A couple of people over at Facebook may have seen it, but here it is for you guys to see now that I have a final graph for it.
Here it is, a 3 part horn, in audible distance form, in a graph:

The Red Line is the near sounds.
The Green Line is the far sounds.
The Blue Line is the stereo echo sounds. Stereo gives you a realistic echo. In real life, echoes do not have the Doppler effect, therefore, a stereo sound creates this effect.
A bit of tweaking is still to be done, as well as actually getting these numbers in to the game.