A nonlinear propagation model-based phase calibration technique for membrane hydrophones

A technique for the phase calibration of membrane hydrophones in the frequency range up to 80 MHz is described. This is achieved by comparing measurements and numerical simulation of a nonlinearly distorted test field. The field prediction is obtained using a finite-difference model that solves the nonlinear Khokhlov-Zabolotskaya-Kuznetsov (KZK) equation in the frequency domain. The measurements are made in the far field of a 3.5 MHz focusing circular transducer in which it is demonstrated that, for the high drive level used, spatial averaging effects due to the hydrophone's finite-receive area are negligible. The method provides a phase calibration of the hydrophone under test without the need for a device serving as a phase response reference, but it requires prior knowledge of the amplitude sensitivity at the fundamental frequency. The technique is demonstrated using a 50-microm thick bilaminar membrane hydrophone, for which the results obtained show functional agreement with predictions of a hydrophone response model. Further validation of the results is obtained by application of the response to the measurement of the high amplitude waveforms generated by a modern biomedical ultrasonic imaging system. It is demonstrated that full deconvolution of the calculated complex frequency response of a nonideal hydrophone results in physically realistic measurements of the transmitted waveforms.     

Hydrophone spatial averaging corrections from 1 to 40 MHz

The purpose of this study was to develop and experimentally verify a practical spatial averaging model for frequencies up to 40 MHz. The model is applicable to focused sources of circular geometry, accounts for the effects of hydrophone probe finite aperture, and allows calibration by substitution to be performed when the active elements of reference and tested hydrophone probes differ significantly. Several broadband sources with focal numbers between 3 and 20 were used to produce ultrasound fields with frequencies up to 40 MHz. The effective diameters of the ultrasonic hydrophone probes calibrated in the focal plane of the sources ranged from 150 to 500 μm. Prior to application of the spatial averaging corrections, the hydrophones with diameters smaller than that of the reference hydrophone exhibited experimentally determined absolute sensitivities higher than the true ones. This discrepancy increased with decreasing focal numbers and increasing frequency. It was determined that the error was governed by the cross-section of the beam in the focal plane and the ratio of the effective diameters of the reference and tested hydrophone probes. In addition, the error was found to be reliant on the frequency-dependent effective hydrophone radius. After applying the spatial averaging correction, the overall uncertainty in the hydrophone calibration was on the order of ±1 dB. The model developed is being extended to be applicable to frequencies beyond 40 MHz, which are becoming increasingly important in diagnostic ultrasound imaging applications     

Broadband PVDF membrane hydrophone for comparisons of hydrophone calibration methods up to 140 MHz

A PVDF membrane hydrophone has been constructed in particular for comparisons of broadband ultrasound hydrophone calibration methods and of the results obtained by different laboratories. Intercomparisons have to accompany the efforts currently undertaken to enhance the calibration frequency ranges and to implement the extension from the determination of amplitude-only to complex-valued calibration data. It can be expected that such hydrophone data will be used much more frequently in the future for exposure measurements on medical ultrasound equipment, in particular for the detection of nonlinearly distorted waveforms. The hydrophone design chosen has a foil thickness of 9 microm and an electrode diameter of 210 microm. A broadband differential preamplifier (-3 dB roll-off frequency: 95 MHz) is integrated to achieve a high signal-to-noise ratio over a broad frequency range (e.g., 26 dB-30 dB in the range 50 MHz to 140 MHz for measurements of nonlinearly distorted pulses). The hydrophone response was characterized by means of a primary interferometric calibration technique, by substitution calibration using time-delay spectrometry, and by complex broadband pulse calibration using nonlinear sound propagation. The results show a flat frequency response up to 40 MHz (maximum variations below +/-0.6 dB) and a thickness mode resonance at about 105 MHz. They indicate a useable bandwidth up to 140 MHz. The effective diameter as derived from directional response measurements is 240 microm at frequencies beyond 15 MHz.     

Amplitude and phase calibration of hydrophones by heterodyne and time-gated time-delay spectrometry

Two methods for the amplitude and phase calibration of hydrophones based on time-delay spectrometry are presented: a time-gating technique and heterodyne frequency conversion that allows the phase-locked (i.e., coherent) detection of hydrophone signals. Amplitude and phase of the hydrophone sensitivity are obtained in the frequency range 1-20 MHz. In addition, to show the broad-band capabilities of the methods, amplitude values are measured between 4 and 50 MHz.     

