I have a complicated cylindrical geometry with a speaker mounted on somewhere middle of the rig. The one end is closed, the other is open. Sweeping the speaker frequency from 0 to 1000 Hz, I tried to find the acoustic resonance frequency of this geometry from several sensors at different location. It is found that there are some frequencies that give the local peak amplitudes. I thought those frequencies are the resonance frequencies, but when I compared the signals from two different sensors, the phase difference was neither in phase nor out of phase, but like 120 degree. This result is counter-intuitive to me because I thought the resonance always happens with the standing wave, which gives only in phase or out of phase between two sensors. If the end boundary conditions are hard or soft BCs, the resonance theoretically occurs with the standing wave pattern. So I think my open end is not actually sound soft BC, but rather have low impedance BC.
Anyway, the below is my questions:
- Does resonance always happen with the standing wave?
- What properties affect the end impedance. Is it function of temperature? Frequency?