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The response of a resonant chain of spheres to changes in holder material and precompression is studied at ultrasonic frequencies. The system is found to be very sensitive to these parameters, with the creation of impulsive waveforms from a narrow bandwidth input seen only for certain chain lengths and holder materials. In addition, careful experiments were performed using known amounts of precompression force, using a calibrated stylus arrangement. At negligible precompression levels, impulses were generated within the chain, which were then suppressed by increased precompression. This was accompanied by large changes in the propagation velocity as the system gradually changes from being strongly nonlinear to being more linear. Simulations using a discrete model for the motion of each sphere agree well with the experimental data. [ABSTRACT FROM AUTHOR]

A study has been undertaken of ultrasonic communications methods in air, using a quadrature modulation method. Simulations were first performed to establish the likely performance of quadrature phase shift keying over the limited bandwidth available in an ultrasonic system. Quadrature phase shift keying modulation was then implemented within an experimental communication system, using capacitive ultrasonic sources and receivers. The results show that such a system is feasible in principle for communications over distances of several meters, using frequencies in the 200 to 400 kHz range. [ABSTRACT FROM AUTHOR]

Air-coupled ultrasonic spectroscopy is described, whereby the output from a pulse compression system is used. It is demonstrated that the cross-correlation operation used within a pulse-compression system preserves amplitude and phase information. This approach allows he signal-to-noise ratio and, hence, signal-detection capability to be improved by the cross-correlation, while allowing noncontact spectral information for solid samples to be obtained. Results are presented for chocolate samples, where measurements of interest to the food industry have been obtained. [ABSTRACT FROM AUTHOR]

The use of ultrasound in air as a means of communicating digital signals is demonstrated. The work uses capacitive transducers with a useful bandwidth to transmit digitally coded signals across an air gap in the laboratory, using three of the common methods used in digital communications. These are on-off keying (OOK), binary frequency-shift keying (BFSK), and binary phase shift keying (BPSK). All three methods are simulated numerically using the available bandwidth of the transducer systems and are compared to results obtained experimentally. It is demonstrated that BPSK can be used to transmit signals with a low bit error rate. [ABSTRACT FROM AUTHOR]

A new signal processing method, which uses a modified chirp signal for air-coupled ultrasonic imaging, is described. A combination of the elliptical and Tukey window functions has been shown to give a better performance than the Hanning windowing used in most pulse-compression algorithms for air-coupled applications. The elliptical-Tukey chirp signal provides an improvement in both the resolution of images and signal-to-noise ratios. In addition, this type of signal also reduces the level of signal voltages required to drive the source transducer while maintaining the performance of the system. This approach, thus, may have wide interest in all forms of wide bandwidth ultrasonic imaging. [ABSTRACT FROM AUTHOR]

The performance of capacitive micromachined ultrasonic transducers (cMUTs) was investigated at low pressures in various gases such as air, carbon dioxide, and helium. The aim was to replicate the pressure conditions likely to meet on the surface of other planets such as Mars, where ultrasonic wind velocity measurements might be possible. It is demonstrated that cMUTs are capable of operating at low pressures, and the response to pressures below terrestrial atmospheric values is observed experimentally and compared to theoretical predictions. The center frequency of Operation and sensitivity are both observed to be affected by changing pressures. [ABSTRACT FROM AUTHOR]

Air-coupled ultrasound has been used for the nondestructive evaluation of concrete, using broad bandwidth electrostatic transducers and chirp excitation. This paper investigates the benefits of using time-frequency analysis in such situations, for both waveform retrieval and imaging in the presence of low signal levels. The use of the short-term Fourier transform, the Wavelet transform, and the Wigner-Ville distribution all are considered, in which accurate tracking of the ultrasonic chirp signals is demonstrated. The Hough transform then is applied as a filter. An image of a steel reinforcement bar in concrete has been produced to illustrate this approach. [ABSTRACT FROM AUTHOR]

