Acoustic antennas designed for ultrasonic imaging – generally referred to as array transducers – are used for a wide variety of applications including medical diagnostics and non-destructive testing. Their success is linked to the non-ionizing character, low cost and versatility of ultrasound and to the fact that such images and measurements contain data linked to several physical and structural parameters of the explored media. The antenna properties – both electroacoustic, such as sensitivity and bandwidth, and acoustic, i.e. radiated field – determine most of the imaging system’s performance. The exceptional improvements in image quality and development of various ultrasonic imaging modalities in the past 30 years have been made possible by the higher performance of new acoustic antennas. Technological breakthroughs, particularly in new functional materials, are at the center of such improvements.
This chapter will first review the operation principles of simple ultrasonic transducers; then more complex antennas, namely array transducers, will be considered. Material issues will be discussed, particularly those of the piezoelectric materials which are the core of most acoustic antennas. Finally, recent developments such as high frequency arrays and devices based on electrostatic forces (capacitive micro-machined ultrasonic transducers) will be addressed.