Re: how to detect occurrence of standing wave by microphone in kundt tube

Well, I guess that putting the microphone at the location of a
node is not the best place to put it.  But there is no reason why you need
to put the microphone at the location of a node.  When you have tuned the
frequency of your input signal to a standing-wave frequency of the tube,
the intensity of the sound wave in the tube gets very large.  So, by
measuring the sound intensity as a function of frequency, it is easy to
find the frequencies of the standing waves.

I do not understand how you intend to use a Kundt tube to locate objects
under water.  If you want to use active sonar (i.e. emit a pulse and then
listen) to locate objects under water, the way to do it is by time of
flight.  You get directional information by using an array of detectors,
and measuring the differences in time of flight for the different
detectors.