That sound.

Abstract

Sound is a vibration or pressure wave that transmits energy from its source through a medium such as air or water. Sound pressure waves will alternately compress and decompress as they travel away from their source through a medium, such as air or water. The compressions and decompressions associated with sound waves are detected as changes in pressure and are then perceived as sound by a human ear or other acoustic receiver (such as a microphone or hydrophone) that may be in the path. The pressure fluctuations (amplitude or height of the sound pressure wave) determine the loudness. The decibel (dB) system is used to express the relative loudness (amplitude) of sound. The decibel system is logarithmic, which results in an exponential scale being represented as a linear scale, like the Richter scale that expresses the strength of earthquakes. Decibel is not a measuring unit, but a ratio that must be expressed using a reference (benchmark) value (See Sound in Water in Not the Same as Sound in Air section). Also, sound pressure levels are not to be confused with sound intensity (power), which is the acoustical energy emitted by a sound source over a specified unit of time. Sound pressure is what our ears hear and what sound meters measure.