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## Physics Tutorials

### Sound Waves and Universal Wave Equation

Sound is a mechanical wave that spreads through a physical medium. Mechanical waves can not travel through a vacuum because they actually consist of molecules jostling and bumping into each other, sending their energy "down the line." Imagine, if you will, what happens to a set of dominoes standing on end when someone topples the first domino. Now imagine what would happen if each domino was attached to a wobbly spring. The dominoes would wave back and forth as the energy went "down the line." These kinds of waves are called longitudinal.

When something vibrates, it causes molecules of air to bump into and bounce off each other. Sound waves reach our ears as alternating high and low air pressure. These pulses of pressure on our ears have the same effect as somebody pounding on a drum. The result is we hear sound.

If the pulses are spaced far apart, our eardrums beat slowly. If the pulses are close together, our eardrums beat quickly. The distance between the pulses is called wavelength. How quickly our eardrums vibrate is called frequency (or pitch). Pulses that are close together have short wavelength and a high frequency. Pulses that are far apart have a long wavelength and a low frequency. The wavelength of sound can be measured in meters (m). The frequency of sound can be measured in pulses per second, or Hertz (Hz).

The relationship between the speed, frequency, and wavelength of sound is given by the universal wave equation:

speed = frequency x wavelength

For example, sound travels at about 340 m/s in room temperature air. If somebody strikes the "A" key near the middle of a keyboard, our ears vibrate at 440 Hz. What is the wavelength of this note?

(340 m/s) = (440 Hz) x wavelength.

So, wavelength = 340/440 = 0.77 m

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