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This review was compiled as part of a project to formulate a UK strategy for the development and standardisation of measurement methods for high power/cavitating ultrasonic fields. It reviews the scientific literature relating to various methods of measuring high power fields which have been developed for application in health care, sonochemistry and industrial ultrasonics, and compares these methods in terms of attributes such as spatial resolution, bandwidth and sensitivity.

A systematic study has been undertaken of the influence of diffraction on ultrasonic absorption coefficient and phase velocity measurements made using the through‐transmission substitution technique. A diffraction correction model dealing explicitly with the interaction of the sample with the plane‐ and edge‐wave components of the acoustic field of a piston transducer is introduced. The National Physical Laboratory ultrasonic materials characterization facility has been used to validate the model using a pulsed technique covering the frequency range 1–15 MHz. In the validation, absorption coefficient and phase velocity measurements are presented for test specimens of cross‐linked polystyrene of various thickness. Strong diffraction effects have been demonstrated in the measurements, particularly for the absorption coefficient where measured values can be typically a factor of 2–3 higher than plane‐wave values. Excellent agreement is demonstrated between theory and measurement for both absorption and phase...

A key parameter for coated microbubbles in diagnostic and therapeutic applications is the non-linear response of their shell to acoustic pressures. Different measurements of this parameter exist, but most rely on the bubbles to be stuck on a surface or on the absence of acoustic excitation. In this work, we use the dynamic of coated microbubbles in an acoustofluidic device to measure the acoustic forces acting on them and the dynamic response of their shell to an increasing acoustic pressure. The device comprises of a microfluidic glass chip, where acoustical tweezers (operating in the range of 160-180 kHz) and optical tweezers can be used simultaneously. Comparing results from the calibrated optical tweezers, laser vibrometry and particle tracking allows a precise characterization of the acoustic field as a function of the driving voltage, at fixed frequencies and for pressures up to 4 kPa. Building on this result we extend the tracking technique to polymer-coated microbubbles and measure the acoustic fo...

The key compariosn CCAUV.U-K4 involved measurement of end-of-cable loaded sensitivity in units of volts/pascal of two travelling standards, 1 mm element diamater medical hydrophones at medical ultrasound frequencies. This is a repetition of key comparison CCAUV.U-K2 but the scope has been extended upwards to 20 MHz and downwards to 0.5 MHz. The reduction in the lower frequency provided an overlap with the underwater acoustics key comparison CCAUV.W-K1 which covers the range 1 kHz to 0.5 MHz. The results are analysed and presented in terms of degrees of equivalence, suitable for entry in the BIPM key comparison database. Main text 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 kcdb.bipm.org/. 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).

Significant nonuniformities in the acoustic intensity distribution generated by physiotherapy ultrasound treatment heads are not uncommon, potentially leading to significant localised temperature rises and tissue damage. An acoustic absorber tile containing a thermochromic pigment has been developed to provide rapid quality assurance of physiotherapy ultrasound treatment heads by virtue of a thermochromic colour change, indicating the time-averaged intensity distributions generated by these devices. As a bench-top device, the use of the tile is designed to mimic the nature of the physiotherapeutic application, requiring minimal training. Two designs where thermochromic pigments are added to the various polymeric layers of the tile are presented. Testing has been conducted with two physiotherapy treatment heads of differing performance, one of them notably exhibiting a strong “hot-spot” in localised acoustic time-averaged intensity. Findings show good qualitative agreement with classical hydrophone scans. Techniques are explored for the correction of nonlinearities in the thermochromic relationship, to enhance the accuracy of quantitative assessment.

