“Pipe organ” air-coupled broad bandwidth transducer

Air-coupled transducers are used to conduct fast non-contact inspections in NDT. Normally, the bandwidth of a conventional transducer can be enhanced, but with a cost to its sensitivity. However, low sensitivity is very disadvantageous in air-coupled devices. This paper presents a methodology for improving the bandwidth of an air-coupled micro-machined ultrasonic transducer (MUT) without sensitivity loss by connecting a number of resonating pipes of various length to a cavity in the backplate. The design is inspired by the pipe organ musical instrument, where the resonant frequency (pitch) of each pipe is mainly determined by its length.

I. Introduction

Air-coupled transducers are widely used to conduct fast noncontact inspections for many materials including wood, electronic packages, aero-space carbon fiber, honeycomb structures, sandwich laminates and so on [1]. This technology can also improve the ease and the speed of non-destructive testing (NDT) [2]. The most common air-coupled transducers are piezoelectric bulk and micro-machined transducers (MUT). Many research studies [1]–[3] indicate that compared with piezoelectric devices, MUTs have a greater bandwidth and are better coupled to air, but have a lower sensitivity. Moreover, the bandwidth of a conventional piezoelectric bulk transducer can also be further enhanced by a backing layer, but with a cost to its sensitivity [1], [2]. The aim is therefore to design high sensitivity devices for air-coupled NDT. This paper presents a methodology for improving the bandwidth of air-coupled MUT transducers without sensitivity loss by using a novel design of backplate.