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.

Assembly

So I finally got my Ruben's tube working. As there are many instructions online, I will not go into many details. Basic steps I took:

1. Drill 96 holes each 2mm in diameter and 20mm distance apart alongside the top of the pipe.
2. Weld one side of the steel pipe shut.
3. Weld two small steel pipes, 10mm in diameter and approximately 50mm in length, into  the steel pipe for connecting the gas tubes.
4. Sealed the other end of the steel tube with a surgical glove and duct tape forming a rubber membrane.
5. Connected grill gas supply to the steel pipe by splitting one gas tube coming from the pressure adapter into two and connecting each of them to the small welded-in pipes.
6. Placed a Bluetooth speaker in front of the membrane.

Tube in action

Parts

First I did some shopping and basic preparation. I got myself:

1. 2000mm length, 76mm diameter steel tube.
2. 5000mm length of gas tube.
3. Regular grill gas.
4. Gas valve pressure adapter.
5. Couple of fittings for connecting gas tube to the pipe.
6. A steel plate for sealing off one end of the steel pipe.
7. A t-connector for splitting the gas tube into two.