This document discusses noise control for HVAC systems. It provides background on how noise is measured and perceived by humans. Common noise sources in HVAC systems are identified along with recommended noise level limits from ASHRAE. Methods for controlling airborne noise include duct liner, silencers, acoustic barriers and isolating noisy equipment. Proper location and integration of noise control elements into the overall system design is emphasized to prevent noise issues from arising.

1. HVAC Noise Control
By Russell Hawkins
russellstewarthawkins@gmail.com
Fundamentals

2. ± 1 dB not noticeable
± 3 dB noticeable change
± 10 dB perceived as twice as loud
Example: 60db seem twice as loud as 50db
± 20 dB perceived as four times as loud
Example: 70db four times as loud as 50db
Loudness Level Differences

3. dB
140
120
100
80
60
40
20
0
Jet take-off
Rock concert
Construction site
Street Traffic
Busy office
Conversation Speech
Living Room
Library
Bedroom
Examples

4. Frequency

5. 10 20 50 100 200 500 1k 2k 5k 10k 20k 100k
5 – 12,000 Hz
50 – 45,000 Hz
2,000 – 120,000 Hz
1,000 – 120,000 Hz
75 – 6,000 Hz
125 – 7,500 Hz
500 – 5,000 Hz
Broadband
1 – 10 Hz

6. Sources of Sound in HVAC Systems
• These sound sources are usually analyzed in
frequency bands.
• Frequency bands are specific groupings of
frequencies that are generally used in HVAC noise
control.

7. HVAC Acoustics
16 31.5 63 125 250 500 1,000 2,000 4,000 8,000
RUMBLE ROAR WHISTLE HISSTHROB
VAV Unit Noise
Diffuser Noise
Fan & Pump Noise
Chiller Noise
Fan Instability
Turbulent Airflow

8. Sound Rating Methods
dB(A) – Internationally Utilized.
Correlates well to human judgement
of loudness.
dB(C) – More sensitive to low
frequency noise
Noise Criteria (NC) – This rating
method has has been used for more
than 50 years. It is generally
preferred by engineers and
acoustical consultants.
Room Criteria (RC) – Similar to the
NC but includes sound quality rating
and low frequency evaluation.
Source: National Park Service
USA Sound Map

9. ASHRAE Recommended Levels
From 2011 ASHRAE Handbook, Chapter 48 Noise and Vibration Control

10. Where Is Noise Control Needed?
NC 20

11. NC 30

12. NC 25

13. NC 20

14. NC 30

15. NC 30

16. NC 30

17. dB(A)
A weighted Decibels - dBA
• At 63 hertz we hear about 26
db less than what’s really
there
• Each octave band is weighted
per the values shown and
logarithmically summed to
produce a single number –
dBa
• Used in high noise level
environments (industrial) and
Outdoor noise codes – LEED
requirements

18. NC
• Each octave band is plotted
in the noise criteria chart.
• The highest point on the
curve indicates the noise
criteria rating.
2011 ASHRAE Handbook, Chapter 48 Noise and Vibration Control

19. NC
How to use NC Chart
• NC 40 with the second
octave band being the
highest.
• If this was a noise sensitive
space and needed to meet
a lower noise criteria, the
low frequency noise would
need to be controlled.

20. RC
• Each octave band is plotted
(similar to NC)
• Using the Quality
Assessment Index (QIA), the
spectral deviations are
accounted for.
• In the example shown it
would be LF (indicating more
low frequency noise)

21. Source
Determining the SPL
The noise can be controlled at any of these points
- Path - Receiver

22. Sound Power vs Sound Pressure
•It is important to note that the values
on these charts (NC, RC, dBA) are
sound pressure levels.
•This is very different from a sound
power level that is used to rate fan
equipment.
•The difference between them is
inherent in their name. Power vs
Pressure. It is also evident in the
reference.
•Since both are on a logarithmic
scale, they each reference a value,
but while sound power level
references Watts (10^-12 Watts)
Sound pressure level references
pressure (20 microPascals).

23. Sound Propagation
• The position of the receiver is
essential for determining the
sound pressure level because
sound decays over time.
• Outdoors this follows an inverse
square law.
• Most often, for smaller spaces
indoors though, the sound level
decays at 3dB per doubling of
distance. Osha.gov

24. Noise paths

25. Controlling Airborne Noise

26. Longer Path
Attenuation due
to propagation
Least cost effective
Least energy efficient

27. Duct Liner
0.0
10.0
20.0
30.0
40.0
50.0
60.0
63 125 250 500 1000 2000 4000
InsertionLoss
Octave Bands
Advantages:
Very effective at controlling
mid-high frequency noise.
Cost-effective
Disadvantages:
Almost no low-frequency
attenuation
Not ideal for all environments
(cleanrooms, hospitals, etc.)
Example - 36”x36” Straight Duct
20’ long – 1” duct liner

