Acoustical Presentation to the
Rocky Mountain ASHRAE Chapter
April 16, 2010
Discussion Topics
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Acoustics Overview
Frequency Ranges of
Mechanical Noise
Design Criteria for Typical Spaces
Mechanical Noise Control : Areas of Interest
Sound Transmission Paths
Typical Ductborne Mitigation Methods
Typical Duct Breakout Mitigation Methods
Typical Structure-borne Noise and Vibration Mitigation
Methods
Mechanical Design affecting Sound Isolation
LEED for Schools
Acoustics - A Brief Overview
1.
 2.
 3.
 4.
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2
Sound Isolation
Noise Control
Vibration Control
Interior Acoustics
3
1
4
Acoustics 101
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Frequency is the rate of repetition of a periodic event.
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Most sound sources, except pure tones, contain energy ever a wide
range of frequencies.
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For measurement and analysis of sound, the frequency range is divided
into sections labeled as octave bands
Acoustics 101
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
Decibel (dB): Measure on logarithmic scale of the magnitude of
sound pressure, sound power, or sound intensity level with respect
to a standard reference value.
 L = 20 log (Prms/Pref)
Pref = 20µPa
Human Hearing

Threshold of Audibility: 0 dB
 Threshold of Pain: 120 dB
 Ear cannot differentiate less than 1 dB of change
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Due to log scale, dB does not add algebraically
1 Vacuum = 90 dB
2 Vacuums ≠ 180 dB
2 Vacuums = 93 dB
Acoustics 101
Definitions of Terms: Sound Power vs
Sound Pressure
Definitions of Terms: dBA
A-Weighted Sound Levels (dBA)
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dBA does not completely
represent human perception of
noise.
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dBA is used primarily in
environmental noise studies and
LEED for Schools Requirements.
A-Weighting Curve
Octave Band A-Weighting
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0
-10
-20
-30
-40
31.5 63
125 250 500 1000 2000 4000 8000
Octave Band Center Frequency (Hz)
Definitions of Terms: NC
Noise Criteria
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Noise Criteria Level (NC)
90
80
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Industry Standard
Does not address frequencies
below 63 Hz
Does not provide sound quality
assessment.
Sound Pressure Level (dB)
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70
NC 70
60
NC 65
NC 60
50
NC 55
NC 50
40
NC 45
NC 40
30
NC 35
NC 30
20
NC 25
NC 20
10
Approxima t e t hre shold of
he a ring for c ont inuous noise
32
63
125
250
500
1k
2k
Octave Band Center Frequency (Hz)
4k
8k
Definitions of Terms: RC
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Room Criteria (RC)
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Probable industry standard for
future
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Addressed frequencies below
16 and 31.5 Hz
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Provides sound quality
assessment.
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N, R, H, RV
Excerpted from Chapter 7, “Sound and
Vibration,” of the 1993 ASHRAE
Fundamentals Handbook
Comparison of dBA, NC, RC
NC 36
39 dBA
RC 18 (R)
39 dBA
Perception of Sound
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Decrease of 3 dB represents a
halving of sound energy but is
a just noticeable difference.
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Decrease of 10 dB represents
a halving of perceived sound
levels
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Decrease of 20 dB represents
¼ of the perceived sound
levels
Picture from Bell Telephone Laboratories
Frequency Ranges of
Mechanical Noise
Frequency (Hz)
Perceptible Sound
Possible Reason for Mechanical Noise
0.8 to 31.5
Throb
Turbulent Airflow and Fan Instability
31.5 to 125 Hz
Rumble
Turbulent Airflow and Poor Vibration Isolation
125 to 500 Hz
Roar
Fan Noise, Turbulent Airflow, VAV Boxes
125 to 1000 Hz
Hum & Buzz
Poor Vibration Isolation, Fan Powered VAV Boxes
500 to 2000 Hz
Whine and Whirr
Pumps and Chillers
1000 to 8000 Hz
Hiss and Whistle
Grilles, Diffusers, Water Valves
Design Criteria for Typical Spaces
Space
NC Level
RC (N) Level
15
15
20 to 25
20
25
20 to 25
Tele/Videoconferencing; Distance Learning Classrooms
25 to 30
25 to 30
Conference Rooms; Classrooms
30 to 35
25 to 30
Private Offices; Residences
35
30 to 35
Lobbies, Corridors, Computer Classrooms; Retail
40
35 to 45
Laboratories; Toilets
45
40 to 50
Kitchens, Laundry Rooms, Computer Equipment Rooms
50
45 to 55
Recording Studios; Concert Halls
Studios
Auditorium; Sanctuary
Mechanical Noise Control : Areas of
Interest
 Equipment Selections
 Type of Fans, Variable vs Constant, Diffusers/Grilles
 Noise Data for Equipment Selections
 AHU’s, RTU’s, VAV Boxes, Cooling Towers, Fan Coil Units, etc…
 Ductwork layouts
 Overhead Ducted, Displacement, Under Floor Distribution
 Ducted vs. Plenum Return
 Airflow Velocities
 Plumbing noise
 Vibration Isolation
Sound Transmission Paths
Excerpted from Chapter 7, “Sound and Vibration,” of the 2003
ASHRAE Fundamentals Handbook
Sound Transmission Paths
Excerpted from Chapter 7, “Sound and Vibration,” of the 2003
ASHRAE Fundamentals Handbook
Sound Transmission Paths
Excerpted from Chapter 7, “Sound and Vibration,” of the 2003
ASHRAE Fundamentals Handbook
Sound Transmission Paths
Excerpted from Chapter 7, “Sound and Vibration,” of the 2003
ASHRAE Fundamentals Handbook
Sound Transmission Paths
Excerpted from Chapter 7, “Sound and Vibration,” of the 2003
ASHRAE Fundamentals Handbook
Sound Transmission Paths
Excerpted from Chapter 7, “Sound and Vibration,” of the 2003
ASHRAE Fundamentals Handbook
Typical Ductborne Mitigation Methods
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Internal Ductliner
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Attenuates Mid to High
Frequencies
Distance of ductwork from
mechanical equipment
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Sound Attenuators
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Most effective at attenuating
Mid to High Frequencies
 Increases Static Pressure
Drop
Lined Plenum
 Most effective method for
attenuating low frequencies
 Can be incorporated into
AHU and RTU Casing
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Typical Ductborne Mitigation Methods
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Double Wall Ductwork
 Utilized when internally lined
ductwork is not allowed.
