The aim of this work was to investigate theoretically and experimentally the fact that it is possible to create sound by using the nonlinearity in air. A speaker was built verifying the theory that if at least two high frequencies at high amplitude is propagating through a nonlinear medium, for example air, the difference frequency of these two is created in mid air. Measurements and simulations were performed on what the pressure distribution from a ultrasonic transducer looks like.
Often when one thinks about creating sound one thinks about a speaker that uses a piston movement to create the sound. This way of creating sound is not the only way. The audible sound can be created in midair by using ultrasound and the non-linearity in air.
Source: Blekinge Institute of Technology
Author: Fredin, Johan