Mufflers are designed to alter the noise emitted by your vehicle, which usually means making it quieter. The way they work depends upon whether you choose a glasspacked or chambered design, but both mitigate sound waves one way or another:
Glasspacked
Arguably the simpler design, glasspacked mufflers are packed with a sound-absorbing material, usually fiberglass but sometimes steel wool, so that sound energy is dissipated as it moves through the system.
Chambered
These mufflers work through a phenomenon called destructive interference. As sound waves move through the component, they are bounced through strategically placed tubes and chambers, which direct them in such a way that some waves cancel each other out. This means that fewer sound waves end up being emitted from the system, and the overall sound produced is quieter. The tubes and chambers inside these mufflers can be adjusted to ‘tune’ the sound emitted, offering excellent diversity.