Silencers and Mufflers 11
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Transcript of Silencers and Mufflers 11
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:
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