Acoustic propagation in corrugated pipes offers interesting features in presence of a mean flow, which are investigated experimentally in this paper. The scattering matrix of the corrugated section is measured with and without a mean flow. The analysis of the results is based on the extraction of the wavenumbers of propagation within the corrugated pipe. Without flow, a small decrease of the speed of sound and a slight increase of the attenuation are observed. In presence of mean flow, oscillations against the frequency of both the real part and the imaginary part of the wavenumber occur. For sufficiently large flow velocities, the oscillation caused by the sound-flow interaction is such that the acoustic waves are amplified by the flow. The Strouhal number for maximum gain shows a small dependency on the Reynolds number, and converges to values of 0.4–0.5 depending on the presence of rounded or sharp upstream edges of the cavities. Non-linear effects begin to appear for an acoustic amplitude such that the acoustic velocity is about 1% of the flow velocity.