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AcousticsAcoustics

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IntroductionIntroduction

Whatissound?

Howdo

we

measure

sound?

Soundpowervs.soundpressure

Soundquality

AHRI880/885

NC

vs.

RCInstallationeffects

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TheSound

RoomThe

Sound

Room

Productsaretestedinaqualified

reverberantchamber(perAHRI

220)

Reverberantchambersareusedfor

quietproducts

Anechoicchambers

are

used

for

noisyproducts

Thereverberantfieldeliminatesall

directionalityfromasoundsource

Soundlevelswithinthereverberant

fieldareequalatallpoints

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TheComparison

MethodThe

Comparison

Method

Determinethesoundpower(Lw)bycomparisontoaknownreferencesoundsource(RSS)

Measurethesoundpressure(Lp)oftheRSSinordertodeterminetheroomattenuation

Lp=Lw roomattenuation

Lw=Lp

+room

attenuation

IfweknowthattheRSScreatesLw=80dBinthefirstoctaveband(63Hz),butwereadonlyLp=70dB,weknowthatwehave10dBofroomattenuationinthat

octaveband

Roomattentionisconstant

Allsoundmetersmeasuresoundpressure(Lp)

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TheDecibel

(dB)The

Decibel

(dB)

Becauseofthegreatdifferencesinenergy(or

pressure)available,

the

log

of

the

actual

valueisused

Referencepoweris1012watts

Referencepressure

is

0.0002

microbars

dBismeasuredvs.frequency

Aninfinitenumberoffrequencies,sotheyare

averagedinto

bands,

typically

called

Octave

Bands

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OctaveBandsOctaveBands

Octavebandsarecenteredabout

increasinglywider

frequency

ranges,

startingwith63cycles/second(Hz)

Each

band

doubles

in

frequencyBandsaretraditionallynumbered,

inourindustry,asshown

Octave Band DesignationsCenter Frequency 63 125 250 500 1000 2000 4000 8000

Band Designation 1 2 3 4 5 6 7 8

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OctaveBandsOctaveBands

Fanpoweredproductsusuallycreatetheirhighestsoundlevelsinoctaveband2(125Hz),butsometimesoctave

band

3(250

Hz)

Grilles,registersanddiffuserscreatetheirhighestsoundlevelsinoctavebands4(500Hz),5(1000Hz)or6(2000Hz)

Octavebands46areknownasthespeechinterferencebands

Itsindustryconventiontoreportsounddatafor

octavebands

27only

Soundroomsizeanddesigncancauseproblemswithreadingsinoctavebands1and8

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To add two decibel values:

80 dB

+ 74 dB

DecibelAddition

ExampleDecibel

Addition

Example

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154 dB (Incorrect)

DecibelAddition

ExampleDecibel

Addition

ExampleTo add two decibel values:

80 dB

+ 74 dB

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To add two decibel values:

Difference in Values: 6 dBFrom Chart: Add 1.0 dB

to higher Value

80 dB+ 1 dB

81 dB81 dB (Correct)(Correct)

80 dB- 74 dB

= 6 dB

Difference In Decibels Between TwoValues Being Added (dB)

Co

rrec

tion

To

Be

Added

To

Higher

Va

lue

(dB)

0

0.5

1

1.5

2

2.5

3

0 2 4 6 8 10

DecibelAddition

ExampleDecibel

Addition

Example

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GoodTo

KnowGood

To

Know

Anysoundsource10dBlowerthanbackground

level

will

not

be

heardAdd3dB(or3NC)todoubleasoundsource

TwoNC40terminalunitsoveranofficewould

probablycreate

an

NC43

sound

level

TwoNC20diffusersinaroomwouldcreateaworst

casesoundlevelofNC23(iftheyareclosetogether)

Donttry

to

add

up

dissimilar

products

in

this

manner

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Sound

Power

ChangesSound

Power

Changes

Equationfor

sound

power

changes

=10lognEquation

for

sound

power

changes

=10logn

1Fanon vs.2Fanson n=2 Add3dB



1Fanon vs.100Fanson n=10

0

Add20dB

50Fanson vs.100Fanson n=2 Add3dB

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ProximityTo

Sound

SourcesProximity

To

Sound

Sources

Wouldyoureallyexpecttohear100fansrunningat

thesame

time?

