Hydrophone Calibration Research Articles

Verification of the results of the calibration of needle hydrophones on the measurement standard NDETU AUV-02-2018

The results of the verification of the calibration of needle hydrophones using the national primary measurement standard of the unit of ultrasonic pressure in the aquatic environment (NDETU AUV-02-2018) are presented. The main objective of the study was to confirm the accuracy of the calibration of needle hydrophones No. 2538 and No. 2515 by comparison with the calibration results obtained on the measurement standard at the Physikalisch-Technische Bundesanstalt (PTB), Germany. The paper describes the calibration methods used by laboratories to determine the sensitivity of hydrophones, including the two-transducer reciprocity method and the comparison method. The results of the calibration of needle hydrophones in the frequency range from 1 to 10 MHz obtained by the National Metrology Institute of Germany (PTB) and Ukraine (SE Research Institute “Systema”) were evaluated. Furthermore, the level of uncertainty of the calibration results was compared. The results of the study demonstrated a high degree of consistency in the measurement results. Added to this, the results confirmed the reliability of the results of the calibration of needle hydrophones using the NDETU AUV-02-2018 measurement standard by the calibration method selected and with the measurement equipment used. The conclusions confirmed the effectiveness of using the national measurement standard NDETU AUV-02-2018 and its conformity with international measurement standards, thus increasing the confidence in the results at the international level. Additionally, the study provides the ground for future international comparisons with measurement standards of other countries.

Performance-Enhanced Piezoelectric Micromachined Ultrasonic Transducers by PDMS Acoustic Lens Design.

This paper delves into enhancing the performance of ScAlN-based Piezoelectric Micromachined Ultrasonic Transducers (PMUTs) through the implementation of Polydimethylsiloxane (PDMS) acoustic lenses. The PMUT, encapsulated in PDMS, underwent thorough characterization through the utilization of an industry-standard hydrophone calibration instrument. The experimental results showed that the ScAlN-based PMUT with the PDMS lenses achieved an impressive sensitivity of -160 dB (re: 1 V/μPa), an improvement of more than 8 dB compared to the PMUT with an equivalent PDMS film. There was a noticeable improvement in the -3 dB main lobe width within the frequency response when comparing the PMUT with PDMS encapsulation, both with and without lenses. The successful fabrication of high-performance PDMS lenses proved instrumental in significantly boosting the sensitivity of the PMUT. Comprehensive performance evaluations underscored that the designed PMUT in this investigation surpassed its counterparts reported in the literature and commercially available transducers. This encouraging outcome emphasizes its substantial potential for commercial applications.

Application of 2E-2U method for free-field underwater calibrations of hydrophones and projectors in a reverberant laboratory test tank

In this paper, application of a Two-Equations Two-Unknowns (2E-2U) method is described for calibration of hydrophones and projectors below 1 kHz in a laboratory test tank. At low frequencies, amplitude and phase measurements for the calibration of the hydrophones and projectors in the test tank are difficult to perform since echo-free time of the laboratory test tank is not large enough due to transducer initial transients and tank wall boundary reflections. To overcome these difficulties, the 2E-2U method is applied to received (windowed) signals obtained during calibration measurements. Thus, calibration measurements become possible at a frequency down to 250 Hz. These measurements in the test tank are performed for a hydrophone and a developed flextensional projector. First, the received sensitivities for the hydrophone are calculated and validated by comparisons with pressure calibration in a closed chamber. Good agreements are obtained between the two measurement platforms, with a maximum difference of 0.5 dB and uncertainty of 1.3 dB. Then, the transmitting voltage response (TVR) of the flextensional projector is calculated and compared with the calibration data obtained from the method defined in the relevant standards. Good agreements are obtained between the two TVR data with a maximum difference of 1.1 dB and uncertainty of 1.7 dB.

