A train moves towards a stationary observer with speed $34 \mathrm{~m} / \mathrm{s}$. The train sounds a whistle and its frequency registered by the observer is $f_1$. If the speed of the train is reduced to $17 \mathrm{~m} / \mathrm{s}$, the frequency registered is $f_2$. If speed of sound is $340 \mathrm{~m} / \mathrm{s}$, then the ratio $\frac{f_1}{f_2}$ is