9-11 November 2005, Busselton, Western Australia
General Design Principles for an Automotive Muffler
Day Design Pty Ltd, Acoustical Consultants, Sydney, NSW
This paper discusses the general principles of muffler design and explains the main advantages of various styles of mufflers. When designing a muffler for any application there are several functional requirements that should be considered, which include both acoustic and non-acoustical design issues as detailed in this paper.
INTRODUCTION TO AN AUTOMOTIVE
with time. They have the ability to reduce noise at various
frequencies due to the numerous chambers and changes in
geometry that the exhaust gasses are forced to pass through.
The sole purpose of an automotive muffler is to reduce
engine noise emission. If you have ever heard a car running
without a muffler you will have an appreciation for the
significant difference in noise level a muffler can make. If vehicles did not have a muffler there would be an unbearable amount of engine exhaust noise in our environment. Noise is
defined as unwanted sound.
The down side to reactive mufflers is that larger
backpressures are created, however for passenger cars where
noise emission and passenger comfort are highly valued
reactive mufflers are ideal and can be seen on most passenger vehicles on our roads today.
Sound is a pressure wave formed from pulses of alternating
high and low pressure air. In an automotive engine, pressure waves are generated when the exhaust valve repeatedly opens
and lets high-pressure gas into the exhaust system. These
pressure pulses are the sound we hear. As the engine rpm
increases so do the pressure fluctuations and therefore the
sound emitted is of a higher frequency.
All noise emitted by an automobile does not come from the
exhaust system. Other contributors to vehicle noise emission include intake noise, mechanical noise and vibration induced noise from the engine body and transmission.
The automotive muffler has to be able to allow the passage of exhaust gasses whilst restricting the transmission of sound.
There are numerous variations of the two main types of
muffler designs commonly used, namely absorptive and
reactive. Generally automotive mufflers will have both
reactive and absorptive properties.
Figure 1: Typical reactive automotive muffler
An absorptive or dissipative muffler, as shown in Figure 2,
uses absorption to reduce sound energy. Sound waves are
reduced as their energy is converted into heat in the
absorptive material. A typical absorptive muffler consists of a straight, circular and perforated pipe that is encased in a
larger steel housing. Between the perforated pipe and the
casing is a layer of sound absorptive material that absorbs
some of the pressure pulses.
The reactive or reflective mufflers use the phenomenon of
destructive interference to reduce noise. This means that they are designed so that the sound waves produced by an engine
partially cancel themselves out in the muffler. For complete destructive interference to occur a reflected pressure wave of equal amplitude and 180 degrees out of phase needs to
collide with the transmitted pressure wave. Reflections occur where there is a change in geometry or an area discontinuity. A reactive muffler, as shown in Figure 1, generally consists of a series of resonating and expansion chambers that are
designed to reduce the sound pressure level at certain
frequencies. The inlet and outlet tubes are generally offset and have perforations that allow sound pulses to scatter out in numerous directions inside a chamber resulting in destructive interference.
Reactive mufflers are used widely in car exhaust systems
where the exhaust gas flow and hence noise emission varies
Australian Acoustical Society
Figure 2: Typical absorptive automotive muffler...