A decibel is the scaled logarithm of the ratio of a measured value with respect to a reference value. Decibels are useful because they can show a huge range of values in a small space. For instance a sound pressure scale going from 0 - 120 dB can represent sound pressures from 20 μPa (micro-pascals) to 120,000,000 μPa. This roughly represents the lowest SPL a human can hear all the way up to uncomfortably loud sounds. The generally accepted value for “Pref” in the formula shown here is 20 μPa.