The most common method is to chamber the exhaust gases. This is used by most of the OEM as it is simple and inexpensive. The sound waves hit a partition, or baffle, and bounce back into themselves or another wall to kill the sound. Effective, but this method also creates backpressure and reduces power. This is almost a parasitical power loss and is incorporated by numerous aftermarket muffler builders. It doesn't take a degree in physics to understand that while the sound wave is being interrupted, so is the pressure or power wave. Thus a power loss will incur. It also makes a very hot muffler and often is the reason for resonance inside the cockpit.