Properlyselecteddiffusersshouldntbeheardfrom

morethan10feetaway

Althoughthere

may

be

multiple

diffusers

in

aspace,

itsunlikelythatmorethanoneortwoarewithin10

feetofanoccupant

Wewould

only

expect

to

be

able

to

hear

a10

foot

sectionofcontinuouslineardiffuserfromanysingle

location

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1dB notnoticeable

3dB justperceptible

5dB noticeable

10dB twiceasloud

20dB fourtimesasloud

For

High

FrequenciesFor

High

Frequencies

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3dB noticeable

5dB twiceasloud

10dB four

times

as

loud

For

Low

FrequenciesFor

Low

Frequencies

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WhatWe

HearWhat

We

Hear

Ourearscanbefooledbyfrequency

Bothtones

sound

equally

loud

65 dB65 dB

63 HZ63 HZ 1000 HZ1000 HZ

40 dB40 dB

A Difference of 25 dB16

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Acoustic

QualityAcoustic

Quality

Not

too

quiet Dont

destroy

acoustic

privacy

NottooloudAvoidhearingdamage

Dontinterferewithspeech

Nottoo

annoying

Norumble,nohiss

Noidentifiablemachinerysounds

Notime

modulation

Nottobefelt Nonoticeablewallvibration

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10

20

30

40

50

60

70

80

63 125 250 500 1K 2K 4K 8K

MID - FREQUENCY, HZ

OCTAVEBAN

D

LEVEL

_dB

RE0

.0002MICR

OBAR

APPROXIMATETHRESHOLDOF HUMANHEARING

NC-70

NC-20

NC-60

NC-50

NC-30

NC-40

NC

CurvesNC

Curves

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Typical

NC

LevelsTypical

NC

Levels

ConferenceRooms< NC30

Privateoffices< NC35

Openoffices=NC40

Hallways,utilityrooms,restrooms< NC45

NCshouldmatchpurposeofroom

DifficulttoachievelessthanNC30

SelectdiffusersforNC2025

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SoundPower

Vs.

Sound

PressureSound

Power

Vs.

Sound

Pressure

Soundpower(Lw)cannotbemeasureddirectly

Soundpressure(Lp)ismeasuredwithaveryfast

pressuretransducer(i.e.amicrophone)

Calculatesound

power

(Lw)

by

correcting

sound

pressure(Lp)readingsinareverberantchambertoa

knownpowersource

ReferenceSound

Source

(RSS)

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ReferenceSound

SourceReference

Sound

Source

Correctiondevicefora

reverbroom

is

the

RSS

(per

AHRI250)

Calibratedinananechoic

chamberto

simulate

afree

fieldcondition

Usedinareverberantfield,

sothere

is

aknown

error

calledtheEnvironmental

Effect21

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InaReverb

RoomIn

aReverb

Room

Soundpower(Lw)iscalculatedfrommeasured

soundpressure

(Lp)

and

corrected

for

background

Productsoundshouldbe10dBabovebackground

RSSis

used

to

calibrate the

room

Dataisrecordedperoctaveband(or1/3octave

bandifpuretonesareanticipated),foreachoperatingcondition

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Catalog

DataCatalog

Data

Soundpressuredataiscollectedbya

frequencyanalyzer

that

samples

microphones

viaamultiplexer

Dataiscollectedandsoundpowerrecorded

Spreadsheetsareusedtocheckthelinearity

ofdatasets

Catalogdata

is

prepared

from

actual

sound

powerdatasheetsusingacceptedregression

techniques23

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Diffuser

TestingDiffuser

Testing

Currentteststandardfor

diffusers

ASHRAE702006

Nosignificantchangesinmany

years

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Terminal

Unit

TestingTerminal

Unit

Testing

Currentteststandardfor

terminal

unitsASHRAE1302008

ASHRAE130iscurrentlyunder

review

SPC130

Itwillbeupdatedtoinclude

more

products

including

exhaustboxes

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Sound

TestsSound

Tests

Dischargesound,VAVterminals

Unitmounted

outside

room

Dischargingintoreverbroom

Radiatedsound,VAVterminals

Unitmounted

inside

room

Dischargingoutsidereverbroom

Allductworklaggedtoprevent

breakoutDiffusersupply/returnsound

Unitmountedflushtoinsidethe

reverbroomwall26

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Performance

Rating

Performance

Rating

CurrentratingstandardforterminalsunitAHRI8802011(effective

Jan1,2012)

