Sound pressure level ($L_p=20{\log }_{10}(p_{\mathrm{rms}}/p_0)$)

The logarithmic level of root-mean-square sound pressure relative to a reference, conventionally $20\mu \mathrm{Pa}$ in air. A pressure ratio $r$ changes level by $20{\log }_{10}r$ decibels. The factor twenty follows from the square dependence of acoustic intensity on pressure in a fixed medium, combined with the power-level convention $10{\log }_{10}$.