A muffler is a device for reducing noise propagated by a moving stream of gas. Although mufflers are used to reduce noise from such sources as air compressors, aircraft engines, ventilating systems, and industrial plants, their most important use is the reduction of exhaust-gas noise from an internal-combustion engine in an automobile or other motor vehicle.

Every time an automobile engine fires there is an explosion. The explosions produce irregular high-pressure sound waves with peaks at various frequencies over the range from about 40 to 4,000 Hertz. These sound waves and the exhaust gases travel through the exhaust manifold and exhaust pipe to the muffler, which discharges the exhaust through a tail pipe to the atmosphere.

A typical present-day automobile muffler is shown in the cutaway diagram. Its shell is an oval steel can roughly 20 inches (50 cm) long, 10 inches (25 cm) wide, and 6 inches (15 cm) high. It contains three perforated steel tubes, two short tuning tubes, and five chambers separated by steel partitions. The combination of tubes and chambers acts as a broad-band acoustic filter that smooths sound-pressure peaks within the frequency range from 40 to 4,000 Hertz.

In the muffler illustrated most of the sound attenuation occurs in the middle chamber, where sound peaks are smoothed by causing interference of the sound waves from the perforated tubes. A single muffler reduces the sound-pressure level of the noisiest parts of the exhaust-noise spectrum roughly from 90 to 60 decibels, a thousandfold decrease. Some expensive cars have a dual exhaust and four mufflers, making the exhaust very quiet. See also noise control.

In this "reverse-flow" muffler, exhaust gases enter on the left, pass back and forth through the three midsection rubes, and exit on the right. Perforations (exposed in bottom cutaway) filter out the higher frequencies. Resonator sections at both ends contain short tuning tubes that reduce the lower frequencies.

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