A Kundt’s tube apparatus has a steel rod of length 1.0 m clamped at the centre. It is vibrated in its fundamental mode at a frequency of 2600 Hz. The lycopodium powder dispersed in the tube collects into heaps separated by 6.5 cm. Calculate the speed of sound in steel and in air.

Sol. A tuning fork produces 4 beats with a known tuning fork whose frequency = 256 Hz So the frequency of unknown tuning fork = either 256 – 4 = 252 or 256 + 4 = 260 Hz Now as the first one is load its mass/unit length increases. So, its frequency decreases. As it produces 6 beats now original frequency must be 252 Hz. 260 Hz is not possible as on decreasing the frequency the beats decrease which is not allowed here.