So i finally got around to test the rubens tube more. What I did this time, was noting where are the peaks and the bottoms of the waves when a 400hz is played into the tube.  Then I mounted cheap-ass microphones from ebay into three of the positions. Played the 400hz signal into the tube again and inspected the microphone output on an oscilloscope. I was very glad to find out that a clear difference between these points could measured!  This measn that I should be able to digitize the tube.