Hydrophone Calibration Research Articles

R/V Marcus G. Langseth seismic source: Modeling and calibration

This paper presents analyses of calibration data of the R/V Marcus G. Langseth seismic sources that were collected in 2007–2008. The analysis includes measurements of the levels and azimuthal directivity of the Langseth two‐string source, which is used for 3‐D seismic surveys and compares these data with multichannel seismic hydrophone array data in shallow water. The analysis also shows the important contribution of seafloor and subseafloor reflected and refracted arrivals and the associated effects of water depth and seafloor slope. As expected and predicted by modeling, azimuthal directivity depends greatly on the “footprint” dimensions of the seismic array. In shallow water, where the acoustic field is dominated by near‐vertically traveling reflected and refracted waves, data recorded using a towed MCS array are a useful proxy for single hydrophone calibration data. In deeper water, the easily modeled direct arrivals are exceeded in amplitude by seafloor reflected and subseafloor refracted energy at offsets which depend upon the water depth, limiting the applicability of a priori modeling that does not include these interactions. In addition, in a seafloor sloping environment, amplitudes depend greatly on whether the receivers are upslope or downslope from the source.

Use of a complex moving weighted averaging method for receiver nonuniform frequency response restoration

One of the limitations of the complex moving weighted averaging method is the requirement of a small nonuniformity of the desired frequency response in a weighted averaging frequency interval. A multistep procedure in which an averaging result is used for initial frequency dependence editing is proposed. An example of use of the proposed procedure during continuous radiation in a nonanechoic water tank is given for free-field calibration of hydrophone with a sensitivity frequency response that is nonuniform at low frequencies.

Reducing the influence of a transition process in field calibration of hydrophones at low frequencies with the use of quadrature-added harmonic signals

A method of reducing the signal settling time in a transmitter-receiver system in field calibration of a hydrophone at low frequencies in a non-anechoic water tank based on correction of the radio frequency response is considered. The advantage gained with the use of the radiation of quadrature-added harmonic signals in precise low-frequency calibration of hydrophones by comparison with pulse-tone radiation is demonstrated.

Lower frequency of free-field calibration of hydrophones in emission of tonal signals in a non-anechoic water tank

An approach to estimation of the lower free-field calibration frequency as well as the required magnitude of the water tank for calibration of hydrophones in a specified frequency range from the cepstrum of a segment of the amplitude response of the electroacoustic system, projector - hydrophone - non-anechoic water tank - measurement path of standard facility, is proposed. Estimates of the lower calibration frequency of hydrophones in a water tank with minimal dimension of 6 m in the case of radiation of quadrature-added harmonic signals are presented.

Reciprocity Measurements in Acoustical and Mechano-Acoustical Systems. Review of Theory and Applications

In 1873 Lord Rayleigh defined the General Reciprocity Theorem for stable, finite, lumped, passive, linear dynamical systems which only contain reversible (bilateral) elements. In spite of Rayleigh's authority, there have been a number of papers expressing doubts about the validity of the theorem for certain acoustical and mechano-acoustical systems. It can be shown however, that these papers are either based on too primitive experiments or on too simple theoretical models. On the other hand there are many publications showing the validity of the theorem. It can be concluded that Rayleigh's theorem is correct. Flow disturbs reciprocity because it introduces non-reversible elements. Nevertheless, in many systems with flow, the disturbing effect is limited and reciprocity can still be applied. The most straightforward application of reciprocity is the measurement of transfer functions. Second in order, because it requires two measurements instead of one, is the measurement of acoustical or mechanical source strength by reciprocal substitution. Third in order, because it needs three measurements, is the reciprocity calibration of microphones and hydrophones. Until 1970, only the latter application was known. In the period between 1970 and 1988 the author of this paper and his colleagues developed a number of methods for the reciprocal measurement of transfer functions and source strength in mechano-acoustical and acoustical systems. It was shown that the reciprocal measurement of transfer functions and of source strength has often great advantages over the direct alternatives. Since 1985, others joined the efforts. Particularly J. W. Verheij, F. J. Fahy and I. L. Ver have done very much for the further development and dissemination of the methods. Nowadays they are widely applied in the automotive industry and in the shipbuilding industry, where they are used for transfer path analysis, data gathering for prediction methods, source identification, source characterisation and for radiation measurements. The methods are also applied to aircraft, trains and buildings, but the dissemination of the methods for those systems is less than for cars, trucks and ships. Further applications concern the development of quiet gearboxes, the development of quiet fans, the prediction of the effects of sonic booms and the design of musical instruments.

