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Metamaterials exhibiting Fabry–Pérot resonances are shown to achieve ultrasonic imaging of a sub-wavelength aperture in water immersion across a broad bandwidth. Holey-structured metamaterials of different thickness were additively manufactured using a tungsten substrate and selective laser melting, tungsten being chosen so as to create a significant acoustic impedance mismatch with water. Both broadband metamaterial behavior and sub-wavelength imaging in water are demonstrated experimentally and validated with finite element simulations over the 200–300 kHz range. [ABSTRACT FROM AUTHOR]

A theoretical model is described for the field resulting from reflection of a plane piston source from a curved surface. The approach is based on an impulse response model, which treats the surface of the mirror as a grid of point reflectors to give an overall reflective response. The model was verified by comparison to experimental measurements in air, using a polymer-filmed capacitive transducer source fitted with an optical-grade mirror, at frequencies up to 1 MHz. The design parameters of such a device were investigated in order to determine the effect on the focal area, and to indicate how this model could be used to optimize the focusing effect of such a system. [ABSTRACT FROM AUTHOR]

This paper presents a method of calculating the pressure at any point emerging from a plane piston of polygonal shape in an infinite baffle. This is achieved by calculation of the impulse response as a time-stepped function, and considering the effect of each edge of the polygon in turn. The pressure is found from the convolution of this function with the time derivative of the piston velocity multiplied by the density of the propagation medium. Calculated pressure fields are shown for both a series of sources shaped as regular polygons, and for a source with an irregular shape. It is shown that the approach could be useful for the calculation of radiated fields from sources of arbitrary shape. [ABSTRACT FROM AUTHOR]

A new formulation is presented for the prediction of the signals transmitted by a rectangular acoustic source, and detected by a finite-sized rectangular receiver. This problem is encountered in measurements where the receiver is of a finite size compared to the shortest wavelength emitted by the source. The paper examines this problem theoretically, and a new formulation is presented in which the geometrical aspects are considered in some detail. The result is a simplified approach, in which the overall impulse response can be obtained from a combination of certain contributions defined geometrically. Theoretical field predictions are compared to experimental measurements for ultrasonic signals radiated into air, to demonstrate the usefulness of the approach. [ABSTRACT FROM AUTHOR]

This paper presents a study of the radiated ultrasonic fields of capacitive micromachined ultrasonic transducers (often referred to as cMUTs) in air. These fields were modeled theoretically and then compared to the experimental near-field amplitude variations and directivity patterns of square cMUTs. The good agreement between theory and experiment indicates that the devices can be approximated to plane piston radiators. The fields of multiple elements driven in phase on the same silicon substrate are presented, where again comparison is made to theory. The results indicate that individual elements are unaffected by radiation through the silicon substrate from adjacent devices. It will also be demonstrated that it is possible to use the knowledge of these fields to develop air-coupled ultrasonic surface imaging systems. © 2003 Acoustical Society of America. [ABSTRACT FROM AUTHOR]

This paper describes the use of air-coupled ultrasonic tomography for the simultaneous measurement of flow and temperature variations in gases. Air-coupled ultrasonic transducers were used to collect through-transmission data from a heated gas jet. A transducer pair was scanned in two-dimensional sections at an angle to the jet, and travel time and amplitude data recorded along various paths in counter-propagating directions. Parallel-beam tomographic reconstruction techniques allowed images to be formed of variations in either temperature or flow velocity. Results have been obtained using heated jets, where it has been shown that it is possible to separate the two variables successfully. [ABSTRACT FROM AUTHOR]

A pair of air-coupled ultrasonic capacitance transducers with polished metal backplates have been used to image temperature and flow fields in gases using ultrasonic tomography. Using a filtered back-projection algorithm and a difference technique, cross-sectional images of spatially variant changes in ultrasonic attenuation and slowness caused by the presence of temperature and flow fields were reconstructed. Temperature fields were produced in air by a commercial soldering iron, and the subsequent images of slowness variations used to reconstruct the air temperature at various heights above the iron. When compared to measurements made with a thermocouple, the tomographically reconstructed temperatures were found to be accurate to within 5%. The technique was also able to resolve multiple heat sources within the scan area. Attenuation and velocity images were likewise produced for flow fields created by an air-jet from a 1-mm-diam nozzle, at both 90 and 45 degrees to the scanning plane. The fact that temperature and flow fields can be measured in a gas without the need to insert any measuring devices into the image region is an advantage that may have many useful applications. © 1998 Acoustical Society of America. [ABSTRACT FROM AUTHOR]

The radiated pressure fields of axisymmetric ultrasonic transducers have been measured experimentally at a single frequency to a high resolution. Results in both the nearfield and farfield have been compared to the prediction of theory in three dimensions, developed to examine transducers vibrating with an arbitrary amplitude and phase distribution across their front face. Good correlation was obtained for polyvinylidene fluoride (PVDF) radiators, assuming plane-piston-type behavior. For lead zirconate titanate (PZT) transducers, marked amplitude variations with radius across the disk face were encountered, and these needed to be modeled theoretically to obtain good agreement with experiment. Other predictions of theory which are of interest to applications involving acoustical imaging are presented. [ABSTRACT FROM AUTHOR]

