This document discusses noise control measures for HVAC systems. It begins by outlining the effects of noise on humans and some common mistakes in noise control. It then describes basic noise control methods like absorption, damping, decoupling, mass, and flow control. Specific examples of decoupling measures like resilient channels, clips, and hangers are provided. The document also includes a case study of noise control applied to an HVAC system, identifying common noise sources and effective control approaches like equipment location, sealing, isolation, attenuation, and ductwork design. Active noise control is discussed as a supplement to traditional passive methods.

1. Noise control measures
R.Narasimha Swamy
Senior consultant
narasimhaswamy@yahoo.com

2. Contents
• Effect of noise on human beings
• Basic measures of noise control
• Case study of a HVAC system

3. Effect of noise on humans

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

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.

9. Noise Reduction by Absorption
Increased absorption reduces ambient noise

10. Noise reduction
by absorption and decoupling

11. Ceiling tiles

12. Fabric-wrapped glass wool panels absorb sound

13. Noise Reduction
• Combined effect of TL and absorption
• NR = TL - 10 log (S/AR)
 NR: noise reduction (db)
 TL: transmission loss (db)
 S: area of barrier wall (ft2)
 AR: total absorption of receiving room (Sabins, ft2)

14. Decoupling measures

15. Directivity index

17. Pressure increases at walls

20. Resilient Channel

21. Resilient Clip

22. Resilient Duct Hangers
Elastomeric Hanger
From Kirkegaard Associates

23. Resilient Duct Hangers
Spring-and-Neoprene-in Series
Isolator (Hanger)
From Kirkegaard Associates
Pre-compressed Spring-and-
Neoprene-in-Series Isolator (Hanger)

26. Dampers

28. Isolator Types
Elastomeric Pads

29. Isolator Types
Elastomeric Pads

30. Isolator Types
Neoprene-In-Shear Floor Mount

31. Isolator Types
Open Spring Floor Mount

32. Isolator Types
Restrained Open Spring Floor Mount

33. Flow control measures

35. Plenum chamber

36. Wind generated tones can be avoided
by profile changes or spoilers

37. Smooth ducts and pipes create less
turbulence noise

38. LF exhaust noise transformed
to HF is easier to attenuate

40. Flow noise in pipes is formed by
sudden pressure changes

47. Case study of noise control in HVAC system

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

49. Main HVAC Noise Sources
• Fans (for air circulation)
– Axial
– Centrifugal
– Propeller
• Compressors (that convert
gas to liquid)
– Piston
– Rotary
– Scroll
– Centrifugal
– Screw
• Pumps (to circulate
liquids)
• Diffusers and
Ductwork (to distribute
air)
– Turbulent aerodynamic
noise
– “Break-out” noise

50. 50

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

53. Typical Air-Handler Design

54. Equipment Location: Rooftop

55. Equipment Location:
Mechanical Equipment Room
• Noise inside the
MER
• Noise outside the
MER
• Duct Breakout
• Active Noise Control

56. Fan Noise Components
• 1 duct length
• 3 duct length
• 5 duct length
• Aerodynamic noise
• Blade-passage noise
 fB = (RPM/60) ·N
 N = number of blades

57. Fan Noise
Fan noise depends on the fan
operation point on the fan curve

58. Fan Noise
Fan noise depends on the fan
operation point on the fan curve

59. Fan and Compressor 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)

61. How Ductwork Radiates Noise (Break Out)

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

63. Duct Liner
MJR Figure 9.5 and 9.7, pp. 193 and 194

64. Duct Lagging
Make the ducts stiff using lagging, typically fire-rated drywall.

65. Duct Lagging
MJR Figure 9.14, p. 200

66. Duct Lagging
From Kirkegaard Associates

67. Duct Lagging

69. Resilient Hangers
From Kirkegaard Associates

70. Resilient Hangers
From Kirkegaard Associates

71. Flexible Duct Connections
From Kirkegaard Associates

72. Application of Duct Liner in Underfloor Plenum
From Kirkegaard Associates
Lined Plenum
(For under-floor air supply)

73. Silencer Location
From Paul Henderson, Acoustics for Mechanical Engineers, ASHRAE Expo 2005

74. Duct Sound Attenuators
From Kirkegaard Associates

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

76. Improvements in Design for Noise Performance
Poor Design
Better Design

77. Duct Liner Data

78. Airflow: Turbulent Noise in Ductwork

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

80. A Sample Interior Noise Prediction

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.

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.

88. Thank you

89. Air-Borne Sound
Transmission Loss (TL): sound energy lost through a construction assembly