# Decibel

## A logarithmic scale for audio levels

A decibel is a logarithmic scale commonly used to express differences in signal levels. It is useful in audio because it can express a wide dynamic range with relatively small numbers (or a small movement on a meter), and it more closely matches how we perceive sound.

The measurement quoted in dB describes the ratio between the quantity of two levels, the level being measured and a reference.

The absolute quantity of the signal is not relevant. This means that decibels are always comparing one quantity to another. For example, when we measure gain in dB, we are comparing the output level to the input level.

To describe an absolute value, the reference point must be known. There are different reference points defined.

dBV represents the level compared to 1 Volt RMS. 0 dBV = 1 V with no reference to impedance. dBu represents the level compared to 0.775 Volt RMS on an unloaded, open circuit.

dBm represents the power level compared to 1 mWatt. This is a level compared to 0.775 Volt RMS across a 600 Ohm load impedance.

1 dBV equals +2.2 dBu, +4 dBu equals 1.23 Volt RMS, the reference level of -10 dBV is the equivalent to a level of -7.8 dBu.

Headroom is a measure (usually in dB) of how much higher the peaks of a signal can be compared to the nominal level without clipping. That is, it compares the peak level (in volts RMS) to the nominal level (in volts RMS). The difference between the two (in dB) is the headroom.

• Infobox
• Bittner Audio
September 2011