A unit used to describe sound levels. The decibel quantifies sound levels relative to some 0 dB reference. The reference level is typically set one of several ways: 1. when referring to sound pressure levels (SPL) the reference is set to the threshold of perception of an average human; 2. In digital recording, you set the level in a recording system relative to as 0 dBfs where fs refers to full scale, or the strongest signal that can be recorded without distortion, digital level meters read in negative numbers from left to right like -20dB, -12dB, -6dB, -3dB, 0dB; 3. when adjusting audio levels in audio clips in a non-linear editing system, typically 0dB for each clip is the normal level and you go plus or minus in terms of dB in order to make the clip softer or louder. Decibels are actually ratios. The ratio of the sound pressure at the threshold of hearing to the limit that ears can hear without harm is above a million. Because the power in a sound wave is proportional to the square of the pressure, the ratio of the maximum power to the minimum power is above one trillion. To deal with such a range of numbers, logarithmic units are useful: the log of a trillion is 12, so this ratio represents a difference of 120 dB. Its easier to deal with numbers between 0 dB and 120 dB to talk about the dynamic range of sound rather than a trillion. We typically work with sound adjustments in 3dB (for a small change) and 6 dB (more noticeable change) increments. Even though an increase of 3 dB represents a doubling of the intensity of the sound, we dont perceive it that way. Perception studies have shown that a 3 dB change in sound level is barely noticeable. Most listeners dont report a significant change unless its 6 dB and it requires a big change of 10 dB before the average listener hears a doubling of the sound.