Increasesdischargesound

levelsdue

to

end

reflection

Thisaffectsallpublisheddataandselectionsoftware

Theboxeswillstillsound

thesame,

but

now

the

acousticalconsultantswillbehappier

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Sound

Path

Determination

Sound

Path

Determination

Currentstandardforestimatingsoundlevelsinrooms

AHRI8852008

Providessoundpathdata

fromASHRAE

research

Attenuationfactorsforductlining,ceilingtiles,roomvolume,elbows,flexduct,

etc

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Industry

Standardization

Industry

Standardization

AHRI8852008containsAppendixERecommendsstandard

attenuationstobeusedbyallmanufacturersforcatalogdata

FirstpresentedinARI88598

Makescomparingcatalog

NClevels

much

less

risky

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AHRI

885AHRI

885

2008

Catalog

Assumptions2008

Catalog

AssumptionsRadiatedSound

OctaveBandOctaveBand

22 33 44 55 66 77

EnvironmentalEffect 2 1 0 0 0 0

Ceiling/Space

Effect 16 18 20 26 31 36

TotaldBAttenuation 18 19 20 26 31 36

mineral fiber tile5/8 in thick

20 lb/ ft3

density

5 ft, 1 in fiberglass lining8 in flex duct to diffuser

2500 ft3

room volume5 ft from source

The following dB adjustments are used for the calculation of NC above 300 CFM

DischargeSound OctaveBandOctaveBand

22 33 44 55 66 77

EnvironmentalEffect 2 1 0 0 0 0

DuctLining 3 6 12 25 29 18

EndReflection 9 5 2 0 0 0

FlexDuct 6 10 18 20 21 12

SpaceEffect 5 6 7 8 9 10

TotaldB

Attenuation 25 28 30 53 59 40

OctaveBandOctaveBand

22 33 44 55 66 77

300700CFM 2 1 1 2 5 1

Over700

CFM 4 3 2

2

7

1 30

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Certified

Performance

DataCertifiedPerformanceData

AHRIProgram

Directory

of

Certified

ProductPerformance

www.ahrinet.org

Randomsamplessubjected

toannual

third

party

lab

testing

Verifiesthatperformanceiswithinestablishedtest

tolerancesFailuresresultinpenalties

Voluntaryprogram

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AHRI

SourceAHRISource

PathPath

Receiver

ConceptReceiverConcept

DB(SNDPOWER) DBPATHLOSS = DB(SNDPRESSURE)

Source Path Receiver

AHRI 880Air Terminals

AHRI 885Attn Factors

NC or RCSound Pressure Level

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NCSpecifyingNCSpecifying

SpecifyingandunqualifiedNCvalueisan

open specificationSpecifyinganNCwithspecificpath

attenuationelementscouldresultin

acceptablesound

quality

Itisfarpreferabletosetmaximumallowable

soundpower

levels

than

to

specify

NC

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ReverbRoom

InstallationsReverb

Room

Installations

Testsareunderidealconditions

Boxeshave

only

the

unit

exposed

to

the

room

Diffusershavewonderful inletconditions

Actualconditions

will

always

be

louder

thanreverbroomtestsindicate

CoveredbyAHRI8852008

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SingleNumber

RatingsSingle

Number

Ratings

Anumberofsinglenumberratingschemes

havebeen

developed

to

deal

with

aspectrum

ofsound

Theseinclude:

NC,NRandRC

dBA(Ascale,BscaleandCscale)

Sones,Bels

STC,NRC

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The

dBA

ScaleThedBAScale

Usedforoutdoornoiseevaluation

Also

used

for

hearing

conservation

measurementsBasisofmostnonterminalsoundratings

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ExampleExample

10

20

30

40

50

60

70

80

63 125 250 500 1K 2K 4K 8KMID - Frequency, HZ

Oc

tave

Band

Leve

l_dB

RE

0.0

002Micro

ba

r

Approximatethresholdof humanhearing

NC-70

NC-20

NC-60

NC-50

NC-30

NC-40

Sound Power

Sound Power less 10 db

in each band

NC rating given is NC-30

since this is highest point

tangent to an NC curve

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ExampleExample

10

20

30

40

50

60

70

80

90

63 125 250 500 1K 2K 4K 8K

MID - FREQUENCY, HZ

Octave Band Level

dB RE 0.0002 Microbar

NC-70

NC-20

NC-60

NC-50

NC-30

NC-40

Approximate threshold

of human hearing

NC rating given is NC-45

since this is highest point

tangent to an NC curve

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Both noise spectrums would be

rated NC-35, However, they would

subjectively be very di fferent!

