Sound is any disturbance that travels through an elastic medium such as air, ground, or water to be heard by the human ear. When a body moves back and forth (vibrates), the oscillation causes a disturbance of the surrounding medium, it radiates outward in straight lines in the form of a pressure wave. The pressure wave is perceived by the ear as a sound. In physics, sound is considered to be waves of vibratory motion, whether or not the human ear hears them.
Sound waves are generated by and vibrating body. When something is moved, like a violin sting, the sting moving pushes molecules of air into a clump, which also creates a space. The clump's energy of motion causes it to move into other air molecules. Then air molecule move back into the empty space. Sound waves cannot travel in a vacuum because there is no air.
The human ear can hear most sounds. We can generally hear between 20 and 20,000 vibrations per second, but changes for each individual. The sound's volume is measured in decibels. The number of vibrations per second is called the frequency. Sounds below 20 vibrations per second are called subsonic. Sounds above 20,000 vibrations per second are called an ultrasonic.
A sound wave is usually represent graphically by a wavy, horizontal line. For a Picture see the bottom of page 6. The upper part of the wave is called the crest. It represents condensation. The lower part is called the trough. It represents the rarefaction. The graph is not a picture of the wave; it is just a representation of the wave. The wavelength, length of the sound wave, is measured as the distance from one point of the greatest condensation to the next following it or from one point to the next corresponding point on another set of waves.
The velocity of the sound is not constant; it varies because of temperature. Sounds travel better in warmer temperatures. Sound travels more slowly in gases than in liquids and more slowly in liquids then in solids.