Acoustic Treatment Two – Noise and Vibration Control

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Acoustic Treatment Two Noise and Vibration
Control
MEBS 6008
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Balanced Flow
4.7 cub.m
Balanced vs unbalanced air flows
31.9 deg C
Unbalanced flow increase effectiveness of heat
exchanger Heat exchanger transfer less overall
heat, why? The following example illustrates the
reasons
25.6 deg C
35.7 deg C
4.7 cub.m.
28.9 deg C
Exhaust air flow/ supply air flow (EAF/SAF) Sensible Effectiveness
1 0.5 90 or 0.9
2 0.6 85 or 0.85
3 0.7 79 or 0.79
4 0.8 76 or 0.76
5 0.9 70 or 0.7
6 1.0 66 or 0.66
EAF/SAF 4.7cub.m. /4.7cub.m. 1
Unbalanced Flow
3.3 cub.m.
33.2 deg C
25.6 deg C
35.7 deg C
30.0 deg C
4.7 cub.m.
EAF/SAF 3.3 cub.m./4.7 cub.m. 0.7
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Addition of two sound pressure levels one
example to illustrate
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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.
Note the unit of the equation
Sound power is proportional to the square of
sound pressure ? multiplier 20 is used (not 10).
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Why 42dBA as stated in previous lecture???
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Noise Control in Ventilation System
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Noise Control in Ventilation System
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Noise Control in Ventilation System
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Noise Control in Ventilation System
End reflection loss   The change in propagation
medium (when sound travels from duct termination
into a room)? reflection of sound back up the
duct.   The effect is greatest at long wavelength
(i.e. low frequencies) This leads to a
contribution to the control of low frequency
noise from the system.  
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Noise Control in Ventilation System
Determination of sound level at a receiver point
When a source of sound operates in a room, energy
travels from a source to the room boundaries,
where some is absorbed and some of it is
reflected back into the room.
The relation between sound pressure level and
sound power level in real room may be found by
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Noise Control in Ventilation System
Determination of sound level at a receiver point
(continued)
For a normally furnished room with regular
proportions and acoustical characteristics
between average and medium-dead and room
volume lt 430 m3, a point source of source could
be found by -
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FUNDAMENTALS OF VIBRATION A rigidly mounted
machine transmits its internal vibratory
forces directly to the supporting structure.
Vibration isolators is resilient mountings By
inserting isolators between the machine and
supporting structure, the magnitude of
transmitted vibration can be reduced
(). Vibration isolators can also be used to
protect sensitive equipment from disturbing
vibrations. Vibration energy from mechanical
equipment ? transmitted to the building structure
? radiated as structure-borne noise.
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Vibration isolation    Any residual,
out-of-balance force in the rotating parts as a
weight located eccentrically.
eventually appear as noise energy
The weight rotates ? each part of the machine
structure subjected to a cyclic force from
inertia of the rotating off-centre height
Vertical component of the force is concerned
?acting alternately upwards and downwards, at a
frequency equal to the shaft rotational
frequency.
Assumption machine is rigid for every point
(includes mounting feet).
Other case of cyclic force example Combustion
loads in reciprocating engines
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SINGLE-DEGREE-OF-FREEDOM MODEL The simplest
example is the single-degree-of-freedom model.
Only motion along the vertical axis is
considered Damping is disregarded Valid only
when the stiffness of the supporting structure gtgt
the stiffness of the vibration isolator.
(mechanical equipment on G/F or basement
locations) Natural frequency of the isolator
deflect the spring a little more suddenly
release it ? the machine oscillate vertically
about its rest position at natural
frequency. The natural frequency fn of the
system is
where k is the stiffness of vibration isolator
(force per unit deflection) M is mass of the
equipment supported by isolator.
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This equation simplifies to (try yourself, noting
the unit of g)
where dst is the isolator static deflection in
mm, k/M g /dst. Static deflection
incremental distance the isolator spring
compressed under the equipment weight. Isolator
static deflection supporting load ? achieve the
appropriate system natural frequency
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Undamped Vibration Use the steel spring as
vibration isolator.
The machine settles under its own weight The
machine deflects the spring by a certain amount ?
static deflection of the isolator Static
deflection determines the eventual performance of
the spring as an isolator when the machine is
running. Static deflection depends only upon the
static stiffness of the spring, and weight of the
machine.
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Damped Vibration Real isolators have a certain
degree of internal damping Energy is
progressively removed from the system Amplitude
of vibration steadily reduces Large amount of
damping? movement of the mass back to its rest
position after initial deflection will be very
sluggish Neither overshoot nor oscillate   Critica
l damping Amount of damping just sufficient for
mass to return to its mean position in min. time
without overshoot   
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Damping equation
Figure shown D20 and D100
Effects of Damping Frequency ratio for maximum
transmissibility lt Equivalent undamped amount At
high forcing frequency, transmissibility varies
with amount of damping
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Transmissibility T and Displacement x
Transmissibility T is inversely proportional to
the square of the ratio of the disturbing
frequency fd to the system natural frequency fn,
or
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Vibration isolation begin after fd /fn gt 1.4 .
At fd fn, resonance occurs (the denominator of
Equation equals zero)
Vibration transmissibility rapidly decreases.
At resonance ?theoretically infinite transmission
of vibration. In practice, some limit on the
transmission at resonance exists due to some
inherent damping.
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A frequency ratio of at least 4.5 is often
specified, which corresponds to an isolation
efficiency of about 90, or 10
transmissibility. Higher ratios may be specified,
but in practice this is difficult to
achieve. Nonlinear characteristics cause typical
isolators to depart from the theoretical curve.
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Equipment mass increased ? the resonance
frequency decreases? increasing the isolation.
In practice, the load-carrying capacity of
isolators requires their stiffness or number be
increased. The use of stiffer springs leads,
however, to smaller vibration amplitudesless
movement of the equipment.
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TWO-DEGREE-OF-FREEDOM MODEL When heavy mechanical
equipment is installed on a structural floor(
especially roof), the stiffness of the supporting
structure may NOT be gtgt the stiffness of the
vibration isolator. Significantly softer
vibration isolators are usually required in this
case. Two-degree-of-freedom model for the
design of vibration isolation in upper-floor
locations.
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The precise behavior of this system with respect
to vibration isolation is difficult to determine.
The objective is to minimize the motion of the
supporting floor Mf in response to the exciting
force F. Evaluating the interaction between two
system natural frequencies and the frequency of
the exciting force? complicated Fraction of
vibratory force transmitted across an isolator to
the building structure (transmissibility) depends
in part the isolator stiffness comparing with
that of supporting floor.
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Stiffness is inversely proportional to deflection
under the applied load, this relationship is
shown as a ratio of deflections.
To optimize isolation efficiency?static
deflection of the loaded isolatorgtgt incremental
static deflection of the floor under added
equipment weight.
Floor deflection?excessive vibration is
attributable to upper floor or rooftop mechanical
installations.
Ideally, this ratio should be on the order of
101 to approach an isolation efficiency of about
90.
Static deflection of the vibration isolator is
incremental deflection of the supporting floor
under the added weight of the equipment ?50 of
the vibratory force
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Selection of vibration isolators on the basis of
the single-degree-of-freedom model ?neglected
floor stiffness? inadequate Steps to choose
vibration isolators with consideration of floor
stiffness Asking structural engineer to estimate
the incremental static deflection of the floor
due to the added weight of the equipment at the
point of loading Choose an isolator that will
provide a static deflection of 8 to 10 times that
of the estimated incremental floor
deflection. Consider also building spans,
equipment operating speeds, equipment power,
damping and other factors Remarks The type of
equipment, proximity to noise-sensitive areas,
and the type of building construction may alter
these choices.
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Fans and Air-Handling Equipment Fans with wheel
diameters lt or 560 mm and all fans operating at
speeds to 300 rpm NOT generate large vibratory
forces. For fans operating under 300 rpm, select
isolator deflection so that the isolator natural
frequency is 40 or less of the fan speed. Fan
operating at 275 rpm, an isolator natural
frequency of 110 rpm (1.8 Hz) or lower is
required (0.4 275 110 rpm). A 75-mm
deflection isolator can provide this isolation.
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Pumps Concrete bases (Type C) should be
designed for a thickness of one tenth the longest
dimension with minimum thickness as follows
For up to 20 kW, 150 mm For 30 to 55 kW, 200
mm For 75 kW and higher, 300 mm.
Pumps over 55 kW and multistage pumps may exhibit
excessive motion at start-up ? supplemental
restraining devices can be installed if
necessary.
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ISOLATION OF VIBRATION AND NOISE IN PIPING
SYSTEMS All piping has mechanical vibration
(equipment and flow-induced vibration and Noise)?
transmitted by the pipe wall and the water
column. Equipment installed on vibration
isolators exhibits motion or movement from
pressure thrusts during operation. Vibration
isolators have even greater movement during
start-up and shutdown (equipment goes through the
isolators resonant frequency). The piping
system must be flexible enough to