This paper describes theoretical and experimental methods for characterising the performance of a 25 kHz sonochemical reactor (RV-25), which is being developed as a reference facility for studying acoustic cavitation at the National Physical Laboratory (NPL). Field measurements, acquired in different locations inside the sonoreactor, are compared with finite element models at different temperatures, showing that relatively small temperature variations can result in significant changes in the acoustic pressure distribution (and consequent cavitation activity). To improve stability, a deeper insight into the way energy is transferred from the power supply to the acoustic field is presented, leading to criteria – based on modal analysis – to dimension and verify an effective temperature control loop. The simultaneous use of measurements and modelling in this work produced guidelines for the design of multi-frequency cylindrical sonoreactors, also described.

A significant source of error in measurements of the acoustic output of medical ultrasonic equipment can arise from spatial averaging of the acoustic pressure over the active area of the hydrophone. Although criteria exist that quantify the maximum permissible effective hydrophone diameter, these are frequently violated for measurements on medical diagnostic systems, even for 0.4‐mm‐diam hydrophones that currently constitute the smallest commercially available device. In such circumstances, corrections have to be applied to the measured acoustical parameters. In the past, these correction procedures have been based on idealized models of the transducer pressure field distribution and have not taken account of finite amplitude effects which lead to nonlinearly distorted waveforms. This paper presents a systematic study of the influence of nonlinear distortion on spatial‐averaging corrections. A 5‐MHz focused transducer has been used to generate a range of acoustic waveforms suffering from varying degrees o...

An investigation into the influence of finite amplitude distortion on narrow‐band ultrasonic attenuation measurements is described. Measurements have been made using a through‐transmission substitution technique in the nonlinear field of a 5‐MHz plane‐piston transducer driven under tone‐burst conditions. Various transducer excitation levels were used to generate a range of shock parameters σ at the position of the measuring hydrophone up to a maximum of 3. The influence of the resulting loss in amplitude at the fundamental frequency has been studied by measuring the transmission properties of reference attenuators consisting of Dow Corning‐710 fluid‐filled cells of various thickness. The presence of nonlinear distortion in the acoustic waveform produces overestimates of the measured transmission coefficients. The magnitude of the error has been shown to depend on the value of σ, the small signal transmission loss of the sample and its position in the acoustic field. In some situations, the error was as hi...

This paper describes the effects of temperature changes on the operation of a 25 kHz sonochemical reactor, which is being developed as a reference facility for studying acoustic cavitation at NPL. Field measurements, acquired using a hydrophone in different locations inside the cavitation reactor, are compared with Finite Element Models at different temperatures, showing that significant changes in the acoustic pressure distribution (and consequent cavitation activity) can result from relatively small temperature variations. Modal analysis was used in this work as a tool to explain the physical reasons behind this behaviour and the effects of a preliminary temperature control system will be described, both in terms of temperature and for the improved stability of the acoustic pressure field. This work also highlights some of the limitations of modal analysis for the design of more complex reactors and associated temperature control methods.

Quantifying the acoustic output of diagnostic and therapeutic medical ultrasound devices is an established practice, using measurement methods based on applying miniature hydrophones to determine acoustic pressure distributions. However, specification standards require manufacturers to describe and declare acoustic output information in the form of intensity values, as these are more relevant to the possibility of adverse bioeffects. Simple relationships exist to calculate intensities from pressure data, but under many circumstances, such as away from the acoustic axis and in the transducer near-field, the underpinning assumptions break down. This paper describes the design, development and testing to proof-of-concept of a novel design of ultrasound sensor which can determine intensity directly. The sensor uses the pyroelectric properties of the piezoelectric polymer PVDF, and takes the form of a conventional membrane hydrophone backed with a highly attenuating polyurethane-based material. Ultrasound incident on the backing material is quickly absorbed, and the rate of temperature increase over a short timescale is proportional to the intensity in the beam, and produces a pyroelectric voltage response in the PVDF film. The new sensor also behaves as a conventional hydrophone, and can be used to derive acoustic pressure profiles. Intensity results are compared with pressure-squared data obtained from beam-plotting a range of simple transducer fields, and suggest differences in the beam profiles, particularly in the acoustic near field.

