Intensity of a wave: $I = \frac{1}{2}\rho v \omega^2 A^2 \propto A^2$. When two waves of amplitudes $A_1$ and $A_2$ interfere: $I_{\max} = (A_1+A_2)^2$ and $I_{\min} = (A_1-A_2)^2$ (proportional). Ratio: $I_{\max}/I_{\min} = (A_1+A_2)^2/(A_1-A_2)^2$. In decibels: sound level $\beta = 10\log_{10}(I/I_0)$ where $I_0 = 10^{-12}$ W/m$^2$.