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9 Natural Frequency

Natural frequency is the frequency at which a material vibrates when hit, plucked, strummed, or somehow set into motion. When a tuning fork is struck, it vibrates at a specific frequency: its natural frequency. As a tuning fork vibrates, it causes the air around itself to vibrate, which produces the sound waves you hear. A simple motion, such as rubbing the rim of a glass, can cause it to vibrate at its natural frequency and produce sound.

See for Yourself

Materials

 stemmed glass (this will work best if the glass is thin)

 dish detergent

 small bowl of water

What to Do

1 Remove excess oil from your hands and clean the rim of the glass by washing with dish detergent and rinsing well. Dry your hands and the stemmed glass thoroughly.

2 Place the glass on a table.

3 Hold the base of the glass against the table with your hand.

4 Wet the index finger of your free hand with water and move your wet finger in one direction around the rim of the glass pressing gently.

FIG 1

What Happened?

The glass starts to “sing.” Washing your hands removes any oil that might act as a slippery lubricant. Rubbing a wet finger around the rim causes the glass to vibrate. Due to friction, your finger skips and pulls at the glass as it moves around the rim. Just in the way a tuning fork begins vibrating when struck, this irregular touching on the glass rim actually acts like tiny taps that cause the glass to begin vibrating. In turn, the air inside and outside the bowl of the glass is struck by these vibrations and begins to move back and forth in a wavelike pattern. These sound waves spread out in all directions from the vibrating glass. A musical tone can be heard. The pitch of the sound you hear is due to the natural frequency of the glass.