Two sound  waves of slightly different frequencies $f_1$ and $f_2$   $(f_1>f_2)$ with zero initial phase difference, same amplitude a, travelling in the same direction superimpose:

Phenomenon of beats is always observed by human ear

Intensity of resultant wave is a constant

At a given point intensity of resultant wave varies periodically with time with maximum intensity  $4a^2$ and minimum intensity zero

A maxima appears at a time  $\frac{1}{2(f_1-f_2)}$  later (or earlier) than a minima appears