Using metamaterial impedance (liner) on the walls of a waveguide offers various possibilities to control and manipulate sound waves. In particular, it is possible to lower the local speed of sound. When adding a mean ow with velocity U larger than the effective speed of sound Ceff , supersonic regimes can be reached at low Mach numbers U/Co. By trapping the waves in a certain region of space, one can realize acoustic analogues of black hole. We will discuss a particular configuration of a double transition subsonic/supersonic flow that displays the analogue of the "black hole laser" effect. Due to the presence of negative energy waves, the ow can provide gain for the acoustic propagation, which leads to a whistling phenomenon at very low frequency.
Reference : A. Coutant, Y. Aurégan, V. Pagneux, "Slow sound laser in lined flow ducts" arxiv.org/abs/1904.03079 (2019)
