Sound waves are produced by vibrating objects which could be:
· folds in the larynx forced to vibrate by air from the lungs
· a string which is plucked (guitar), bowed (violin) or hit (piano)
· a column of air in a wind instrument or organ pipe
· a taut membrane which is struck (drum)
· a solid object which is struck (cymbals)

Objects vibrate faster if they are smaller, thinner and more taut.
The vibrating object affects the air near it by pushing air molecules closer together, forming a region of high pressure, and then drawing back leaving a region of low pressure. The alternating regions of high and low pressure which pass through the air away from the vibrating object constitute the sound wave.
A sound wave is a longitudinal wave consisting of a series of compressions and rarefactions.

Figure 1.7 The vibrations of the loudspeaker set up a series of compressions and rarefactions
in the air. The graph of air pressure as a function of distance from the speaker is a sine curve.

Note that sound wave illustrations usually look like sine waves, as in Fig 1.7, because the horizontal displacements of the particles or the pressure variations of the medium are represented on the vertical axis, with distance or time on the horizontal axis. When a sound input is converted to an electrical signal, as in a CRO, the visual representation of the electrical signal is a sine wave.

The frequency of sound is the number of waves or the number of compressions passing a giving point in the medium each second. It is the same as the frequency of the vibration which is producing the sound. The greater the frequency the higher the pitch of the sound.

For any musical notes which are an octave apart, the higher note has twice the frequency of the lower note.

The wavelength of the sound is the distance between successive compressions, or between two successive points in phase.

The period of the sound wave is the time taken for successive compressions to pass a given point.

The amplitude of the sound is the maximum displacement of the air particles from their rest position. It is also the difference between the maximum air pressure in a sound wave and normal air pressure. The greater the amplitude, the louder the sound.