Acoustical Presentation to theRocky 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

Acoustics 101

Definitions of Terms: Sound Power vs Sound Pressure

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

Comparison of dBA, NC, RC

NC 36 RC 18 (R)

39 dBA39 dBA

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 Sound Isolation Penetrations Full Height Partitions

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

Case Studies