Key comparison CCAUV.W-K2: features of hydrophone calibration

Key comparison CCAUV.W-K2: features of hydrophone calibration

A calculable pistonphone for the absolute calibration of hydrophones in the frequency range from 0.5 Hz to 250 Hz

The demand for traceable hydrophone calibrations at low frequencies in support of ocean monitoring applications requires primary standard methods that are able to realise the acoustic pascal. In this paper, a new method for primary calibration of hydrophones is described based on the use of a calculable pistonphone to cover frequencies from 0.5 Hz to 250 Hz. The design consists of a pre-stressed piezoelectric stack driving a piston to create a varying pressure in an air-filled enclosed cavity, the displacement (and so the volume velocity) of the piston being measured by a laser interferometer. The dimensions of the front cavity were designed to allow the calibration of reference hydrophones, but it may also be used to calibrate microphones. Examples of calibration results for several sensors are presented alongside an uncertainty budget for hydrophone calibration with expanded uncertainties ranging from 0.45 dB at 0.5 Hz to 0.30 dB at 20 Hz, and to 0.35 at 250 Hz (expressed for a coverage factor of k = 2). The metrological performance is demonstrated by comparisons with results for other calibration methods and an independent implementation of primary calibration methods at other institutes.

Key comparison CCAUV.W-K2: features of hydrophone calibration

The results of key comparison are the most convincing and objective indicator of confirmation of the correctness and accuracy of the newly developed measurement method. The results of comparison and measurement methods applied at the second key comparison of national standards of the unit of sound pressure in water CCAUV.W-K2, organized by the Consultative Committee for Acoustics, Ultrasound and Vibration of the International Committee for Weights and Measures, are described. A feature of the comparison is that the calibration range has been expanded into the low frequency region by two octaves compared to the first key comparison CCAUV.W-K1. The results of the CCAUV.W-K2 comparison were considered successful. Important scientific results of participation in the comparison were confirmation of: the correctness of the hydrophone free-field calibration methods developed at VNIIFTRI (at ultra-low frequencies for a sound-reflecting water tank and in the noise reverberant sound field of the water tank), the equivalence of the calibration results when radiating signals of various types: chirp, noise and ton-burst. In the process of performing control calibrations, the pilot laboratory discovered violations of the stability of the reference hydrophone of comparison. When comparing the results obtained by participants, there was an increase in the discrepancy between the results in the frequency range 60–100 kHz, which is considered the least problematic.

An overview of the calibration process for a digital hydrophone

Digital hydrophones are instruments for measuring underwater sound that incorporate the entire data chain into a single unit. Calibration of analog and digital hydrophones is largely similar but differs in measurement signal type. Digital hydrophone calibration is not currently captured in the standards outlining hydrophone calibration (such as IEC 60565), so they currently cannot be calibrated to a recognized standard. The testing setup and calibration methods are the same as for an analog hydrophone but without needing external digitization or analog signal conditioning. Potential issues regarding digital hydrophone calibration: careful calculation of signal amplitudes is required at upper frequency band; the lack of analog output requires different units of measure or information on counts/volt conversion. The first point here is also a current issue for analog hydrophones where the digitization and the processing of data during the calibration are not prescribed. Ideally, both of these issues would be addressed in future standards updates, which would improve the operation of both analog and digital hydrophones. This paper will expand on the issues highlighted here involving digital hydrophone calibration and offer recommendations for best practice. It will also provide an outline of the current calibration procedure as used at Ocean Sonics.

STUDY OF THE EFFECTIVE RADII OF ULTRASONIC TRANSDUCERS

The study on the effective radii of ultrasonic transducers for hydrophone calibration at the National State Primary Standard of the Unit of Ultrasonic Pressure in Water (NDETU AUV-02-2018) is presented and the appropriate method is described. The impact of the effective radius on measurement distance and diffraction loss coefficients is evaluated. The uncertainty calculation of the effective radius measurement of ultrasonic transducers is provided, and its influence on the accuracy of hydrophone calibration is assessed. Considerable attention is given to estimating the measurement distance between the hydrophone and the ultrasonic transducer, which significantly affects the accuracy of hydrophone calibration.

