When a high velocity jet is directed toward the mouth of a tube closed at the downstream end, large, nonlinear flow oscillations may occur within the tube. Shock waves propagate up and down the tube and generate strong heating of the tube walls. The device is called a Hartmann–Sprenger tube (in short, H–S tube). It has been proposed by several investigators to strengthen the shock waves and consequently the heating effects by tapering the tube. Conical and multistepped configurations have been investigated, but also tubes having a sudden area contraction (single step). This latter geometry produces remarkable pressure and thermal heating amplification compared to a constant area tube. The paper presents theoretical and experimental results obtained for the single‐step H–S tube. It is shown that several oscillation modes can exist. These are separated by intervals of instability. The influence of geometrical parameters (upstream to downstream cavity diameter and length ratios) and of jet Mach number on oscillation mode is investigated.