Acoustical Presentation to the Rocky Mountain ASHRAE Chapter April 16, 2010.
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Transcript of Acoustical Presentation to the Rocky Mountain ASHRAE Chapter April 16, 2010.
Discussion Topics 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. Sound Isolation 2. Noise Control 3. Vibration Control 4. Interior Acoustics
1
2 3 4
Acoustics 101 Frequency is the rate of repetition of a periodic event.
Most sound sources, except pure tones, contain energy ever a wide range of frequencies.
For measurement and analysis of sound, the frequency range is divided into sections labeled as octave bands
Acoustics 101
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
Due to log scale, dB does not add algebraically
1 Vacuum = 90 dB 2 Vacuums ≠ 180 dB
2 Vacuums = 93 dB
Definitions of Terms: dBA
A-Weighted Sound Levels (dBA)
dBA does not completely represent human perception of noise.
dBA is used primarily in environmental noise studies and LEED for Schools Requirements.
A-Weighting Curve
-40
-30
-20
-10
0
31.5 63 125 250 500 1000 2000 4000 8000
Octave Band Center Frequency (Hz)O
ctav
e B
and
A-W
eigh
ting
Definitions of Terms: NC
Noise Criteria Level (NC)
Industry Standard
Does not address frequencies below 63 Hz
Does not provide sound quality assessment.
Noise Criteria
10
20
30
40
50
60
70
80
90
32 63 125 250 500 1k 2k 4k 8k
Octave Band Center Frequency (Hz)
So
un
d P
ress
ure
Lev
el (
dB
)
Approximate threshold of
hearing for continuous noise
NC 20
NC 70
NC 65
NC 60
NC 55
NC 50
NC 45
NC 40
NC 35
NC 30
NC 25
Definitions of Terms: RC
Room Criteria (RC)
Probable industry standard for future
Addressed frequencies below 16 and 31.5 Hz
Provides sound quality assessment. N, R, H, RV
Excerpted from Chapter 7, “Sound and Vibration,” of the 1993 ASHRAE Fundamentals Handbook
Perception of Sound
Decrease of 3 dB represents a halving of sound energy but is a just noticeable difference.
Decrease of 10 dB represents a halving of perceived sound levels
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
Recording Studios; Concert Halls 15 15
Studios 20 to 25 20
Auditorium; Sanctuary 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
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 Internal Ductliner
Attenuates Mid to High Frequencies
Distance of ductwork from mechanical equipment
Sound Attenuators 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
Typical Ductborne Mitigation Methods
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
FAN-POWERED, SERIES FLOW, VAV
Integral Sound Attenuators
Manufacturer NC Ratings
CONSTANT OR VARIABLE AIR VOLUME
Good Design Practices Fan Discharge Configurations
Inlet Configuration
Excerpted from Chapter 7, “Sound and Vibration,” of the 2003 ASHRAE Fundamentals Handbook
Typical Duct Breakout Mitigation Methods Lagging or Wrapping
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
Typical Structure-borne Noise and Vibration Mitigation Methods 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 Rooftop Isolation Curb
RTU’s
Spring/Neoprene Hangers Ductwork/Piping 30 foot critical distance
Flex Connections Double Bellows
Mechanical Design affecting Sound Isolation Crosstalk between Spaces
Length of ductwork
Junctions and Elbows
Internal Ductliner
Plenum Return Z or U Shaped Internally
Line Transfer Ducts
Excerpted from No Noise Classroom Acoustics Publication
Mechanical Design affecting Environmental Noise Control Most states, counties, cities, and towns have property line noise ordinances.
Typical Day/Night level of 55/50 dBA
Typical Equipment Culprits Emergency Generators
Radiator, Exhaust, Intake
Cooling Towers Fans
Rooftop Units Alignment of Compressor/Condenser Section
Mechanical Design affecting Environmental Noise ControlMitigation Measures
Equipment Locations Adjacent Properties
Barrier Walls/Screens Materials Height
Louvers Type
LEED for Schools
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
Any Questions????
Additional Resources ASHRAE Application Handbook Chapter 47 Architectural Acoustics: David Egan
Case Studies
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 FacilityVibration Issues in NMR and Crystallography
Growth ChambersAcoustical testing: Issues at 32 HzShort circuited spring isolators in AHU fans:
Still bolted down for shipping