Previous experiments on collapsed-tube self-excited oscillation have shown that the resistance to flow downstream of the tube controls whether a given operating point is stable, oscillatory or divergent. This paper extends the documentation of those experiments by reporting the results of measuring the frequency-dependent complex impedance to flow up- and downstream of the collapsed tube. Both up- and downstream there was a shallow resonant impedance peak. To test whether such resonance might have governed the frequency of self-excited oscillation through entrainment, both resonant frequencies were increased approximately three times, maintaining as exactly as possible the same fluid inertia and flow resistance, and approximately the same sharpness of resonant peak. Whereas the frequency of the original resonances corresponded to the broad fundamental-frequency range of the dominant mode to self-excited oscillation, the altered resonances did not attract self-excited oscillations at those frequencies. With the increased resonant frequency downstream, the transient collapse and recovery once per cycle of the self-excited oscillation gave rise to rapidly decaying ringing, clearly visible in pressure and cross-sectional area recordings. Nevertheless, the higher-frequency system was not conducive to entrained oscillation. This suggests, in turn, that entrainment was unlikely to have been responsible for setting the frequency of self-excited oscillation in the previous experiments.