Peter D. Theobald

The application of scanning vibrometer in mapping ultrasound fields

Optical interferometry has been used for calibrating hydrophones at frequencies greater than 500 kHz for many years with very small uncertainties. Optical method have been found to be an effective non-perturbing method for detecting ultrasound fields with very fine spatial and temporal resolution; ultrasound fields can be mapped using a membrane and a scanning vibrometer, and two-dimensional fields can be imaged when the optical beam from a Laser Doppler Vibrometer (LDV) is aligned normal to the axis of the sound beam from a arbitrarily shaped projector. This paper presents a theoretical analyses for using a LDV system to detect ultrasound fields and further presents the result of some initial optical scan measurements performed using an LDV system. Results are presented of the particle velocity distributions produced by a focused ultrasound projector measured using a scanning LDV on an acoustically compliant, optically reflective membrane using a scanning LDV. Further measurement results are also presented of two-dimensional field maps obtained by positioning the laser beam perpendicular to the acoustic axis of a projector and detecting the line-integral acousto-optic effect. Visual maps of the acoustic waves radiating from the projector and the reflected and scattered propagating field by objects in water were obtained using this method and are presented in this paper.

Underwater noise generated from marine piling

Marine piling impact piling is a source of high-amplitude impulsive sound that can travel a considerable distance in the water column and has the potential for impact on marine mammals and fish. It involves steel piles being driven into the seabed using powerful hydraulic hammers, and is a commonly used construction method for fixing structures to the sea-bed in the offshore industry, and for the installation of offshore wind turbines in shallow coastal waters such as those around the UK. This paper describes methodologies developed for measurement of marine piling including estimation of the energy source level. Measurement results are presented of measurements made during the construction of an offshore windfarm, involving piles of typically 5 m in diameter driven by hammers with typical strike energies of around 1000 kJ. Acoustic data were recorded as a function of range from the source using hydrophones deployed form a vessel, allowing the transmission loss to be confirmed empirically. The use of fixe...

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.

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 Can...

Experimental validation of dispersion curves in plates for acoustic emission

This paper presents the findings of an investigation to determine theoretically and empirically the wave speeds and frequency content of the two primary Lamb wave modes, the symmetric (S0) and anti-symmetric (A0). A 2 mm thick steel plate measuring 700 mm by 700 mm was used to perform all measurements. A broadband pulse propagated through the plate and detected by a conical type piezoelectric receiver was used to show how the dispersive properties of the plate influenced the detected AE signals. It was shown that the two primary Lamb wave modes cover a very broad range of velocities, leading to a severe spreading of arrival times. A further investigation was completed using four acoustic emission sensors to record a pencil lead fracture, which was used as an artificial source. Reflections in the plate were shown to cause interference in the signal that can complicate the interpretation of the arrival modes. A recorded signal 400mm from the source was filtered into frequency bands. The arrival times of the wave modes were determined for each frequency band and the appropriate velocities calculated allowing a dispersion curve to be plotted experimentally. The plotted curve was shown to be a very close approximate to the calculated curve.

Significance of dynamic variation in renewable energy device noise to background noise levels under varying conditions

Assessment of the potential impact of underwater acoustic noise from both construction and operation of almost all human marine activities is now commonplace within many international regulatory frameworks. More recent industries like offshore renewables (wind, wave and tidal) have experienced an increasing requirement to understand how their energy conversion devices interact with existing acoustic environments and what their potential for impacts might be. Assessing the potential for impact, however, results in a number of measurement challenges, for example, many of these devices represent highly complex, multi-modal, distributed sources which can be anything from arrays of many small (few meters) devices to larger systems with dimensions of hundreds of meters. The systems can also vary in nature from a floating device, for example a wave energy surface attenuator, or can be distributed throughout the water column, as can be the case for both wave and tidal systems, or wind turbine. In the case of an offshore wind turbine for example, the device is coupled to the seabed, and extends throughout the water column and in to the air. This distribution and configuration can make the measured acoustic characteristics strongly dependent on the environment, with the potential for propagation through the seabed, water and air, with complex interactions, particularly between the seabed and waterborne components of the sound. The environments themselves are also often acoustically complicated. For example, many of the devices are being developed in relatively shallow water where there is often strong bathymetric variation, and other natural and manmade noise sources. The devices themselves often also exhibit time variant characteristics. They can become more energetic with tidal speed, wave height wind speed increase, for example. Furthermore, these properties also alter the `natural acoustic environment or baseline condition. The relationship between device noise and background or ambient noise is also likely to be fundamental to the effect these devices have on marine species, for example, by causing a change in behaviour or creating a masking effect. It may effect the detection capability of an animal, which may be important for collision avoidance. This paper discusses analysis of some of these challenges in the context of a number of emerging technology types. In particular, the paper considers the dynamically varying nature of the ratio of the noise radiated from the device to the background noise. Data is presented of the operational noise levels against the background noise levels, under different sea-states for a full-scale attenuator wave energy system.

