Transcript Slide 1

Acoustic Treatment –- one (Noise and Vibration Control)
MEBS 6008
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Heat radiation operation (all cooling operation)
VRV with
Heat Pump
Heat absorption operation (all heating operation)
Heat recovery operation (cooling & heating operation)
Heat absorption tendency heat recovery operation (mainly heating, part cooling operation)
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Balanced Flow
Balanced vs unbalanced air flows
4.7 cub.m
31.9 deg C
25.6 deg C
Unbalanced flow increase effectiveness of heat exchanger
Heat exchanger transfer less overall heat, why?
35.7 deg C
The following example illustrates the reasons:
4.7 cub.m.
Exhaust air flow/
supply air flow
Sensible
Effectiveness
1
0.5
90%
2
0.55
85%
3
0.6
80%
4
0.65
75%
5
0.7
70%
6
0.75
65%
Unbalanced Flow
33.2 deg C
3.3 cub.m.
25.6 deg C
35.7 deg C
4.7 cub.m.
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28.9 deg C
30.0 deg C
3
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Hot gas Bypass ?????
Do you want to ask ???
Hot gas Bypass Line
Hot gas bypass diverts hot, high-pressure refrigerant vapor
from the discharge line to low-pressure side of refrigeration system.
This added “false load” to maintain an acceptable suction pressure and
temperature.
Hot gas bypass not reduce energy consumption
(not allow the compressor to shut off at low load)
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What is Sound?
Audible emissions from vibration of molecules within an
elastic medium
Generated by vibrating surface or movement of a fluid
Through air or structure
Noise is unwanted sound
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ACOUSTICAL DESIGN OBJECTIVE
The primary objective for the acoustical design of HVAC systems and
equipment is to ensure that the acoustical environment in a given space
is not degraded.
Sound and vibration are created by a source, are transmitted along one
or more paths, and reach a receiver.
Treatments and modifications can be applied to any or all of these
elements to achieve an acceptable acoustical environment.
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CHARACTERISTICS OF SOUND
Sound is a propagating disturbance in a fluid (gas or liquid) or in a solid.
In fluid media, the disturbance travels as a longitudinal compression wave.
Airborne sound
Sound in air is called airborne sound or simply sound.
Generated by a vibrating surface or a turbulent fluid stream.
Structure borne sound
In solids, sound travels as bending waves, compression waves, torsion
waves, shear waves and others.
Sound in solids is generally called structure borne sound.
In HVAC system design, both airborne and structure borne sound
propagation are important.
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What is frequency?
Comparison of Pitch and Frequency
Frequency
-
an objective quantity
-
independent of sound-pressure level.
Pitch
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subjective quantity
-
primarily based on frequency
-
dependent on sound-pressure level and
composition
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not measured
-
described with terms like bass and tenor
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Speed of Sound and Wavelength
The speed of sound transmission depend on the
physical property of the medium.
For air, the speed varies slightly with temperature
change.
The speed of sound = a constant (344 m/s) in
consideration of narrow temperature range in
HVAC system.
Sound traveling through the air at a frequency of
200 Hz has a wavelength of 1.7 m.
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Sound
Sounds are of a broadband nature,
Sound is composed of several frequencies
and amplitudes, all generated at the same
time.
The sound energy is greater at some
frequencies than at others.
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Octave Bands
Human ear perception: sounds at frequencies 20 to 16,000 Hz,
HVAC system sounds 45 to 11,200 Hz (11,156 data points).
HVAC sounds frequencies smaller ranges (octave bands).
The highest frequency in the band is two times the lowest
frequency.
Center frequency = square root of the product of the lowest and
highest frequencies in the band.
The frequency range (45 to 11,200 Hz)  eight octave bands with
center frequencies of 63, 125, 250, 500, 1,000, 2,000, 4,000, and
8,000 Hz.
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Sound Power and Sound Pressure
Sound power
•
Acoustical energy emitted by the sound
source
•
Unaffected by the environment
•
Expressed in terms of watts (W)
Sound pressure
•
Pressure disturbance in the atmosphere
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Expressed in terms of Pascal (Pa) Can be measured
directly
•
What our ears hear and what sound meters measure
•
Affected by strength of source, surroundings, and distance
between source and receiver
•
Affected by strength of source, surroundings, and distance
between source and receiver
•
Also Affected by room is carpeted or tiled/ furnished or bare
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Sound Level
The loudest sound the human ear can hear =
1,000,000,000 x perceptible sound
A logarithmic scale is used.
