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 $v_1$. If the train's speed is reduced to$17 \mathrm{m} {\mathrm{s}}^{-1}$, the frequency registered is $v_2$. If the speed of sound is $340 \mathrm{m} {\mathrm{s}}^{-1}$, then the ratio $\frac{v_1}{v_2}$ is