I wonder if overlaying a beep and a white noise would have been better. It would have been obviously not just a compressed air hose, and wouldn't get lost in other white noise as much.<br><br>Trucks could have randomly assigned frequencies in case 2 were going at once, and the beep component could have been directional to reduce echoes from random places.<br><br>Or maybe they already studied that and found pure noise to be better?<br><br>I wonder how the studies handled people already knowing what to listen for?