A sound wave, is introduced into a medium by an object that is vibrating. The vibrating object is the source of the disturbance that transports the medium. That vibrating object that creates the disturbance could be the vocal cords of a person, the sound board and vibrating string of a violin or guitar, the vibrating tines of a tuning fork, for example, or the vibrating diagram of a radio speaker. The particles of the medium through which the sound moves is vibrating in a back and forth motion at a given frequency. The frequency of a wave deals with to how open the particles of the medium vibrate when a wave passes through the medium. The frequency of a wave is measured as the number of complete back-and-forth vibrations of a particle of the medium per unit of time.
pitch
The sensation of a frequency is commonly referred to as the pitch of a sound. A low pitch sound corresponds to a low frequency sound wave, and a high pitch sound corresponds to a high frequency sound wave. Amazingly, many people, especially those who have been musically trained, are capable of discovering a difference in frequency between two separate sounds that is as little as 2 Hz. When two sounds with a frequency difference of greater than 7 Hz are played, most people are capable of detecting the presence of a complex wave pattern resulting from the interference and superpostion of the two sound waves. Specific sound waves when played (and heard) will create a particularly pleasant sensation when heard, are said to be consonant. Such sound waves form the basis of intervals in music. For example, any two sounds whose frequencies make a 2:1 ratio are said to be separated by an octave and result in a particularly pleasing sensation when heard. Two sound waves sound are played together if one sound has twice the frequency of the other, similarly two sounds with a frequency ratio of 5:4 are said to be separated by an interval of a third. Such sound waves also sound good when played together.