In this study, we visualized the superposition principle of sound waves in a both-closed-end (BCE) resonance tube. The resonance tube is constructed using an acrylic tube with 2.64 cm internal diameter and 150 cm length, polystyrene beads to illustrate the positions of displacement nodes and antinodes and two identical detachable speakers as source of sound waves at both ends of the tube. The superposition principle of sound waves in a BCE tube is performed for two conditions: speakers at both ends are in-phase and 180° out-of-phase with each other. The findings indicate that the superposition principle of sound waves in a BCE tube changes the modes of harmonic series in the tube depending on the phase of speakers. The harmonic series in BCE tubes occur at the even mode (n = 2, 4, 6...) for in-phase speakers and the odd mode (n = 1, 3, 5...) for out-of-phase speakers. The experimental values of harmonic frequencies differ by less than 6.09% compared to the theoretical values, confirming that the experiment can be used to visualize the superposition principle of sound waves in a resonance tube.