Strike either tuning fork and hold it to the end of the tube. The sound intensity of the fork at the resonant frequency (480Hz) of the tube increases dramatically, as the second harmonic of the tube is excited; whereas the fork with the non-resonant frequency (384Hz) does not become significantly louder.
This illustrates the principle of resonance. One tuning fork's frequency is a multiple of the natural frequency of the air column in the tube, while the other is not.