observed. It was confirmed in a previous study [Y. Nakamura and T. Otani, Jpn. J. Appl. Phys. Suppl. 31-1 31, 266–268 (1992)] that the irregularity can be attributed to the radial resonance of a surface wave induced by the incident pressure on the hydrophone’s backing material. To estimate theoretically the frequency response of the hydrophone, the transient acoustic field on the receiving surface of the hydrophone was studied by time-domain analysis. The field is theoretically described in the time domain as the superposition of an incident wave, a reflected plane wave, a diffracted edge wave, and a leaky elastic wave that originates from a surface wave propagating along the radius on the hydrophone’s backing material. The validity of the theoretical analysis of the acoustic field on receiving surface is verified by experimental observations. Thus it is shown that the frequency response of the piezoelectric polymer film hydrophone is estimated from the analytical results of the acoustic field.

A multichannel digital system for use in calibrating towed arrays

A long-line hydrophone calibrator (LLHC) has been developed at NRL–USRD to serve as a platform for calibrating a variety of towed arrays. The LLHC consists of multiple projectors and hydrophones in a long water-filled pipe. Digital control of the projectors allows a precisely controlled acoustic field to be generated in the pipe. The LLHC makes extensive use of digital signal processing to control and monitor up to 128 acoustic channels. The DSP algorithm for field generation is distributed among 9 signal processors, each controlling 15 acoustic channels. The transfer function of the pipe is characterized by driving each projector separately and recording the responses of all hydrophones to it. To build a specified sound field in the pipe, the inverse of the response matrix is multiplied by the vector of desired hydrophone responses to form the vector of drives required to achieve the desired field. This presentation describes the details of the electronics and software necessary to build the desired field. [Work supported by NAVSEA.]

A finite element method for the analysis of piezoelectric composite hydrophones

A robust finite element method is developed for the analysis of piezoelectric bodies. The method is general enough to account for the losses in material coefficients. The method is used to analyze the behavior of 3:1 PZT-rubber composite hydrophone. The effect of material properties, sizes and locations of the rubber phase on the response of the composite is studied. Also the viscoelastic properties of the rubber phase are studied for its effect on the sensitivity as well as on the dynamic response of the composite hydrophone.

Comparisons of downhole geophones and hydrophones

Receiver tests were conducted to compare the responses of downhole geophones and hydrophones. Commercial receiver tools use a maximum of eight geophone levels; however, we use hydrophones because we can record 48 levels simultaneously. For frequencies above 300 Hz, signal‐to‐background‐noise ratios for hydrophones and geophones in a prototype tool were comparable. (This prototype tool is a lightweight, large‐clamping‐force device that can record higher frequencies than commercial geophone tools.) For frequencies below 300 Hz, signal‐to‐noise ratios were greater for the geophones than for the hydrophones. A commercial geophone tool had lower low‐frequency signal‐to‐background‐noise ratios than the prototype tool, but greater than those of the hydrophones. Further analysis was performed to determine why the signal‐to‐background‐noise ratios for geophones were greater than those for hydrophones at low frequencies. The measured signal level for a hydrophone was 2.4 times that for a geophone, compared with a theoretical prediction of 1.8. Thus, the signal levels do not explain the difference in signal‐to‐background‐noise ratios. The low‐frequency background noise was attributed to coherent noise in the form of tube waves, a noise type to which hydrophones are much more susceptible than are geophones. Thus, the low signal‐to‐background‐noise ratios at frequencies below 300 Hz for hydrophones resulted from ambient noise propagating as tube waves in the borehole. The high‐frequency background noise was attributed to random seismic noise in the environment and not to instrument noise. These results show that hydrophones, which do not need to be clamped to the borehole wall, are preferable to geophones for high‐frequency borehole seismic applications using first arrivals. Geophones are preferable to hydrophones for borehole seismic applications using reflector arrivals, because these later‐arriving events are obscured by source‐generated tube waves in hydrophone data. Development of a method to reduce both the source‐generated and ambient tube‐wave noise detected by hydrophones would result in high‐quality borehole seismic data at a greatly reduced cost.

Lithotripsy pulse measurement errors due to nonideal hydrophone and amplifier frequency responses

When using miniature ultrasonic hydrophones to probe the focal region of extracorporeal shock wave lithotripsy devices, the frequency response of the measurement hydrophone and any associated amplifier must be broad enough to minimize pulse distortion. To study the potential effects of the bandwidth-limited behavior, a mathematical model was used. Several parameters of a simulated lithotripsy pulse were compared before and after being filtered by hydrophone and amplifier response functions. Errors were computed for the peak positive and negative pressures, risetime, pulse duration, and pulse intensity integral as functions of hydrophone and amplifier bandwidths. Although most of the energy in a shock wave pulse lies at frequencies below a few megahertz, it is found that significant errors can occur unless measurement bandwidths are much wider.

Frequency Response of a Piezoelectric Polymer Film Hydrophone and an Elastic Wave Induced on the Backing Surface

The Frequency response of hydrophones using a thin film of piezoelectric polymer of P(VDF-TrFE) as an active element is investigated. An irregularity in the response for which can not be theoretically accounted is always observed. We have verified by means of optical visualization or schlieren method, that the Rayleigh type wave excited on the backing surface of the hydrophone brings a radial resonance and shows a phenomenon of irregularity on the frequency response.

A model of the effects of hydrophone and amplifier frequency response on ultrasound exposure measurements

To examine how less-than-ideal hydrophones and amplifiers could affect the measurement of pulse parameters, a simulated distorted pressure pulse is analyzed before and after being filtered by hydrophone and amplifier response models. The hydrophone model used is similar to the response of piezopolymer membrane hydrophones. The amplifier model is taken from typical responses of integrated circuit operational amplifier-based designs. The pressure pulse, hydrophone, and amplifier are characterized respectively by the pulse center frequency, f (c), the hydrophone resonance frequency, f(h), and the amplifier low-pass corner frequency, f(a).

High frequency response of fiber-optic planar acoustic sensors

The frequency response of planar interferometric fiber-optic acoustic hydrophones is considered. It is shown that such sensors formed from materials whose impedance closely matches that of the fluid generally have acoustically induced acceleration effects which tend to decrease the high-frequency response for the normal incidence case. Planar sensor configurations are identified which reduce this effect, and measurements on one such configuration are presented to confirm this. Since this is achieved by reducing the response of the sensor to normal acceleration, this fiber sensor design should also be useful for other applications requiring a low acceleration response.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>