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FUNDAMENTALSOFNOISE

Dr.ASHISHKDARPE

ASSOCIATEPROFESSOR

DEPARTMENTOFMECHANICALENGINEERING

IITDELHI

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Soundisasensationofacousticwaves(disturbance/pressurefluctuationssetup

name um

Unpleasant,unwanted,disturbingsoundisgenerallytreatedasNoiseandisa

highlysubjectivefeeling

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Soundisadisturbancethatpropagatesthroughamediumhavingproperties

.

transmittedisair.

Basicallysoundpropagationissimplythemoleculartransferofmotional

. .

Guesshowmuchisparticle

displacement??

.

Frequency:Numberofpressure

cycles/time

alsocalledpitchofsound(inHz)

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Providesdefinitequantitiesthatdescribeandrate

sound Permitprecise,scientificanalysisofannoyingsound

(objectivemeansforcomparison)

HelpestimateDamagetoHearing

program:Airports,Factories,Homes,Recording

, , .

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Quantifying Sound

AcousticVariables:PressureandParticleVelocity

RootMeanSquareValue(RMS)ofSoundPressure

Meanener associatedwithsoundwavesisits fundamentalfeature

energyisproportionaltosquareofamplitude

1

221

[ ( )]

T

p p t dtT

= 0.707p p=

7

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SpeedofSoundTherateatwhichthedisturbance(soundwave)travels

Propertyofthemedium

0P Tc =Alternativel ,0

c Speedofsound P0,0 PressureandDensity Ratioofspecificheats R UniversalGasConstant

empera ure n o ecu arwe g

1 =0

2731

+= ccc.25

smc /35540 =

SpeedofLight:299,792,458m/s Speedofsound344m/s

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RangeofRMSpressurefluctuationsthatahumanearcandetectextendsfrom

0.00002 N m2 Pascal

(thresholdofhearing)to

20N/m2 (Pascal)(sensationofpain)1,000,000timeslargerpeakpressureofloudestsoundis3500timessmallerthanatm.pressure

9

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Verylargerangeof

soundintensitywhich

theearcan

accommodate,

fromtheloudest

(1watt/m2)

tothequietest

(10 watts/m ),

10energyreceivedfroma50wattbulb

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Levels

Aunitofalo arithmicscaleof owerorintensit calledthepowerlevelorintensitylevel.

Thedecibelisdefinedasonetenthofabel

One

bel

represents

a

difference

in

level

between

two

intensities(oneofthetwoistentimesgreaterthantheother

Thus,theintensitylevelisthecomparisonofoneintensitytoanotherandmaybeexpressed:

Intensitylevel=10log10 (I1 /Iref)(dB)

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lowones

Theotherreason:Equalrelativemodificationsofthestrengthofa

physicalstimulusleadtoequalabsolutechangesinthesalienceof

thesensoryevents(WeberFechnerLaw)andcanbeapproximated

byalogarithmiccharacteristics

(Earrespondslogarithmicallytostimulus)

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dB SCALE

Acoustic parameters are expressed as logarithmic ratio of themeasured value to a reference value

The Bel (B) is a unit of measurement invented by Bell Labs and

named after Alexander Graham Bell.

The Bel was too large, so the deciBel(dB), equal to 0.1 B,

became more commonl used as a unit for measurin sound

intensity

Power Ratio of 2 = dB of 3

Power Ratio of 10 = dB of 10

Power Ratio of 100 = dB of 20

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Sound Pressure Level

Inlinearvibroacoustics,timeaveragedpowervaluesareproportional

tot esquare rmsamp itu eso t e ie varia es e.g.,pressure,

particlevelocity)

Thustocalculatelogarithmiclevelsfromthefieldvariables,itisthese

squaredrmsamplitudesthatmustbeused.

21

10 210 rms

pSPL Log dB=

11020

rmspSPL Log dB=

In acoustics, the reference pressure

refre

Pref=2e-5 N/m2 or 20Pa (RMS) loudest sound pressure that a

normal person can barely perceive at 1000Hz

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Sound Pressure Level

Corresponding to audio range of Sound Pressure

2e-5 N/m2 - 0 dB

20 N/m2 - 120 dB

Normal SPL encountered are between 35 dB to 90 dB

For underwater acoustics different reference pressure is used

Pref= 0.1 N/m2

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16

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Typical average decibel levels (dBA) of some common sounds.

