In pipe systems, pressure and flow fluctuations below cutoff frequency propagate as plane waves along pipes. Depending on the pipe length and propagation velocity, resonance leading to high amplitude pressure fluctuation may occur. At low pressure, cavitation is an important source of fluctuation. Beside its active role in the mechanism of noise generation, the cavitation reflects partially the incoming plane waves. This may modify the values of the eigenfrequencies of the system consisting of the pipe, the contained fluid and the vapor cavity. The influence of cavitation is experimentally investigated in a hydroacoustic resonator: a straight pipe connecting two tanks. At three quarters of the pipe length, a bluff body is placed cross flow to generate periodic vortex wake cavitation in a limited section of the pipe. The analysis of the wall pressure measurements along the hydroacoustic resonator results is performed with the help of a one-dimensional transient model of the pipe including the compliance of the cavities created in the wake of the bluff body. The results of the numerical simulations enable the determination of both the eigenvalues within the resulting system of equations and the mode shape of the pressure fluctuations corresponding to the experimental results.