Material properties of three syntactic foams

Material properties have been determined for three product samples of syntactic foam manufactured by CMT Materials, Inc., namely HYTAC-W, HYTAC-WF, and HYTAC-XTL. Cuboids of approximate dimensions 60×60×30 mm were fabricated by machining. The material density was determined from measurements of the sample mass and dimensions. The longitudinal- and transverse-wave sound speeds were determined as the quotients of sample thickness and respective time-of-flight measurement of an ultrasonic pulse transmitted through the sample. The possible anisotropy of each sample was investigated by the sound speed measurements. This was found to be weak, less than 0.1% for the W and WF types, but as much as 2% for the XTL type. Mean values for the respective properties of the W control-sample were 716 kg/m3, 2110 m/s, and 1121 m/s; for the WF control-sample: 867 kg/m3, 2708 m/s, and 1389 m/s; for the XTL control-sample: 712 kg/m3, 2672 m/s, and 1361 m/s. The effect of soaking on the properties was also investigated. Measurement methods are described, measurement statistics are given, and results are also expressed in terms of the elastic moduli. Measurements of acoustic backscattering spectra from spheres fabricated from the same product samples were performed, but will be described in a future paper.

Calibration of marine autonomous acoustic recorders

Absolute measurement of sound in the ocean is becoming increasingly ubiquitous, a common driver being the requirement for monitoring marine noise in support of regulation for the protection of the marine environment. In these measurements, the performance of the measurement system is a crucial factor governing the quality of the measured data. In recent years, there has been an increasing use of autonomous acoustic recorders for absolute in-situ measurement of sound in the marine environment. The technology has developed rapidly utilising recent improvements in mobile microprocessors and data acquisition systems, and currently there are a number of commercial off-the-shelf units available to the user. Whilst offering the enhanced ability to monitor acoustic signals autonomously for extended periods, such recorder units introduce a number of measurement and calibration challenges in addition to those associated with the calibration of individual hydrophones. These include the need to treat the autonomous acoustic recorder as a complete system, providing a traceable calibration which includes the hydrophone, hardware signal processing, the digital-to-analogue conversion, and software processing used to produce the sound data file. Because recorders often use archival storage of digital data, there is typically no access to “live” analogue electrical signals, requiring significant modifications to the standardised calibration procedures adopted for individual hydrophones. An additional problem can occur for recorders where the hydrophone is fixed rigidly to the end of the recorder body. In such cases, diffraction and scattering of the sound around the recorder body may influence the frequency response and directivity at kilohertz frequencies. At lower frequencies, the recorder performance may also be influenced by resonances in the recorder body. Other key performance characteristics include the system self-noise (important for measurement of low-level signals such as may be present in ambient ocean noise), and the dynamic range (important for measurement of high-amplitude impulsive sound sources). In this paper, methodologies are presented for the calibration and characterisation of autonomous recorders to determine the key acoustic performance characteristics, including the absolute system sensitivity as a function of frequency and direction, and the self-noise of the hydrophone and system. Consideration is given to effects due to the proximity of the recorder body to the measuring hydrophone on the frequency and directional response of the overall system. Examples of the results obtained are given and a discussion is presented of the implications of system performance on the quality of the measured data. The work described here aims to provide the traceability for the absolute measurement of sound in the ocean using autonomous recorders so that noise monitoring strategies and in-situ source characterisations are underpinned by robust metrology. The need for enhanced traceability is particularly acute at frequencies below 1 kHz where high-amplitude anthropogenic sources of greatest concern radiate much of their sound energy, and NPL now offers a calibration service to provide traceability for users of marine acoustic recorders over this vital low frequency range. Finally, this paper provides a discussion of how the work described here feeds into a European initiative to provide improved traceability and more robust metrology infrastructure to catch up with the rapidly evolving legislative framework.

Underwater sound measurement data during diamond wire cutting: First description of radiated noise

This paper describes the underwater noise characteristics of an underwater diamond wire cutting operation during the severance of a 0.76 m diameter conductor at an oil and gas platform in the North Sea. The conductor was cut approximately 10 m above the seabed using a “36-inch” (0.91 m) diamond wire cutting machine, in a water depth of approximately 80 m. The analysis revealed that the sound radiated from the diamond wire cutting of the conductor was not easily discernible above the background noise, which was present during the cutting operation (it should be noted that the cutting process involved the presence of several operational vessels). Increases of between around 4 dB and up to 15 dB were detectable for one-third octave band spectral levels at some frequencies, during the period which broadly corresponded to the cutting operation, with the higher frequencies showing greater increases. There was generally an observable increase in the spectral level for the one-third octave bands at frequencies ab...

Acousto-optic effect compensation for optical determination of the normal velocity distribution associated with acoustic transducer radiation

The acousto-optic effect, in which an acoustic wave causes variations in the optical index of refraction, imposes a fundamental limitation on the determination of the normal velocity, or normal displacement, distribution on the surface of an acoustic transducer or optically reflecting pellicle by a scanning heterodyne, or homodyne, laser interferometer. A general method of compensation is developed for a pulsed harmonic pressure field, transmitted by an acoustic transducer, in which the laser beam can transit the transducer nearfield. By representing the pressure field by the Rayleigh integral, the basic equation for the unknown normal velocity on the surface of the transducer or pellicle is transformed into a Fredholm equation of the second kind. A numerical solution is immediate when the scanned points on the surface correspond to those of the surface area discretization. Compensation is also made for oblique angles of incidence by the scanning laser beam. The present compensation method neglects edge waves, or those due to boundary diffraction, as well as effects due to baffles, if present. By allowing measurement in the nearfield of the radiating transducer, the method can enable quantification of edge-wave and baffle effects on transducer radiation. A verification experiment has been designed.