Sound pressure level
= Sound pressure level
{title2=$L_p=20\log_{10}(p_{\rm rms}/p_0)$}
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 $10\log_{10}$.