"The Doppler effect … is the change in frequency of a wave … for an observer moving relative to its source … It is commonly heard when a vehicle sounding a siren or horn approaches, passes, and recedes from an observer. Compared to the emitted frequency, the received frequency is higher during the approach, identical at the instant of passing by, and lower during the recession.”
In other words, the deep, throaty roar of loud pipes isn’t really very deep or throaty until the vehicle has passed the listener. "
Another , interesting point to note:
Now, consider the definitive Motorcycle Accidents In-Depth Study from Europe, in which researchers found the vast majority of motorcycle collisions come from the front of the bike. That’s bad news for a motorcyclist, if he or she is relying on loud pipes as an Distant Early Warning system. The Doppler Effect means your exhaust could be creating a dangerous false confidence; by the time the car driver knows you’re there, it may be too late.
So there is Pros and Cons on loud pipes. I just keep to a level where all are comfortable with a smile when i pass them.
*Pilots and Racing Car drivers will vouch for this Doppler Effect.