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Name: Saghir
Status: educator
Grade: 9-12
Country: India
Date: Spring 2012


Question:
In a pipe which is closed at one end, resonance take place because of superposition of incident and a reflected wave from the closed end. How does this happens in a pipe which is open at both the ends? How do or why is a reflected wave comes from?

Replies:
Saghir, Resonance takes place because of reflection at BOTH ends, whether the ends are opened or closed. Reflection of a wave is not bouncing of atoms from a surface. It is setting up a pattern of oscillations that fits the restrictions of the situation. Molecule position moves back and forth in a sound wave, but the molecules do not travel with the wave. Pressure also oscillates, but the pressure does not travel. Only the energy travels, moving from molecule to molecule and from pressure point to pressure point. The wave pattern travels through the air, but the air does not travel.

At a closed end, the molecules cannot move. The pressure oscillates, but position is constant. For this to happen with a sound wave, the wave must reflect so that position oscillations oppose each other. This then results in maximum pressure oscillation.

At an open end, the situation is reversed. There can be as much movement as desired, but the pressure cannot change. Pressure beyond the end of the pipe is determined by the outside air. For this to happen, the wave must reflect so that pressure oscillations oppose each other. This then results in maximum position oscillation.

Dr. Ken Mellendorf Physics Instructor Illinois Central College





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