Optimization of ultrasonic transducers

The so-called KLM-model for ultrasonic transducers is employed to optimize transducer design. Some new performance characteristics are defined which change monotonically with design parameters. These characteristics are based on the area of the envelope of the echo waveform produced by the transducer and of the corresponding amplitude spectrum. The efficiency of the transducer is defined by the round trip energy factor. The performance characteristics are used in a composite performance measure, which is then employed as a criterion in the optimization procedure. Two transducers are investigated: for medical imaging purposes and for spectral analysis of clinical echograms. The influence of electrical matching, backing impedance, matching layer impedance, bond line thickness and series induction on the optimized transducers is investigated.     

A calculation scheme for the optimum design of ultrasonic transducers

A calculation scheme that establishes the transfer functionof a thickness-mode piezoelectric transducer is described. A computer program enables flexible manipulation of the design. Parameters that can be inserted are the properties of the piezo-material, the quarter-wavelength layer(s), the backing load and the electrical tuning and matching.Optimization criteria such as maximum bandwidth, optimum dynamic range, minimum pulse duration and minimum round-trip insertion loss (or maximum efficiency) are discussed.     

Piezoelectric multilayer transducers for ultrasonic pulse compression

This paper describes piezoelectric multilayer transducers for application in ultrasonic pulse compression systems. The transducers are constructed in such a way as to produce binary pulse sequences, in particular Barker-coded sequences. This is achieved by the polarization pattern of the active layers. Pulse compression is effected without any electronic circuitry by using a receiver transducer with a pattern corresponding to that of the transmitter.     

Efficiency of face-plated underwater acoustic transducers

The possibility of obtaining the efficiency of simple disc transducers, broad-banded by use of face-plates, from input electrical conductance measurements under air-loading is explored. For an air-backed ceramic disc with a Perspex face-plate it was found that the main limitation to the accuracy of efficiency estimates is due to departures from uniformity of the face-plate thickness.     

The performance of thick piezoelectric transducers as wide-band ultrasonic detectors

A Q-switched Nd:YAG laser has been used to generate reproducible acoustic displacements in metallic samples. The acoustic waveforms were initially detected by an absolute displacement-sensitive, capacitance transducer. The capacitance transducer was then replaced in turn by two piezoelectric ceramic transducers of different thickness and diameter. Comparison of waveforms from the two types of detector indicate that thick piezoelectric transducers are useful in the detection of fast rise-time displacement waveforms at metal surfaces. Their performance is optimized by making them as thick as possible, with a small area of contact with the surface.     

Acoustic impedance matching of medical ultrasound transducers

Ultrasonic transducers for pulse-echo systems are studied both theoretically and experimentally. For the theoretical calculations the Mason model for thickness-mode disc transducers with and without backing and matching layers is used. By building several of the theoretically investigated transducer configurations it is shown that theory and experiment agree well. Thus the properties of a transducer can be predicted to a good approximation before its experimental realization. To find transducers with good sensitivity and short pulses, the pulse shape and frequency response for the following classes of transducers were studied both theoretically and experimentally: transducers with backing only, transducers with heavy backing and front matching layers, and air-backed transducers with front matching layers.     

A polyvinylidene fluoride needle hydrophone for ultrasonic applications

A new type of piezoelectric polymer hydrophone for the frequency range above 1 MHz is described, which consists of a needle, coated with a thin layer of polyvinylidene fluoride. The sensitive area is a small polarized zone of the polymeric material around the needle point. Special properties, caused by the constructive design of this ‘needle hydrophone’, are its non-directivity in a wide angular range, and high spatial resolution of the ultrasonic sound field pattern. Construction and performance are compared with other types of piezoelectric polymer hydrophones. Some possible applications are discussed.     

Automatic system for dynamic control of resonance in high power and high Q ultrasonic transducers

This paper describes the fundamentals, design criteria and electronic structure of a new frequency control system to keep permanently at resonance high Q ultrasonic transducers whose mechanical resonance band may vary within wide limits under normal operating conditions. The procedure developed is based on keeping constant at its zero value the phase of the motional admittance of the transducer by automatically and instantaneously adjusting the frequency of the driving signal provided by a voltage controlled oscillator. The characteristics of this system, especially the fact that the transducer is not an integral part of the feed-back loop of the oscillatory circuit and the frequency tracking mechanism does not depend directly on the magnitude of the motional variables of the transducer, offer some advantages in construction and performance with respect to the conventional motional positive feed-back systems.     

A programmable pulse generator for piezoelectric multielement transducers

A pulse generator for multielement ultrasonic transducers has been developed with digital control of pulse timing and firing. It can optimize matching to the probe characteristics, and can consequently maximize the amplitude of the received echo.     

