SOUND

Sound is a disturbance in a media in the form of a wave.  Sounds have frequency, which we perceive as pitch, and amplitude, which we perceive as volume.  The frequency of a sound is equal to its velocity divided by its wavelength.  There are two types of  waves, a transverse wave in which the particles vibrate perpendicular to the direction of the wave, and longitudinal waves in which particles vibrate parallel to the direction of the wave.  Sound is a longitudinal wave, traveling as a series of compressions and rarefactions in the air.

When two waves overlap, they interfere with each other.  The net amplitude is the algebraic sum of the two waves.  If two compressions line up, their amplitudes magnify each other, and we hear a louder sound than either of the two sounds on their own.  If a compression and a rarefaction line up, they cancel each other out, and we hear a quieter sound. 

For a simulation on sound waves, visit the simulation at our parent site Mr. Fizzix Longitudinal vs. Transverse Wave

DOPPLER EFFECT

When waves are emitted by a moving object, they are compressed or spread out by the movement of the object.  The waves in front of a moving object are compressed by the moving object, increasing their frequency.  Those behind the object are spread out by the object moving away, decreasing the frequency.  This is why approaching objects make a higher pitched noise, and objects moving away make deeper noises.

For a simulation of the Doppler effect, visit http://www.goodbuddy.net/budco/sware/prods/wavenrg/Doppler/DplrSnd.htm

SOUND BARRIER

In room temperature air, sound moves at approximately 343 m/s.  This velocity is called the "sound barrier".  As the object emitting the sound moves faster, the sound waves ahead of it are more compressed.  Once the object reaches the 343 m/s sound barrier, the sound waves literally are compressed into one wave. The compressions of the sound waves line up and are added together into a single sound; this is called constructive interference.  The amplitude of the new sound is tremendous because it is the sum of many sounds, and causes a high-energy wave, known to most as a sonic boom.

To see a video of a jet breaking the sound barrier, follow the links at http://www.wilk4.com/misc/soundbreak.htm