This is similar to why emergency vehicle sirens are not monotonal. If a police car only blared a clear single tone, your ears would stop being able to tell where the sound was coming from after a few seconds.<br><br>This is because your ears use the time of arrival difference between the ears to figure out which direction a sound has come from. So, after a few seconds of listening to a single tone, the ears can no longer tell which ear is hearing it first, because there are no sound "imperfections", if you will, to measure a difference in arrival time between. Obviously natural echoes, terrain morphing and the doppler effect would help, but it would still be somewhat more difficult to tell.<br><br>Using two tones means your ears can "lock on" to the sound a lot easier because there is a constant, clear change that your ears can measure as a difference between them.