Question

what is the condition for the beats to be audible

Answer 1

Its frequence must be from 20 Hertz to 20 khz..

Answer 2

Beats are produced when two sound waves having same amplitude and frequency interfere with each other travelling in the same medium.

By this phenomenon soft and loud sounds are produced causing beats.

One can hear these beats when constructive wave of one sound wave interfere with destructive wave of the other sound wave and both together reach our ear.