In my experience, there is a difference between something like a loud harley and a loud sportsbike. With a harley, I can't easily tell which direction the sound is coming from, but with a sportsbike I can.

Frankly, I don't understand how it is that there are so many obnoxiously loud bikes on the road. I've always assumed that it has nothing to do with safety and is more a reflection of the (deliberately) inconsiderate nature of some people.