Experimental setup
We used the same experimental setup (Fig. 3A) used by Darias
et al. (2011). To vibrate the tube we used a vibration system composed
by a function generator (GW Instek SFG-2110,) linked to anaudio amplifier (Crown XLS 202) feeding two coupled loudspeakers.
The loudspeakers were 1000WDual Coil 12 in. High Output Sub
Drivers (Lanzar MXSQW12D), and the signal fed to one of them
was 180◦ out of phase with respect to the other. Both speakers
were firmly attached to a solid medium-density fiberboard (MDF)
shelf. The shelf was fixed on a heavy steel plate with three aligning
screws. A Plexiglas plate was fixed to each speaker. Both plates
were mechanically coupled by steel rods