One important characteristic of mufflers is how much backpressure they produce. Because of all of the turns and holes the exhaust has to go through, mufflers like those in the previous section produce a fairly high backpressure. This subtracts a little from the power of the engine.
There are other types of mufflers that can reduce backpressure. One type, sometimes called a glass pack or a cherry bomb, uses only absorption to reduce the sound. On a muffler like this, the exhaust goes straight through a pipe that is perforated with holes. Surrounding this pipe is a layer of glass insulation that absorbs some of the pressure pulses. A steel housing surrounds the insulation.
These mufflers produce much less restriction, but don't reduce the sound level as much as conventional mufflers.