This article presents results for the prediction of in situ exposure levels using a new experimental technique based on the use of acoustical attenuators. These are made of low-density polyethylene and are placed in the ultrasound beam between the transducer face and the measuring hydrophone. The method has been tested for a number of different ultrasonic fields including those generated by diagnostic medical ultrasonic equipment. Using a tissue-mimicking material as a reference, it is shown that it is possible to use this new method to simulate acoustic pressure levels to within ±10% and acoustic intensity and power levels within ±20%. The method is proposed as the basis for a standard test method with wide applicability.

The prediction of the acoustic levels occurring in a patient during ultrasound examination is important for the assessment of equipment safety. While considerable effort has been devoted to theoretical methods of predicting exposure levels, there is a need to develop simple experimental methods which are universally applicable to the wide range of ultrasonic fields generated by medical ultrasonic equipment. This article outlines a number of methods that have been proposed and explores in detail a new experimental method based on the use of acoustical attenuators, made of low-density polyethylene, which are placed in the ultrasound beam between the transducer and the measuring hydrophone. Measurements of important acoustical quantities have been made using a measurement system based on a multielement hydrophone and comparisons made between this new method and other methods such as electrical attenuation and linear derating theory. The acoustical attenuation technique has been systematically studied, and results are compared with reference levels obtained using tissuemimicking material. A procedure involving making measurements with the attenuator placed first at the face of the transducer and then at the hydrophone has been developed. By taking the mean value of these two measurements, it has been shown that this new method can simulate within ±10% the acoustic levels obtained using the reference. It is concluded that it could be the basis of a universally applicable method.

This paper describes a new thermally based method of monitoring acoustic output power generated by ultrasonic transducers. Its novelty lies in the exploitation of the pyroelectric properties of a thin membrane of polyvinylidene fluoride (PVDF). The membrane is backed by a thick layer of polyurethane rubber that is extremely attenuating to ultrasound, with the result that the majority of the applied acoustic power is absorbed within a few millimeters of the membrane-backing interface. Through the resultant rapid increase in temperature of the membrane, a voltage is generated across its electrodes whose magnitude is proportional to the rate of change of temperature with respect to time. Changes in the pyroelectric voltage generated by switching the transducer ON and OFF are related to the acoustic power delivered by the transducer. Features of the technique are explored through the development of a simple one-dimensional model. An experimental evaluation of the potential secondary measurement technique is also presented, covering the frequency range 1 to 5 MHz, for delivered powers up to a watt. Predictions of the sensor output signals, as well as the frequency dependent sensitivity, are in good agreement with observation. The potential of the new method as a simple, rapid means of providing traceable ultrasonic power measurements is outlined.

A new secondary method of determining ultrasound power is presented based on the pyroelectricity of a thin membrane of the piezoelectric polymer, polyvinylidene fluoride (PVDF). In operation, the membrane is backed by a polyurethane-based rubber material that is extremely attenuating to ultrasound, resulting in the majority of the acoustic power applied to the PVDF being absorbed within a short distance of the membrane-backing interface. The resulting rapid heating leads to a pyroelectric voltage being generated across the electrodes of the sensor that, under appropriate conditions, is related to the rate of change of temperature with respect to time. For times immediately after changes in transducer excitation (switching either ON or OFF), the change in the pyroelectric voltage is proportional to the delivered ultrasound power level. This paper describes a systematic evaluation of the measurement concept applied at physiotherapy frequencies and power levels, investigating key aspects such as repeatability, linearity and sensitivity. The research demonstrates the way that heating of the backing material affects the sensor performance, but outlines the potential of the method as a reproducible, rapid, solid-state method of determining power, requiring calibration using a known ultrasound power source.

The acoustic field produced by a 25 kHz, 25 l cylindrical sonochemical processing cell has been characterised systematically using a sonar hydrophone, with the aim of establishing it as a reference test bed on which future investigations into acoustic cavitation activity may be based. Data acquired at sonication levels up to 500 W have shown that though significant cavitation activity is generated throughout the vessel, the acoustic field generated is reproducible, typically to ±12%. The increases in acoustic pressure are shown to be nonlinear with applied power, suggesting an intermediate optimum level for future study.

