Second sound in counter flowing superfluid and normal fluid

From ordinary hydrodynamics it is known that an "entrainment" of sound occurs in a moving fluid. A similar effect is expected to take place in the hydrodynamics of helium II. A measurement has been made of the change in time of flight of a second sound pulse as a function of the relative velocity (v[lowered n] - v[lowered s]), where v[lowered n] is the normal fluid velocity and v[lowered s] is the superfluid velocity. From this measurement we may infer how the reference frame of second sound varies with the motion of the normal fluid and superfluid. The change in time of flight was measured within ±0.14 x lO[raised -6] seconds.