Sound Sound is a longitudinal wave. The source of a sound wave is something vibrating (your vocal cords, some part of a musical instrument, mosquito’s wings, a stereo speaker etc). The typical medium for sound is air, but sound can also propagate through liquids (water) and solids as well. Sound waves are repeating “zones” of compression and rarefaction (expansion) caused by the vibrating object vibrating out and squeezing the air molecules together, then vibrating in and pulling them apart.

Remember, sound travels out in three dimensions from the source Remember, sound travels out in three dimensions from the source. This is difficult to show in a two dimensional picture

Notice that the zones of compression are an increase in atmospheric pressure, and the zones of rarefaction are a decrease in atmospheric pressure. At the equilibrium position, the air pressure is normal air pressure (1.10 x 10^5 Pa), not zero.

As these low and high pressure zones, created by the vibrating object, hit your ear drum, your ear drum starts to vibrate at the same frequency. These vibrations are interpreted as “sound” by our brain. The amplitude of a sound wave is the maximum amount of difference between normal air pressure and the higher or lower air pressure in a zone of compression or rarefaction. The wavelength of a sound wave would be the distance between the middle of a compression and the middle of the next compression. Or the distance between the middle of a rarefaction and the middle of the next rarefaction. The length of either of the two red lines would be the wavelength of the sound wave.

For a sound wave: The amplitude determines the loudness of the sound. Greater amplitude means a louder sound. Also, the louder the sound, the more energy a sound wave carries. The frequency determines the tone or pitch of the sound. Greater frequency means a higher pitch.