The setup: 1.5m transparent tube about 10 cm diameter, set vertically so
that water can be introduced into or drained from the bottom. Loudspeaker
mounted over the open upper end; small microphone just below the
loudspeaker, connected to oscilloscope. The usual exercise consists of
changing the water level to find the resonant lengths of the air column
tones of different frequency, thus finding the speed of sound and the end
correction. The oscilloscope is used to observe when the sound from the
tube is loudest.
One notices that with the water just a centimetre or two below
the resonant position, the position apparently not affected by the
frequency, the trace goes flat, suggesting destructive interference. I have
a student looking for a detailed explanation, but he has run out of ideas.