Acoustical Design Considerations

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