Muffler Acoustics

7/28/2019 Muffler Acoustics

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Noise Attenuation inExhaust Systems


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The noise produced by internal combustion engines,particularly automobiles, is a major contributor toenvironmental noise pollution. In automotive design, externalnoise must be limited according to industry standards as wellas federal and state regulations. Engine exhaust is one of themain sources of sound produced by automobiles, making it a

very important aspect in vehicle design.

Exhaust systems must be engineered to attenuate sound inorder to meet required levels and sound quality while alsobalancing aspects of emissions, performance, and fueleconomy.


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In an internal combustion engine, pressure waves aregenerated when the exhaust valve repeatedly opens and letshigh-pressure gas into the exhaust system. These pressurepulses translate to audible sound. As engine rpm increasesso do the pressure fluctuations and therefore the soundemitted is of a higher frequency. The function of the muffler inan automobile exhaust system is to attenuate the noise

produced by the engines exhaust.

The two different types of mufflers are: absorptive andreactive (reflective)


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Absorptive Muffler Absorptive or dissipative mufflers use absorption to reducesound energy, this is done by converting the energy into heatusing an absorptive material, usually fiberglass or steel wool.

Consists of a straight perforated pipe that is encased by alarger housing. Between the housing and the perforated pipeis a layer of sound absorptive material that absorbs some of the sound energy as it travels through the muffler.

Create less backpressure than reactive mufflers but at thecost of less sound attenuation.

Used in performance applications

Reduce noise over the entire frequency spectrum and moreso at higher frequencies


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Absorptive Muffler


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Reactive Muffler Reactive or reflective mufflers use thephenomenon of destructive interference toattenuate sound. Destructive interferenceoccurs when a reflected sound wave of equal amplitude and 180 degrees out of phase collides with sound waves producedby the engines exhaust.

Consist of a series of resonating andexpansion chambers that are designed toreduce the sound pressure level at certainfrequencies. Inlet and outlet tubes are offsetand have perforations that allow soundwaves to scatter in different directions insidea chamber resulting in destructiveinterference.

Decrease in engine efficiency andperformance due to the backpressurecreated in the exhaust system.


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Reactive Muffler


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Active Exhaust SystemUses the principles of destructive interference coupled with atraditional passive muffler.

Sound attenuation comparable to traditional reactive systemswith increased power and fuel economy.

The design uses a U-shaped bypass pipe attached to the

original exhaust pipe. The bypass pipe has a length variablemechanism so that when two divided noises meet, the pipelength difference is adjusted to make the major component of the noise have a 180 degree phase difference, thereforecancelling the noise by destructive interference eliminating theneed of a secondary noise source such as a speaker.


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Active Exhaust SystemIn the low engine rpm range, only a passive muffler is used andin the high rpm range, where only a short length difference isrequired, the proposed system with a very low backpressure

muffler is used.

With the control of the valve at the left, the exhaust gas andnoise are fed to a conventional muffler at the bottom in a lowrpm range. In the higher rpm range, where active control isengaged, the valve is switched to drive the exhaust gas andnoise to the active muffler system at the top. The bypass pipesare composed of inner and outer tubes and actuators are usedto change the length. A simple muffler, which has the minimumbackpressure, is used to remove the remaining noise.


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Active Exhaust System


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Muffler Acoustics