A decibel is a calculated value
A decibel is based on the ratio of measured and reference
values.
It is defined as shown on the left equation
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Sound Power Level
The reference value used for calculating sound-power level
is 10-12 watts.
Sound-power level (Lw) in dB is calculated using the upper
left equation
Sound Pressure
The reference value used for calculating sound-pressure
level is 2 ×10-5 Pa.
Sound-pressure level (Lp) in dB is shown on the lower left
equation
Reference values are the threshold of hearing.
Sound power is proportional to the square of sound
pressure  multiplier 20 is used (not 10).
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Loudness Contour
The sensation of loudness = f(sound pressure &
frequency).
Each contour approximates an equal loudness
level across the frequency range shown.
Human ear is more sensitive to high frequencies
than low frequencies.
Ear’s sensitivity at a particular frequency
changes with sound-pressure level.
Loudness to human ear : 60 dB 100 Hz = 50 dB at
1,000 Hz.
Human ears are less sensitive to low-frequency sounds.
Contours are flatter at higher decibels  a more
uniform response to “loud” sounds.
Human ear not respond linearly to pressure and
frequency.
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Single-Number Rating Methods
Human ear : sound as loudness and
pitch
Electronic Sound-measuring equipment :
Sound as pressure and frequency.
Most frequent single-number descriptors
to express both the intensity and quality
of a sound :–
1) A-weighting network
2) Noise criteria (NC)
3) Room criteria (RC)
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Steps to calculate A-weighted Sound Pressure Level
A-weighted Sound Pressure Level - Example
1)
List actual sound-pressure levels for the eight
octave bands
2)
Add or subtract the decibel values represented by
the A-weighting curve.
3)
Logarithmically sum all eight octave bands to get
an overall A-weighted sound-pressure level.
A-weighted sound-pressure level is 42 dBA in
this example.
Most sound meters can automatically calculate
and display the A-weighted sound-pressure level.
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Noise criteria (NC)
The steps to calculate an NC rating:-
1) Plot the octave-band sound-pressure levels
on the NC chart.
2) The highest curve crossed by the plotted
data determines the NC rating.
Example: plotting the sound pressure levels
on the NC curves  NC-39
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Room criteria (RC) curves are similar to NC curves in that
they are used to provide a rating for sound-pressure levels
in indoor environments.
Steps to determine RC rating are as follows: 1)Plot the octave-band sound-pressure levels on the RC
chart.
2) Determine the speech interference level = arithmetic
average of the sound pressure levels in the 500 Hz, 1,000
Hz, and 2,000 Hz octave bands.
Perceptible vibration : The sound level in the octave bands
between 16 Hz and 63 Hz regions (A and B).
Region A: High probability that noise-induced vibration
levels in lightweight wall and ceiling constructions.
Anticipate audible rattles in light fixtures, doors, windows.
Region B: Noise-induced vibration levels in lightweight wall
and ceiling constructions may be felt.
Slight possibility of rattles in light fixtures, doors, windows.
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Setting a Design Goal
The first step of an acoustical design is to quantify
the goal.
Several single-number descriptors that designers
commonly use to define the acoustical design goal
for a space.
Each descriptor has its advantages its and
drawbacks.
In general, when defining the acoustical design goal
for an interior space, either an NC value or an RC
value is used.
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Setting a Design Goal
Other goals to be achieved:
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A balanced distribution of sound energy over a broad
frequency range
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No audible tonal or other characteristics such as whine,
whistle, hum, or rumble
•
No noticeable time-varying levels from beats or other system
induced aerodynamic instability
•
No fluctuations in level such as a throbbing or pulsing
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The Noise Control Ordinance deals with the following forms of noise:
1.
Noise from domestic premises and public places (often referred to as
general neighbourhood noise);
2.
Noise from construction activities (including piling);
3.
Noise from places other than domestic premises, public places or
construction sites (for example, noise from industrial or commercial
premises);
4.
Noise from intruder alarm system installed in any premises or vehicle;
5.
Noise from individual items of plant or equipment (referred to in the
Ordinance as Product Noise, for example, noise from hand-held breaker
and air compressor); and
6.
Noise emission from motor vehicles.
When defining the acoustical design goal for an outdoor environment, to
meet noise ordinance for example, the Environmental Protection Department
specified A-weighted scale.
