How Exhaust Baffles Workby Richard Rowe
Acoustics is a strange and fascinating science, and there's plenty of that science in your exhaust system. Exhaust baffles are simple in construction and execution, but many complex principles underpin their operation.
Ripples in a Pond
Sound waves are much like the ripples in a pond when you throw a stone in: they're pulses of pressure moving through a fluid. Cast a stone into a pond, and the force of its rapid displacement of water sends out pressure waves in the form of ripples. That much is easy to see. Look closer and you can see that the water inside those waves isn't actually moving forward. It's just swelling upward as the pressure waves pass through. The pressure waves keep going until they lose their energy and dissipate, or hit something and stop.
Imagine there was a brick wall in the middle of that pond, and you threw your stone in about three feet from it. Now, the pressure waves from the ripple will hit the wall one after the other. As each one impacts, it will add a bit of force until the accumulated pressure pushes back and makes a counter-ripple. The counter-ripple coming back from the wall will hit the incoming ripples, and stop a few of them before they get to the wall. Some ripples will make it through to the wall unscathed, but a certain number will get cancelled out on the counterwaves from the wall.
Sound works in exactly the same way when it hits a "wall," or "baffle." When soundwaves hit the baffle, some of them will form a counterwave that pushes back against the incoming soundwaves. This won't entirely cancel the sound out, but it will reduce it by bouncing some back. In practice, many mufflers will use several baffle walls with a number of staggered holes and openings, creating "chambers." These chambers can contain extra baffle walls set at angle to each other. They're all designed to bounce sound around in precise ways, causing different wavelengths to cancel each other out at different points. The result is not only a reduction in overall sound volume, but a fine-tuned output tone rich with pleasant, low-frequency notes.