Threshold of hearing 0 dB Motorcycle (30 feet) 88 dB

Rustling leaves 20 dB Foodblender (3 feet) 90 dB

Quiet home 40 dB Diesel truck (30 feet) 100 dB

Quiet street 50 dB Power mower (3 feet) 107 dBNormal conversation 60 dB Pneumatic riveter (3 feet) 115 dB

Inside car 70 dB Chainsaw (3 feet) 117 dB

Automobile (25 feet) 80 dB Jet plane (100 feet) 130 dB

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Sound Power

n ens y : verage a e o energy rans er per un area

2W 2

2

4 r0

attr rc

= =

Sound Power Level:1010log

ref

WSWL

W= dB

Reference Power Wref=10-12

Watt

Peak Power out utPeak Power out ut:

Female Voice 0.002W, Male Voice 0.004W,

-9 v Large Orchestra 10-70W, Large Jet at Takeoff 100,000W

15,000,000 speakers speaking simultaneously generate 1HP

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Sound Intensity

1

T

I p u dtT

= 2

PI

c=

For plane progressive waves;

Hold true also for spherical

1010

I

IL Log=

waves far away from source

= -12 2re

2/ c

re

10 10 20

20 102 5 (2 5) /( )

SPL Log dB Log dBe e c

= =

10 10 1012 2 2

0 0

10 1010 10 10

10 (2 5) /( ) (2 5) /( )ref

I ISPL Log dB Log Log

e c I e c

= = +

For air, 0c 415Ns/m3 so that 0.16 dBSPL IL= +

20

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21

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Intensity&pressure measuredusing

Poweriscalculated

Powerisbasicmeasureofacousticenergyit

canproduce

&isindependentofsurroundings

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SoundPressure:

evaluationofharmfulnessand

location&ratingofnoisesourcesrateofenergyflowperunitarea

SoundPower:

fornoiseratingofmachinesuniquedescriptorofnoisinessof

source

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Soundintensitymeasurementallowsinsitu

estimationofnoisesourceranking

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Sound Intensit

Time averaged rate

of energy flow per

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Sound Intensity

Timeaveragedrateof

energyflowperunitarea

1

T

0

T

26

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easur ngsoun owerfrom

intensity

2W/mW

I=4 r

27

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Steadybackgroundnoise

isnotaproblem

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RANKING

29

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Lp1Lp22 2 2

1 2totp p p= +% % %2

110 2

10 rms

ref

pSPL Log dB

p=

12

1 102

10pp

p=

%

%

1 2

2 2 10 1010 10p pL L

tot ref p p = + % %

1 22

10 1010lo 10 lo 10 10

p pL L

totp

= +%

refp %

nLpN

31

10

1

tot

n=

=

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COMBINATIONS OF SOURCES

ntens ty eve s o eac o t e sources s same,1

10

L

T

og=

1T og=

us or en ca sources, o a n ens y eve s og

i.e., 3dB greater than the level of the single source

For 2 sources of different intensities: L1 and L2

L1=60dB, L2=65.5dB

=

L1=80dB, L2=82dB

LT=84dB

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C rr l t n n rr l t r

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Whichsourceto

firsttakecare?

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FREQUENCY & FREQUENCY BANDS

Fre uenc of sound ---- as im ortant as its level

Sensitivity of ear

Sound insulation of a wall

Attenuation of silencer all vary with freq.

< z z to z > z

Infrasonic Audio Range Ultrasonic

35

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2 1.0

1.2

0

1

Amplitude =

Amplitude

0.4

0.6

0.8

0

0.05 30

40-1

F 0 10 20 30 40 50 60

0.0

0.2

.

0.15

0.20

10

requency

TimeFrequency (Hz)

F C i i f S d

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Frequency Composition of Sound

Pure tone

Musical

Instrument

37For multiple frequency composition sound, frequency spectrum isobtained through Fourier analysis

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Complex Noise Pattern

produced by exhaust of Jet Engine, water at base of

Niagara Falls, hiss of air/steam jets, etc

tud

e(dB)

A1

Ampli

No discrete tones, infinite frequencies

Better to group them in frequency bands total strength in

each band gives measure of sound

Octave Bands commonly used (Octave: Halving / doubling)

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Octave Filters

39

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Octave and 1/3rd Octave

an ers

mostly to analyse relatively

smooth varying spectra

If tones are present,

1/10th

Octave or Narrow-bandfilter be used

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42

R di ti f S

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Radiation from Source

22 24 4 Watt

pW r I r = =

Point Source (Monopole)

0c

W: is acoustic power output of the source;

10 10

1 110log 10 log

W WIL

= =

10lo 20lo

refr r

W

IL r

= 4 10

Constant term Depends on distanceInverse S uare Law

rom source

When distance doubles (r=2r0) ; 20log 2 + 20log r0 means 6dB difference in the Sound Intensity/pressure