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Environmental Protection Department, HKSARG
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Source–path–receiver model
This modeling method traces sound from the
source to the receiver.
How the sound travels between the source
and the receiver, and everything it
encounters as it travels along the way,
constitutes the path.
The receiver is the person working in the
adjacent conditioned space.
The supply duct provides one of the paths
for sound to travel from the source to the
receiver.
To specify the maximum allowable
equipment sound power not exceeding the
sound-pressure target for the space.
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Typical Sound Paths
Airborne
Sound that travels through supply ductwork, return
ductwork, or an open plenum
Can travel with or against the direction of airflow
Breakout
Sound that breaks out through the walls of the supply
or return ductwork
Transmission
Sound that travels through walls, floors, or ceilings
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Identifying Sound Sources and Paths
One piece of equipment may contain several sound
sources.
For example,a packaged rooftop air conditioner contains
supply and exhaust (or return) fans, compressors, and
condenser fans.
Sound may travel from a single source to the receiver along
multiple paths: supply airborne, supply breakout, return
airborne, and transmission through the adjacent wall.
The total sound heard = the sum of all the sounds from
various sources traveling along several paths.
Supply airborne path contributes to the total soundpressure level in the space much more than the other three
paths.
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Free field
In theory, a free field is a homogeneous, isotropic medium that is free
from boundaries.
In practice, an example of a free field over a reflecting plane would be a
large open area void of obstructions.
An ideal sound source, that is, one that radiates sound equally in all
directions, placed in a free field generates sound-pressure waves in a
spherical pattern.
At equal distances from the source, the sound pressure is same in all
directions.
As the sound waves travel farther away from the source, the area of the
sphere increases.
Doubling of the distance from the source spreads the sound over four
times as much surface area.
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Near field
The near field is an area adjacent to the source where
sound not behave as in a free field.
Most sound sources, including all HVAC equipment, do not
radiate sound in perfectly spherical waves.
This is due to the irregular shape of the equipment and
different magnitudes of sounds radiating from the various
surfaces of the equipment.
These irregularities cause pressure-wave interactions
the behavior of the sound waves unpredictable.
Sound-pressure measurements should not, therefore, be
made in the near field.
The size of the near field depends on the type of source
and dimensions of the equipment.
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Reverberant Field
A reverberant field is nearly the opposite of a free field.
Reverberant fields exist in rooms with reflective walls, floors,
and ceilings.
When a sound source is placed in an enclosed room, the
sound waves from the source bounce back and forth
between the reflective walls many times.
This can create a uniform, or diffuse, sound field.
In a perfectly reverberant room, the sound-pressure level is
equal at all points within the room.
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Semi-reverberant field
Semi-reverberant field Buildings are somewhere
between a free field and a reverberant field environment.
The walls, floor, and ceiling prevent the sound from
behaving as it would in a free field.
Some of the sound is reflected by these surfaces ( but a
portion of the sound is absorbed or transmitted).
The characteristics of the sound field change with distance
when a small sound source is placed in the center of a
room.
Close to the source, in the near field, sound measurement
is unpredictable.
Near the wall, in the reverberant field, the reflected sound
begins to add to the sound coming directly from the source.
The reduction in sound level due to the distance from the
source tends to be cancelled out by the addition of the
sound reflecting off the wall  a near-constant soundpressure level near the wall.
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Free-Field Method
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This equation is used to determine how loud a piece of equipment will be at a given
distance.
For example, the manufacturer of an aircooled chiller lists the sound-pressure level of
the chiller as 95 dB at a distance of 9.1 m from the chiller.
The sound-pressure level at 36.6 m from the chiller is 83 dB.
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Free-Field Over Reflecting Plane.
In cases of unavailability of completely free field measurements can
only be made in a free field over a reflecting plane.
That is, the sound source is placed on a hard floor or on pavement
outdoors.
Since the sound is then radiated into a hemisphere rather than a full
sphere, the relationship for Lw and Lp for a non-directional sound
source becomes
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The sound pressure level at a given location in a room
The sound power level information on a source to predict the sound pressure level at a
given location due to a source of known sound power level depends on:
(1)
(2)
(3)
(4)
room volume,
room furnishings and surface treatments,
magnitude of the sound source(s), and
distance from the sound source(s) to the point of observation.
