Optical Studies of Acoustic Fields

Obtaining detailed views of complex acoustic and ultrasonic fields is made difficult by the need to use hydrophones and mechanically scan these through the field. The use of optical techniques to measure fields has several advantages: a hydrophone, that can perturb the field, is not required and an optical beam can potentially be scanned much faster than a physical device. Three different optical techniques for measuring fields will be considered, two of which exploit the acousto-optic effect. The first approach is the use of conventional schlieren techniques, which will be illustrated with examples of ultrasound scattering from structures and the observation of resonances in cylindrical shells. This approach is easily extended to more complex situations such as the chaotic fields of a stadium cavity. More recently the availability of Laser Doppler Vibrometers (LDVs) has enabled the fields of ultrasonic transducers to be studied by tomographic techniques or by measuring the surface vibration of the transducer and numerically predicting the field based on the surface velocity. A range of resulting images will be used to illustrate the performance of these various approaches to the optical measurement of ultrasonic fields.