Focusing of Airborne Sound by Heated Wires

In this paper, we discuss the mechanism of focusing airborne ultrasound using heated wires. The heated wire generated a radially stratified temperature field surrounding the wire. The temperature field subsequently caused both the sound speed in the air and the mass density of the air to become radially stratified. This radial profile causes the impinging plane ultrasonic wave to form two strong beams in the forward direction, with a very strong shadow region. When multiple such wires are placed into a grid, the beams from adjacent wires form a sharply focused region. In this paper, experiment details are discussed. The analysis of single and multiple scattering of a planar acoustic wave by such fields are performed using experimentally measured temperature field, and compared with the experimentally measured acoustic field.