Gary Hayman

Acoustic characterization of panel materials under simulated ocean conditions using a parametric array source

A technique for evaluating the underwater acoustic performance of panels under simulated ocean conditions in a laboratory test facility is described. The method uses a parametric array as a source of sound within a test vessel capable of simulating ocean depths down to 700 m and water temperatures from 2 to 35 degrees C. The reflection loss and transmission loss of the test panel may be determined at frequencies from a few kilohertz to 50 kHz. The use of the parametric array enables wideband measurements to be undertaken with short-duration pulses and reduces the effects of diffraction from the panel edges. An acoustic filter is used to truncate the array in order to provide a source-free measurement region and to simplify the measurement process. The difficulties of establishing a parametric array in the confined space of the vessel are outlined, and the experimental procedures adopted are described. The techniques were validated by undertaking measurements on two test objects that have predictable behavior. The potential of the technique is also illustrated with experimental results for test panels for hydrostatic pressures up to 2.8 MPa. An extensive discussion of the measurement limitations is included.

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 us...

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.

Particle velocity measurements using heterodyne interferometry and Doppler shift demodulation for absolute calibration of hydrophones

Underwater hydrophones are calibrated using the three-transducer reciprocity method as primary standard, where their sensitivities are obtained with traceability to electrical standards. However, there are some disadvantages associated with this technique, one of which being the lack of direct traceability to the unit of sound pressure. In the current technique, one of the three transducers needs to be reciprocal to be used as transmitter/receiver, whereas a transmitter (sound source) and receiver (hydrophone under calibration) configuration with no need for reciprocal response would be more straightforward. Optical interferometry provides an alternative method, potentially overcoming these limitations. In this case, a transducer provides the sound excitation and a pellicle strip is placed in the far field. The interferometer uses a frequency-shifted reference beam and also provides a measurement beam probing a fixed point on the pellicle. By analysing the reflected signal mixed with the reference, the Doppler shift is calculated and the acoustic velocity field is measured in a direct and absolute way. This paper presents the results of a hydrophone calibration comparison between reciprocity and interferometry with reasonable agreement between the two methods.

A comparison of hydrophone near-field scans and optical techniques for characterising high frequency sonar transducers

Two potential methods of fully characterising the response of high frequency sonar transducers and arrays operating in the frequency range 100 kHz to 500 kHz are compared. In the first approach two-dimensional planar scans, with a spatial resolution of better than half a wavelength, are performed in the acoustic near-field using a small probe hydrophone. The measured two-dimensional data are propagated numerically using a Fourier Transform method to predict the far-field response. Alternatively the data can be back-propagated to re-construct the pressure distribution at the source, a powerful diagnostic technique which can identify defects in transducers and array elements. The second approach uses a scanning laser vibrometer to measure the velocity of the transducer surface; with the resulting velocity data also being used to predict the far-field response by numerical propagation. The two approaches are compared for a number of devices. Comparison of the propagated hydrophone near-field scan data with direct measurements at these ranges shows very good agreement, indicating the usefulness of the method for deriving far-field transducer responses from near-field measurements in laboratory tanks. The potential limitations introduced to the optical approach by the acousto-optic effect are discussed.

A model for characterizing the frequency-dependent variation in sensitivity with temperature of underwater acoustic transducers from historical calibration data

The performance of underwater electroacoustic transducers often depends on water temperature and, for accurate calibrations, it is necessary to take account of this influence on measurement data. Doing so is particularly important for open-water calibration facilities where the environmental conditions cannot be controlled and seasonal variations in temperature can contribute significant measurement uncertainty. This paper describes the characterization of the sensitivity of underwater electroacoustic transducers in terms of their variation with water temperature. A model containing adjustable parameters is developed for providing frequency-dependent temperature coefficients whose use enables measurement data to be corrected for temperature. Estimates of these coefficients are determined by applying the model to the time history of measurement data obtained over a period of several years. The influence of seasonal temperature variation can then be separated from the slow temporal drift in the transducer sensitivity. The uncertainties associated with the corrected sensitivity values are evaluated.

Calibration and Characterization of Autonomous Recorders Used in the Measurement of Underwater Noise.

The use of autonomous recorders is motivated by the need to monitor underwater noise, such as in response to the requirements of the European Union Marine Strategy Framework Directive. The performance of these systems is a crucial factor governing the quality of the measured data, providing traceability for future underwater noise-monitoring programs aimed at the protection of the marine environment from anthropogenic noise. In this paper, a discussion is presented of measurement methodologies for the key acoustic performance characteristics of the recorders, including self-noise, dynamic range, and the absolute sensitivity as a function of frequency of the hydrophone and recorder system.

A comparison of two methods for phase response calibration of hydrophones in the frequency range 10–400 kHz

A comparison is made of two methods for determining the phase response of hydrophones in the kilohertz frequency range: The three-transducer spherical-wave reciprocity method and the method of optical interferometry. The implementation of the methods and the corresponding experimental systems are described. To facilitate a comparison, the methods are used to determine the phase response of three commercially available measuring hydrophones over the frequency range from 10 to 400 kHz. The results are compared, showing agreement within the estimated uncertainties of the methods. An investigation is conducted into the sources of uncertainties in the methods which increase with frequency. The major sources of uncertainty are residual positioning errors giving rise to phase uncertainties; a significant problem for the reciprocity method is that it requires one hydrophone to be rotated during the measurement procedure. An additional source of uncertainty at higher frequencies may be present if the position of the sensing element is not central within the outer hydrophone boot.

Measurement of radiated ship noise

A methodology is presented for measuring the radiated noise from a ship in shallow water, and deriving the source level spectrum. The method is applied to the measurement of dredgers in UK waters, vessels which are restricted to shallow water during aggregate extraction. Estimation of source level requires an estimate of the transmission loss which accounts for the effect of both the surface and seabed. The measurement method used involves the simultaneous measurement of the radiated noise at a number of measurement stations, each consisting of hydrophones which are either deployed from a stationary survey vessel, or from an autonomous recording buoy. The measurements at up to four ranges from the source vessel allow for empirical estimation of the source level using appropriate transmission loss models, with the data analysed in third-octave frequency bands. Measurement results presented are for trailing suction hopper dredgers, which lower a drag head and suction pipe to the sea floor to extract the sand or gravel, whilst returning unwanted material and water over the side.. Noise levels are shown for the same dredger under different operational modes illustrating that, for these vessels, the noise output level is partially dependent upon the aggregate type being extracted.

Phase calibration of hydrophones by the free-field reciprocity method

In this paper, a description is given of work carried out at the National Physical Laboratory to undertake phase response calibration of hydrophones by the free-field reciprocity method. A co-linear arrangement of hydrophones, based on the method of Luker and Van Buren, is used in order to eliminate some of the difficulties in the measurement of device separation and speed of sound in water. The experimental implementation of this method in the NPL open tank facility is described, along with the use of a laser alignment system to accurately set the position of the central hydrophone. The results of calibrations of three hydrophones are presented and these are compared to phase response measurements, performed on the same three hydrophones, using an optical vibrometry method, at Hangzhou Applied Acoustic Research Institute (HAARI) in China.