Resonators are part of the exhaust system and are designed to absorb a lot of the engine's noise at a certain resonant frequency. Without one, the sound exiting the exhaust is raspy and piercing, sound qualities that even the best mufflers can't handle.

Also unlike mufflers, resonators don't have chambers, and they're usually found close to the engine. When present on a vehicle, the exhaust gas passes through the catalytic converter just before getting to the resonator.