  We consider the propagation of sound in a waveguide with an impedance wall.
In the low frequency regime, the first effect of the impedance is to decrease
the propagation speed of acoustic waves. Therefore, a flow in the duct can
exceed the wave propagation speed at low Mach numbers, making it effectively
supersonic. We analyze a setup where the impedance along the wall varies such
that the duct is supersonic then subsonic in a finite region and supersonic
again. In this specific configuration, the subsonic region act as a resonant
cavity, and triggers a laser-like instability. We show that the instability is
highly subwavelength. Besides, if the subsonic region is small enough, the
instability is static. We also analyze the effect of a shear flow layer near
the impedance wall. Although its presence significantly alter the instability,
its main properties are maintained.
