Demos: 4B-02 Standing Waves in a Vertical Air Column

A variable column of air is obtained by moving a glass tube inside another, slightly larger, tube containing water. A tuning fork is held above the movable tube. When the movable tube and the tuning fork are raised and lowered, strong resonances occur when the length of the air column equals an odd number of a quarter wavelength of the tuning fork, according to the relationship:

Directions: Grasp the movable tube by the lip. Strike the tuning fork and hold it horizontally close to the opening of the tube. Slowly lift the tube and listen for strong resonances. For quantitative measurements, note the length of the column at each resonance.

Suggestions for Presentation: Discuss resonance. Remind students of the “singing in the shower” phenomenon in which certain notes are amplified. Then strike the tuning fork and note that it isn’t audible to most of the class. However, when the tube is the proper length, the sound is reinforced and is clearly audible.

Applications: Resonances in a variety of cylinders such as the tubes in musical instruments.

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