High Power Ultrasonic Transducers for Use in Gases and Interphases

Abstract

The application of ultrasonic energy in gases and interphases (gas/liquid, gas/solid) has been limited by the lack of sufficiently effective transducers. The problems concerning the ultrasonic generation in gases are mainly due to the low specific acoustic impedance and the high absorption of the medium. Therefore, in order to obtain an efficient transmission of energy, it is necessary to achieve a good impedance matching between the transducer and the gas, large amplitudes of vibration and high directivity. There exist different types of ultrasonic transducers for use in gases. However, almost none of them seems to be able to generate high power with good efficiency and directivity. Recently, a new type of transducer has been developed in which these requirements have been attained. It essentially consists of a specially designed stepped plate of extensive area which oscillates flexurally, driven at its centre by a piezoelectrically activated vibrator. The main characteristics of these transducers are: electroacoustic efficiency of about 80%, beamwidth of 2 degrees (at 3 dB) and power capability over 200 W. As the loaded bandwidth is very narrow, the driving electronic device includes a new automatic system for dynamic control of resonance. Among the various possible applications, these new transducers have been successfully employed in industrial defoaming and aerosol precipitation.