Here's some information from Motordyne:
Trying to make their products as quiet as they can, Motordyne used a method by altering the exhaust harmonics of the test pipes which enhanced the performance and sound of the pipe that is now called Advanced Resonance Tuning which provides 15 to 20 ft lbs of more low end torque than your conventional test pipes.
The ART pipes increase power and torque without the drastic increase in loudness normally associated with test pipes. These provide the quietest, smoothest, deepest sound while greatly reducing or eliminating rasp and drone that you would normally here in the cabin.