acoustical design considerations
DESCRIPTION
Slides used in a May 11, 2010 presentation at the monthly meeting of the Illinois Chapter ASHRAE by David G. Paoli, P.E., senior engineer, Shiner + Associates, Inc.TRANSCRIPT
Introduction
Purpose of Course
Provide an Introduction to Acoustics and a Review of Practical Applications
of Mechanical System Noise and Vibration Control
Outline of Presentation
• Basic Acoustical Definitions
• Indoor Criteria
• Mechanical System Design Guidelines• Air handling systems
• Large Built-up Systems• Indoor Package Units• Roof-top Units
• Terminal Boxes
Basic Definitions
• Amplitude (loudness)
• Frequency (pitch)
• Quality (character)
Perceived Loudness Rules of Thumb
• 1 dB change not perceptible
• 3 dB change is perceptible
• 5 dB change is clearly noticeable by all
• 10 dB increase sounds twice as loudAbove discussion applies to
long periods of time
Frequency
• Frequency is Measured in Hz (cps)
• Human Ear is not Equally Sensitive to All Frequencies
Audible Frequency Range
Quality of Sound
• Intermittent vs. Steady-State
• Cyclical, Repetitive, Pulsating• Air compressors, chillers
• Tonal Sources• Fan blade pass frequency, screw chillers
Indoor Noise Criterion
• Noise Criterion Curves (NC)
• Room Criterion Curves (RC)
• RC Mark II
• A-weighted Sound Pressure Level (dBA)
NC Curves
NC Curves
NC 47
RC CurvesRoom Criterion (RC) Curves
10
20
30
40
50
60
70
80
90
16 31.5 63 125 250 500 1000 2000 4000
Octave Band Center Frequency, Hz
So
un
d P
ress
ure
Lev
el, d
B
Region A
Region
Region C
RC 25
RC 30
RC 35
RC 40
RC 45
RC 50
RC CurvesRoom Criterion (RC) Curves
10
20
30
40
50
60
70
80
90
16 31.5 63 125 250 500 1000 2000 4000
Octave Band Center Frequency, Hz
So
un
d P
ress
ure
Lev
el, d
B
Region A
Region
Region C
RC 25
RC 30
RC 35
RC 40
RC 45
RC 50
RC 35 RVa
RC Mark II
RC Mark II
RC Mark II
ASHRAE Recommendations1991
Applications1999
Applications
A-Weighting
• Good Single Number Descriptor
• Correlates Well with Human Perception
• In Widespread Use Primarily for Outdoor Use
A-Weighting
Equipment Source Sound Levels
• Sound Power Levels (dB, re: 10-12 Watts)• Total sound energy of a source
• Not affected by surrounding environment
• Sound Pressure Levels (dB, re: 20 µPa)• Level of sound due to a source
• Levels a function of distance & environment which must be documented.
Sound Power vs. Sound Pressure
Total Radiated Heat/Time (BTU/hr) ~ Sound Power Heat converts to temperature based Sound power converts to pressure based on distance and heat conductance based on distance and sound of room surfaces. absorption of room surfaces.
Room Temperature ~ Sound Pressure Level
This is why we specify noise levels
in terms of Sound Power.
Mechanical Equipment
• Air handling systems sound paths• Ductborne, low frequency duct break-
out, casing, vibration
• Terminal box sound paths• Ductborne, radiated
• Chillers/Pumps sound paths• Airborne, vibration
Air Handling Unit Sound Paths
Air Handling Design Guidelines
• Space Planning
• Good intake and discharge duct conditions• Achieve uniform flow
• Avoid turbulent flow conditions
• Duct attenuators• Pressure drop not to exceed 0.3” w.g.
• Do not located directly upstream or downstream of duct fittings, will significantly increase rated pressure drop and regenerate noise
• Internal duct lining and vibration isolation
AHU Design Guidelines
Roof-top Air Handling Unit
Noise Mitigation forRoof-top Air Handling Units
• Space planning
• Thickened slab below units (5’-8’ around perimeter of unit)
• Duct attenuators
• Duct lining
• Double wall duct
• Vibration isolation curbs
Roof-top Unit Mitigation Concepts
Terminal Boxes
• Fan powered – constant volume or VAV and single duct VAV• Discharge noise from the fan or damper
transmitted through the duct
• Radiated noise from the fan or damper transmitted through the box wall or box opening in the case of fan powered
Terminal Box Design Guidelines
• To Achieve NC 35• Box inlet static pressure ≥1.00” wg
• Box selection
• Radiated noise• Single Duct VAV – 1700 cfm
• Fan Powered – 1300 cfm
• Ductborne noise• 10 to 15 feet of internal duct lining
• Duct attenuators
Terminal Box Selection
Effect of Ceiling Plenum on Radiated Noise of Terminal Boxes
• Distance of box to back of ceiling tile – no effect
• Volume of the plenum has a large impact• Plenum volume depends on:
• Height (from underside of slab to top of ceiling)
• Extent (area of open ceiling plenum)
Terminal Box Noise Example
Room 109 Room 107 Room 105Supply & Exhaust Supply Only Exhaust Only
Supply - 2.2" w.g.NC 54 NC 44 NC 55
Exhaust - 3.0" w.g.Supply - 2.2" w.g.
NC 42 NC 42 NC 49Exhaust - 1.5" w.g.Supply - 1.25" w.g.
NC 37 NC 37 NC 37Exhaust - 1.1" w.g.
Chiller/Pump Design Guidelines
• Chillers• Thickened structural slab (above and
below)
• Floating floor
• Vibration isolation including neoprene flexible connectors
• Pumps• Concrete inertia bases
• Vibration isolation including neoprene flexible connectors
Chiller Vibration Isolation
Pump Vibration Isolation