Auto questions: how mufflers work

A guide to the way mufflers function in automoblies, tips on fixing and maintaining.

On all automobiles with gasoline or diesel powered engines, the by-product of internal combustion are exhaust gases, which are expelled from the engine when the exhaust valves open. These exhaust gases then pass through a network of sealed exhaust pipes. On most modern street-driven automobiles, manufacturers have these exhaust pipes lead into a muffler(s) before exiting to the atmosphere usually at a point by the rear of the vehicle.

So why are mufflers used and how do they work?

Due to the high energy created during the combustion process in the engine, exhaust gases when they first enter the exhaust pipes will contain a variety of sound frequencies most of which are not very acoustically pleasing to the ear. Mufflers are therefore used on vehicles as a way to 'muffle' or reduce the sound level of the exhaust gases leaving the engine. They can also be used to tune the exhaust note (like a musical instrument) by canceling out undesirable sounds leaving behind only those which are most pleasant.

The three most common types of mufflers used on the majority of automobiles today are an Absorption muffler, a Diffusion muffler and a Hemholtz chamber.

Absorption Muffler: This design is most commonly used by performance muffler manufacturers as it gives the least resistance to the exhaust gases that pass through. Absorption mufflers work by using a straight perforated tube that's en-cased or wrapped in sound deadening material. In this design, the exhaust gases are able to pass through with almost no resistance while the various frequencies of sounds contained in the gases are absorbed by the material around the perforated tube. Typical materials used for sound deadening are fiberglass, stainless steel mesh and ceramics.

Diffusion Muffler: A diffusion muffler splits the flow of the exhaust gases up into a series of different paths using a series of plates and tubes called baffles. These baffles cause the exhaust to follow a longer path when passing through the muffler. As the exhaust twists its way through these series of tubes and plates, it looses velocity (energy) and in so doing, several sound frequencies are lost and reflected. This in turn tunes the exhaust note that exits the muffler.

Hemholtz Chamber: Also popularly called a Cancellation Muffler or Resonator. This form of muffler derives its name from the German Scientist, Herman Hemholtz, who made several discoveries in the field of acoustics and medicine. The Hemholtz chamber is designed very carefully to be a specific volume and length. As the sounds in the exhaust gases enter on one end, a resonant frequency is established in the chamber that causes all other sounds at that specific frequency to be cancelled out. As this type of muffler only cancels out one frequency in the exhaust, they are rarely used by themselves, but in conjunction with a Diffusion or Absorption Muffler.

Using these three main types of mufflers, the sound frequencies in exhaust gases are tuned, making the final sound that leaves the vehicle be at a desired noise level and quality for both drivers and pedestrians.