4. • Affects peace of mind, social behaviour and
concentration.
• Causes discomfort, irritation and
annoyance.
• May cause hearing impairment.
• Affects other physiological functioning of
the body.
• In totality, disturbs the life style and affects
the productivity.
Effect of noise and vibration on humans
5.
6. Four noise control mistakes to avoid
1. Thinking that there is no noise problem! NL greater than
85dB in factory environment and level as low as a 55dB in a class
room/Studios should be considered as a problem. In general, if
conversation is difficult, treat it as a serious noise issue.
2. Not considering noise control during the design stage:
Although all source of noise can be treated after installation, it’s
generally twice as expensive and half as effective compared with
designing proper noise control into the system before the noise
source is installed.
3. Not sealing the air leaks: Sound always takes the easiest path
around or through a barrier. Construction gaps or air leaks are the
easiest way for sound to pass from one space to another.
4. Not using a systems approach to noise control: A common mistake
in noise control effort is the failure to consider all possible noise
paths. All airborne and structure borne noise paths must be studied
and treated accordingly.
7. Basic measures of noise control
• Absorption
• Damping
• Decoupling
• Mass
• Flow control
8. • Absorption by installing appropriate sound absorbing solutions
in the form of wall panels, false ceiling, carpeting. This is
necessary for any serious soundproofing project.
• Decoupling to isolate walls from the studs, thereby breaking
the direct path of sound, by using resilient channel and sound
clips. This decoupling technique adds resilience to the walls.
• Damping is the process of sound insulation by installing the non
vibrating dead panels as barriers. Damping is improved by
applying suitable compounds in between two constrained layers.
• Mass simply means creating a heavier wall by using more
(another layer) and/or thicker material.
• Flow control measures to avoid turbulence and resulting noise
and vibration in free flow of fluids in pipe lines, ducts etc.
48. 48
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
51. Noise Control Approaches in HVAC
• Location
• Sealing penetrations
• Resilient mounting & connected services
• Flexible connections to equipment to lower
fluid velocities
• Internal duct lining and attenuators
• Routing of ductwork and piping
• Enclosing ductwork and piping
52. Fan Coil Units
• Opportunity for
significant noise
issues:
– Fan and coil in close
proximity: high
turbulence
– Applications: typically
close to “listeners” (hotel
rooms, etc.)
– Water flow noise
60. Diffuser Noise
• Flow sets the
noise level at a
given static
pressure level
forcing the flow
• Good
aerodynamics are
important to
lower the noise
from air
terminals
From Paul Henderson, Acoustics for Mechanical Engineers, ASHRAE Expo 2005 (Long Fig. 13.23, p. 474)
62. Duct Shape and Noise Control
From Paul Henderson, Acoustics for Mechanical Engineers, ASHRAE Expo 2005
• Stiffness of round
ductwork reduces break-
out noise since motion of
the duct walls is restricted
• However, this means that
more noise energy stays
within the duct and may
produce higher noise levels
at the outlet
75. Air Plenums, Passive and Active Silencers
• Plenum used near
equipment outlet;
promotes laminar
airflow and provides
acoustical insertion
loss (< 12 dB)
• Passive silencers used
when large insertion
loss is required; must
account for pressure
drop
• Active silencer has no
pressure drop.
From Paul Henderson, Acoustics for Mechanical Engineers, ASHRAE Expo 2005
79. Long, p. 486
Frequency
(Hz)
63 125 250 500 1000 2000 4000
Loss (dB/ft)
Circular
0.03 0.03 0.03 0.05 0.07 0.07 0.07
Loss (dB/ft)
Square
0.36 0.20 0.11 0.06 0.06 0.06 0.06
• Data for circular duct from Long, Table 14.1
• Data for square duct from previous equations with P/S = 4
Duct Shape and Noise Control
81. Active Noise Control in Ducts
MJR Figure 9.19, p. 205
Using data from the input microphone, the controller generates a signal
to be played by the loudspeaker which is out of phase (180⁰) with the
duct-borne noise at the loudspeaker position. Feedback from the error
microphone (which ideally senses no noise) helps fine tune the process.
82.
83.
84.
85.
86. Active silencer
• Good LF attenuation.
• HF attenuation remains a challenge.
• Normally combine active/passive for full
range attenuation.
• Lower pressure loss.
• Smaller size and weight than passive for
similar LF performance.
• Higher initial cost?
87. Conclusion
• All measures discussed viz Absorption,
damping, decoupling, adding mass and flow
control needs to be carefully considered
while designing the potential noise emitting
equipment.
• Active silencer is attractive measure but,
not an alternative for the classical
measures, but, can supplement these
efforts.