Standing Waves (original) (raw)
Pressure and Displacement, Air Column
The standing waves associated with resonance in air columns have been discussed mainly in terms of the displacement of air in the columns. They can also be visualized in terms of the pressure variations in the column. A node for displacement is always an antinode for pressure and vice versa, as illustrated below. When the air is constrained to a node, the air motion will be alternately squeezing toward that point and expanding away from it, causing the pressure variation to be at a maximum. This view of resonant modes in terms of pressure waves makes it easier to see why the mouthpiece end of a wind instrument is a node for the resonances. For example, the clarinet is acoustically a closed-end cylindrical air column because the mouthpiece end acts as a pressure antinode.
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One way to demonstrate standing waves in an air column is by stroking a metal rod to set up the longitudinal standing wave in the rod. If a disc is placed on the end of the rod, it can set up standing waves in the air column. The motion of the air at the antinodes is sufficient to move cork dust to produce a pattern in the dust. The illustration at left is part of a Kundt's tube designed to produce the standing waves. |
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