This paper describes investigations of the spatial distribution of cavitation activity generated within an ultrasonic cleaning vessel, undertaken using a novel cavitation sensor concept. The new sensor monitors high frequency acoustic emissions (>1 MHz) generated by micron-sized bubbles driven into acoustic cavitation by the applied acoustic field. Novel design features of the sensor, including its hollow, cylindrical shape, provide the sensor with spatial resolution, enabling it to associate the megahertz acoustic emissions produced by the cavitating bubbles with specific regions of space within the vessel. The performance of the new sensor has been tested using a 40 kHz ultrasonic cleaner employing four transducers and operating at a nominal electrical power of 140 W under controlled conditions. The results demonstrate the ability of the sensors to identify 'hot-spots' and 'cold-spots' in cavitation activity within the vessel, and show good qualitative agreement with an assessment of the spatial distribution of cavitation determined through erosion monitoring of thin sheets of aluminium foil. The implications of the studies for the development of reliable methods of quantifying the performance of cleaning vessels are discussed in detail.

This paper describes a theoretical study of the way in which a circular aperture placed in front of a plane-piston modifies the ultrasonic field it generates. Specifically, the radiated acoustic power transmitted by the aperture and the radiation force experienced by an absorbing target placed in the transmitted beam, are evaluated at a distance from the exit-side of the aperture. The calculations used a finite element (FE) method, in conjunction with a surface Helmholtz integral formulation to solve the fluid/structure interaction problem. The PAFEC (Program for Automatic Finite Element Computation) vibroacoustics software was used for the FE calculations and the implementation of the surface Helmholtz integral formulation method. Acoustic pressures and particle velocities were computed at required points, whilst accounting for the presence of the aperture in the medium, together with its dynamic properties when subjected to an incident sound field. This enabled the calculation of the radiation force on the absorber and its variation with the applied aperture diameter was investigated. As part of the validation process for the new FE aperture model, the ratio of radiation force to radiated acoustic power obtained using the FE method in the unapertured case, through the use of the Rayleigh integral, yielded good agreement with results obtained through an analytical solution. The study has been carried out to provide a better understanding of the factors affecting the measurement uncertainty for the aperture method of determining the effective radiating area (AER) of physiotherapy ultrasound treatment heads.

For Part I see ibid., vol.50, no.10, p.1342 (2003). This paper describes a series of experimental studies to evaluate the performance of newly developed sensors for monitoring broadband acoustic emissions generated by acoustic cavitation. The prototype sensors are fabricated in the form of hollow, open-ended cylinders, whose inner surface is made from a thin film of piezoelectric polymer acting as a passive acoustic receiver of bandwidth greater than 10 MHz. A 4 mm thick coating of special acoustical absorber forms the outer surface of the sensor. The layer functions as a shield to cavitation events occurring outside the hollow sensor body, allowing megahertz acoustic emissions emanating from within the liquid contained in the sensor to be monitored. Testing of the new sensor concept has been carried out within the cavitating field provided by a commercial ultrasonic cleaning vessel operating at 40 kHz whose power output is rated at 1 kW. It is demonstrated that the prototype cavitation sensors are able to record a systematic increase in the level of the high-frequency acoustic spectrum (>1 MHz) as electrical power to the cleaning vessel is increased. Through careful control of the experimental conditions, reproducibility of the high frequency "energy" associated with the cavitation spectrum was found to be typically /spl plusmn/25%.

