Energy in Sound Waves
Wave motion of all kinds possesses energy of varying degrees. Electromagnetic waves from the Sun, for instance, bring us the enormous quantities of energy that are necessary for life on Earth. By comparison, the energy in sound is extremely small. That's because producing sound requires only a small amount of energy. For example, 10,000,000 people talking at the same time would produce sound energy equal only to the energy needed to light a common flashlight. Hearing is possible only because of our remarkably sensitive ears. Only the most sensitive microphone can detect sound that is softer than what we can hear.
Sound energy dissipates to thermal energy while sound travels in air. For waves of higher frequency, the sound energy is transformed into internal energy more rapidly than for waves of lower frequencies. As a result, sound of low frequencies will travel farther through air than sound of higher frequencies. That's why the foghorns of ships are of a low frequency.
Forced Vibrations
If we strike an unmounted tuning fork, the sound from it may be rather faint. If we hold the same fork against a table after striking it, the sound is louder. This is because the table is forced to vibrate, and, with its larger surface, the table will set more air in motion. The table will be forced into vibration by a fork of any frequency. This is a case of forced vibration.
The mechanism in a music box is mounted on a sounding board. Without the sounding board, the sound the music-box mechanism produces is barely audible. Sounding boards are important in all stringed musical instruments.
Natural Frequency
When someone drops a wrench on a concrete floor, we are not likely to mistake its sound for that of a baseball bat hitting the floor. This is because the two objects vibrate differently when they are struck. Tap a wrench and the vibrations it makes are different from the vibrations of a baseball bat, or of anything else. Any object composed of an elastic material when disturbed will vibrate at its own special set of frequencies, which together form its special sound. We speak of an object's natural frequency, which depends on such factors as the elasticity and shape of the object. Bells and tuning forks, of course, vibrate at their own characteristic frequencies. And, interestingly enough, most things from planets to atoms and almost everything else in between have a springiness to them and vibrate at one or more natural frequencies.
Resonance
When the frequency of forced vibrations on an object matches the object's natural frequency, a dramatic increase in amplitude occurs. This phenomenon is called resonance. Literally, resonance means "resounding," or "sounding again." Putty doesn't resonate because it isn't elastic, and a dropped handker- chief is too limp. In order for something to resonate, it needs a force to pull it back to its starting position and enough energy to keep it vibrating.
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