The relationship between source sound power level & room sound pressure level :
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Insertion loss (IL)
The difference in sound pressure measured in a single
location with and without a noise-control device located
between the source and receiver.
The difference in the sound pressure measured in the
occupied space with the wall versus without the wall is the
IL of the wall
Sound transmission loss (TL) of a partition or
other building element
It equals 10 times the logarithm (base 10) of the
ratio of the airborne sound power incident on the
partition to the sound power transmitted by the
partition and radiated on the other side.
The quantity so obtained is expressed in
decibels.
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Transmission Loss of Partition
Transmission loss (TL)
It also depends on material properties, such as stiffness
and internal damping.
16 mm gypsum boardTL depends mainly on the surface
mass of the wall at frequencies below about 1 kHz (match
with mass law)
At higher frequencies, there is a dip in the TL curve
(coincidence dip) the the wavelength of flexural
vibrations in the wall coincides with the wavelength of
sound in the air.
Critical frequency
The frequency where the minimum value of TL occurs in
the coincidence dip
The stiffer or thicker the layer of material, the lower the
critical frequency.
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Sound transmission through partitions
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Noise reduction due to enclosure
Reduction in reverberant sound pressure level in a room due to the
enclosure of a source in the same room:
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Sound Barriers
A sound barrier is a solid structure that intercepts the
direct sound path from a sound source to a receiver.
It reduces the sound pressure level within its shadow
zone.
Figure illustrates the geometrical aspects of an outdoor
barrier where no extraneous surfaces reflect sound into
the protected area.
Scattering and refraction of sound into the
shadow zone formed by the barrierthe
limiting value of about 24 dB
Practical constructions, size and space
restrictions often limit sound barrier
performance to 10 to 15 dB.
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Attenuation refers to the reduction in sound level as sound
travels along the path from a source to a receiver( through
a duct system).
Straight ducts, elbows, junctions, and silencers are
examples of elements that attenuate sound.
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Regenerated sound results from components of the duct
system that create turbulence in the air stream.
An abrupt change in airflow direction or velocity with a
corresponding static-pressure lossTurbulence
Regenerated sound increases with air velocity or when the
air is forced to make sharp turns.
Elbows, junctions, diffusers, silencers, and dampers
regenerate sound.
Notice that some elements can both attenuate and
regenerate sound.
Air makes a 90-degree turn in a rectangular duct elbow,
some of the sound is reflected back upstream, attenuating
the airborne sound downstream of the elbow.
The turbulence created by the air turning the sharp corner
causes some regenerated sound.
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The total sound energy that strikes a surface
is either reflected, absorbed by the material,
or transmitted through the material.
•
A material provides a barrier to the incident
sound energy when it reduces the amount of
sound energy that is transmitted through the
material.
•
Materials that are dense (such as masonry
block or wallboard) or stiff (such as glass) are
generally better at reducing transmitted sound
than materials that are lightweight or flexible.
•
Increasing the thickness of a material reduces
the amount of sound transmitted through it.
•
High-frequency sound is more easily reduced
than low-frequency sound when it pass
through material.
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Noise reduction (NR)
The difference between sound-pressure measurements taken on each side
of a barrier.
The NR for this same door is the difference in the sound-pressure level
inside the office space, with the door closed, and on the other side of the
door inside the equipment room.
Sound Absorption of Materials
Absorptive materials work by converting acoustical energy into heat
energy.
The absorbed energy is the portion of the incident sound energy that is
neither transmitted through the material nor reflected off the material.
The absorptivity of a material depends on several factors, including
thickness, frequency of the sound, and whether there is a reflective
surface located behind the absorptive material.
The absorptivity of a material is typically described in terms of an
absorption coefficient.
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BASIC DESIGN TECHNIQUES
Selecting fans (or other related mechanical equipment) and designing air distribution systems +
minimization of sound transmitted from different components to the occupied spaces :
Step 1- Design the air distribution system to minimize flow resistance and turbulence.
High flow resistance increases the required fan pressure, which results in higher noise being
generated by the fan.
Turbulence increases the flow noise generated by duct fittings and dampers in the air distribution
system(especially at low frequencies).
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BASIC DESIGN TECHNIQUES
Step 2 - Select of a fan
A fan operating close to its rated peak efficiency at design quantity of air and static pressure.
A fan generates the lowest possible noise at design conditions
Oversized or undersized fan not operate at or near rated peak efficiency  substantially
higher noise levels.
