SILENCERS AND MUFFLERS

By,ARUN B(06M606),

IV BE SW MECHANICAL.

Acoustic silencers or mufflers are devices designed to

attenuate and/or absorb air-borne sound waves

propagated in a flowing medium.

Typical applications include air handling systems,

exhaust and intake units, pumps, compressors etc..

Most powerful weapons available to the acoustical

engineer.

Despite the terms and myriad of configurations,the

device can be broken into three fundamental groups.

INTRODUCTION:

Most powerful weapons available to the acoustical engineer.

Despite the terms and myriad of configurations , the device can be broken into three fundamental groups.

The Dynamic Insertion Loss - DIL - is the difference between the sound power or intensity levels measured in the same point of the duct work before and after the insertion of the silencer.

INTRODUCTION:

Absorptive/dissipative silencers- contain

fibrous materials and depend on dissipation

of acoustical energy.

Reactive silencers – self destruction as the

basic noise reduction mechanism.

Dispersive/diffuser silencers-diffusion

action.

CLASSIFICATION:

DISSIPATIVE SILENCERS:

Dissipative silencers employ a sound absorbing material

to attenuate the sound waves.

The thickness of acoustical linings should be selected

based on the predominant frequency of the noise.

The incident sound energy is partially transformed into

heat by causing motion in the fibers during its passage

through the material.

DISSIPATIVE SILENCERS:

Absorptive silencers include,

Lined duct attenuators,

Packaged cylindrical and rectangular attenuators,

Acoustic louvers and

Lined plenum chambers.

DISSIPATIVE SILENCERS:

LINED DUCTS:

Lining the inside walls of a duct with sound

absorbing materials is one example of a

dissipative silencer.

Lined ducts are commonly used to attenuate fan

noise and duct transmission between adjoining

spaces for noise control in buildings.

The material may either be bonded to the duct

wall or held in place using special fasteners.

LINED DUCTS:

For better insertion loss performance, cylindrical and

rectangular splitter silencers are use.

Addition of splitters into an airway can greatly

improve the static insertion loss performance of a

silencer, but this is achieved at the expense of a

greater pressure drop.

Parallel or annular baffles are common types of

absorptive silencers.

RECTANGULAR SILENCERS:

CYLINDRICAL SILENCERS:

The acoustical performance of such silencers is a

function of the length and thickness of the baffle

sections, space between the baffles, and absorption

coefficients of the material used in the baffles.

Higher frequencies are attenuated more than lower

frequencies.

In cylindrical silencers, the splitter consists of a

tubular baffle in the middle of a lined cylindrical

duct.

RECTANGULAR AND CYLINDRICAL SILENCERS:

REACTIVE OR REFLECTIVE SILENCERS:

Reactive silencers consist of one or more non-

dissipative elements arranged either in

parallel or in series.

Typical elements include expansion chambers,

side-branch resonators, and perforated tubes.

The primary function of reactive silencers is to

reflect sound waves toward the source.

REACTIVE OR REFLECTIVE SILENCERS:

Energy is dissipated in the extended flow

path resulting from internal reflections, and

by absorption at the source.

Reactive silencers are used for low-

frequency applications.

In general reactive silencers are used for

fixed speed machinery.

REACTIVE SILENCERS:

A reactive or cavity resonator is a vessel containing

a volume of air that is connected to a noise source

such as a piping system through a neck.

When sound wave is propagated along the pipe, the

air in the vessel expands and contracts.

By proper design of the area and length of the neck,

and volume of the chamber, sound wave

cancellation can be obtained, thereby reducing the

tone.

Reactive resonators:

Dispersive or diffuser silencers are pressure-

reducing devices that fit in the downstream of an

orifice.

They are also called pneumatic silencers.

Dispersive silencers act to drop the gas pressure,

hence reducing the velocity and straighten the

flow, reducing the turbulence which is the prime

cause of aerodynamically induced noise.

DISPERSIVE OR DIFFUSER SILENCERS:

DIFFUSER SILENCERS:

Since the jet sound intensity is proportional to

the eighth power of the jet velocity, a small

reduction in velocity can reduce the noise

levels substantially.

These silencers usually have the form of a

slotted or perforated metal cage or a

covering of porous materials around the exit

of an air line.

DISPERSIVE OR DIFFUSER SILENCERS:

Active control of noise is the process of reducing existing

noise by the introduction of additional noise.

The additional noise may be introduced by any one or a

combination of different mechanisms.

The most common mechanism is that of noise

cancellation, where the introduced control sound is anti-

phase to the original sound and cancellation results.

A reference or input microphone monitors the primary

noise and converts the sound pressure to an electrical

signal.

ACTIVE NOISE CONTROL:

This signal is then sent to a controller where a new

signal is generated.

This control signal activates a control source

(loudspeaker) which produces destructive

interference with the undesired noise.

An error sensor detects the residual sound after

control and feedback an error signal to the controller,

which adjusts or modifies the control signal to

optimize the cancellation process.

ACTIVE NOISE CONTROL:

Active noise control:

THANK YOU