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 (dB SPL).
