# Engineering Acoustics/Specific application-automobile muffler


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**General information about Automobile muffler**

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## Introduction

A muffler is a part of the exhaust system on an automobile that plays a
vital role. It needs to have modes that are located away from the
frequencies that the engine operates at, whether the engine be idling or
running at the maximum amount of revolutions per second.A muffler that
affects an automobile in a negative way is one that causes noise or
discomfort while the car engine is running.Inside a muffler, you\'ll
find a deceptively simple set of tubes with some holes in them. These
tubes and chambers are actually as finely tuned as a musical instrument.
They are designed to reflect the sound waves produced by the engine in
such a way that they partially cancel themselves out.( cited from
www.howstuffworks.com )

It is very important to have it on the automobile. The legal limit for
exhaust noise in the state of California is 95 dB (A) - CA. V.C. 27151
.Without a muffler the typical car exhaust noise would exceed 110 dB.A
conventional car muffler is capable of limiting noise to about 90 dB.
The active-noise canceling muffler enables cancellation of exhaust noise
to a wide range of frequencies.

## The Configuration of A automobile muffler

![](ProRacer.jpg "ProRacer.jpg")

## How Does automobile muffler function?

### General Concept

The simple and main part of designing the automobile muffler is to use
the low-pass filter. It typically makes use of the change of the cross
section area which can be made as a chamber to filter or reduce the
sound wave which the engine produced.

### Low-Pass Filter

A low-pass filter is a circuit that passes low frequency signals but
stops the high frequency signals. Once the low pass filter is set by the
user at a specific cutoff frequency, all frequencies lower than that
will be passed through the filter, while higher frequencies will be
attenuated in amplitude. This circuit is made up of passive components
(resistor, capacitors and inductors) capable of accomplishing this
objective.
![](inductive_law_pass_filter.jpg "inductive_law_pass_filter.jpg")

the formula to be used:

### Human ear sound reaction feature

When these pressure pulses reach your ear, the eardrum vibrates back and
forth. Your brain interprets this motion as sound. Two main
characteristics of the wave determine how we perceive the sound:

1.sound wave frequency. 2.air wave pressure amplitude.

It turns out that it is possible to add two or more sound waves together
and get less sound.

### Description of the muffler to cancel the noise

The key thing about sound waves is that the result at your ear is the
sum of all the sound waves hitting your ear at that time. If you are
listening to a band, even though you may hear several distinct sources
of sound, the pressure waves hitting your ear drum all add together, so
your ear drum only feels one pressure at any given moment. Now comes the
cool part: It is possible to produce a sound wave that is exactly the
opposite of another wave. This is the basis for those noise-canceling
headphones you may have seen. Take a look at the figure below. The wave
on top and the second wave are both pure tones. If the two waves are in
phase, they add up to a wave with the same frequency but twice the
amplitude. This is called constructive interference. But, if they are
exactly out of phase, they add up to zero. This is called destructive
interference. At the time when the first wave is at its maximum
pressure, the second wave is at its minimum. If both of these waves hit
your ear drum at the same time, you would not hear anything because the
two waves always add up to zero.

### Benefits of an Active Noise-Canceling Muffler

1\. By using an active muffler the exhaust noise can be easily tuned,
amplified, or nearly eliminated.

2\. The backpressure of a conventional muffler can be essentially
eliminated, thus increasing engine performance and efficiency.

3\. By increasing engine efficiency and performance, less fuel will be
used and the emissions will be reduced.

## Absorptive muffler

![](Open-twister.gif "Open-twister.gif")

### Lined ducts

It can be regarded as simplest form of absorptive muffler. Attach
absorptive material to the bare walls of the duct.( in car that is the
exhaustion tube) The attenuation performance improves with the thickness
of absorptive material.

The attenuation curves like a skewed bell. Increase the thickness of the
wall will get the lower maximum attenuation frequency. For higher
frequency though, thinner absorbent layers are effective, but the large
gap allows noise to pass directly along. Thin layers and narrow passages
are therefore more effective at high frequencies. For good absorption
over the widest frequency range, thick absorbent layers and narrow
passages are best.

### Parallel and block-line-of-sight baffles

Divide the duct into several channels or turn the flow channels so that
there is no direct line-of-sight through the baffles. Frequently the
materials line on the channels. Attenuation improves with the thickness
of absorptive material and length of the baffle. Lined bends can be used
to provide a greater attenuation and attenuate best at high frequency.
Comparatively, at low frequency attenuation can be increased by adding
thicker lining.

### Plenum chambers

They are relatively large volume chambers, usually fabricated from sheet
metal, which interconnect two ducts. The interior of the chamber is
lined with absorbing material to attenuate noise in the duct. Protective
facing material may also be necessary if the temperature and velocity
conditions of the gas stream are too severe.

The performance of a plenum chamber can be improved by: 1.increase the
thickness of the absorbing lining 2.blocking the direct line of sight
from the chamber inlet to the outlet. 3.increase the cross-sectional
area of the chamber.
