Please help with this question Imagine an experiment to calc

Imagine an experiment to calculate the depth of a vertical, cylindrical well in the ground. When * very low frequency sound (the speed of sound in the well is 335 m/s) is directed into the well resonances are detected at two successive frequencies: 4.5 Hz and 6.3 Hz.

Solution

The beat frequency

        f = f 2 - f1

           = 6.3 Hz - 4.5 Hz

           =1.8 Hz

Assume the well as closed tube at one open end

frequency

           f = v / 4 L

          L = v / 4f

              = (335 m/s ) / 4 (1.8 Hz)

             =46.5227 m