Acoustical Engineering Solutions

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

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Learning Objectives

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

Noise Measurement Parameters

Significant Sources

Air bornevs.

Structure borne

Noise Control Schemes

Mechanical Noise

FSTC dBA

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”

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”

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

Rating Mechanical Noise • Loudness

• Soft• Quiet• Loud• Noisy

• Quality of Noise• Rumble• Roar• Hiss• Tonal

• 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

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

NC vs. RC(N)

Mechanical Noise Measures

Method Loudness Quality

dBA • ‐NC • ‐RC • •

RC(N) • •

ASHRAE* Recommended Background Levels

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.

SignificantSources

Find the big stuff because….

Bigger is louder!

Emergency Generators

Cooling Towers

Chillers

Rooftop AHU’s

ExhaustFans

Elevator Machinery

Rooftop Condensing Units

VAV Boxes

Chiller with compressor

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

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.

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:

To Control Air Borne Noise…

• Use noise barriers to control or block air borne noise from other locations.

(remember this…quiz later)

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:

Pumps on the floor above…

…or Pumps on the floor above.

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)

A little quiz…

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?

Path A: Structure‐borne through floor

Path B: Airborne through supply air system

Not this for duct borne noise

Duct Silencers

Courtesy of ASHRAE

Branching

Elbows

Side track…

• 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.

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

Path C: Duct breakout from supply air duct

Mass Loaded Vinyl Wrap

Path D: Airborne through return air system

Duct Silencers

Courtesy of ASHRAE

Case Study

• Roof top exhaust fan impacting neighbors….

‐1.0 dBA

‐ 16 dBA

Path E: Airborne through mechanical equipment room wall

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)

• Barrier or Absorber?

• Barrier

• Absorber

• Barrier

• Absorber

Quiz• Barrier or Absorber?

• This is of absolutely no use to the acoustician except to keep beer cold!

How do we rate noise barriers (partitions)?

Sound Transmission Class

or STC

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

Room to Room

STC Applies to BothFloor/Ceiling SystemsAnd Walls

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

Room to Room Quiz…

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

Bring it all together…

• Structure Borne• Proper Isolation

• Springs• Pads• Roof Curbs

Mechanical Vibration

• Equipment Type• Structural Support• Base Type• Isolator Type

…or Pumps on the floor above.

Typical Pump Isolation

Not just the “unit”, ducts and piping too!

As an audience, how quiet can you be?

In Review…

Significant Sources

Air bornevs.

Structure borne

Mechanical Noise

Thank You for the Attention!

Questions?Questions?

Discussion?Discussion?Earplugs?Earplugs?

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