This paper describes a new concept for an ultrasonic cavitation sensor designed specifically for monitoring acoustic emissions generated by small microbubbles when driven by an applied acoustic field. Its novel features include a hollow, open-ended, cylindrical shape, with the sensor being a right circular cylinder of height 32 mm and external diameter 38 mm. The internal diameter of the sensor is 30 mm; its inner surface is fabricated from a 110 /spl mu/m layer of piezoelectrically active film whose measurement bandwidth is sufficient to enable acoustic emissions up to and beyond 10 MHz to be monitored. When in use, the sensor is immersed within the liquid test medium and high frequency (megahertz) acoustic emissions occurring within the hollow body of the sensor are monitored. In order to shield the sensor response from events occurring outside the cylinder, the outer surface of the sensor cylinder is encapsulated within a special 4 mm thick polyurethane-based cavitation shield with acoustic properties specifically developed to be minimally perturbing to the 40 kHz applied acoustic field but attenuating to ultrasound generated at megahertz frequencies (plane-wave transmission loss >30 dB at 1 MHz). This paper introduces the rationale behind the new sensor, describing details of its construction and the materials formulation program undertaken to develop the cavitation shield.

This paper describes a quick and cost-effective method for constructing a radiation force balance for measuring ultrasonic output power. It utilises a target manufactured from a high-quality acoustical absorber material. The target geometry is in the form of a cup or well that is water-filled and placed directly on the pan of a top-loading chemical balance, thus overcoming the need for the traditional gantry arrangement found in the majority of commercially available balances. The face of the transducer is placed directly in the water contained within the well. This simplification reduces time spent in setting up a balance for measurement, and targets can be manufactured to any required geometry and used on any suitable top-loading balance to measure output power. Within this study, the performance of the absorbing well method was evaluated over the frequency range of 1 MHz to 5 MHz, for acoustic power levels up to 1 W. Power measurements on three transducers were compared with measurements made on the National Physical Laboratory (NPL) primary standard radiation force balance and good agreement is demonstrated between the two systems. At a power of 50 mW, using a chemical balance of resolution 0.1 mg, typical type A (random) uncertainties were +/- 2.0% when expressed at the 95% confidence level.

This paper describes the application of a novel broadband acoustic sensor to evaluating the acoustic emissions from cavitation produced by a typical commercial 20 kHz sonochemical horn processor. Investigations of the reproducibility of the processor, and of the variation in cavitation emissions as a function of output setting and sensor location are described, and resulting trends discussed in terms of the broadband integrated power in the megahertz frequency range. Companion studies with a conventional membrane hydrophone have illustrated for the first time that cavitation emissions produced by a sonochemical horn processor can extend to frequencies beyond 20 MHz, and the sensor shows that significant nonlinearity can be seen in measured cavitation activity with increasing nominal output power.

This paper describes a novel acoustic sensor of potential application in monitoring cavitation occurring within ultrasonic cleaning vessels. The sensors, fabricated in the form of hollow cylinders, are manufactured from a thin (110 /spl mu/m) piezoelectric polymer whose acoustic bandwidth (10 MHz) is sufficient to detect the high-frequency signals generated by cavitating bubble collapse. One of the key characteristics of the sensors is their spatial resolution: acoustic signals detected at high frequencies originate from bubble events occurring within the hollow cylinder itself. To isolate these, the cavitation sensors are encapsulated in a 4 mm thick layer of a specially-developed acoustical absorber acting as a shield to bubble events occurring outside of the sensor volume. The construction of the new sensor is described, and early results of its response in the acoustic field of an ultrasonic cleaning vessel are presented.

