Imaging of low frequency sound fields in air

Imaging of acoustic fields has been of interest for a long time. To see with sound waves instead of ordinary light waves is an intriguing concept. Naturally, humans are not capable of doing image processing with acoustic data like some kinds of animals.

However, the idea of imaging with sound waves in animals was not recognized until 1793 by the Italian scientist Spallanzani.^ The idea was first used to build an imaging device called SONAR during World War I. Later many devices for imaging through optically opaque objects were invented. The motivations for acoustic imaging are the capabilities to see through opaque objects and test some mechanical properties of sample materials by introducing mechanical perturbations into them. Generally, there are two basic methods used to image acoustic fields: utilizing piezoelectric or optical devices.

Devices that employ piezoelectric transducers are very compact and efficient. They are also very sensitive. The high sensitivity property is very important for medical applications in which a high level of insonification, the input strength, is not allowed. On the other hand, a device that employs a laser beam can be quite bulky and less sensitive. The advantages the laser device has over piezoelectric devices are the variable operating frequency and the ability to detect phase and amplitude at the same time.

This thesis will describe a system that uses a laser beam to image an acoustic field in air. Usually when we talk about sound or acoustic waves, we think of audio frequencies, (in the range of 20 - 20,000 Hz). However, in this thesis we will not put that limit on any acoustic wave, i.e., an acoustic field can be in any frequency range.