A train moves towards a stationary observer with speed $34 \mathrm{m} {\mathrm{s}}^{-1}$. The train sounds a whistle and its frequency registered by the observer is ${\mathrm{f}}_1$. If the train's speed is reduced to $17 \mathrm{m} {\mathrm{s}}^{-1}$, the frequency registered is ${\mathrm{f}}_2$ . If the speed of sound is $340 \mathrm{m} {\mathrm{s}}^{-1}$ then the ratio $\frac{{\mathrm{f}}_1}{{\mathrm{f}}_2}$ is