A project has been completed to develop reference methods for the measurement of ultrasonic power with a validated measurement uncertainty of < 7% at power levels of 1 to 20 W over the frequency range 1 to 3 MHz of collimated beams. The project is the result of collaborative research between the Physikalisch-Technische Bundesanstalt, Germany (PTB, DE), the National Physical Laboratory, UK (NPL, UK) and the Netherlands Organisation for Applied Scientific Research, Prevention and Health (TNO-PG, NL). The work has been undertaken under the 4th Framework Programme of the European Community (EC). Primary standard designs of radiation force balances based on both absorbing and reflecting targets have been constructed. To avoid heating effects, the measurements should be done relatively quickly (10 to 20 s). The methods have been validated using ultrasound (US) transducers that demonstrated an adequate short and long-term stability; a method to detect cavitation based on monitoring the acoustic signals produced by bubble oscillation and collapse has been confirmed. It has been shown that only the detection of the subharmonic can be used in practice as cavitation detector. Different procedures for obtaining degassed water have been investigated. A method showing significant promise to be used in a clinical or manufacturer's environment involves the addition of sodium sulphite (Na2SO3). During the validation process, commercially available radiation force balances and ultrasonic physiotherapy devices have also been evaluated. Limitations of current measurement methods and practices, including power measurements made on transducers exhibiting a diverging beam, have been identified. It has been shown that a reflecting target is not appropriate to measure powers of transducers with a ka-value < 30. Based on beam shape and target distance, it has been shown also that proper power measurements using a 45 degrees convex-conical reflecting target can never be performed for transducers with a ka-value < 17.4.

This paper describes a newly developed material with acoustic properties that make it ideal for applications as radiation force balance-absorbing targets. The material is now commercially available from National Physical Laboratory (NPL) and is based on a polyurethane rubber. It exhibits an echo reduction of 45 dB, and single-pass transmission loss of 30 dB, both determined at an acoustic frequency of 1 MHz. The composition and structure of the new NPL absorber are presented, along with values for the frequency and temperature variation of the echo reduction and transmission loss. Over the frequency range 1 to 10 MHz, its acoustic properties comply with the requirements for force balance-absorbing targets specified in IEC 61161.

This paper describes the investigation and validation of a new method for measuring the effective radiating area (AER) of physiotherapy ultrasound treatment heads. The method is based on the use of a conventional radiation force balance, but employs special attenuating apertures that are used to selectively mask off different areas of the treatment head. The resultant reduction in the radiating surface is accompanied by a decrease in output power that is measured using the force balance. The AER of the treatment head is derived from an analysis of the measurements, which essentially involves initially evaluating the minimum area through which 75% of the acoustic power is transmitted. AER values derived using the new method are presented for 17 treatment heads representative of the range of physiotherapy systems commonly used in clinical practice. These are compared to reference values derived using hydrophone scanning, according to the recently published International Standard, IEC 1689. Typical levels of agreement between values of AER derived using the two techniques are ± 11%. The potential of the method as a rapid, relatively low-cost, means of measuring treatment head AER, applicable in both manufacturing and hospital environments, is assessed.

A multi-frequency reference vessel (RV-multi) has been established at NPL, with the ultimate aim of establishing a standard source of acoustic cavitation in water. The vessel is a cylindrical cavity and can work at 6 frequencies, from 21 kHz up to 140 kHz, in controlled temperature conditions. RV-multi is capable of self-tuning its driving frequency to lock onto a fixed modal pattern, based on the maximised phase derivative of the driving voltage signal. The ability of system self-locking was tested under various power levels. As part of the validation process for the new reference vessel, its performance has been investigated using various methods. A small hydrophone was used to determine the acoustic pressure distribution along selected planes of symmetry inside the vessel. The generated cavitation distribution was also monitored using sonoluminescence, erosion and acoustic emission. In a novel step, broadband laser vibrometry was used to measure the motion of the vessel wall to investigate the feasibility of a non-invasive method of cavitation detection.

Ultrasonic Computed Tomography (UCT) has been unable to rival its x-ray counterpart in terms of reliably distinguishing different tissue pathologies. Conventional piezoelectric detectors deployed in UCT are phase-sensitive and it is well established that their use can give rise to phase-cancellation artefacts that mask true tissue structure. In contrast, phase-insensitive detectors are more immune to this effect, although sufficiently sensitive devices for clinical use are not yet available. This paper establishes proof-of-concept for a novel phase-insensitive transducer for UCT. The detector employs an acoustic absorber to convert received acoustic intensity into heat that is subsequently detected using the pyroelectric response of a thin piezoelectric membrane bonded intimately to the absorber. The paper explores UCT application of the phase-insensitive detectors, comparing with traditional detection methods. Results are presented for a range of detector apertures; tomographic reconstruction images bein...

