Logarithm of the ratio of a @P04792@ to a reference @P04792@. Levels are expressed in different ways: \(L_{P}=\frac{1}{2}\ \ln (\frac{P}{P_{0}})\ \mathrm{Np}\), where \(\mathrm{Np}\) is the symbol for the unit @N04106@ coherent with the SI, or \(L_{P}=10\ \log _{10}(\frac{P}{P_{0}})\ \mathrm{dB}\), where \(\mathrm{dB}\) is the symbol for the unit @D01546@.