I'm with Joe. Last time I checked base was non-directional (thus the need for only one speaker in a home theater setup) and treble tones are directional. The lower pitched the tone the more likely it is to travel in all directions.
I don't agree on the loud pipes save lives thing though. I think they just like the pipes and are looking for some practical excuse for them. The only time I hear them coming is when they are right next to me. That is usually when they decide to rev the engine and scare the crap out of me as they are cutting lanes in traffic. The douche bag attitude of these folks is that everyone in traffic should be constantly checking their mirrors for bikes cutting lanes and move over or they will rev the bike right next to your car.
Sorry, but so many of these folks are insanely self centered.