A deciBel is a relative unit of measurement expressing the ratio of two values of power or root power in a logarithmic scale. When using measures of **power** the level in decibels can be expressed as: $ L_{P} = 10 \log_{10}\left( \frac{P}{P_{0}} \right) $ When using measures of **root-power** such as RMS, current, voltage, or sound pressure: $ L_{p} = 20 \log_{10} \left( \frac{p}{p_{0}} \right) $ The [[sound pressure level (SPL)]] is an example of a root-power measure that is commonly expressed in decibels. In this case, the reference of 0 dB is the [[threshold of hearing]] at 1000 Hz.