Exhaust pipes are far more than simple tubes; they're highly-engineered systems designed to ferry gases away from the engine, reduce emissions and create a melodic exhaust note. Resonators are one tool that engineers use to accomplish the latter, and are an important part of any sound control strategy.
Resonators are functionally similar to mufflers but exist for a different purpose. A resonator is essentially a muffler designed to reduce sound only in a certain frequency range instead of reducing overall sound volume. Think of your exhaust system like you would a stereo system; if the primary muffler is a volume knob, then the resonator is a graphic equalizer.
Expansion chambers are essentially just wide spots on the exhaust tube; they're the simplest and cheapest type of resonator, and are fairly effective at controlling certain frequencies. Fill an expansion chamber with fiberglass or steel wool packing and you have a dissipative muffler or "cherry bomb."
Helmholtz resonators use a series of tubes, cavities and chambers to bounce sound around and cancel out certain frequencies. Helmholtz resonators are extremely effective but have a very narrow range of frequency control, which makes them best suited as a supplemental sound control device.