Multivariate control charts for calibration of hydrophones using the Mahalanobis statistic

Statistical analysis of data to ensure the validity of calibrations can be challenged when the measurement model describes a process and result that spans a high-dimensional parameter space. Calibration of hydrophones presents one such case whereby the sensitivity to acoustic pressure is calculated using measured values for several different quantities and the result is needed at many excitation frequencies distributed over a large bandwidth. We addressed this challenge by developing a multivariate statistical analysis procedure that supports efficient and effective process control for hydrophone calibrations. We first characterize the in-control condition by the mean and covariance of measurements expressed in a p-dimensional parameter space using a curated training data set. We then test subsequent measurements against a control limit using Hotelling’s T 2-test to compare the current measurement process to the training data. We report a case study for secondary calibration of hydrophones spanning a frequency range of 1 kHz–200 kHz where we detected an emergent defect in the laboratory’s calibrated reference hydrophone by inspection of data presented in multivariate control charts. Process control data collected during calibration services exceeded the control limits in intermediate and high-frequency bands by factors of 5.1 and 11.2, respectively. The process remained in control for the low-frequency band. These observations were consistent with misalignment of the reference hydrophone’s sensitive element, which we confirmed upon disassembly, inspection and repair of the instrument.

Ultrasonic needle hydrophone calibration in air by a parabolic off-axis mirror focused beam using three-transducer reciprocity

An acoustic field distribution investigation in air requires a small receiving sensor. Needle hydrophones seem to be an attractive solution, and it has previously been demonstrated that needle hydrophones designed for use in water can be used in air. The metrology problem is that an absolute sensitivity calibration is needed, because needle hydrophones are not characterized in air, especially for frequencies below 1 MHz, which is of interest for air-coupled ultrasound. Conventional, three-transducer/microphone reciprocity calibration requires measurements to be done in the far field. However, when transducer diameter is large and the frequency is high, the required measurement distance becomes very large: 3 m for a 20 mm source, transmitting at 1 MHz. Large propagation distance leads to high attenuation and nonlinear effects in air propagation, and distortion and losses accumulate. Small needle hydrophones have low sensitivity, so that high excitation amplitudes would be required, which can lead to transducer heating and increase nonlinearity effects. A derivative of the three-transducer reciprocity calibration method is proposed, where a large aperture transducer is focused onto a hydrophone, using hybrid of plane wave and spherical wave reciprocity. Use of a focused source minimizes the frequency-dependent diffraction effects, and the spherical wave approximation is valid at the focal distance, and low level excitation signals can be used. Focusing is accomplished using a parabolic off-axis mirror. Calibration is in transmission, which reduces the complexity of the electrical measurements. The corresponding equations have been derived for this setup. Calibration of the transducer and needle hydrophone absolute sensitivity is obtained.

Hydrophone calibration between 1 kHz and 10 kHz using elastic waveguides

Modern techniques for hydrophone calibration from IEC 60565:2020 typically require sensors to be placed in a free field or within a hydrostatically varying chamber. At mid-frequencies (defined as 1 kHz–10 kHz), the wavelength is too long for free-field conditions in tanks available to most manufacturers and academics. A novel technique to calibrate hydrophones is investigated to address the measurement gap between very-low-frequency (0.1 Hz–1 kHz) and high-frequency (10 kHz–200 kHz) techniques. The measurement environment consists of a 12-meter length of copper tubing that is coiled into a 1-meter diameter helix. Propagation in the elastic waveguide decreases the speed of sound within the apparatus and the wavelength relative to its free-space equivalent. This provides a longer reverb-free time within which to make the calibration measurements using a small (<1 m3) volume. To increase the gain of the system, the recorded files are matched filtered against normalized transmission replicas to determine the signal energy at the receiver. The propagation within the waveguide is studied, including investigating the modal dispersion and channel gain. Calibrations are performed on multiple Ocean Sonics icListen hydrophones, including independently calibrated reference units, using an uncalibrated source to determine the accuracy and precision of the system.

