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When a long metal rod is held in the center with one hand and stroked with the other, a high-pitched sound is produced.
Explanation: Every material has a set of natural vibrations. When you hold the aluminum rod in the center and stroke it with rosin coated fingers, your fingers slip and stick as they slide along the rod. This causes the rod to start moving with one of its natural frequencies of vibration – a half-wave tone. As you continue to stroke the rod, the vibrations increase and the loudness increases. The node is a place on the object that is not moving. An anti-node is a place on the object with maximum vibration. Touching a node will not dampen the sound; touching an anti-node will.
You can calculate the frequency of the note you are hearing by dividing the velocity of sound in the rod (ca. 5100 m/s for aluminum) by the wavelength which is twice the length of the rod (remember: a rod held in the center vibrates at a half-wave frequency). For the 24-inch aluminum rod, the frequency is about 4200 cycles per sec or Hz.
Frequency = velocity / wavelength = (5100 m/s) / (2 x .61 m) = 4200 Hz
Holding the rod at a point from the end that is a quarter of its length, will produce a higher pitched sound, a full-wave tone, about 8400 Hz.
While holding the rod at one node, touch the other node and notice that the sound is not dampened.
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National Science Education Standards
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