A noncontact acoustical system has been developed which measures Young's modulus of solid materials at high temperature with <=0.05% accuracy. The system employs capacitive (or electrostatic) transducers to excite and detect vibrations of millimeter-sized resonant tuning forks, whose resonance frequencies alter with changing temperature and material properties. The use of tuning forks in their fundamental symmetric modes of vibration provide resonances of high Q's and eliminate irreversible frequency and drift effects that occur with other forms of resonator. The use of noncontact capacitive transducers reduces the damping and stresses that otherwise occur with contacting transducers, and allows the system to be simply and accurately modeled. Both single crystal silicon tuning forks, and those manufactured from hydrided and unhydrided Zr-2.5%Nb, were investigated at temperatures up to 700 °C. The measured responses of silicon forks confirmed the system accuracy, and suggested that single-crystal silicon be standardized as a calibration material for acoustical measurement systems. Results obtained with Zr-2.5%Nb tuning forks allowed an accurate quantitative analysis of the effects of hydride dissolution and precipitation on Young's modulus, and confirmed that Young's modulus of hydrided Zr-2.5%Nb decreases in proportion to free hydrogen concentration. This experimental system should prove valuable for accurate modeling of high-temperature material transformations in solids. © 1997 Acoustical Society of America. [ABSTRACT FROM AUTHOR]

Investigations into the field characteristics of an air-coupled ultrasonic capacitance transducer have been performed for a range of transducer configurations. The field of a 2-MHz bandwidth silicon backplate capacitance transducer has been scanned in air using a 1-mm-diam miniature detector at frequencies of up to 1.5 MHz. The radiated peak sound-pressure field is compared to theory based on a plane piston approach for various driving signals, namely pulsed and tone burst excitation. Aperture modifications, such as an annulus and a zone plate, have also been investigated and the devices have been shown to behave as predicted by theory. © 1997 Acoustical Society of America. [ABSTRACT FROM AUTHOR]

This paper presents numerical simulations and experimental measurements of longitudinal and shear bulk waves in composite materials, generated and detected with an air-coupled ultrasonic system. The transfer function of an anisotropic absorbing layered plate immersed in air is computed with the transfer matrix method, and the waveforms produced by transmission through a plate are simulated from the convolution of a reference waveform and the impulse response of the plate. The calculated waveforms compare well to those recorded experimentally, confirming that the waveforms recorded experimentally have the correct characteristics. It is demonstrated that it is possible to identify longitudinal and shear modes. Results are presented showing how this data may be used to recover the stiffness matrix of composite materials, using the angular variation of velocity of both modes. © 1996 Acoustical Society of America. [ABSTRACT FROM AUTHOR]

An air-gap capacitance transducer with a guard ring has been used as a standard source of wideband ultrasound in solid material. Theoretical and experimental displacement waveforms have been compared, indicating that this capacitance source is characterized by a simple transduction mechanism. The bandwidth can be altered from a sharp transient to a steplike response, while the polarity can be reversed, all through simple changes in the excitation voltage and without damage to the sample. An example of the transducer acting as a source in a dielectric material is also presented. [ABSTRACT FROM AUTHOR]

Wide bandwidth capacitance devices have been designed which have a high transduction efficiency in air at frequencies into the MHz range. They are sufficiently sensitive to allow transmission through solid samples, including polymers and fiber-reinforced composites. Examples are presented of experiments showing how such transducers may be used to measure material properties and to perform ultrasonic imaging. [ABSTRACT FROM AUTHOR]

An air-coupled micromachined silicon electrostatic transducer has been used to detect ultrasound generated by a pulsed laser in metals and polymers. Bulk longitudinal and shear wave modes have been observed, as have Rayleigh waves in thick samples and Lamb waves in thinner plates. The results are in good agreement with surface displacement waveforms obtained using a contact capacitance device as a reference. The result is a flexible noncontact ultrasonic system with potential uses in materials characterization. [ABSTRACT FROM AUTHOR]

Thin and flexible electrostatic loudspeakers have been constructed, which can be curved into different shapes. This allows some interesting geometries to be explored, at frequencies in the range 10 kHz to 100 kHz. The research has compared the radiated fields measured experimentally with those predicted using established theory. The theoretical approach uses a new geometrical formulation to more efficiently predict radiated field patterns, where the curved surface is considered as a series of elements at specific angles to the beam axis. These elements can be of increased width compared to previous methods, making the approach computationally efficient. It is also a more robust approach than the traditional impulse response methods, in situations where the source size is comparable to the radiated wavelengths in air. Predictions could be made for both continuous and transient excitation. A comparison between theory and experiment has demonstrated that interesting field patterns can be generated. For instance, focussed sound fields can be produced using concave shapes, with predictable focal positions which are not at the center of curvature. These and other geometries will be described. [ABSTRACT FROM AUTHOR]

Ultrasonic tomography may be used as a method for imaging the internal structure of a wide range of solid materials. To do so requires tomographic reconstruction programs that can deal with irregular geometries, acoustic raybending and the possible presence of velocity anisotropy. Image reconstruction techniques are now available to deal with such problems, and have been applied to several situations of interest to materials characterization. The first of these involves the study of samples in bulk solid form, where a scanning system involving sliding contact transducers has been used to collect ultrasonic data and image materials that have been subject to thermal cracking. Additional studies have investigated the failure of fiber-reinforced polymer composites under applied load. In both cases, correlation with acoustic emission (AE) data has been performed. Other work has used immersion and noncontact ultrasonic transduction to image the structure of thin plate material using Lamb waves. In all cases, images will be presented and conclusions drawn on the type of information available from such images. [Work supported by SERC.] [ABSTRACT FROM AUTHOR]

This paper describes the use of air-coupled ultrasonic tomography for the simultaneous measurement of flow and temperature variations in gases. Air-coupled ultrasonic transducers were used to collect through-transmission data from a heated gas jet. A transducer pair was scanned in two-dimensional sections at an angle to the jet, and travel time and amplitude data recorded along various paths in counter-propagating directions. Parallel-beam tomographic reconstruction techniques allowed images to be formed of variations in either temperature or flow velocity. Results have been obtained using heated jets, where it has been shown that it is possible to separate the two variables successfully.