On the Importance of Consistent Insonation Conditions During Hydrophone Calibration and Use.

Hydrophones are generally calibrated in acoustic fields with temporally localized (short pulse) or long duration (tone burst) signals. Free-field conditions are achieved by time gating any reflections from the hydrophone body, mounting structures, and surrounding water tank boundaries arriving at the active sensing element. Consequently, the sensitivity response of the hydrophone is a result of direct waves incident on its active element, free from any contaminating effects of reflections. However, when using tone bursts below 400 kHz to calibrate hydrophones, it may not be possible to isolate the direct wave from reflection artifacts. This means that the sensitivity responses derived at these frequencies using short pulse and tone burst signals might not be comparable as they can be characteristic of the acoustic field interaction with either/both the hydrophone active element alone or the hydrophone active element and body. Therefore, there is a need to consider an appropriate calibration method for a given hydrophone type, depending on whether the eventual application employs short pulse or tone burst acoustic fields. This article presents the findings from a short study comprising four needle-type hydrophones of active element diameters in the range of 1-4 mm. These hydrophones were calibrated from 30 kHz to 1.6 MHz using established calibration methodologies within the underwater acoustics (UWA) and ultrasound (US) areas employed at the National Physical Laboratory (NPL), Teddington, U.K. In UWA tone, burst acoustic fields are used, while in US, it is short pulses. The 2- and 4-mm-diameter needle hydrophones showed the largest variation at the overlapping frequencies, in which the maximum disagreement of UWA calibration was 30% relative to US calibration. For the 4-mm hydrophone, UWA calibration exhibited resonant sensitivity structure between 100 and 450 kHz, but which was absent in US calibration. This observed behavior was further investigated theoretically by using a validated acoustic wave solver to confirm the resonant sensitivity structure seen in the case of UWA calibration. The work contained within illustrates the need to ensure that the method of calibration is carefully considered in the context of the duration of the acoustic signals for which the hydrophone is intended.

Dissemination of the Acoustic Pascal: The Role and Experiences of a National Metrology Institute.

Hydrophones are pivotal measurement devices ensuring medical ultrasound acoustic exposures comply with the relevant national and international safety criteria. These devices have enabled the spatial and temporal distribution of key safety parameters to be determined in an objective and standardized way. Generally based on piezoelectric principles of operation, to convert generated voltage waveforms to acoustic pressure, they require calibration in terms of receive sensitivity, expressed in units of [Formula: see text]Pa-1. Reliable hydrophone calibration with associated uncertainties plays a key role in underpinning a measurement framework that ensures exposure measurements are comparable and traceable to internationally agreed units, irrespective of where they are carried out globally. For well over three decades, the U.K. National Physical Laboratory (NPL) has provided calibrations to the user community covering the frequency range 0.1-60 MHz, traceable to a primary realization of the acoustic pascal through optical interferometry. Typical uncertainties for sensitivity are 6%-22% (for a coverage factor k = 2), degrading with frequency. The article specifically focuses on the dissemination of the acoustic pascal through NPL's calibration services that are based on a comparison with secondary standard hydrophones previously calibrated using the NPL primary standard. The work demonstrates the stability of the employed dissemination protocols by presenting representative calibration histories on a selection of commercially available hydrophones. Results reaffirm the guidance provided within international standards for regular calibration of a hydrophone in order to underpin measurement confidence. The process by which internationally agreed realizations of the acoustic pascal are compared and validated through key comparisons (KCs) is also described.