10

20

30

40

50

60

70

80

90

63 125

250

500

1K 2K 4K 8K

Mid - Frequency, HZ

Oc

tave

Ban

dLeve

l_dB

RE0

.0002Mic

ro

bar

NC-30

NC-70

NC-20

NC-60

NC-50

NC-40

Typical grille noise

at a distance of 10FT

(high-frequency)

Typical fan noise from

adjacent mechanical

room (low-frequency)

Approximate threshold

of human hearing

ExampleExample

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NCvs.RCNCvs.RC

NCratesspeechinterference

andputs

limits

on

loudness

NCgivesnoprotectionforlowfrequency

fannoiseproblems

NCstopsat63Hzoctaveband

RCincludesthe31.5Hzand16Hzoctaveband

RCratesspeechinterferenceanddefines

keyelementsofacousticalquality

40

C i iR C i i

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RoomCriteriaRoomCriteria

(RC)Curves(RC)

Curves

RC

50

45

40

35

30

25

C

ADAPTED FROM 2009 ASHRAE FUNDAMENTALS HANDBOOK - ATLANTA, GA

Region AHigh probability that noise

induced vibration levels inlight wall and ceiling structures

will be noticeable. Rattling

of lightweight light fixtures,

doors and windows should

be anticipated.

Region BModerate probability that

noise-induced vibration will be

noticeable In lightweight light

fixtures, doors and windows.

10

20

30

40

50

60

70

80

90

1631

.5 63 125

250

500

1K 2K 4K

Octave Band Center Frequency, HZ

A

B

Threshold

of audibilityOc

tave

Ban

dSoun

dPress.

Leve

l,d

B

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TwoPartsofRCTwoPartsofRC

Example RC40N

Thenumber

is

the

speech

interference

level

Thelettertellsyouspeechquality

(N)

=

neutral

spectrum (R)=toomuchrumble

(H)=toomuchhiss

(V)=too

much

wall

vibration

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RCNumberCalculationRCNumberCalculation

Averageoflevelofthenoiseinthe

octavebands

most

important

to

speech

500HzOctaveband=46dB

1000HzOctave

band

=40

dB

2000HzOctaveband=34dB

RC=(46+40+34)/3=40dB

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RCLetterDeterminationRCLetterDetermination

PlotroomsoundpressureonRCchart

Determinerumble

roof

5dBgreaterthenlowfrequency

Determinehissroof

3dB

greater

then

high

frequency

Rroomsoundpressurecrossesrumbleroof

H

roomsound

pressure

crosses

hiss

roof

Vroomsoundpressuregoesintovibrationzone

Nroomsoundpressuredoesnotcross 44

R blRumbly

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Measured data is outside the

reference region by >5 dB,

below the 500 Hz octave band,therefore the noise is l ikely

to be interpreted as rumbly

PSIL=(38+35+29) / 3 = 34

RC-34(R)

RumblyRumbly

Spectrum(R)Spectrum

(R)

10

20

30

40

50

60

70

80

90

1631

.5 63 125

250

500

1K 2K 4K


Oc

tave

Ban

dSoun

dPress.

Leve

l,dB

45

Rumbly & InducedRumbly & Induced

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A

B

RC-33(RV)10

20

30

40

50

60

70

80

90

1631

.5 63 125

250

500

1K 2K 4K


Octa

ve

Ban

dSoun

d

Press.

Leve

l,dBEven though the PSIL

Is only 33 dB, the

noise spectrum

falls within regions

A & B indicating a

high probability of

noise-induced

vibration in lights,

ceilings, air dif fusers

and return air gr illes

Rumbly&InducedRumbly&Induced

Vibration(RV)Vibration

(RV)

PSIL= (38+32+29) / 3 = 33

46

NeutralNeutral

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Oc

tave

Ban

dSoun

dP

ress.

Leve

l,dB

NeutralNeutral

Spectrum(N)Spectrum

(N)

C

Measured data must

not lie outside the

reference region by>5 dB, below the 500 Hz

octave band

Measured data must

not lie outside the

reference region by

>3 dB, above the 1000 Hzoctave band

RC-34(N)10

20

30

40

50

60

70

80

90

1631

.5 63 125

250

500

1K 2K 4K

PSIL=(38+35+29) / 3 = 34

47

HissyHissy

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C

Measured data is

outside the referenceregion by >3 dB, above

the 1000 Hz octave band,

therefore the noise

is likely to be

interpreted as hissy

RC-35(H)10

20

30

40

50

60

70

80

90

1631.5 63 12

525

050

01K 2K 4K

PSIL = (35+36+34) / 3 = 35

HissyHissy

Spectrum(H)Spectrum

(H)


Oc

tav

eBan

dSoun

dP

ress.

Leve

l,dB

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WhoUsesRC?WhoUsesRC?

