Two identical wire under same tension emits a note of frequency 100 Hz. If tension in one of the wire is changed by 4%, the beat frequency is

$\mathrm{v}=\frac{1}{2l}\sqrt{\frac{\mathrm{T}}{\mathrm{\mu }}}$

$\frac{\mathrm{dv}}{\mathrm{v}}=\frac{1}{2}\frac{\mathrm{dT}}{\mathrm{T}}$

When tension in one of the strings is increased by 4%, frequency changes by 2%

i.e frequency becomes $100\pm 2\mathrm{Hz}$

$\therefore$Beat frequency = 2 Hz.