Each tube, in this collection of colored plastic tubes, is cut to sound at a different note on the C scale. This is especially clear if you bang one tube and then another or two tubes at the same time. You may also listen at the end of a tube and the note it represents is magnified in the background noise. Listen to one tube and then another to notice the effect. There are caps for the tubes which allow tuning down by an octave.
These tubes are similar to the PVC tubes. It may be possible to use a tube like a didgeridoo, as well, to get a beating between the voice and tube.