Level

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If the point source is placed on ground,

it radiates over a hemisphere,

the intensity is then doubled and

1W = 10 22ref

r I

W

=

Pressurelevelgetsdoubled10 10122 10 att esamepoint

20log 8PL L r dB= Vs 20log 11PL L r dB= Forsourcenoton

44

ground

VALIDITY OF POINT SOURCE

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VALIDITY OF POINT SOURCE

In free field condition,

the wavelength of the sound generated is considered a point

source

Alternatively a source is considered point source if the receiver is

Some small sources do not radiate sound equally in all directions

Directivity of the source must be taken into account to calculate

power from the sound pressure

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DIRECTIVITY OF SOUND SOURCE

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oun sources w ose mens ons are sma compare o e wave eng o

the sound they are radiating are generally omni-directional;

,

power Wsoundradiatingsourceldirectionaa

fromrdistanceatandangleanatIntensitySound

=Q

47power Wsoundsametheradiatingsource

rec ona-omnaromrs ancean ens youn

Directivity Factor & Directivity Index

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DirectivityFactor&DirectivityIndex

Directivity Factor Directivity Index

2

p

I

IQ

==

thus

QDI = 10log10

Q

Ir24

=

pSp LLDI = Rigid boundaries force an omni-directional source to radiate sound in preferential direction

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49

EFFECT OF HARD REFLECTING GROUND

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EFFECT OF HARD REFLECTING GROUND

Radiated Sound Power of the source can be affected by a

ri id reflectin lanes

Strength and vibrational velocity of the source does not

pressure and four-fold increase in sound intensity compared to

monopole (point spherical source) in free space

If source is sufficiently above the ground this effect is reduced

50

SoundFields

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ISO3745

51ISO3741

MWLLab,KTHSweden

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,

Finding

sound

power

(ISO

3745)

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Measurements made in semi-reverberant and free field conditions

are in error of 2dB

53

SoundPowerEstimationfromPressure

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eve

measurements

Free

e

con t ons

r= 2I 12 1210 10

11 20logIL L r = + +ogPL L r B = + + I Pwith L L

equation changes to

54

ogI

r = + +

Measurement of Power in Reverberant

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MeasurementofPowerinReverberant

Room

4

10lo

Q

L L

= + +4 r R

( )1avg

avg

S

R

=

constant team used todescribe acoustic

c arac er s c o a room

,L

=Lp +10logV 10logT60 14

55

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ensoun e sne t erfreenorcompletelydiffuse.

Usecalibratedsoundsourcew t nownpower

spectrum.

Thenuse

L

=Lr Lpr + Lp

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Totakecareofnearbyreflecting

surfacesandbackgroundnoise,

Measureatnumberoflocationson

r

Lpd =Lp 10log10(d/r)2

Thenuse

L is

e uivalent

sound

ressure

level

at

the

pd+ og10 referenceradiusd,andLp ismeansoundpressurelevelmeasured

57

,

(S/2)

Backgroundnoise

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Environmental

Effects

WindGradient

HotSunny

Day

e oc y ra en

()

TemperatureGradient

Wind&Tempeffectstendto

cancelout

Increaseordecreaseof56dB

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NoiseMapping

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Noise

Contours

HUMANPERCEPTION

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TheHumanEar

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OuterEar:Pinnaandauditorycanalconcentratepressureontodrum

MiddleEar:Eardrum,SmallBones

connectingeardrumtoinnerear

,

withbasilarmembranerespondto

stimulusofeardrumwiththehelpof

cells,differentportionsresponding

differentfrequenciesofsound.

emovemen o a rce s s

conveyedassensationofsound to

thebrainthroughnerveimpulses

Masking takesplaceatthe

membrane;Higherfrequenciesare

maskedbylowerones,degree

dependsonfreq.differenceand

relative

magnitudes

of

the

two

sounds

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EffectsonHumans:Hearing

Actuallyhearwithourbrains,notourears

Haircellssendimpulsestobrain

Damagetohaircellscanresultinhearingloss

Thresholdshiftseveralhours

Noiseinducedgraduallybecomepermanent

TheHearingMechanism

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gsound waves enter

the outer eartravel through external

auditory canalreach the tympanic

membranemembranesand ossicles vibrate

waves are set up in thefluids of cochlea

basilar membranevibrates

Sensory cells of organof corti are stimulated

Nerve impulses aresent to the brain

TheHearingMechanism

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external auditorycanal

tympanicmembrane

sound wavesenter the outer

mem ranes

and

ossicles vibrate

waves are set upin the fluids of

Nerve impulsesare sent to the

membrane

vibrates

organ of corti

are stimulated

PathogenesisofNoiseInducedHearing

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Transductionofvibrationduetonoiseto.

