US Patent 7343819 Flow meter for gaseous media

For the purpose of metering a flow of gaseous media, there is arranged a perturbation body in a measuring chamber which has a medium flowing through it, and an ultrasonic excitation signal is transmitted through the measuring chamber transverse to the direction of flow and at a point downstream of the perturbation body. The ultrasonic excitation signal after having traversed the measuring chamber is captured together with the ultrasonic excitation signal transmitted as structure-borne noise. The captured signal is squared first and then demodulated with a filter. An indication for the flow through the measuring chamber is derived from the frequency of the demodulated signal.