28. Use perforated baffles
- Perforation is acoustically transparent
- Fiberglass or Recycled cotton absorbs the noise
- Can also be produced without fill as reactive or active
Can be “tuned” to control different octave bands
Circular Rectangular Elbow
Silencers

29. • Silencers are rated for 3 things
–Insertion Loss
–Pressure Drop
–Generated Noise

30. Insertion Loss The difference in the sound pressure level
with a silencer inserted into the system.

31. Pressure Drop
• Any component added to a duct
system will introduce a pressure
differential.
• Even the ductwork itself introduces
some pressure drop due to friction.
• Silencers are also rated for
pressure drops.
• Elbow silencers have higher
pressure drops and circular
silencers have the lowest pressure
drops.

32. Generated Noise
This is the noise that is
generated from air
passing through the
silencer.
Imagine that the fan is
making no noise and
only moving air. This is
the noise that would
result from the
turbulence of going
through the silencer.

33. Why is generated noise usually not a problem?
Logarithmic Scale:
L(p)total = 10*Log(10Lp1/10+10Lp2/10)
Differences between sound sources
combine differently because of the
scale
Example:
Sound Power Level
Difference between two
Sound Sources (dB)
Added Decibel to the
Highest Sound Power Level
(dB)
0 3
1 2.5
2 2
3 2
4 1.5
5 1
6 1
7 1
8 0.5
9 0.5
10 0.5
>10 0
Air Handling Unit
Sound Power Level
Generated
Noise Level
Resulting
Level
70 + 70 = 73
75 + 70 = 76
95 + 55 = 95

34. Relationship Between Ratings
Attenuation Pressure Drop

35. Silencer Performance
0
5
10
15
20
25
30
35
40
45
50
16 31.5 63 125 250 500 1000 2000 4000 8000
InsertionLoss-dB
1/1 Octave Band Center Frequency - Hz

36. Pressure Drop Considerations
ASHRAE (Chapter 47 – Sound & Vibration Control):
Do not locate the silencer in close proximity (3 to 5 duct
diameters) to any fan, elbow, plenum, fitting, or other flow
disturbing device

37. Calculation for optimal straight duct length
Length of straight duct required (Feet)
0 10 20 30 40 50 60 70
24 24
36 36
48 48
60 60
84 84
26.2
39.4
52.4
65.6
91.8
Duct Dimensions (Inches)
W x H EDD
17
26
35
44
61

38. Accounting for System Effects
Excluding System Effects
can cause pressure drops
to be 2-3x higher than
schedule values
However, the straight duct
is not necessary IF the
selection is made with
system effects in mind
ASHRAE Handbook Chapter 48

39. Noise that radiates through the duct walls and
into the surrounding areas

40. Thicker Ceiling
Breakout Noise usually consists of a lot of low
frequency noise
Adding mass will attenuate this noise

41. Duct Lagging
Limp or stiff
Effective but it can
be expensive

42. With a silencer
Silencers can come with a HTL (High Transmission Loss) casing
But the best way to control breakout is to eliminate the noise before it even goes over the space

43. Outdoor Noise Control

44. Location, Location, Location
- the further away, the more attenuation due to propagation
Compressor Blankets
- Not very effective. Only provides approximately 3dB of attenuation
Silencers
- If the sound is coming from discharge or exhaust, silencers can
control a portion of it
Full Enclosure
- In extreme situations, a full enclosure may be nece$$ary
Acoustical Barrier
- Can provide approximately 10-15 dB of attenuation
Solutions

45. Example
2 AHU’s near multiple
other buildings
Without a barrier, the
noise would be
problematic

46. With a barrier around
them, the noise is
greatly reduced

47. Why Do Noise Problems Exist?
1. Noise control was not
considered at the design
stage
2. Post design cost-cutting 3. Noise control products
may have been selected
but not integrated
correctly

48. Why Do Noise Problems Exist?
Noise Control is usually less than
5% of the mechanical budget
The cost to come in and fix it after
completion can be 5-10x as much
as including it in the design phase

49. What do I do?
Identify all possible noise paths:
– Airborne, structure-borne, breakout, radiated, flanking, etc
Figure out how much noise control you need?
– There are free software programs that have all the
calculations built in (AIM, V-A Select, TAPS, Price All-in-One)
Identify project specific requirements
– Space, IAQ, budget, material selection, etc
Develop a solution that not only address noise, but
also meets project specific requirements
Here are the right steps to an effective noise control design

50. Prevent noise problems before they ever happen!

51. Thank you
For questions, please contact Russell Hawkins
russellstewarthawkins@gmail.com