Sound Wave Propagation in Gases at Low Pressure

We have investigated sound wave propagation in helium and argon gases at low gas pressure. At pressure around 50 mtorr for argon and 30 mtorr for helium and sound source frequency of 40 kHz, the phase velocity of sound wave increases with a decrease in pressure, which agrees well with the theoretical prediction based on Navier‐Stokes equation. At a very low pressure ( 5 mtorr ) of helium gas, the wave propagation shows pseudo wave property that the phase velocity is increasing as distance from sound source increases.