Two identical whistles, each sounding at $340\ \text{Hz}$, stand still beside a straight railway track, some distance apart. A passenger on a train moving at $10\ \text{m/s}$ between them, from one whistle towards the other, hears both. The air is still and the speed of sound is $340\ \text{m/s}$. The number of beats per second the passenger hears is ____.