You are waiting for the bus when you notice Christopher Lloyd and Michael J. Fox down the street. They are travelling on hoverboards at a speed of \(9.0 m/s\) towards you with Christopher in front of Michael. The two of them seem to be doing some sort of science experiment as they are each carrying a sound generator that is set to the same frequency. You notice that during the time when you are in between Christopher and Michael (such that Christopher is travelling away from you and Michael is travelling towards you), you hear a beat with a frequency \(3.3 Hz\). What is the frequency of their sound generator? (Assume the speed of sound is \(343 m/s\).)