Ultrasound attenuation dependence on air compression or expansion processes

Abstract

In this work variation of ultrasonic attenuation coefficient is analyzed in terms of air compression or expansion processes. In closed spaces changing air volume, the ultrasound attenuation coefficient depends on thermodynamic processes which occur during the air volume change. Two limiting cases are possible: 1) if the change of air volume is very slow or intensive heat exchange occurs between the system and surrounding environments, so the system stays in a thermodynamic equilibrium; therefore an isothermal process occurs; 2) if the change of air volume is very fast or the working environment has a good thermal insulation, so the heat exchange between the system and the surrounding environment does not occur. In this case an adiabatic process is presented. The attenuation coefficient of ultrasound varies very differently depending on the process (isothermal or adiabatic) that occurs during the change of air volume. In particular, these differences occur when measurements are carried out in a frequency range above 500 kHz during air compression. Initial relative air humidity has high influence on the ultrasonic signals attenuation. Carrying out ultrasonic measurements in such systems, due to reliability of the measurements it is necessary to evaluate thermodynamic process and ultrasound attenuation variation during the process. Oversaturated water vapour may occur during the measurement process, therefore the measurement conditions become more complicated.