 Hospitals, Laboratories
Diffuser/Grille Selection
 Diffusers/grilles should be
selected 5 NC points below
room criteria.
 Flex duct connection
 Airflow velocity
Ductwork
 Airflow velocity
 Number of elbows and junctions
Terminal Units
CONSTANT OR
VARIABLE AIR
VOLUME
 Integral Sound Attenuators
 Manufacturer NC Ratings
FAN-POWERED, SERIES
FLOW, VAV
Good Design Practices
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Fan Discharge Configurations
Inlet Configuration
Excerpted from Chapter 7, “Sound and Vibration,” of the 2003
ASHRAE Fundamentals Handbook
Typical Duct Breakout Mitigation
Methods
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Lagging or Wrapping
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Attenuates Mid to High
Frequencies
Utilized primarily for
plumbing noise
Ductwork Enclosures
Most effective at attenuating
low frequencies
 Primarily used for RTU’s
 Utilized as an extension of
Mechanical Room
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Typical Structure-borne Noise and
Vibration Mitigation Methods
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Concrete Inertia Bases
 Pumps/Large Fans
Spring Isolators
 Pumps
 Rotating Equipment
 Above Grade
Chillers/Cooling Towers
Neoprene Pads
 On Grade Chillers/Cooling
Towers
Typical Structure-borne Noise and
Vibration Mitigation Methods
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Rooftop Isolation Curb
 RTU’s
Spring/Neoprene Hangers
 Ductwork/Piping
 30 foot critical distance
Flex Connections
 Double Bellows
Mechanical Design affecting Sound
Isolation
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Crosstalk between Spaces
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Length of ductwork
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Junctions and Elbows
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Internal Ductliner
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Plenum Return
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Z or U Shaped Internally
Line Transfer Ducts
Excerpted from No Noise Classroom Acoustics
Publication
Mechanical Design affecting Sound
Isolation
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Penetrations Full Height Partitions
Mechanical Design affecting
Environmental Noise Control
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Most states, counties, cities, and towns have property line noise ordinances.
 Typical Day/Night level of 55/50 dBA
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Typical Equipment Culprits
 Emergency Generators
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Cooling Towers
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Radiator, Exhaust, Intake
Fans
Rooftop Units
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Alignment of Compressor/Condenser Section
Mechanical Design affecting
Environmental Noise Control
Mitigation Measures
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Equipment Locations
 Adjacent Properties
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Barrier Walls/Screens
 Materials
 Height
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Louvers
 Type
LEED for Schools
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Acoustics is now a mandatory
LEED credit for Schools
Prerequisite 3 Background Noise Requirements:
 Max BNL of 45 dBA OR
 Achieve an RC (N) Mark II level of 37
EQ Credit 9: Enhanced Acoustical Performance Background
Noise Requirements
 Max BNL of 40 dBA (1 point) or 35 dBA (2 points)
 2: Achieve an RC (N) Mark II level of 32 (1 point) or 27 (2
points)
Thanks for Attending
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Any Questions????
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Additional Resources
 ASHRAE Application
Handbook Chapter 47
 Architectural Acoustics: David Egan
Case Studies
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Ritz Carlton Denver
 Boardroom
and Conference Areas
 NC 45+ due to breakout noise (125 Hz)
 NC 35 Criteria
 Remedial Measures: Incorporated ductwork
enclosure around high pressure running over spaces.
 NC 34 after implementation of remedial measures
Case Studies

UCDHSC Research 1 Facility
 Vibration
Issues in NMR and Crystallography
Growth Chambers
 Acoustical testing: Issues at 32 Hz
 Short circuited spring isolators in AHU fans:
Still bolted down for shipping
Case Studies