A Teflon pressure-transducer-specimen bond for ultrasonic measurements over a wide temperature range

A major limitation for the application of megahertz ultrasonic techniques to the study of the properties of solids is the problem of finding transducer-specimen bonds useful over a wide temperature range. A Teflon pressure bond has been developed and tested. It appears to overcome most of the problems, and promises to allow for many studies previously more difficult or impracticable.     

Design of thermistor probes for measurement of ultrasound intensity distributions

Thermistors coated with absorbing material provide small omnidirectional probes for measurement of ultrasound intensity distributions. They are simple and inexpensive to construct and can be made with dimensions less than 1 mm. The use of various glues, varnishes and resins for the ultrasound absorbing coating is considered. Heat is generated by shear viscosity in the neighbourhood of the boundary between the thermistor and coating material. The rate of temperature rise over the first one or two hundred milliseconds after the ultrasound is switched on is a reasonably linear function of intensity over the range used in physiotherapy.     

The influence of pulser parameters on the transmission response of piezoelectric transducers

A model is presented for the analysis of thickness-mode piezoelectric transmitters under practical operating conditions. The systems feedback approach is employed to clearly isolate the individual quantities relating to electromechanical interaction within the system. As a result, important parameters are highlighted and their influence on transducer response is demonstrated using a number of simulation diagrams. Wherever possible, performance is related to practical excitation devices and a summary of their relevant properties is included.     

Comparative study of wide-band ultrasonic transducers

The relative performance of several types of ultrasonic transducer are assessed using reproducible acoustic transients generated from Q-switched Nd:YAG laser pulses incident on an aluminium alloy sample. A laser interferometer, a capacitance transducer, two types of electromagnetic acoustic transducers (EMATs), and a broad-band piezoelectric transducer are examined as detectors. The comparison includes a study of their rise-times, and typical signal-to-noise ratios. In the case of the interferometer and capacitance transducer, displacement measurements are shown to be highly consistent with theory.     

Effect of bevelling on the thickness vibration of high-frequency Pb(ZrTi)O3 ceramic transducers

The effects of bevelling on the thickness vibration of Pb(ZrTi)O₃ piezoelectric-ceramic disc transducers for generating high-frequency ultrasonic power were studied by measuring electrical characteristics and displacement distributions. A laser heterodyne system was employed for the contact-free measurement of vibration. It was shown for a wide range of diameter-to-thickness ratios that bevelling circular edges was effective in reducing the coupled radial vibrations and enhancing the uniform thickness vibration. The vibration characteristics required for generating high-frequency ultrasound were substantially improved by slight bevelling. These results were accounted for by energy trapping of thickness vibration and by the consequent decrease in the equivalent mass of the transducers.     

Development of methods of precise ultrasonic measurements in small volumes of liquids

The application of ultrasonic methods to investigation of problems of molecular physics and molecular biology is greatly limited by the necessity of precise measurements of ultrasound velocity and absorption in small volumes of liquids. Fixed path interferometric methods are the most adequate for such measurements in the low MHz region.In this paper disadvantages of the present interferometric methods of ultrasonic measurements are considered and new resonator cells and a simple device for such measurements are described. Accuracies of 10^?4% for the velocity change and to better than 1% for the attenuation change, at frequencies of about 7 MHz, are obtained by the device described. The volumes of the resonator cells vary within 0.1 to 0.8 ml depending on the purpose of investigation.     

A new ultrasonic technique for the measurement of liquid level

A new design of pulse-echo ultrasonic liquid level gauge is described in which the ultrasonic pulse (here a Rayleigh or a Lamb wave) travels down a metal bar or strip towards the liquid surface, where, through a double mode-conversion process and a special reflecting structure, a strong echo is generated which travels back up the bar to the receiving transducer.The geometry of the special reflecting structure is outlined, and it is shown how this affects the basic accuracy of measurement. Inaccuracies arising from other effects like temperature variation, viscosity and poor signal-to-noise ratio are also discussed. Some mechanical variants, like using the wall of the tank in place of the metal bar carrying the ultrasonic wave and the use of corner reflectors as part of the reflecting structure, are also considered.A note in the Appendix briefly describes one method which has been used successfully for the generation of Rayleigh and Lamb waves.     

A new method for the demodulation of ultrasonic signals for cross-correlation flowmeters

Cross-correlation flowmeters using clamp-on ultrasonic transducers are particularly attractive for use in hostile or abrasive fluids. However, a phase difference is frequently found to exist between the envelopes of the received ultrasonic signals, which causes the correlation peak to become degraded, distorted, or even inverted. In this paper, an explanation for the phase errors is offered, and a new demodulation method, which eliminates these errors, is described.