Primary calibration of hydrophones with extended frequency range 1 to 70 MHz using optical interferometry

A technique for the primary calibration of hydrophones based on an optical principle is presented. An interferometer determines the displacement of a pellicle mounted on the surface of the tank fluid perpendicular to the sound propagation axis; the known acoustic field is then measured using the hydrophone. The application range extends from 1 to 50 MHz and, with a lesser accuracy, to 70 MHz. The loaded and the open-circuit sensitivity are obtained pointwise, and the frequency and the number of the measurement points can be arbitrarily chosen. As an example, two membrane hydrophones with a spot diameter of 0.5 mm are calibrated, and the comparison with a theoretical model describing the acoustic and electrical properties of the hydrophone shows good agreement.     

Probing acoustic fields of clinically relevant transducers: the effect of hydrophone probes' finite apertures and bandwidths

The influence of finite aperture and frequency response of piezoelectric ultrasonic hydrophone probes on the free-field pulse intensity integral (PII) and mechanical index (MI) was investigated using a comprehensive acoustic wave propagation model. The model developed was capable of predicting the true pressure-time waveforms at virtually any point in the field. The input to the model used pressure amplitude data measured in the immediate vicinity of the acoustic source or transducer considered. The experimental verification of the model was obtained using a commercially available, 8 MHz, dynamically focused linear array and a single element, 5 MHz, focused rectangular source. The verification was performed at low and high excitation levels, corresponding to linear and nonlinear acoustic wave propagation, respectively. The pressure-time waveforms were recorded using piezoelectric polymer hydrophone probes that had different sensitivities, frequency responses, bandwidths, and active element diameters. The nominal diameters of the probes ranged from 50 to 500 microm, and their useable bandwidths varied between 55 and 100 MHz. The PII, used to calculate the thermal index (TI), was found to increase with increasing bandwidth and decreasing effective aperture of the probes. The MI, another safety indicator, also was affected, but to a lesser extent. The corrections predicted using the model were used to reduce discrepancies as large as 30% in the determination of PII. The results of this work indicate that, by accounting for hydrophones' finite aperture and correcting the value of PII, all intensities derived from the PII can be corrected for spatial averaging error. The results also point out that caution should be exercised when comparing acoustic output data. In particular, hydrophone's frequency characteristics of the effective diameter and sensitivity are needed to correctly determine the MI, TI, and the total acoustic output power produced by an imaging transducer.     

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.     

Extending the frequency range of the National Physical Laboratory primary standard laser interferometer for hydrophone calibrations to 80 MHz

A method for the primary calibration of hydrophones in the frequency range up to 60 MHz is described. The current National Physical Laboratory (NPL) primary standard method of calibrating ultrasonic hydrophones from 500 kHz to 20 MHz is based on optical interferometry. The acoustic field produced by a transducer is detected by an acoustically transparent but optically reflecting pellicle. Optical interferometric measurements of pellicle displacement at discrete frequencies in tone-burst fields are converted to acoustic pressure, and the hydrophone for calibration is substituted at the same point, allowing sensitivity in volts per pascal to be obtained directly. For calibrations up to 60 MHz, the interferometer is capable of measuring the displacement of the pellicle as a function of frequency in a harmonically rich nonlinear field up to and including the 12th harmonic of the shocked field generated by a 5 MHz focusing transducer, allowing hydrophones to be calibrated by substitution in the same field. Sources of uncertainty in the new method have been investigated. Best combined random and systematic uncertainties at the 95% confidence level for the new method are 7% at 20 MHz, 11% at 40 MHz, and 16% at 60 MHz.     

Harmonic ultrasound fields through layered liquid media

Harmonic field generation through a layered liquid media is studied experimentally and theoretically. Lateral and axial beam profiles of the fundamental to the 4th harmonic component of the field from a focused, 19-mm diameter transducer were measured using a calibrated hydrophone in a water tank. Measurements were performed before and after the insertion of a cylindrical phantom containing vegetable oil. A frequency domain numerical solution to the "KZK" equation was used to calculate the beam profile, taking into account the acoustic properties of the medium and phantom. Effects of nonlinear propagation, diffraction, attenuation, and reflection are included in the calculation. Agreement within 5% was obtained between measurements and theoretical predictions throughout the mid- and far-field of the transducer for both the uniform path and the layered media. Measurements also were carried out using an unfocused transducer as a receiver. The shape of the axial beam profile using this receiver agreed very well with the theoretical prediction using the "KZK" equation, after accounting for phase variations over the finite-sized detector in the calculated field.     