1Pb-16 Development of a hydrophone calibration system based on the reflection method using optical interferometry(Poster Session)

1Pb-16 Development of a hydrophone calibration system based on the reflection method using optical interferometry(Poster Session)

Calibration of hydrophones in a field with continuous radiation in a reverberating pool

We examine a method for estimating the transient impedance of an emitter and a receiver in a free field based on sliding complex averaging of the frequency dependence of the transient impedance of the emitter and the receiver obtained under continuous emission in an undamped pool. The method makes it possible to weaken the influence of reflected signals and to obtain frequency characteristics almost coinciding with measured ones in free-field conditions. We present results from experimental studies of the method and its application for calibration of hydrophones by field with the reciprocity method in a reverberating field.

Absolute calibration of hydrophones immersed in sandy sediment

An absolute calibration method has been developed based on the method of three-transducer spherical-wave reciprocity for the calibration of hydrophones when immersed in sandy sediment. The method enables the determination of the magnitude of the free-field voltage receive sensitivity of the hydrophone. Adoption of a co-linear configuration allows the acoustic attenuation within the sediment to be eliminated from the sensitivity calculation. Example calibrations have been performed on two hydrophones inserted into sandy sediment over the frequency range from 10 to 200 kHz. In general, a reduction in sensitivity was observed, with average reductions over the frequency range tested of 3.2 and 3.6 dB with respect to the equivalent water-based calibrations for the two hydrophones tested. Repeated measurements were undertaken to assess the robustness of the method to both the influence of the sediment disturbance associated with the hydrophone insertion and the presence of the central hydrophone. A simple finite element model, developed for one of the hydrophone designs, shows good qualitative agreement with the observed differences from water-based calibrations. The method described in this paper will be of interest to all those undertaking acoustic measurements with hydrophones immersed in sediment where the absolute sensitivity is important.

Two approaches to the continuous-radiation field calibration of a hydrophone in a nonanechoic water tank

A comparative analysis is carried out of a time-delay spectrometry method and a proposed method for the sliding complex weighted averaging of the frequency dependence of the transfer impedance of a radiator and receiver, which provide different approaches to the free-field continuous-radiation calibration of hydrophones in a nonanechoic water tank. The advantages of the second method are demonstrated.

A hydrophone calibration centre for the Mediterranean area

The Underwater Acoustics Laboratory of the Institute of Acoustics O. M. Corbino in Rome, Italy, has recently achieved accreditation for hydrophone calibration. It can offer hydrophone calibration services operating under a management system compliant with UNI EN ISO/IEC standard and traceability of measurements to NPL primary standards. Precise, traceable measurements are needed today in all applications of underwater acoustics, where a faithful response of measuring devices is essential for a correct evaluation of the system performance and for a rigorous assessment of the environmental impact of man‐made activities at sea. To attain these requirements the Laboratory has a water tank 6 m long, 4 m wide and 5,5 m deep, equipped with a motorized two‐trolley positioning system capable of handling loads of up to 100 kg. A variety of test and measurement equipment is available, controlled by a computer acoustic calibration system, and standalone instruments for signal generation, acquisition and analysis. B...

Validation of primary hydrophone calibrations by inter‐laboratory comparisons and by independent calibration methods

A description is presented of two approaches which may be used to validate primary calibration methods for hydrophones and transducers. Firstly, a comparison may be made with another independent absolute calibration method, preferably one based on a different physical principle (and therefore with few common sources of uncertainty). Secondly, an inter‐laboratory comparison of calibrations may be undertaken between different institutes operating at a similar level. This paper describes the results of such exercises for free‐field calibration of hydrophones in the range from 1 kHz to 500 kHz. Firstly, two independent calibration methods are compared: the three‐transducer reciprocity method and a method based on optical interferometry. The differences observed in the results are typically less than 0.5 dB, which is of the same order as the overall uncertainties of each of the methods. Secondly, the results are shown of a recent international comparison of hydrophone calibrations involving institutes from Canada, China, Germany, Russia, South Africa, UK, and USA. Here, the agreement was generally within quoted uncertainties, the results generally lying within a ±0.5 dB band for frequencies up to 300 kHz. A discussion is given of the general sources of uncertainties in the calibrations.

Hydrophone calibration in the presence of reflecting elements with the use of matched spatial filtering

A method of determining the parameters of sources of reflected waves is considered. The method is used for precise free-field hydrophone calibration. It is based on expanding the dependence of the transfer impedance of the projector and the hydrophone on the distance between them in the spatial functions of sources of spherical waves. The expansion of the dependence of the transfer impedance is performed with the use of the matched filtering. It is shown that the method makes it possible to determine not only the parameters of a remote source, which is related to the underwater structure of the standard, but also the parameters of a closely located source formed by the hydrophone itself. The location of the source formed by the hydrophone determines the possibility of a precise calibration of the hydrophone in a standard calibration tank with specified dimensions of the working zone.