CCAUV.W-K2 final report - key comparison CCAUV.W-K2: calibration of hydrophones in the frequency range from 250 Hz to 500 kHz

Main textThis report is the final report for key comparison CCAUV.W-K2. This key comparison covers primary free-field standards for sound in water at frequencies between 250 Hz and 500 kHz. This project is one of the key comparisons organised under the auspices of the Consultative Committee on Acoustics, Ultrasound and Vibration (CCAUV) of the CIPM.This report has the status of a Final Report and has been submitted to the Key Comparison Database (KCDB). In the report, the results of participants are presented with the Key Comparison Reference Values and Degrees of Equivalence. The results are calculated according to the procedures agreed after consideration of the Draft A1 and A2 reports, and the Draft B report has been approved by the CCAUV. All participants have had the opportunity to give final agreement on the contents and amendments have been made to account for their comments.In many respects, the comparison has been a success with good agreement achieved over an extended lower frequency range compared to the previous CCAUV.W-K1 comparison, the lower frequency limit for CCAUV.W-K2 being extended down by two octaves to 250 Hz. The generally more difficult frequency range from 100 kHz to 500 kHz has also shown very good agreement between the participants. However, in the range 60 kHz to 100 kHz the agreement was not as good, with three participants exhibiting some discrepant results.To reach the main text of this paper, click on Final Report. Note that this text is that which appears in Appendix B of the BIPM key comparison database https://www.bipm.org/kcdb/.The final report has been peer-reviewed and approved for publication by the CCAUV, according to the provisions of the CIPM Mutual Recognition Arrangement (CIPM MRA).

Design and Characterization of an Aluminum Nitride-Based MEMS Hydrophone With Biologically Honeycomb Architecture

In this article, we present a high-performance aluminum nitride (AlN)-based microelectromechanical system (MEMS) hydrophone sensor, in which the sensing cells are designed and arranged as an innovative honeycomb architecture for achieving large acoustic pressure sensitivity and high fill-factor. An 8×9 array of 360 μm in characteristic size, 0.92-MHz MEMS hydrophone sensor is developed based on an AlN-on-cavity silicon-on-insulator (CSOI) platform. The size of the MEMS hydrophone sensor is 3.2 mm ×3.2 mm. The MEMS hydrophone sensor and its preamplification circuit are integrated on a printed circuit board and packaged together with an acoustically transparent material, in order to meet the stringent requirements of underwater applications. The packaged MEMS hydrophone is fully characterized through using an industry-standard hydrophone calibration instrument. The MEMS hydrophone achieves an acoustic pressure sensitivity of -178 dB (re: 1 V/ μ Pa), with a maximum nonlinearity of 0.1%, and a noise resolution of 58.7 dB (re: 1 μPa/Hz). The measurement results show that the reported MEMS hydrophone sensor with bioinspired honeycomb structure is compared favorable with advanced commercially available bulky hydrophones.

Application of Sound-Conducting Polymeric Pellicle for Hydrophone Calibration by Means of Optical Interferometry

Application of Sound-Conducting Polymeric Pellicle for Hydrophone Calibration by Means of Optical Interferometry

Design and implementation of beaded cilia MEMS vector hydrophone

In view of the problem that the increased difficulty of underwater moving targets detection caused by the development of acoustic stealth technology, a beaded cilia MEMS vector hydrophone (BCVH) is proposed in this paper. Compared with previous cilia structures, it can enlarge the receiving area of acoustic wave to improve sensitivity without the introduction of additional structures. Meanwhile, the introduction of additional mass and the loss of working frequency band are reduced, and the structure is manufactured at one time. In this paper, the design scheme and optimal size of BCVH are determined via theoretical analysis and simulation, then vector hydrophone calibration system is used to verify it. The experimental results show that the sensitivity of BCVH reaches −183.3 dB (@1kHz, 0dB=1V/μPa), 13.7 dB higher than bionic cilia MEMS vector hydrophone (CVH). Moreover, the depth of concave point of BCVH is more than 30 dB, which has a good directivity.