A noncontact ultrasonic inspection technique has been developed to study the properties of wood samples in air. The system makes use of two broad bandwidth capacitive transducers, combined with signal processing techniques. A coded chirp signal was used in the current application to provide a waveform that could he post processed to provide sufficient sensitivity for transmission across samples of wood. It will he shown in this paper that the signal-to-noise ratio (SNR) can be greatly improved using two signal recovery techniques, namely pulse compression and swept frequency multiplication (SFM). A simulation of both techniques is presented and compared to experimental data. As will be seen from the experimental results, it is possible to perform noncontact ultrasonic experiments to extract a range of useful information such as ring density and the presence of microcracks. [ABSTRACT FROM AUTHOR]

A new technique has been investigated for improving the signals that can be obtained in air-coupled nondestruction evaluation (NDE). This relies on the wide bandwidth available from polymer-filmed capacitive transducers. The technique relies on a swept-frequency "chirp" signal, which is transmitted from a transducer in air. The new technique differs from existing time-domain correlation techniques, such as pulse compression, in that a single multiplication process is performed in the time domain to give a difference frequency signal. This then can be isolated easily in the frequency domain. It will be demonstrated that this new swept frequency multiplication (SFM) approach gives the potential for rapid air-coupled imaging. [ABSTRACT FROM AUTHOR]

This paper describes the use of a focused aircoupled capacitance transducer combined with pulse compression techniques to form high-resolution images of thin materials in air. The focusing of the device is achieved by using an off-axis parabolic mirror. The lateral resolution of the focused transducer, operating over a bandwidth of 1.2 MHz, was found to be less than 0.5 mm. A combination of the focused transducer as a source and a planar receiver in through-transmission mode has been developed for the measurement of different features in paper products, with a lateral resolution in through-transmission imaging of ∼0.4 mm. Images in air of thin samples such as bank notes, high-quality writing paper, stamps, and sealed joints were obtained without contact to the sample. [ABSTRACT FROM AUTHOR]

An ultrasonic waveguide has been produced in air by using a gas jet. This uses the fact that a lower acoustic velocity can be produced within the jet, relative to the air surrounding it. The lower velocity is achieved by mixing carbon dioxide with air within the jet at a concentration that is a compromise between lower acoustic velocity and increasing attenuation. Using a capacitance transducer placed within the flowing gas, it is shown that improvements in the beam width can result when the gas jet is used. Air-coupled images of solid samples have been produced in through transmission, which demonstrate that an improved lateral resolution can result when a comparison is made to images from conventional air-coupled testing. [ABSTRACT FROM AUTHOR]

Illustrates the use of air-coupled ultrasonic tomography for the measurement of a high-temperature flame from a natural gas burner using capacitative ultrasonic transducers. Measurement of propagation times; Effects of the variations in temperature within the flame; Application of the tomographic reconstruction approach.

Presents a study which fabricated surface micromachined, capacitive ultrasonic transducers using the complementary metal oxide semiconductor-compatible process. Description of the device fabrication; Characterization of the devices; Testing of the micromachined devices in liquids; Conclusions.

Describes the experiments carried out using a pair of high-frequency, broadband, air-coupled capacitance transducers and various paper and card samples. Experimental method; Results of the thorough-thickness experiment; Use of air-coupled ultrasound.

Reports a study which described the construction of a pair of electrostatic air-coupled ultrasonic transducers using a membrane capable of withstanding high temperatures. Information on the injection molded ceramic; Experimental procedures of the study; Conclusions.

State that tomographic reconstruction is a method of imaging by illuminating the object in many different directions in the plane of interest, usually using X-rays or ultrasound. Two main groups of tomographic image; Use of filtered back project theorem; Equipment and experimental technique; Results of the study; How the ultrasonic imaging was performed.

Presents the use of pulsed laser source and capacitance transducer detector to record the propagation of guided waves in carbon fiber-reinforced polymer composite laminates. Comparison of experimental dispersion curves and the phase velocity predictions from a global matrix theory; Reasonable correlation between theory and experiment; Demonstration of the validity of the calculations and experimental method.

Investigates the characteristics of micromachined water-coupled ultrasonic capacitance transducers. Applications of water-coupled ultrasonic transducers; Popular element used in the transducer; Problem with conventional type of immersion transducers.