Abstract
In liquid helium II second-sound waves with rectangular profiles degenerate ultimately into triangular form and the speed of the wave front is no longer constant. This effect gives rise to a non-linear dependence of the shock speed on the initial wave amplitude, but its magnitude is insufficient to account for the experimental results above a certain limited range of amplitudes. However, the degeneration into triangular form is the dominant effect in the dependence of the wave speed on decreasing pulse width, at constant amplitude, and here the authors analysis agrees well with the experimental results.

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