Ultrasonic transducers for wideband attenuation measurements

An application of ultrasound presently being investigated at the Council for Mineral Technology {MINTEK) is the measurement of the concentration of graphite particles in suspension in an agitated pulp. The principle of the measurement is based on the attenuation characteristics of the medium over a frequency range extending from 0.1 MHz to 10 MHz. This wide range and the highly attenuating medium (typically 3 dB/cm) require therefore that the ultrasonic transducers exhibit efficient wideband characteristics. Piezoceramic transducers vibrating in thickness-mode are the most suitable to generate ultrasound at these frequencies. However, their high acoustic impedances relative to their liquid loads make them inherently narrowband and inefficient. To overcome these undesirable features, impedance matching techniques are used whereby the impedance mismatch between a transducer and its load is reduced. In this thesis, various electrical and acoustic impedance matching schemes were implemented on a number of conunercial PZTSA transducers which were then evaluated in terms of bandwidth and efficiency. At 1 MHz, the 3-dB bandwidth of a PZTSA transducer operating in pulse-echo mode in water was improved from 0. 1 MHz for the unmat.ched case to 0. 83 MHz when 2 quarter-wave matching sections were bonded to the radiating surf ace and an electrical matching section was inserted at the electrical port. The resulting 2-way Insertion Loss over the pass band was 16 dB. Similarly, the 3-dB bandwidth of a 500 kHz PZTSA transducer improved from 50 kHz to 138 kHz with an accompanying Insertion Loss of 12 dB when both electrical and acoustic impedance matching were implemented. Certain radiation characteristics of thickness-mode transducers such as beam directivity and radiation resistance were investigated as functions of the "diameter to wavelength" ratio, D/)... , of the radiating transducer. Maximum radiation resistance (and therefore maximum acoustic coupling) was observed when D/ .A =13. The same ratio also produced the best performance in terms of directivity and sensitivity to parallelism between transmitter and reflector {or receiver). A comprehensive study of the resonance spectra of flexural, in-plane and thickness-mode vibrations of thin disks was also performed using various materials commonly used in transducer construction. The elastic constants of the materials were then determined by three different methods, each method making use of a different resonance spectrum.

Grantor dc:publisher.institution

Department of Electrical Engineering

Year dc:date.issued

1987

Author dc:creator

• Mayer, J Bertrand

Advisor dc:contributor.advisor

• Bell, J F W

Language dc:language.iso

eng

Handle dc:identifier.uri

http://hdl.handle.net/11427/8359

OAI identifier oai:identifier

oai:open.uct.ac.za:11427/8359