Speed of sound measurement from spontaneous Brillouin scattering

Abstract

In order to overcome the problem of poor accuracy of resonant interferometer method in the measurement of thermodynamic sound speed and hypersound speed of liquids an experimental setup for measuring the sound speed of liquids is established based on the principle of spontaneous Brillouin light scattering. A Fabry-Perot interferometer is used to filter the scattered light and a data acquisition card as well as a photon counting head is used to detect and analyze the scattered light, then a data acquisition and analysis method of scattered light is presented. This method overcomes the limitation of the signal distortion in conventional Brillouin light scattering and increases the measuring accuracy of the sound speed of liquids remarkably. The sound speed of saturated liquid CCl4 is measured in the frequency range of 308.6 to 906.2 MHz at 298.15 K. Results agree well with the data reported in the literature, and show that the experimental method is feasible. In addition, the method for measuring the ultrasonic speed is proposed by adding several free spectral ranges to the measured Brillouin frenquency-shift. The ultrasonic speed of CCl4 measured is in the frequency range of 5406.1–5521.0 MHz. It is shown that the thermodynamic sound speed does not change with the sound frequency, while the hypersound speed increases with the increase of sound frequency and it is much greater than the thermodynamic sound speed, which proves the dispersion phenomena of CCl4.