Interlaboratory comparison of hydrophone calibrations

The results of an interlaboratory comparison of hydrophone calibration techniques in the frequency range 1-10 MHz are reported. Two membrane hydrophones were calculated to six laboratories, and each laboratory determined the end-of-cable loaded sensitivities using their normal calibration methods; these included optical interferometry, planar scanning, reciprocity combined with time-delay spectrometry, and suspended-sphere radiometry. After converting the results to end-of-cable open-circuit sensitivities, in most cases agreement between the various values was within +/-10% at all frequencies.     

Calibration of a focusing transducer and miniature hydrophone as well as acoustic power measurement based on free-field reciprocity in a spherically focused wave field

The free-field transmitting voltage response at the pressure focus of a spherically focusing transducer was defined and calibrated based on the reciprocity theorem of a free-field spherically focused acoustic wave. The acoustic power, the radiation conductance, and the pressure at the pressure focus were derived and measured accordingly from the transmitting current response on the imaginary mirror symmetric spherical surface of the radiating surface. A miniature hydrophone was calibrated by the self-reciprocity of the spherically focusing source. Comparison results show that the measured acoustic power deviation between the reciprocity method and the radiation force balance method are within +/- 5% for two air-backed focusing transducers at 1.53 MHz and 5.27 MHz, respectively, and the maximum deviation of a hydrophone calibration between the new method and the free-field plane wave reciprocity method is within 1.4 dB in the frequency range from 1 MHz to 2 MHz in experiments.     

Frequency response of fiber-optic multilayer hydrophones: experimental investigation and finite element simulation

The frequency response of a fiber-optic hydrophone that uses a dielectric multilayer system as the sensing element for ultrasound detection is investigated. A primary interferometric calibration technique is applied to determine by experiment the frequency-dependent pressure-voltage transfer function up to 45 MHz. The interaction between an incident pressure wave and the fiber end is analyzed by finite element methods. The simulation yields the response of the sensor to a short Gaussian impulse in the time domain from which the transfer function is calculated. The results of the model simulations allowed the transfer function obtained to be interpreted as the result of the superposition of longitudinal, edge diffraction and lateral waves with a resonant vibration mode of the fiber body representing an elastic rod     

Pulse-echo field distribution measurement technique forhigh-frequency ultrasound sources

A simple technique for the determination of the spatial and temporal transmit-receive field distributions of spherically focused high-frequency transducers is described. Instead of a point-like target, tungsten wires (line-like targets) with diameters less than the acoustic wavelength are used as pulse-echo targets. Spatial and temporal field quantities were determined for spherically focused transducers in the frequency range from 3 to 17 MHz, and a comparison with hydrophone measurements showed that both techniques yielded comparable results for the low-frequency transducer. However, for the higher frequency transducers, hydrophone measurements did not yield satisfactory results compared to the wire-target technique due to the hydrophone's aperture size, while the results from the wire-target technique were in general agreement with theory     

Automation and Evaluation of Two Different Techniques to Calibrate Precision Calibrators for Low Frequency Voltage Using Thermal Devices

Low frequency (LF) voltage and current are important parameters in electrical metrology. The standards for LF voltage and current are established by assigning AC–DC transfer difference to thermal devices, i.e. thermal converters or thermal transfer standard along with current shunts. Automated calibration systems have been developed based on Null method and measurement technique developed by Budovsky for calibration of precision calibrator in LF voltage and current against thermal devices. The technique based on the Algorithm developed by Dr. Ilya Budovsky (National Metrology Institute (NMI), Australia) has been compared with the conventional null technique. Indigenously developed software has been used to calibrate the precision calibrator in the entire LF voltage and current range using Holt thermal converters and current shunts. Calibration results at 1 V, 10 V in the frequency range from 10 Hz to 1 MHz as well as calibration results of 1 A in the frequency range from 40 Hz to 10 kHz are presented in this paper. These result shows that the measurement technique developed by Budovsky has reduced the complexity of AC–DC transfer measurements, measurement time and the uncertainty in measurement.     