NCremainsthebestwaytomakeproduct

selectionsRCispreferredasananalysistool

Acousticalconsultantswilltypicallyreport

whetherornotequipmentmeetsNCspecbutwilldescribetheresultingsoundspectrumin

terms

of

RCYoushouldcontinuetoseecatalog

applicationdataintermsofNC49

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TerminalUnitInstallationsTerminalUnitInstallations

Soundcharacteristics

Optimalinstallation

Attenuators

Liners

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SoundCharacteristicsSoundCharacteristics

Radiatedsoundisprimaryissuewithfanpowered

terminalsDischargesoundisprimaryissuewithnonfan

terminals

Fanpowered

sound

is

typically

set

in

2nd

(125

Hz)

and3rd(250Hz)octavebands

Longsoundwaves

Harderto

attenuate

Dischargesoundiseasilyattenuatedwithlined

ductworkandflexduct51

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IdealTerminalUnitInstallationIdealTerminalUnitInstallation

VAV

UNIT

Lined Sheet Metal Plenum(Max velocity 1,000 FPM)

Lined Flexible Ducts

To Diffusers

Flexible Connectors

For Fan-powered Units

D

> 3 D

Ceiling

Maximize Height

Above Ceiling

4' Min.

52

Max velocity 2,000 FPM

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AttenuatorsAttenuators

Singleduct

Equivalentto

lined

ductwork

Dualduct

Providesamixingareaforunit,butnotmuch

soundattenuation

Fanpowered

Lined

elbow

or

boot may

provide

2dB

attenuationbyremovinglineofsighttomotor

Carefullyengineeredattenuatorscanprovide

additional

sound

reductions53

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LinersLiners

Differentlinersinsingleductsdonotaffectdischarge

sound

much Unitistooshortfortheairtointeractwithliner

1"linerdoesnotsignificantly

decrease

sound

compared

to

"Foilfacedlinersadd68dB

Fiberfreeadds46dB

Doublewall

is

variable

Kettledrumeffectincreasessound,butitisdirectional

54

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FlexDuctFlexDuct

Dontforgetaboutflexduct

5'of

flex

can

reduce

mid

frequencies

by20dBormore

Flexisbetterthanlinedductorattenuators

inreducing

low

frequencies

Youcanhavetoomuchofagoodthing

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Diffuser

Tests

Diffuser

Tests

ASHRAE

ConditionsASHRAE

Conditions

10 equivalent Diameters, min

Pressure

Measured Air Flow

Discharge VelocitySound 56

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Inlets:

3

Equivalent

Diameters

Inlets:

3

Equivalent

Diameters

IdealIdeal~1NCaddtocatalogdata

Pressure

Measured Air Flow

3 equivalent Diameters

Discharge VelocitySound

Flex Duct, 1 radius bend

57

I l t L 90 t DiffI l t L 90 t Diff

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Inlets:Long90atDiffuserInlets:Long90atDiffuser

~3NCaddtocatalogdata

Pressure

Measured Air Flow

Discharge Velocity

Sound

Flex Duct

58

I l t H d 90 t DiffI l t H d 90 t Diff

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Inlets:Hard90atDiffuserInlets:Hard90atDiffuser

~5NCaddtocatalogdata

Pressure

Measured Air Flow

Discharge Velocity

Sound

Flex Duct

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Inlet

Inlet

KinkedKinked~79NCaddtocatalogdata

Pressure

Measured Air Flow

Discharge VelocitySound

2 equivalent Diameters

Flex Duct

60

Summary of ResultsSummary of Results

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SummaryofResultsSummaryofResults

Minimumaddforflexduct=1NC

Worstcase

add,

Kinked =

79NC

Airdistributionpatterncanbegreatlyeffected

Plaque/Perforatedshowsmosteffect

MultiCone/Louveredshowsleasteffect

Resultswerenotthesameforalldiffusertypes

Dontforget

that

catalog

NCs

are

based

on

typicaloffices(10dBacrossallbands)

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Some

Diffuser

SolutionsSome

Diffuser

SolutionsLocatebalancingdampersatbranchtakeoff

Keepflexible

duct

bends

as

gentle

as

possible

Flexductisagreatattenuatorofupstreamnoise

sources

Keepduct

velocities

as

low

as

possible

Butoversizingcanresultinhigherthermalloss

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Additional

ResourcesAdditional

ResourcesASHRAEFundamentals

Chapter8,

2009

Edition

ASHRAEHVACApplications

Chapter48,2011Edition

CognizantTechnicalCommittee

TC2.6

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SummarySummaryNCremainsthepreferredsoundspecification

RCis

often

used

after

the

fact

Maxsoundpowerlevelsaresafest

Liningmaterialsaffectsoundlevels

Carefulselection,designandinstallationare

requiredtoavoidproblems

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2012 Acoustics

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