Phase calibration of hydrophone and its application in the reconstruction of underwater acoustic waveforms

Abstract: In this paper, both the reciprocal method and the optical method for the calibration of plural sensitivities of hydrophones are reviewed firstly. Then an experimental system is introduced in which with a 1.5MHz focusing transducer acting as a position indicator, phase responses of hydrophones can be calibrated easily and accurately. Measurements are carried out for different types of hydrophones and results are given in the frequency range 20kHz to 400kHz. Finally, with the plural responses of hydrophones included in the construction for the transfer function of the receiving system, acoustic waveforms produced by a projector are reconstructed in wide band from received signals, and a few conclusions are summarized from the experiment.

Low frequency hydrophone calibration with using tensometric pressure sensor

Usually when calibrating a hydrophone in an acoustic coupler the piezoelectric hydrophones use an auxiliary transducer. Widely applicable reciprocity calibration is high‐ly accurate, but difficult in realization and laborious. The method of comparison with a reference transducer is more simple and quick, but less accurate. In the paper the method of low frequency hydrophone calibration using the auxiliary transducer with a flat frequency response up to 3 kHz is considered. The auxiliary transducer is calibrated accurately by changing the water column in the acoustic coupler. It finally allows us to calibrate measuring hydrophones more accurately too. The measuring equipment on the basis of tensometric pressure sensor type MPX 5010 is considered.

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.

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.

An ultrasonic time-delay spectrometry system employing digital processing

Time-delay spectrometry (TDS) is a swept-frequency technique that has proven useful in several ultrasonic applications. Commercial TDS systems are available, but only in the audio frequency range. Several ultrasonic research TDS systems have been constructed, and they have been used effectively for substitution calibration of hydrophones and for measurement of attenuation and sound velocity in materials. Unfortunately these systems depend on features of commercial equipment no longer manufactured, so a new system has been designed using modern equipment and straightforward signal processing. This system requires a frequency source with a reasonably linear sweep of frequency versus time, audio frequency filters, a standard double-balanced mixer, a power splitter, a waveform digitizer capable of handling audio frequency signals, and a personal computer. An optional implementation that shifts the signal to a lower frequency for more convenient digitization and easier velocity measurements additionally requires an audio frequency oscillator and an audio-range analog multiplier. The processing steps are performed with standard signal processing software. To demonstrate the operation of the system, substitution calibration measurements of hydrophones as well as attenuation measurements on a tissue mimicking material were obtained and compared to a custom TDS system previously described by the authors. The data from these two TDS systems agree to within +/- 0.5 dB in the 1-10 MHz frequency range used. Higher frequency source transducers could be used to extend this range.

Influence of nonlinear propagation on calibration of hydrophone sensitivity using a two‐transducer reciprocity method

This paper describes the phenomenon by which the measured value depends on the ultrasound pressure in the measurement field when the hydrophone sensitivity has been calibrated using the two‐transducer reciprocity method. This dependence is attributable to nonlinear ultrasonic propagation. However, the international standard, IEC 60866:1987, ‘‘Characteristics and calibration of hydrophones for operation in the frequency range 0.5 MHz to 15 MHz,’’ provides a correction factor for diffraction and attenuation derived assuming linear propagation of sound. We attempted to derive the correction factor from the ultrasound pressure distribution calculated using the Khokhlov‐Zabolotskaya‐Kuznetsov (KZK) equation, which is well known as a nonlinear ultrasonic model. Results demonstrated that the dependence of the calibrated sensitivity value on measured sound pressure is quantitatively explainable. The hydrophone sensitivity can be calibrated properly by compensating for the influence of ultrasonic nonlinear propagation using the KZK equation.

An international key comparison of free-field hydrophone calibrations in the frequency range 1 to 500kHz

A description is given of the results of a Key Comparison of primary free-field standards for underwater acoustics at frequencies from 1 to 500kHz. This is the first such Key Comparison exercise in the field of underwater acoustic calibration and measurement. Laboratories from UK, Germany, USA, Russia, China, Canada, and South Africa participated by calibrating three reference hydrophones, with project coordination provided by the National Physical Laboratory, UK. The agreement between the results obtained from the comparison was generally encouraging, with the calibration values reported by the laboratories agreeing within quoted uncertainties over the majority of the frequency range, and the results generally lying within a ±0.5-dB band for frequencies up to 300kHz. A discussion is given of the general sources of uncertainties in the calibrations, in particular those which are thought to have contributed to the differences in the results between laboratories. The results of the participants have been used to estimate the equivalence of national measurement standards within this field.

FIRST ACTIVITIES IN ACOUSTIC DETECTION OF PARTICLES IN UPV

The first activities related to acoustic detection of particles by DISAO research group in the Univesitat Politècnica de València are described. We are applying some techniques from physic, engineering and oceanographic acoustics to face the high energy neutrino underwater acoustic detection challenge. The work is focused mainly in two topics: design, characterization and calibration of hydrophones, and simulation of the propagation of the signal in the sea. We present also some examples for these two topics: piezoelectric modelling and transducer simulation, calibration of hydrophones using MLS signals, and evaluation of the contribution of the sea surface noise to the deep water noise in the Mediterranean Sea by means of simulations of propagation of sound.