Carbon nanofibers and nanotubes are currently being utilized as active elements in acoustic sensors for emerging microelectromechanical systems and nanoelectromechanical systems technologies. A methodology for measuring the displacement of carbon nanofibers in combination with heterodyne interferometry is reported here. Experimental results show that ultrasonic field detection is possible using this technique, and results are presented for measurements in the ultrasonic frequency range. This approach could potentially lead to new calibration methods for ultrasonic sensors. A different approach to that of interferometry is also reported for future investigation.

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A central objective of the Mutual Recognition Arrangement (MRA), signed by national measurement institute (NMI) directors in 1999, is the establishment of the degrees of equivalence of national measurement standards held by each institute. International comparisons, known as key comparisons, represent the sole mechanism for establishing these degrees of equivalence. In this paper we describe a key comparison, undertaken under the auspices of the BIPM/CIPM Consultative Committee for Acoustics, Ultrasound, and Vibration, related to the realization of the acoustic pascal in water at ultrasonic frequencies. This is most appropriately achieved through a comparison of calibrations of stable transfer standard hydrophones; 1 mm active element bilaminar membrane hydrophones, being chosen for this purpose. With NPL acting as the pilot laboratory, two hydrophones were calibrated using the NPL primary standard laser interferometer and circulated sequentially to participant NMI laboratories in Germany, China, The Neth...

Ultrasonic hydrophones continue to play a pivotal role in assessing the safety of medical ultrasonic equipment. International and national standards require measurements of the acoustic fields generated by this type of equipment to be made using hydrophones of specified properties such as bandwidth, spatial resolution, etc. A critical property of the hydrophone is the absolute calibration of its receive sensitivity and its ultimate traceability to national standards of measurement. At NPL, the primary standard laser interferometer has been in operation for over 10 years, providing this traceability for the majority of hydrophones currently used worldwide. This presentation will give an outline description of the interferometer, describing how the primary standard is disseminated to the user community through secondary standard hydrophones using a nonlinear intercomparison method. A number of key results related to the long‐term stability of reference hydrophones will be presented. The paper will then concentrate on recent developments in the application of the interferometer which have sought to increase its working frequency range up to 60 MHz.

This paper describes imaging of breast phantoms using a prototype phase-insensitive ultrasound computed tomography (piUCT) system. The piUCT technique generates quantitative maps of acoustic attenuation with potential application to the detection of breast cancer. The effectiveness of the piUCT technique has been previously demonstrated in the laboratory through imaging of small, cylindrical, polyurethane phantoms. This paper presents images generated by the new system, which was designed to image breasts and breast phantoms up to 200 mm in diameter at 3.2 MHz. piUCT images are compared with XCT images and laboratory measurements of the attenuation of the constituent materials of the phantom.

The International Photoacoustic Standardisation Consortium (IPASC) emerged from SPIE 2018, established to drive consensus on photoacoustic system testing. As photoacoustic imaging (PAI) matures from research laboratories into clinical trials, it is essential to establish best-practice guidelines for photoacoustic image acquisition, analysis and reporting, and a standardised approach for technical system validation. The primary goal of the IPASC is to create widely accepted phantoms for testing preclinical and clinical PAI systems. To achieve this, the IPASC has formed five working groups (WGs). The first and second WGs have defined optical and acoustic properties, suitable materials, and configurations of photoacoustic image quality phantoms. These phantoms consist of a bulk material embedded with targets to enable quantitative assessment of image quality characteristics including resolution and sensitivity across depth. The third WG has recorded details such as illumination and detection configurations of PAI instruments available within the consortium, leading to proposals for system-specific phantom geometries. This PAI system inventory was also used by WG4 in identifying approaches to data collection and sharing. Finally, WG5 investigated means for phantom fabrication, material characterisation and PAI of phantoms. Following a pilot multi-centre phantom imaging study within the consortium, the IPASC settled on an internationally agreed set of standardised recommendations and imaging procedures. This leads to advances in: (1) quantitative comparison of PAI data acquired with different data acquisition and analysis methods; (2) provision of a publicly available reference data set for testing new algorithms; and (3) technical validation of new and existing PAI devices across multiple centres.

