Wavelet analysis of sound signal in fluid-filled viscoelastic pipes

In viscoelastic pipes, where the material properties depends on a complex bulk modulus as well as on a complex shear modulus, the sound field within the fluid is affected. Therefore, the dispersion of flexural waves occurs in the pipe, while the speed of flexural waves decreases due to the coupled fluid mass. Coupling between the pipe wall and the fluid also decreases the sound speed in the fluid. Likewise, the speed of sound in fluid is frequency-dependent, just as the group velocity of bending waves depends on the frequency. Wavelet transform of non-stationary sound signal was used to identify the frequency-dependent fluid sound speed. A time-frequency map, constructed by plotting the wavelet coefficient against the translation and scale parameters, shows an alteration in the low frequency waves. The so called fluid mode and pipe mode resonant frequencies are also clearly evident. Lastly, the impact of different pipe wall material properties is also shown. Results suggests that the wavelet transform gives in general more information from measured results.