mel314 jan2012
TRANSCRIPT
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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=
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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
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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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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= +
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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
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easur ngsoun owerfrom
intensity
2W/mW
I=4 r
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Steadybackgroundnoise
isnotaproblem
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RANKING
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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
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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
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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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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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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
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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
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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
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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