What happens to a sound wave when it interferes with another sound wave having the same frequency but traveling in the opposite direction?

Question

anonymous · Answer

They form a stationary wave..(having loops).

This effect is most noticeable in musical instruments where, at various multiples of a vibrating string or air column's natural frequency, a standing wave is created, allowing harmonics to be identified. 

Nodes occur at fixed ends and anti-nodes at open ends.
 
If fixed at only one end, only odd-numbered harmonics are available.