An investigation of sound fields based on the acousto-optic effect

Various types of transducers are nowadays capable of translating different properties of sound waves into mechanical/electrical quantities, which can afterwards be reinterpreted into acoustical ones. However, in certain applications, for example when using microphone arrays, the presence of bulk transducers can bias the acoustic measurement. Although this influence can often be either neglected at low frequencies or compensated for (typically in the form of a frequency response), the present work alternatively explores the interaction between sound and light as a means to characterize an acoustic field. This non-invasive technique is based on the so-called acousto-optic effect, i.e., the variations of the refractive index of a medium caused by density fluctuations, which follow sound pressure fluctuations. In the current study, this phenomenon is investigated in air, within the audible frequency range, and in two different measurement scenarios where the sound field is well-known: in a rectangular duct and in an anechoic room. Models for predicting the acousto-optic effect in such scenarios are derived and measurements are carried out with a laser Doppler vibrometer. The results show a fairly good agreement between the experimental and simulated data.