JEE Main · 2026 · 28 Jan 2026 · Shift 2 · Waves & Sound

The two forks give $8$ beats in $2$ s, a beat frequency of $4$ Hz, so $|f_A-f_B|=4$ Hz. With $f_B=380$ Hz, $f_A$ is either $384$ Hz or $376$ Hz. Loading fork A with wax lowers its frequency, after which the beat frequency falls to $4$ beats in $2$ s, i.e. $2$ Hz. If $f_A$ were $376$ Hz, lowering it would increase $|f_A-f_B|$ beyond $4$ Hz, contradicting the observation. If $f_A=384$ Hz, lowering it brings $f_A$ towards $380$ Hz and reduces the beat frequency, as observed. Hence the original frequency of fork A is $384$ Hz.