Dow Corning 710 silicone fluid has been investigated to establish its suitability as a reference liquid for ultrasonic attenuation. Measurements made in the megahertz frequency range with overall uncertainties of 4–5% at a confidence level of 95% are compared with previous values given in the literature. The results of studies made to assess the long-term stability of the fluid are also presented.

PONTIFÍCIA UNIVERSIDADE CATÓLICA DO RIO DE JANEIRO COORDENAÇÃO DE APERFEIÇOAMENTO DO PESSOAL DE ENSINO SUPERIOR PROGRAMA DE SUPORTE À PÓS-GRADUAÇÃO DE INSTS. DE ENSINO Esta dissertação de mestrado tem por objetivo avaliar e determinar a influência da cavitação no fenômeno da erosão de materiais de engenharia à luz das recomendações da norma ASTM G32:2010. O trabalho foi motivado pelo interesse do Programa de Metrologia em participar, no âmbito da cooperação celebrada com o Instituto de Metrologia do Reino Unido, de um novo desafio no campo da metrologia científica relacionado à medição de cavitação. A metodologia utilizada baseou-se em trabalhos experimentais realizados com o medidor de cavitação recentemente desenvolvido pelo Laboratório de Acústica do NPL, que foi pioneiramente utilizado para mapear os níveis de cavitação nos experimentos de erosão de materiais de engenharia para diferentes amplitudes (delta) de pulsação do transdutor e diferentes posicionamentos (lambda) da haste do transdutor em relação ao espécime submetido à erosão. Dentre os resultados destaca-se a perda de massa mensurada para cada um dos três materiais de engenharia submetidos à erosão por cavitação: 101,4 mg no Alumínio; 66,0 mg na Liga Alumínio-Bronze e 6,80 mg no Aço inoxidável 304. Três são as conclusões mais relevantes do trabalho (i) no que concerne a perda de massa (medida da erosão), o parâmetro delta mostrou-se mais crítico que o parâmetro lambda; (ii) os materiais de menor dureza são os mais susceptíveis à erosão por cavitação e (iii) quanto maior a resistência à erosão menor é a uniformidade da cavitação. The aim of this dissertation is to evaluate and assess the influence of the cavitation on the erosion of engineering materials in line with recommendations of the ASTM G32:2010 standard. The work was motivated by the interest of the postgraduate Metrology Programme to participate, within the framework of the cooperation celebrated with the National Metrology Institute of the United Kingdom (National Physical Laboratory, NPL), in a new metrology challenge related to measurement of cavitation. The methodology used was based on experimental work carried out with the cavitation meter recently developed by the Laboratory of Acoustics of the NPL, which was first used to map the levels of cavitation inducing erosion in engineering materials for different amplitude of the transducer (delta) and displacement (lambda) of the transducer horn relative to the specimen subjected to erosion. Among the results is emphasized the mass loss measured for each of the three engineering materials submitted to cavitation erosion: 101.4 mg for the aluminium; 66.00 mg for the Alloy Aluminium-Bronze and 6.80 mg for the stainless steel 304. Three are the main findings of the work (i) with respect to loss of mass (a measure of erosion), the parameter delta was more critical that the parameter lambda; (ii) the lower the hardness of the engineering material eroded, the most susceptible it becomes to cavitation erosion and (iii) the higher the erosion resistance the lower the uniformity of cavitation.