Logarithm of the ratio of two values of a field quantity, usually amplitude. Levels are expressed in different ways: \(L_{F}=\ln (\frac{F}{F_{0}})\ N_{\text{p}}\), where \(N_{\text{p}}\) is the symbol for the unit @N04106@ coherent with the SI, or \(L_{F}=20\ \log _{10}(\frac{F}{F_{0}})\ \mathrm{dB}\), where \(\mathrm{dB}\) is the symbol for the unit @D01546@.