Sound is caused by vibrations and carries energy as a wave
through all media – air, water, walls, windows etc. – but not through vacuum. In
most situations sound is a complex sum of many signals from different sources
which may interfere with each other. Like all waves, a particular sound can be
described by a combination of individual frequencies which together form a
pattern called the spectrum of the sound waves.
When sound waves reach the ear or measuring instrument the resulting change of
pressure can be measured. Sound intensity is usually expressed in decibels of sound pressure level
Source: GreenFacts, based on SCENIHR
"Potential health risks of exposure to noise from personal music players"
Decibel - Frequency (in the context of sound) - Noise