acoustical engineering solutions
TRANSCRIPT
Acoustical Engineering Solutions.
Phoenix Noise & Vibration LLC
Consider Mechanical NoiseIncorporating mechanical noise and vibrationcontrol concepts in the building design process.
By Scott Harvey, PE, INCE Bd. Cert.Phoenix Noise & VibrationFrederick, Maryland
Phoenix Noise & Vibration LLC
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© Phoenix Noise & Vibration, LLC 2017
Learning Objectives
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After attending this course, participants will be able to:
• Understand basic mechanical noise measurement parameters;
• Recognize significant mechanical noise sources in the building design phase;
• Differentiate between air borne and structure borne noise paths;
• Design rudimentary mechanical noise control schemes to avoid negatively impacting building occupants with mechanical sounds.
Overview
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Noise Measurement Parameters
Significant Sources
Air bornevs.
Structure borne
Noise Control Schemes
Mechanical Noise
Noise Measurement Parameters
ASTC
STC
NC
FSTC dBA
RC
OITC
Terminology• Loudness measured in decibels (dB)
• dB = 10 x log(Pressure2/refPressure2)
• Human Range: 0 to 130 dB
• “A‐weight” to simulate human hearing… “dBA”
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Phoenix Noise & Vibration LLC
Why dB???
• Reduces wide range of numbers to a smaller range:• 20 to 20,000,000 microPascals becomes 0 to 120 dB
• Example of the logarithmic nature:
• Eases Comparisons:30 to 33 = “Barely Perceptible”90 to 93 = “Barely Perceptible”
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Change in dB Subjective Response
3 dB Barely Perceptible
5 dB Clearly Perceptible
10 dB Twice as Loud
Terminology
•Everything is based upon the decibel scale
•It’s the fundamental unit of measure
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Rating Mechanical Noise • Loudness
• Soft• Quiet• Loud• Noisy
• Quality of Noise• Rumble• Roar• Hiss• Tonal
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• Single number ratings describing the relative loudness and speech interference properties of a given noise spectrum
•Used to specify background noise levels for various space uses, mainly HVAC and mechanical noise
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Background Noise Measures
Background Noise Measures
•Noise Criteria (NC)• Simple but,• No assessment of sound quality
•Room Criteria (RC(N))• More difficult to comprehend• Evaluates sound quality• Diagnostics
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Phoenix Noise & Vibration LLC
NC vs. RC(N)
Mechanical Noise Measures
Method Loudness Quality
dBA • ‐NC • ‐RC • •
RC(N) • •
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ASHRAE* Recommended Background Levels
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Room Type NC/RC dBA
Residences, Apartments, Condominiums
Living areas 30 35
Bathrooms, kitchens, utility rooms 35 40
Hotels/Motels Individual rooms or suites 30 35
Meeting/banquet rooms 30 35
Office buildings Executive and private offices 30 35
Conference rooms 30 35
Open Plan offices 40 45
But wait. What about vibration?
• Vibration: an oscillation of the parts of a fluid or an elastic solid whose equilibrium has been disturbed.
• Sound: vibrations that we hear
• Noise: my neighbor’s stereo
• It’s all vibration.
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SignificantSources
Find the big stuff because….
Bigger is louder!
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Emergency Generators
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Cooling Towers
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Chillers
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Rooftop AHU’s
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Pumps
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ExhaustFans
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Elevator Machinery
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Rooftop Condensing Units
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VAV Boxes
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Chiller with compressor
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Bigger is relative…
• The smaller stuff can be annoying too but it is easier to fix after the fact….
…..unless you built 300 units with noisy bathroom exhaust fans
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Air Bornevs.
Structure Borne
Air Borne and Structure Borne
• Air borne noise and structure borne noise are really descriptors of the pathways by which sound arrives at the listener from the noise source.
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Air Borne Noise Path
• The sound heard in the room which contains the source is transmitted through partitions and becomes air borne noise in other rooms.
• Generally air borne is sound or noise
• Example:
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Phoenix Noise & Vibration LLC
To Control Air Borne Noise…
• Use noise barriers to control or block air borne noise from other locations.
(remember this…quiz later)
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Structure Borne Noise Path
• Vibration from the source is transmitted directly to the building structure, propagates through that structure and becomes audible sound in another part of the building.
• Generally structure borne is vibration
• Examples:
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Pumps on the floor above…
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…or Pumps on the floor above.
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Structure Borne Noise
1. Ideally, if the vibrating object did not touch anything there would be no structure borne noise
2. Since #1 is pretty much a “no go”, proceed to #33. Introduce vibration isolation
• Springs• Neoprene (rubber)• Polyurethane Foam
To ISOLATE the machine from the structure. (more later)
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A little quiz…
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Noise Control Schemes
Snidely Whiplash
By Jay Ward
The Best Method of Noise Control is at the source…• Buy the quieter unit to begin with…• Increase the distance between source and receiver• Schedule operations of the unit during non‐noise sensitive times
But then what?
