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Vibrations in a Column of Air

Here is a 50 cm long tube filled with air. It is open at one end and, at the other end, there is speaker that emits a pure sound wth a frequency of 680 Hz.  This frequency corresponds to the resonant frequency of second rank harmonic for this tube. We see here the stationary (or standing) wave characteristic of this mode. We say that the column of air is resonating.

The layers of air that never move are located at the nodes of vibration.

The layers of air that vibrate with the greatest amplitude are located at the antinodes of the vibration.

The curve s(x,t) measures the horizontal displacement  of a layer of air with abcissa x.

A microphone is not sensitive to displacement, but to pressure. We see that a node of vibration corresponds to an antinode of pressure, and vice versa. An antinode of vibration is a node of pressure.

Click on "microphone" and then drag the mike around in the interior of the tube.

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