Pete D. Theobald

Technique for the calibration of hydrophones in the frequency range 10 to 600 kHz using a heterodyne interferometer and an acoustically compliant membrane

A technique for the calibration of hydrophones using an optical method is presented. In the method, a measurement is made of the acoustic particle velocity in the field of a transducer by use of a thin plastic pellicle that is used to reflect the optical beam of a laser vibrometer, the pellicle being acoustically transparent at the frequency of interest. The hydrophone under test is then substituted for the pellicle, and the hydrophone response to the known acoustic field is measured. A commercially available laser vibrometer is used to undertake the calibrations, and results are presented over a frequency range from 10 to 600 kHz. A comparison is made with the method of three-transducer spherical-wave reciprocity, with agreement of better than 0.5 dB over the majority of the frequency range. The pellicle used is in the form of a narrow strip of thin Mylar©, and a discussion is given of the effect of the properties of the pellicle on the measurement results. The initial results presented here show that th...

Gated photon correlation spectroscopy for acoustical particle velocity measurements in free-field conditions.

The measurement of acoustic pressure at a point in space using optical methods has been the subject of extensive research in airborne acoustics over the last four decades. The main driver is to reliably establish the acoustic pascal, thus allowing the calibration of microphones with standard and non-standard dimensions to be realized in an absolute and direct manner. However, the research work so far has mostly been limited to standing wave tubes. This Letter reports on the development of an optical system capable of measuring acoustic particle velocities in free-field conditions; agreement within less than 0.6 dB was obtained with standard microphone measurements during these initial experiments.

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.

Characterization of solid complex multiphase systems based on oscillatory photon correlation spectroscopy

The particle loading and dispersion profiles are two significant properties directly affecting the engineering properties and performance characteristics of polymer nanocomposites. Current measurement techniques are often destructive, require special sample preparation, are limited to small, unrepresentative sample size, and/or cannot discriminate between the two aforementioned parameters. This Letter demonstrates the application of photon correlation spectroscopy on mechanically oscillated solids; experimental results show that this technique can discriminate between different grades of such materials.

COMPILE—A Generic Benchmark Case for Predictions of Marine Pile-Driving Noise

The prediction of underwater noise emissions from impact pile driving during near-shore and offshore construction activities and its potential effect on the marine environment has been a major field of research for several years. A number of different modeling approaches have been suggested recently to predict the radiated sound pressure at different distances and depths from a driven pile. As there are no closed-form analytical solutions for this complex class of problems and for a lack of publicly available measurement data, the need for a benchmark case arises to compare the different approaches. Such a benchmark case was set up by the Institute of Modelling and Computation, Hamburg University of Technology (Hamburg, Germany) and the Organisation for Applied Scientific Research (TNO, The Hague, The Netherlands). Research groups from all over the world, who are involved in modeling sound emissions from offshore pile driving, were invited to contribute to the first so-called COMPILE (a portmanteau combining computation, comparison, and pile) workshop in Hamburg in June 2014. In this paper, the benchmark case is presented, alongside an overview of the seven models and the associated results contributed by the research groups from six different countries. The modeling results from the workshop are discussed, exhibiting a remarkable consistency in the provided levels out to several tens of kilometers. Additionally, possible future benchmark case extensions are proposed.

Novel non-contact optical characterisation methods of polymeric nanocomposite structures based on their particle loading and dispersion profile

Current methods to characterise specific properties of polymeric nanocomposites (PNCs), such as particle loading and dispersion profile, rely on a number of techniques that require special sample preparation and treatment, are very expensive, require long measurement times and quite often produce ambiguous results that are difficult to evaluate and interpret. In addition, given their complexity, they are not entirely suited for in-situ industrial environments. This paper presents alternative techniques based on optical diffraction and diffusion mechanisms combined with signal processing that can successfully discriminate between different particle loadings and levels of dispersion. The techniques discussed in this paper are Fourier-domain optical coherence tomography in the infra-red, Fraunhofer wavefront correlation in the visible red and oscillatory photon correlation spectroscopy in the visible green parts of the spectrum. Most importantly, they are non-invasive, are compact, fast and efficient, can potentially analyse large areas of the material and therefore suited for a wide variety of research and industrial situations.

Calibration of hydrophones and electroacoustic transducers: A contribution to the OES standards initiative

Underwater acoustic measurements are made to provide validation and qualification for a wide range of applications and developments in the off-shore industry, oceanography, defense, fisheries, and geophysics. However, measurements are only meaningful if they are performed in a technically sound manner and if they can be related to common standards of measurement. In this paper, a summary is given of the existing standards for hydrophones and electroacoustic transducers used in underwater acoustics. A description is given of how primary standards are currently realized and disseminated, how they are validated by international comparisons, and of new developments which may lead to the next generation of standards. A report is also given of the status of recent developments in specification standards, for example, within the International Organization for Standardization (ISO) and the International Electrotechnical Commission (IEC). Finally, a description is provided of the current initiative within the Oceanic Engineering Society (OES) to compile and disseminate information on standards, protocols, quality assurance (QA) procedures, and best practices that are important in ocean engineering.

Primary ultrasonic interferometer photodiode characterization using frequency-modulated laser wavefront radiation

Photodiodes play an important role in many optical systems and, as such, their stability and performance are of great importance. Manufacturers of such detectors typically only provide information regarding their sensitivity as a function of the optical wavelength and this is often sufficient for applications such as telecommunications. However, in certain applications such as primary ultrasonic standards based on interferometry, the frequency response of the photodiodes is of critical importance because, for accurate calibration, a correction factor must be applied, which contributes a major source of measurement uncertainty. Most optical calibration systems reported in the literature operate either in the GHz range or a very limited MHz range. This work reports on the development of a system based on rotational optical components that produces light patterns scanned through a slit at varying speeds using different deflection mechanisms. This results in the generation of spatially dependent interference fringes in the range 1 Hz up to 115 MHz with an expanded uncertainty of less than 5% for the majority of its frequency range of operation.

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.