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Acoustic field and vibration visualisation is important in a wide range of applications. Laser vibrometry is often used for such visualisation, however, the equipment has a high cost and requires significant user expertise, and the method can be slow, as it requires scanning point by point. Here we suggest a different approach to visualisation of acoustic fields in the kHz - MHz range, using paint-on or removable film sensors, which produce a direct visual map of ultrasound displacement. The sensors are based on a film containing thermochromic liquid crystals (TLC), along with a backing/underlay layer which improves absorption of ultrasound. The absorption generates heat, which can be seen by a change in colour of the TLC film. A removable sensor is used to visualise the resonant modes of an air-coupled flexural transducer operated from 410 kHz to 7.23 MHz, and to visualise 40 kHz standing waves in a Perspex plate. The thermal basis of the visualisation is confirmed using thermal imaging. The speed and cost of visualisation makes the new sensor attractive for use in condition monitoring, for fast assessment of transducer quality, or for analysis of acoustic field distribution in power ultrasonic systems., Competing Interests: Declaration of competing interest The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: [Dave Hutchins, Oksana Trushkevych reports financial support was provided by UK Research Centre in Non-Destructive Evaluation. Rachel Edwards, Oksana Trushkevych has patent #PCT/GB2022/052279 pending to University of Warwick. R Edwards is on the editorial board for this journal. If there are other authors, they declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.]., (Copyright © 2024 The Author(s). Published by Elsevier B.V. All rights reserved.)

In this paper a new method of fabricating cylindrical resin microcantilevers using the Direct Digital Manufacturing (DDM) technique of Micro-stereolithography (MSL) is described. The method is rapid and commercially viable, allowing the fabrication of atomic force microscope (AFM) cantilevers which exhibit much larger spring constants than those currently commercial available. This allows for experimentation in a force regime orders of magnitude higher than currently possible using the AFM. This makes these cantilevers ideally suited for AFM-based depth sensing indentation. Due to their geometry, the assumptions used in the standard Euler-Bernoulli beam theory usually used to analyse AFM cantilevers may no longer be valid. Therefore approximate analytical solutions based on Timoshenko beam theory have been derived for the stiffness and resonant frequency of these cantilevers. Prototypes of the cantilevers have been fabricated and tested. Results show good agreement between experiment and theory., (Copyright © 2011 Elsevier B.V. All rights reserved.)

This paper presents results from a theoretical model of the ultrasonic fields radiated by a 3x3 assembly of capacitive micromachined ultrasonic transducer (cMUT) sources on the same silicon substrate. These predictions have, for the first time, been compared directly to the fields measured experimentally using a scanned miniature detector. This work indicates that there is minimal cross-coupling between source elements, and demonstrates that it is possible to predict successfully the field characteristics of various geometries of such cMUT elements, with a view to the development of future imaging systems.

Polymer-membrane capacitive ultrasonic linear and 2-D arrays have been fabricated for use in air-coupled imaging. By using arrays as receivers, there is a possibility of much faster imaging as the need for physically moving the receiver to scan a sample can be replaced by electronic multiplexing. In order to utilise this, a through-thickness air-coupled image of a composite plate has been made using a 2-D array as a receiver and a comparatively large planar source in air. This was made possible by the use of a chirp drive signal and cross-correlation on the measured waveform. Larger 2-D arrays with an increased number of elements have been simulated using a small scanned single receiver, and excellent imaging potential demonstrated. In addition two array receivers have been used in conjunction with two methods of post-processing, SAFT and ellipse crossing, to locate objects accurately.

This paper describes the construction and operation of an air-coupled capacitance transducer with a conical backplate. This was designed to produce a focal region over an extended distance along the transducer axis. Measurements were performed to examine both the frequency response of the transducer in pulse-echo mode, and the lateral resolution for imaging purposes. The radiated field was measured and compared to theory, and the extent of the focal region determined. Images of surface topography are presented, to illustrate the range of application of the transducer.

Surface micromachined, capacitive ultrasonic transducers have been fabricated using a low thermal budget, CMOS-compatible process. They exhibit interesting properties for transduction in air at frequencies in excess of 1 MHz, when driven from a standard ultrasonic voltage source. Experiments are described using 1 mm square devices in air, operating in both pitch-catch and pulse-echo modes. The dependence on d.c. bias voltage is examined, together with calibration measurements using 1/8 in. microphones. The radiated beam profile, and the farfield directivity pattern, have been measured for both broad bandwidth and one-burst excitation, using a scanned miniature receiver. A 16 element square array is also presented, which has been used to measure the beam cross-sections from a focussed source.

The directional characteristics of an ultrasonic signal have been studied during propagation within an axial gas jet. The effects of nozzle shape, nozzle diameter, and variations in jet velocity, temperature and gas composition have been investigated. At high flow velocities of an air jet, divergence of the ultrasonic beam was observed. This was attributed to the effects of refraction, caused by increased acoustic velocities in the direction of the flow. An effective waveguide was also demonstrated by cooling the air jet to below ambient temperatures, so that the acoustic velocity in the air jet was lower than that in the surrounding atmosphere. This could also be achieved by using carbon dioxide mixed with air, whereas the use of helium led to increased divergence. The result is likely to be of use in air-coupled ultrasonic materials inspection.

Surface micromachined, capacitive ultrasonic transducers have been fabricated using a low thermal budget, CMOS-compatible process. This process allows inherent control of parameters such as membrane size and thickness, cavity size and the intrinsic stress in the membrane to be achieved. Devices fabricated using this process exhibit interesting properties for transduction in air at frequencies in excess of 1 MHz when driven from a standard ultrasonic voltage source. Experiments have been performed with devices containing silicon nitride membranes of variable thicknesses over a 2 microm thick air cavity and with device dimensions of up to 5 mm square. This is much larger than has been reported for a device with a single membrane. Calibration measurements using 1/8 inch microphones in air, and miniature PVDF hyrdophones in water, have been performed. The dependence on d.c. bias voltage is examined, involving static membrane deflection measurements and received peak voltages. Pulse-echo and pitch-catch mode operation have been achieved. Interferometric measurements of membrane displacement have been performed in air to illustrate the membrane deflection characteristics. Operation in liquids is also discussed.

Capacitance transducers containing a thin polymer membrane have been used to transmit ultrasonic signals with frequencies in excess of 1 MHz through various paper products such as paper and cardboard. At normal incidence, a resonance was visible in thicker samples, the frequency of which could be correlated to parameters such as the thickness of the paper sample and the moisture content. It has also been demonstrated that images can be obtained of changes in structure across paper and card samples.

A pulse-compression technique has been applied to air-coupled testing of solid materials. Capacitance transducers were used to generate wide bandwidth swept-frequency (chirp) signals in air, which were then used to measure and image solid samples in through transmission. The results demonstrate that such signal processing techniques lead to an improvement in the signal to noise ratio and timing accuracy for air-coupled testing. Measurements of thickness and spectroscopic experiments are presented. Images of defects in a wide range of materials, including metals and carbon-fibre composites have also been obtained. This combination of capacitive transducers with pulse-compression techniques is shown to be a powerful tool for non-contact air-coupled ultrasonic measurements.

A neural network-based approach to the correction of C-scan images is presented. This allows the effects of a finite ultrasonic beam diameter in an immersion experiment to be considered, by training the network on a series of defects of known characteristics. The result is an image which is a better representation of the actual defect.