Sound

Sound is a wave, and like all waves, sound carries energy. Sound transmits that energy through a medium by making small regions of greater (and lesser) pressure many times per second. The fact that sound is a form of energy is most obvious when two objects collide; they lose some of their mechanical energy to sound (in the form of a crash or bang). Likewise, when the surfaces of two objects scrape together, friction makes the molecules vibrate, which creates the pressure waves that produce sound. That means that sounds coming from machinery can tell you that it's losing mechanical energy, like with grinding gears.

The vibrations of molecules can create waves of varying frequencies - this is what creates different pitches in sound. Higher frequencies lead to higher pitches, and lower frequencies to lower pitches.

Every object has a resonance frequency, which is the frequency at which the molecules vibrate the strongest. If a sound wave is perfectly tuned to match the resonance frequency, the vibrations can build up in the material until it cannot withstand the stress from the sound waves. This is what causes wine glasses to shatter when an opera singer sings, as shown in Figure 1.

Figure 1. Wine glass breaking due to resonance frequency

Speakers turn electricity into sound, like music or a phone conversation. This means that sound can be an energy service that can use electricity as the energy currency to create.

These pressure waves bouncing off of walls and other objects make the sounds less distinct and ordered (the human ear can no longer figure out what's being said after the sound bounces around). Eventually, sound turns into thermal energy, warming the system (usually air). There are exceptions, but usually this small amount of warming is generally hard to detect without special equipment.

There are many interesting things to learn about sound that are beyond the scope of Energy Education but still worth learning. A great place to start is hyperphysics or playing with the phet from the University of Colorado.

For Further Reading

• Wave
• Frequency
• Pressure
• Energy service
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