Glasspack mufflers (also called 'cannons' or 'hotdogs') are straight-through design mufflers which consist of an inner perforated tube, an outer solid tube and fibreglass sound insulation between the two tubes. They often have less back pressure than original equipment mufflers, but are relatively ineffective at reducing sound levels. Another common type of muffler is the chambered muffler, which consists of a series of concentric or eccentric pipes inside the expansion chamber cavity. These pipes allow sound to travel into them and cause the sound waves to bounce off the closed, flat, ends of the pipe. These reflections partially cancel each other out, reducing the sound level.