Exhaust sounds different in canyons or when riding with other HDs, that rumble whan bouncing in and out of harmony with the other bikes is a sound truly like rolling thunder. I think the term Jack and Joy are thinking is "Doppler effect" that change in pitch higher or lower as vehicle gets closer to you or farther from you. I also like the sound reflections when you pass sound barrier walls or metal bridge trestles with cross beams the metal scrolled grating humming underneath. I think it is kinda neat, a virtual concert goes on, especially with the half helmet. Enhanced sensations of sounds AND smells...you know your really enjoying the ride if that is what you remember, that first grove of tall trees and you here the echos reflecting back...hey its' Monday, and I am stoked...tomorrow is another motorcycle ride away...ENJOY!