[aknovaman]

Empty tube that's capped at the end. Think of how a coke bottle resonates then you blow across the open end of it. It acts just like a pipe organ. The guy who studied and calculated the math had a last name of Helmholtz. That's why they are named after him.

The exhaust sound 'pulse' enters the tube, bounces off the closed end and is 180 degrees out of phase and that cancels the noise pulse which quiets the exhaust.