• Reduce vibration transmission along the connected
  piping,
• Permit equipment movement without reducing the
  performance of vibration isolators, and
• Accommodate equipment movement or thermal
  movement of the piping at connections without
  imposing undue strain on the connections and
  equipment.

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Flow noise in piping

• Minimized by sizing pipe so that
• the velocity is 1.2 m/s maximum for pipe 50 mm
  and smaller and
• using a pressure drop limitation of 400 Pa per
  metre of pipe length with a maximum velocity of 3
  m/s for larger pipe sizes.
• Flow noise and vibration can be reintroduced by
• turbulence,
• sharp pressure drops, and
• entrained air.

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Resilient Pipe Hangers and Supports Resilient
pipe hangers and supports are necessary to
prevent vibration and noise transmission from the
piping to the building structure and to provide
flexibility in the piping.
Suspended Piping. Isolation hangers described in
the vibration isolation section should be used
for all piping in equipment rooms. The first
three hangers from the equipment the same
deflection as the equipment isolators (a max.
limitation of 50 mm deflection) Remaining hangers
spring or combination spring and rubber with 20
mm deflection.
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Floor Supported Piping. Floor supports for
piping in equipment rooms and adjacent to
isolated equipment The first two adjacent floor
supports should be the restrained spring type,
with a blocking feature that prevents load
transfer to equipment flanges as the piping is
filled or drained. Where pipe is subjected to
large thermal movement, a slide plate should be
installed on top of the isolator
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Piping Penetrations Most HVAC systems have many
points at which piping must penetrate floors,
walls, and ceilings. Risk for a path for
airborne noise?destroy the acoustical integrity
of the occupied space. Seal the openings in the
pipe sleeves by an acoustical barrier such as
fibrous material and caulking (between noisy
areas, such as equipment rooms, and occupied
spaces)
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• Flexible Pipe Connectors
• They provide piping flexibility to permit
  isolators to function properly,
• They protect equipment from strain from
  misalignment and expansion or contraction of
  piping, and
• They attenuate noise and vibration transmission
  along the piping .

The most common type of connector are arched or
expansion joint type, a short length connector
with one or more large radius arches, of rubber
or metal. All flexible connectors require end
restraint to counteract the pressure thrust.
Overextension will cause failure.
Manufacturers recommendations on restraint,
pressure, and temperature limitations should be
strictly adhered to.
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Noise Control in Practice
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Exercises would be provided later as this file
size is too large. Good Luck in the coming Exam.