Acoustic and ultrasonic techniques for condition monitoring

Ultrasound may be used to gain information about the quality of a sample in two distinct ways: by listening to the sound produced by the sample or by listening to echoes of sound coupled deliberately into the sample. The former technique is a passive technique, and is known as acoustic emission, while the latter is an active technique, and is known as nondestructive testing ultrasound. Ultrasound has been shown to be a useful tool when dealing with insulating materials. In this paper, the authors discuss particular applications of ultrasound techniques, both active and passive, to insulating structures and the benefits which may be derived from their use. The remainder of the paper is split into investigations of particular materials, and concluding remarks about the techniques