Most objects have a natural frequency—a rate of vibration at which they tend to oscillate. For example, the sound wave emitted by a tuning fork is the same each time it is struck. Some objects have a set of natural frequencies; rather than producing sound waves at a single frequency, they create a more complex sound.

When two objects have the same natural frequency, the vibration of one of the objects can set the other object in motion as well. For example, if one tuning fork is struck, and a second tuning fork placed nearby has the same natural frequency, it will also start to vibrate. The sound wave produced by the first tuning fork travels through the air and induces the second tuning fork to vibrate; this is an example of resonance.

Many musical instruments use the principle of resonance. For example, the air column inside a trumpet has a natural frequency that can be changed by opening and closing the valves of the instrument. When a musician blows into the trumpet, this forces the air inside to vibrate at the frequency produced by the musician's lips. The trumpeter changes the way he or she blows into the instrument so that the frequencies produced match the natural frequencies of the air column in the trumpet; the resulting resonance produces the clear sounds associated with the instrument.

Resonance causes the amplitude of any vibration to increase. For example, when pushing a child on a swing, properly timing the pushes to the natural frequency of the child's swinging will cause her to swing higher. Similarly, when large numbers of people walk across a bridge, they can amplify the swaying motion if they walk in synchronization with the natural frequency of the bridge's sway. Likewise, during an earthquake, if the frequency of the ground movement matches the natural frequency of a particular building, resonance occurs and that building will shake more violently than other buildings.

Most materials can be stretched or stressed to a certain point, known as their elastic limit. Because resonance causes the amplitude of vibration to increase, the oscillations may exceed the elastic limit of the material and damage it—for example, buildings may fall or bridges may break. Even resonance produced by sound waves can cause a material to break, such as when a glass goblet is shattered by sound. While it is unlikely that a singer could produce a sound that is loud and pure enough to break a glass, it is possible to use equipment to produce such a sound at exactly the natural frequency of the glass. The resonance produced by directing the sound at the glass distorts the material beyond its elastic limit and causes it to shatter.

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