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