ThehaircellsintheorganofCortimaybedamageddirectlybynoise,orindirectlyby

very

high

levels

of

continuous

sound

which

causesvasoconstrictionofthevesselsofthestriavascularisinthecochleabloodsupply.

Thisrendersthehaircellsrelativelyanoxic

a)

Theamountandtypeofdirecthaircell

damagedependsontheintensityofthesound.

A oveacertainminimumo requencyan intensity,theouterhaircellsshowsignsofmetabolicexhaustionwithdroopingofthestereocilia.

Highersoundlevelsdamagetheouterhaircellstereociliafurther,includingdestructionoftheintercilialbridges,andrecoverytakes

b)

fig. Hair cells photograph-ed with a sweeping electron. .

hair cells. a) Normal,uninjured hair cells. b) Haircells injured by noise. (Source: Gran Bredberg,

Hrselkliniken, Sdersjukhuset.)

SOUNDBITS

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Unlessthereisa3dBdifferenceinSPL,humanbeingscan

Soundisperceivedasdoubledinitsloudnesswhenthereis

erence nt e .

(Remember6dBchangerepresentsdoublingofsound

pressure!!)

Earisnotequallysensitiveatallfrequencies:

highlysensitiveatfrequenciesbetween2kHzto5kHz

lessatotherfreq.

on

SPL!!!!

RESPONSEOFHUMANEAR

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LoudnessLevel

(Phon)

valueofSPLat

1000Hz

0Phon:thresholdof

hearing

ou ness eve

(Phon)usefulfor

comparingtwo

forequalloudness

But,60Phonisstill

nottwiceasloudas

30Phon

Doublingofloudness

EqualLoudnessContoursforpuretones,FreeField

conditions

correspondsto

increaseof10Phon

LOUDNESSINDEX

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DirectrelationshipbetweenLoudness

eve ons an ou ness n ex

(Sones)

40

102P

S

=

8Sonesistwiceasloudas4Sones

WeightingCharacteristics

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Aweighting:40Phonequalloudnesslevelcontour

Cweighting:90Phonequalloudnesslevelcontour

DweightingforAircraftNoise

Micro hones

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measurement transducer to measure noise

Condenser Microphone

Ceramic Microphone

Micro hone

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The change in the Smaller the diaphragm higher the

converted into electrical signal

sensitivity

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Can be used in Very expensive

ex reme con on

Insensitive to Sensitive to

humidity &

vibrations moisture

Measurements range can be from the

0.01 Hz to 140 KHzDynamic range up to 140 dB

Othertypesincludedynamic/piezo microphones

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HearingDamagePotentialtosoundenergy

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level & durationofexposure

EquivalentContinuous

SoundLevel(Leq)

jforwhichSPLofLjwas measured

Totaltimeintervalconsideredis

dividedinNparts

witheachparthasconstantSPLofLj

1010

1

10 10j

eq j

j

L Log t dB=

=

100 70

10 1010

1 710 10 10 91

8 8eq

L Log dB= + =

IntegratingSoundLevelMeterforrandomlyvarying

sound e.g.,60secLeq

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Sound

Ex osure

Level

SELConstantlevelactingfor1secthathas

thesameacousticenergyasthe

Vehiclepassingby;

Aircraftflyingover

Measurements

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Measurements

Regulations:

Basisof90dB(A)for8hraday.

ISO(1999):IncreaseinSPLfrom90to

93dB(A)mustreducetimeofexposure

from8to4hours

OSHA:withevery5dB(A)increase,reduce

exposurebyhalf

OccupationalSafetyandHealthAdministration

NoiseRatingCurves(ISOR1996)

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LevelofNoise

Annoyance

NR78

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Errorsoftheorderof6dBaround400Hzduetoreflections

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SoundPressure,IntensityandPower

,

MultipleSoundSources

oun ourcesan soun e s

Directivity SoundLevelMeter

Humanresponse

81

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Sources:

VibrationandNoiseforEngineers,KPujara

FundamentalsofAcoustics,Kinsle,Frey,Coppens andSanders

FundamentalsofNoiseandVibrationAnalysisforEngineers,MNortonandD

Karczub

,

Measuring

Sound,

B&K

Application

NotesSoundIntensit B&KA licationNotes

BasicConceptsofSound,B&KApplicationNotes