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Phoenix Noise & Vibration LLC
Path A: Structure‐borne through floor
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Path A: Structure‐borne through floor
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Path B: Airborne through supply air system
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Phoenix Noise & Vibration LLC
Not this for duct borne noise
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Phoenix Noise & Vibration LLC
Duct Silencers
Courtesy of ASHRAE
Branching
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Elbows
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Side track…
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• What about self generated noise?
Self Generated Noise
• If the speed of the air in the duct work gets too high, the air movement itself will make noise. (Roar, rattle, hum, whistle) which is referred to as “self‐generated noise.”
• Keeping air speed slow by having larger ducts reduces the risk for self generated noise.
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Phoenix Noise & Vibration LLC
Table 6: ASHRAE recommended duct velocities.
Main Duct Location NC Level Maximum Airflow Velocity (fpm)Rectangular Duct Circular Duct
In Shaft or above gypsum board ceiling
45 3500 5000
35 2500 3500
25 1700 2500
Above suspended acoustic ceiling
45 2500 4500
35 1750 3000
25 1200 2000
Duct located within occupied space
45 2000 3900
35 1450 2600
25 950 1700
Back on the main track
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Path C: Duct breakout from supply air duct
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Phoenix Noise & Vibration LLC
Mass Loaded Vinyl Wrap
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Phoenix Noise & Vibration LLC
Path D: Airborne through return air system
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Phoenix Noise & Vibration LLC
Phoenix Noise & Vibration LLC
Duct Silencers
Courtesy of ASHRAE
Case Study
• Roof top exhaust fan impacting neighbors….
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Phoenix Noise & Vibration LLC
Phoenix Noise & Vibration LLC
‐1.0 dBA
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‐1.0 dBA
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‐ 16 dBA
Path E: Airborne through mechanical equipment room wall
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Blocking Noise
• There are noise barriers (blockers) and there are noise absorbers.
• Barriers are not absorbers. • Absorbers are not barriers. • Many times the absorber is somewhat acoustically
transparent.
• Barriers are generally composed of dense, heavy material layers. (Quiz Later)
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Quiz
• Barrier or Absorber?
• Barrier
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Quiz
• Barrier or Absorber?
• Absorber
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Quiz
• Barrier or Absorber?
• Barrier
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Quiz
• Barrier or Absorber?
• Absorber
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Quiz• Barrier or Absorber?
• This is of absolutely no use to the acoustician except to keep beer cold!
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How do we rate noise barriers (partitions)?
Sound Transmission Class
or STC
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Terminology
• Sound Transmission Class (STC) – a single number rating of the partition’s ability to block speech frequencies from one side to another.
• This is blocking AIRBORNE noise, not structure borne
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Room to Room
STC Applies to BothFloor/Ceiling SystemsAnd Walls
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Room to RoomSTC ASTC Subjective Description
30 22‐25 Most sentences clearly understood
40 32‐35 Speech can be heard with some effort
50 42‐45 Loud speech can be heard with some effort
60 52‐55 Loud speech essentially inaudible. Minimum for mechanical rooms.
70 62‐65 Loud music heard faintly
75+ Most noises effectively blockedCredit: Architectural Acoustics: Principles and Design 1999
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Room to Room Quiz…
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STC not always right…
• STC developed for speech, applied to many other noise sources such as:
• Mechanical• Transportation• Music
• Warning: Not always accurate for these other sources
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Bring it all together…
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Phoenix Noise & Vibration LLC
• Structure Borne• Proper Isolation
• Springs• Pads• Roof Curbs
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Mechanical Vibration
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• Equipment Type• Structural Support• Base Type• Isolator Type
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…or Pumps on the floor above.
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Phoenix Noise & Vibration LLC
Typical Pump Isolation
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Phoenix Noise & Vibration LLC
Phoenix Noise & Vibration LLC
Phoenix Noise & Vibration LLC
Not just the “unit”, ducts and piping too!
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Phoenix Noise & Vibration LLC
Phoenix Noise & Vibration LLC
Phoenix Noise & Vibration LLC
Phoenix Noise & Vibration LLC
Phoenix Noise & Vibration LLC
Phoenix Noise & Vibration LLC
Phoenix Noise & Vibration LLC
As an audience, how quiet can you be?
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In Review…
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Noise Measurement Parameters
Significant Sources
Air bornevs.
Structure borne
Noise Control Schemes
Mechanical Noise
Thank You for the Attention!
Questions?Questions?
Discussion?Discussion?Earplugs?Earplugs?
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Acoustical Engineering Solutions
Phoenix Noise & Vibration LLC