Synthesizing the frequency-dependent sound relaxational absorption by three-frequency sound speeds in excitable gases

Abstract

Capturing the frequency-dependent sound relaxational absorption in excitable gases has the potential of wide applications in gas detection; however, it is still challenging to accurately measure it in real time. With the aid of the relationship between effective compression coefficient and acoustic relaxation process, an approach to synthesize the frequency-dependent sound relaxational absorption is proposed by using the sound speeds at three frequencies under a single pressure that are far easier to be precisely measured in real time. The proposed approach includes three steps. Firstly, the relaxation strength is calculated from the low-frequency and the high-frequency sound speeds measured at two selected frequencies; secondly, the adiabatic constant pressure relaxation time is calculated from the sound speed measured at the third frequency within the range where the sound relaxational absorption is significant; finally, the frequency-dependent sound relaxational absorption and sound speed dispersion are synthesized by the real and imaginary parts of the normalized high-frequency effective adiabatic coefficient. For gases containing CH4, CO2, CL2 and N2, the synthesized results are consistent with the experimental data. Our approach provides a real-time acoustic solution with higher accuracy and implementation convenience for gas detecting.