Decibel (dB) - The measuring unit of sound pressure, and hence loudness. The decibel is a numerical ratio between the sound pressure of a given sound and the sound pressure of a reference sound (usually .0002 microbar). Common decibel levels encountered vary from the rustling of grass (15 dB) to conversation (50 dB), to live rock groups (110 dB) to jet plane engines at close range (130 dB). See Threshold of Hearing.