Baffled mufflers are generally used in performance applications that require high flow with minimal disruption. They are calibrated to cancel out certain frequencies of sound waves, while allowing others to escape. Many times these muffler types are chosen specifically for their ability to cancel out an engine's high-pitched whine and emphasise its more aggressive, lower-pitched registers. This stands in sharp contrast to fibreglass packing mufflers, which absorb all sound indiscriminately.
In order to understand how muffler baffles work, one must first understand the differences between pressure waves and fluid flow. A fluid follows the path of least pressure resistance to flow to a point of zero resistance.
A pressure wave is a harmonic vibration that passes through a fluid and is unaffected by the movement of the fluid vector; it travels only in straight lines until it meets some external resistance. A good metaphor would be that of a flashlight beam in a hurricane. No matter how hard the wind blows, the beam of light will never change until it dissipates or reflects off of something.
The inside of a baffled muffler is consciously designed to reflect high-frequency sound waves back onto themselves. It does this by employing a series of angled plates, usually in a chevron pattern in the centre of the muffler that allow the exhaust to flow smoothly, but reflect the waves back into the wall. Large areas are intentionally left open to allow the exhaust gases to expand, and to allow the longer waved sound to pass through unhindered.
Muffler Sound Tuning
Every turn inside of a muffler is designed to cancel out a certain sound frequency, which is dictated by the distance from one turn to the next. A competent sound engineer will include a number of these turns inside the muffler, tuning it to deliver the highest volume of sound in the lowest bass register possible.
In order to duct the exhaust through the path of least resistance without being unduly slowed by the baffles, the exhaust must be chambered to provide several opportunities for expansion. Once the exhaust stream has been slowed sufficiently, it is less affected by the sharp angles and less aerodynamic features of the intermediate baffles.
Baffled vs. Packed
Baffled mufflers are generally lighter than fibreglass-packing types and are better geared to performance applications. Some straight-through packed designs flow better than their baffled counterparts. But the best of them are in fact a hybrid design, as they include a few baffles within the fibreglass itself to bias the exhaust note toward the bass register.