Step 3- design duct connections (the fan inlet and outlet) for uniform & straight air flow.
Failure to do this can result in severe turbulence at the fan inlet and outlet and in flow separation
at the fan blades significantly increase the noise generated by the fan.
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BASIC DESIGN TECHNIQUES
Step 4 - Selection of Silencer
Select duct silencers that do not significantly increase the fan total static
pressure.
Duct silencers can significantly increase the required fan static pressure if
improperly selected.
Selecting silencers with maximum static pressure losses of 87 Pa. 
minimize silencer airflow regenerated noise.
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BASIC DESIGN TECHNIQUES
Step 5 – fan powered mixing boxes
Place fan-powered mixing boxes associated with variable-volume air
distribution systems away from noise-sensitive areas.
Step 6 – Flow at elbow/ duct branch takeoff
Locating elbows or duct branch takeoffs at least five duct diameters
apart  Minimize flow-generated noise by them (10 dia for high vel
and critical areas).
Step 7 – Flow velocity and duct area
Keep airflow velocity in the duct as low as possible (max. 7.5 m/s) near
critical noise areas by expanding the duct cross-section area.
Do not exceed an included expansion angle of greater than 15°(Flow
separation, resulting from expansion angles greater than 15°, may
produce rumble noise).
Expanding the duct cross-section area  reduce potential flow noise
associated with turbulence in these areas.
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BASIC DESIGN TECHNIQUES
Step 8 – Turning vanes at elbows
Use turning vanes in large 90° rectangular elbows and branch takeoffs.
This provides a smoother transition for air smoothly change flow direction
reducing turbulence.
Step 8 – Turning vanes at elbows
Place grilles, diffusers and registers into occupied spaces as faras possible from
elbows and branch takeoffs.
Step 9 - volume dampers near grills, diffusers and registers
Minimize the use of volume dampers near grills, diffusers and registers in
acoustically critical situations.
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BASIC DESIGN TECHNIQUES
Step 10 – Vibration Isolators/ Flexible Connectors
Vibration isolation to all vibrating reciprocating and rotating equipment (mechanical
equipment on upper floors or is roof-mounted).
Vibration isolation to piping supported from the ceiling slab of a basement, directly
below tenant space.
Use flexible piping connectors and flexible electrical conduit between rotating or
reciprocating equipment, pipes and ducts connecting to equipment.
Vibration isolate ducts and pipes, using spring and/or neoprene hangers for at
least the first 15 m from the vibration-isolated equipment.
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Sound Pressure Level of Some Equipment
If manufacturer’s data are not available, the sound pressure level LpA, in dBA, for
centrifugal chillers at a distance (1 m) from the chiller can be calculated as
For reciprocating chillers, LpA in dBA at a distance of 1 m is
The sound pressure level Lp, in dB, at the center frequency of various octave
bands can be obtained by adding the following values at each octave band to the
calculated LpA:
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Circulating pumps
LpA at a distance of 1 m is
where hp power input to the pump, hp.
For fan, predicting equation was found giving very inaccurate result use manufacturer data
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Silencer
A silencer is for reduction of the
sound power level of a fan, an
airflow noise, or other sound
source transmitted along a ductborne path or airborne path.
Rectangular and cylindrical
silencers: (a) rectangular silencer
and (b) cylindrical silencer.
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Silencers used in HVAC&R systems :
Dissipative silencers.
These silencers often use face-covered or encapsulated acoustic material ( fiberglass, mineral wool, and
acrylic polymers) to attenuate noise over a broad range of frequencies.
The facing material can be made of Galvanized or aluminum sheet with perforation.
Packless silencers.
There is no fibrous fill. Noise is attenuated by means of acoustically resistive perforations in the splitters.
They are often made of sintered aluminum or acrylic plastics.
Reflection-dissipative silencers.
These silencers use the combined effect of sound reflection and dissipation in airflow passages of
successive square elbows.
Active silencers.
These silencers produce low-frequency inverse sound waves to cancel the unwanted noise.
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Silencer
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Silencers can be classified into rectangular, cylindrical, and
sound-attenuating plenum according to their configuration.
•
Inside the rectangular casing are a number of flat splitters,
depending on the width of the silencer.
•
These splitters direct the airflow into small soundattenuating passages.
•
The splitter is made from an envelope containing soundattenuating material, such as fiberglass or mineral wool,
with protected non-eroding facing.
