The shortest length of open-air column (open at both ends) for the first resonance sound is the distance between node and another node position, which is 1/2 λ. The shortest length of air column for one side closed for the first resonance sound is the distance between node and anti-node position, which is 1/4 λ. When comparing the lengths of open air-column (open at one end) and open air-column (open at both ends), the length of open air-column (open at both ends) is longer than the length of open air-column (open at one end).