Sound travels through materials as a wave of pressure. As a sound wave passes through the air, molecules in some regions are temporarily pushed closer together (higher pressure) while adjacent molecules are pulled farther apart (lower pressure). (You can visualize this by stretching out a slinky and then quickly bringing one end closer to the other to see a pulse travel from one end to the other.) Sound energy is the energy associated with the vibrations of sound waves. Although we cannot use sound energy to power our cars or light our homes, we can use sound energy to learn about our surroundings. The simplest and most obvious use of sound energy is for hearing. Humans can hear frequencies between about 20 Hz and 20,000 Hz. Sound waves with frequencies less than 20Hz (i.e. too low-pitched for humans to hear) are called "infrasonic" sound. Infrasonic sounds can be produced by earthquakes, volcanoes and avalanches. Thus, scientists can monitor infrasound to learn more about these processes, and potentially provide early warnings just before a volcanic eruption. Another way that we use sound waves to learn about our surroundings is SONAR (short for Sound Navigation And Ranging), which is used to map objects on the ocean floor. SONAR works by sending out a pulse of sound and then measuring how long for that sound to bounce off an object and return to the source. That time can then be converted to a distance by knowing the speed of sound.

Sound energy is typically not used for electrical power or for other human energy needs because the amount of energy that can be gained from sound is quite small. Energy is the amount of work that can be performed by a given force, system, object, or anything else capable of performing work. Work is simply defined as the ability to cause change in a system; this can involve anything from a change in location to a change in heat energy. The amount of work that can be performed by common, day-to-day sounds is quite small, so sound is not often thought of in terms of the energy it contains. Sound energy does, however, exist as the vibrational waves of sound cause a change in energy. The change, however, is very small. Because sound is not like other forms of energy, it is also given an entirely different unit of energy. Rather than measuring sound in typical units of energy, such as joules, scientists and others tend to measure it in terms of pressure and intensity using units such as Pascal and Decibel. Sound measurements are, by their very nature, relative to other sounds that cause more or less pressure.

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