光纤水听器调制激光光源的一种实现方法

在干涉型光纤水听器的信号检测技术中,采用相位载波零差检测方式,直接调制激光光源,具有明显的优越性。提出了一种实现方法,通过产生高频正弦波,对激光光源(多量子阱分布反馈激光器),进行直接驱动调制。光源调制输出波形精确多样(正弦波、三角波(包括锯齿波)和方波),输出频率大范围(0～20MHz),连续可调。实验表明达到了水听器对光源的要求。此调制技术也可在光纤通信等其它领域得以应用。     

High-frequency membrane hydrophone

A membrane hydrophone with a 37-μm diameter spot poled electrode has been fabricated on a 4-μm-thick film of the piezoelectric copolymer, polyvinylidene fluoride trifluoroethylene (PVDF-TrFE), and initially characterized. The hydrophone has an effective spot size of less than 100 μm, an on-membrane +7-dB gain buffer amplifier, and a -3-dB bandwidth of 150 MHz. The acoustic properties of the hydrophone were investigated with a transducer equivalent circuit model, the electric fringe fields due to poling were characterized with a finite difference electrostatic field model, and the effective spot diameters 2a₃ and 2a₆ were estimated. Measurements on the bandwidth, effective spot size, and sensitivity are presented. This hydrophone appears suitable for the characterization of both the frequency and spatial parameters of high-frequency transducers such as intravascular ultrasound (IVUS) catheter transducers operating in the 10-40 MHz range     

Fiber optic ultrasonic sensor using Raman-Nath light diffraction

A novel fiber optic ultrasonic sensor using the principle of Raman-Nath light diffraction has been developed. The sensor does not perturb the acoustic field and exhibits a wideband frequency response. In addition to the remote sensing of the field, it is suitable for measurements of both continuous and pulsed ultrasonic waves. The experimental results obtained with the sensor were compared to those measured using a calibrated PVDF needle hydrophone, showing excellent agreement. The sensor's frequency response in the range from 3 to 15 MHz, typical of that used in medical ultrasound imaging, was determined using the time delay spectrometry (TDS) technique. It appears that the fiber optic sensor provides a useful alternative to the widely used PVDF ultrasonic probes in specific applications where any perturbation between acoustic field and sensor is undesirable. Also, since the active element diameter of the sensor can be made comparable to the core diameter of an optical fiber, the fiber optic sensor minimizes the spatial averaging effects and offers significant improvement in comparison with the present state-of-the-art hydrophones which have a minimum diameter on the order of 300 μm     

Calibration of hydrophones based on reciprocity and time delay spectrometry

A brief survey is given of the calibration methods for hydrophones in the ultrasonic frequency range. The methods presently used in the Physikalisch-Technische Bundesanstalt (PTB), Braunschweig, West Germany, for hydrophone calibrations in the frequency range from 1 to 15 MHz are the primary subject of concern. These methods are the two-transducer reciprocity method for the calibration at discrete frequencies, the time-delay-spectrometry substitution method for quasifrequency continuous calibrations, and the two-transducer reciprocity method with time-delay spectrometry, also for quasi-frequency continuous calibration. Compared with the calibration at discrete frequencies, the expenditure of time for a calibration is considerably reduced in the case of the last-mentioned method. The influencing parameters which affect the evaluation of the measurement uncertainty are briefly discussed for the calibration methods applied at the PTB.     

矢量水听器低频绝对校准装置研究

提出了矢量水听器低频绝对校准装置。首先给出了矢量水听器绝对校准的原理,通过测量声源辐射面的加速度,利用驻波管中平面驻波声场中的声压、质点振速和质点加速度在垂直方向上的分布规律,得出了校准计算公式;之后对校准装置中平面驻波场在垂直方向上的分布规律进行了测量验证,并对测量用加速度计进行了校准;最后对待校矢量水听器的灵敏度和指向性进行了测试。结果表明:在10~315 Hz频带内,矢量水听器低频绝对校准装置在±1 d B误差允许范围内,可用于矢量水听器的绝对校准。     

Wide-band piezoelectric polymer acoustic sources

The design of a wideband acoustic source made of the piezoelectric polymer polyvinylidine fluoride (PVDF) is described. The source was developed for the characterization and absolute calibration of ultrasonic hydrophone probes. Construction details are described and performance characteristics of the wideband PVDF transmitter, including its transmitting voltage response and directivity patterns, are compared with theoretical predictions in the frequency range up to 40 MHz. The Krimholtz-Leedom-Mattaei (KLM) model was used to examine the influence of the PVDF polymer film thickness, the backing acoustic impedance, the cable length, and the electrical source resistance on overall transmit transfer characteristics. A comparison is made with traditional piezoelectric ceramic acoustic sources, and it is shown that piezopolymer transmitters exhibit some improved properties and are well suited for certain ultrasound dosimetry applications. In particular, the polymer sources have been found useful in measurements based on swept-frequency excitation. Those measurements allow characterization of transmitters and receivers to be performed as a virtually continuous function of frequency.