•
The thickness of a splitter is often between 25 and 100 mm.
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Silencer
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Splitters often have a round instead of a flat nose, to
reduce their airflow resistance.
•
A rectangular silencer is often connected with
rectangular ducts or sometimes with rectangular fan
intakes and discharges.
•
A cylindrical silencer has an outer cylindrical jacket
and an inner concentric center body.
•
Both the cylindrical jacket and the center body contain
sound-attenuating material and non-eroding facing.
•
A cylindrical silencer is often used in conjunction with
vane-axial fans and in round duct systems.
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Characteristics of Silencers
The acoustic and aerodynamic characteristics of a silencer are mainly indicated by four
parameters.
Insertion Loss.
Meaning - capacity to reduce the sound power level noise at various frequencies.
Affected by - air- flow direction (especially air velocity > 1 m/s).
A sound wave that propagates in the same direction forward flow,
Opposite to the airflow  reverse flow.
Low frequencies reverse flow has a longer contact time higher IL
High frequencies, sound waves tend to refract toward the absorptive surface in a silencer
under reverse flow
Large high-frequency attenuation in the forward flow and less in reverse flow.
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Silencer
Typical free air ratio = 0.3 to 0.8.
The actual mean air velocity inside the airflow passages in a silencer vfree, in m/s, may be 1.25 to
3.3 times the face velocity vsil.
The face velocity of a silencer and sound-attenuating plenum is 2.5 -10 m/s.
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Silencer
Self-Noise.
This is the lower limit of sound power level, in dB, that a specific silencer can approachat
various octave band center frequencies.
Pressure Drop psil.
This is the total pressure drop of airstream when it flows through a silencer.
Pressure drop psil is a function of its face velocity, free area ratio, length, and the configuration of the splitter or the center body.
In general, if psil > 87Pa, both vfree and airflow noise should be investigated.
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Location of Silencers
A minimum distance Lsil kept between the upstream fan discharge outlet or other
duct fittings and the silencerensure a uniform approach velocity at silencer inlet or
an undisturbed discharging velocity at the silencer exit.
From the fan discharge, Lsil must be equal to or greater than the distance of one
duct diameter for every 5 m/s average duct velocity.
From the fan intake, Lsil should be equal to or greater than 0.75 duct diameter for
every 5 m/s average duct velocity.
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Active Silencers
Active silencers use ducted enclosure to cancel duct-borne,
low-frequency fan noise (including rumbles)
This is done by producing sound waves of equal amplitude
and opposite phase.
The primary sound source is the unwanted fan noise.
The secondary sound source cancel;ing the unwanted
source comprises the inverse sound waves from a
loudspeaker.
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Operating-Characteristics.
An active silencer consists of microphones, a microprocessor-based controller, a loudspeaker, and a
ducted enclosure.
Fan noise is propagated along a duct, an input microphone measures noise and sends an electric signal
proportional to the sound wave to the controller.
The microprocessor-based controller calculates the amplitude, frequency, and phase of the propagating
sound and sends a cancel signal to the loudspeaker.
The loudspeaker broadcasts sound waves of the same amplitude and frequency as the unwanted noise
(180° out of phase).
The destructive interference between these two sound sources results in the cancellation of the incident
fan noise by the secondary source broadcasted by the loudspeaker.
The amplitude of the fan noise is reduced downstream of the loudspeaker.
An error microphone measures the residual noise optimize the performance of the active silencer.
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System Characteristics.
In an active silencer, sound energy is added to the fan noise to cancel it, whereas in a traditional
passive silencer, sound energy is converted to heat and removed from the system.
Because the microphone in an active silencer cannot distinguish the fan noise and the turbulent airflow noise, air velocity in the ducted enclosure of an active silencer not exceed 7.5 m/s.
When the duct velocity exceeds 12.5 m/s, the turbulent noise may completely mask fan noise, and
the effect of an active silencer is reduced to nil.
Pressure drop of a passive silencer is often 50 to 63 Pa.
The electric energy required to produce the canceling sound is only 40 W, a substantial saving
compared to a passive silencer.
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Performance.
It was found that :
Active silencer has a good sound attenuation in frequencies between 31 and 125 Hz
Duct liner provides effective sound attenuation in frequencies of 500 Hz and more
A say 2m length prefabricated silencer is effective in frequencies between 63 and 4000 Hz
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Lined Plenums
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