PaulRB:
This thread twists and turns! First it was how to measure frequency of vibration but the sensor could not be attached to the pipe. Later it turns out to be allowed to attach things to the pipe. Now the question is how to induce the vibrations in the first place, and a motor can't be used. What next?
Question is still the same - which is to measure frequency.
(I have a solution for tube oscillation, which will eventually work.)