Ensuring the validity of results of hydrophone calibration by a mutual comparison calibration method

Hydrophone calibration is one of the most important basic metrology activities in ultrasound. Hydrophones are used in many different calibrations and testing methods in industry, for instance, the evaluation of medical electrical equipment basic safety and essential performance. Ensuring the validity of results of metrological services is a major concern for any laboratory. Primary calibration of measuring instrument or standard is a widely understood way to assure good results, despite it is not enough, it is not the unique way, neither it is easy to get in some circumstances. For instance, hydrophone calibration is a service restricted to a few laboratories worldwide, which demands a logistic effort and may be quite expensive. An alternative method to guarantee the traceability by in-house check is of interest. This paper presents a method to ensure the traceability of hydrophones after mutual calibrations. Eight previously calibrated hydrophones from different manufactures and characteristics (membrane and needle) were mutually calibrated, by comparison, all of them acting as standards and calibration items. Calibrations were performed from 1 MHz to 10 MHz and the results were evaluated by pair-difference chi-squared statistics. Uncertainties typically laid between 6% to 13% and sensitivities were between 33 to 318 nV Pa−1. After statistics analyses, 4 hydrophones were considered reliable to be used as standards, as their sensitivity keep on stable even after several years since the last calibration. The method can be used by the industry, accredited laboratories, or national metrology institutes to perform functional checks for virtually any measuring instrument or standard, being the limit the number of available devices to properly apply the statistical analysis.

Application of a sound-conducting polymer foil for calibration of hydrophones by optical interferometry

Modern calibration of hydrophones at megahertz frequencies is based on the method of optical interferometry, in which the vibrational velocity of an acoustic wave is measured using a thin sound-transparent polymer foil (membrane), metallized on one side to improve light reflection and installed in an ultrasonic field, followed by its replacement with a hydrophone to be calibrated. The main problem of implementing this calibration method is to assess the adequacy of tracking vibrations of the metallized side of the foil to vibrational displacements of water particles under the action of a sound wave incident on the opposite side of the foil. On the basis of the simplified theory of acoustic plane waves passing through layers of dissimilar materials, methods for measuring the speed of sound in the applied foil and an algorithm for calculating the frequency dependence of the sound wave transmission coefficient (in terms of vibrational velocity) from water through the foil to water or air, introduced as a correction to the results of hydrophone calibration, were developed. The uncertainty of the introduction of this correction is estimated.

A Comparison of Different Calibration Techniques for Hydrophones Used in Medical Ultrasonic Field Measurement.

The acoustic output characterization of medical ultrasonic equipment requires regular calibration of the hydrophones used to ensure the reliability of measurements. Such hydrophone calibration is offered as a service by several institutions. Various calibration techniques using a variety of ultrasonic excitation pressure waveforms comprising different pressure amplitude ranges and frequency compositions as well as different reference measurement systems have been proposed and applied over the past decades. Currently, four different setups for hydrophone calibration are available at the Physikalisch-Technische Bundesanstalt (PTB). This internal comparison study addresses the consistency of all four methods, including direct primary calibration and substitution calibration using reference hydrophones. The methods apply single-frequency tonebursts and swept tonebursts in the kPa amplitude range of quasi-linear acoustics as well as impulse excitation including nonlinear propagation. In recent years, a new primary calibration setup using a high-frequency vibrometer has been implemented at PTB, enabling the characterization of hydrophone frequency responses in modulus and phase and extending the upper frequency limit to up to 100 MHz. For the comparison in the frequency range from 0.5 MHz to 60 MHz, two passive membrane hydrophones with well-known characteristics gained from many years of measurements were used. Another membrane hydrophone with a nominal diameter of 0.2 mm and an integrated preamplifier was applied to address the frequency range up to 100 MHz. The results obtained with the different setups showed good agreement with average root-mean-square (rms) deviations of 3% (primary calibrations, 1-60 MHz) and 4% (1-100 MHz). The consistency of the implementations was thus verified in this comparison.