A muffler is the exhaust system of an engine, designed to reduce emissions and noise. In the case of a vehicle, it is usually located underneath the car at the rear, though freighter trucks often use a vertically mounted system behind the cab. A muffler consists of a backbox and one or more tailpipes. When a vehicle is smogged, the inspector will insert a probe inside the tailpipe to read smog emissions emanating from the engine.
A muffler helps to reduce the tremendous amount of noise that an engine produces. Although internal combustion accounts for some of this noise, most of it comes from pressure waves generated by the rapid opening and closing of the engine's valves. The muffler's backbox is internally designed to receive the pressure waves and bounce them around inside carefully designed chambers and cylinders. The shape and length of these baffles creates pressure waves of a roughly equal nature moving in opposite directions. When they collide with one another, the waves of equal but opposite amplitude cancel each other out. One drawback of this design is that it causes backpressure, which impacts performance.
A second type of muffler, known as a high performance muffler, consists of a straight steel pipe with perforations, which allows some sound canceling while creating very little backpressure. The tube is wrapped in glass insulation, protected by a steel shell, and referred to as a glass pack. This muffler was very popular in the 1960s and 1970s for souping up muscle cars like the GTO or classic older Chevy. The muffler has a characteristic explosive sound that earned it the street name cherry bomb. These mufflers are still popular for hot rods and classics.
Because a muffler is located on the undercarriage, it is often neglected. A muffler can rust out over time, rot and even fall off the vehicle. It's a good idea to take a peek at your muffler from time to time to make sure it looks sound and is securely clamped. If a muffler becomes rotted, it's time to replace it. A muffler in poor condition may keep a car from passing state-required smog tests. Many local automotive shops do this work, or if you're good with a wrench, mufflers can be purchased from parts houses and automotive chains.