A metallic rod of length $1 \mathrm{m}$, Young's modulus $3\times {10}^{11} \mathrm{N} {\mathrm{m}}^{-2}$ and density $7.5\times {10}^2 \mathrm{kg} {\mathrm{m}}^{-3}$ is clamped at its middle. Longitudinal stationary vibrations are produced in the rod with total number of displacement nodes equal to $3$. The frequency of vibrations is