Acoustic phonetics.pdf

7/25/2019 Acoustic phonetics.pdf

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Acoustic phonetics


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The science to describe

sound is known asacoustics.

The study of the physical

properties of sound waves.


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Acoustic phonetics is concernedwith describing the dierent

kinds of acoustic signal that themovement of the vocal organsgives rise to in the production of

speech.


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Speech Production system


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COPO!"!TS O# SP""C$P%O&'CT(O!

).The system below the laryn*+S',-OTTA/.

0.The laryn* and thesurrounding structure.

1.The structure and theairways above the laryn*+S'P%A-OTTA/.


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T$" S',-OTTA S2ST"

The trachea3

0.4cm0 cross5sectional area )65)0 cmin length +adults/

The bronchi3

Alveolar sacs3

ungs3

0nos

lies within the lungs

vital capacity 166654666 cm1

ma*imum range of lungvolume availablee*cursionsduring normal breathing )666cm1


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(!SP(%AT(O!

The principal muscles for inspirationare3

).&iaphragm

0. "*ternal intercostals

3 contraction lowers the

diaphragm3 contraction raisesthe ribcage


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"7P(%AT(O!

The principal muscles for e*piration are3

). (nternal intercostals 3

0. Abdominal muscles

The elastic recoil of the lungs always

contributes an e*piratory force8 but thisforce is augmented or reduced by theaction of e*piratory or inspiratory muscles.

contraction pulls theribcage downwards


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T$" A%2!7

The principal structure in the laryn* thatplay a direct role in the production ofspeech are the vocal folds.

9ocal folds3

ength3 ).6 to ).4 cm

thickness 0 to 1 mm roughly parallel to each other in an

antero5posterior direction.

0 bands or cordlikesegments of tissue


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9ocal folds and ventricular folds areadducted+appro*imated/ orabducted+separated/ leaving a

space between the vocal folds.


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SO'!&S

Sound is a pattern of pressure variationthat moves in wave from a source.

Sound waves are the means of acoustic

energy transmission between a soundsource and a sound receiver.

Pressure :uctuations move through

space but each particle moves only asmall distance.


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Sound is e*perienced whenpressure :uctuations reach theeardrum and the auditory system

translate these movement intoneural impulses.

Sound waves perceived byhuman ears range from06Pascal to 06Pascal.


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Propagation of sound

A sound produced at a source sets upa sound wave that travel through theacoustic medium.

Sound waves are small dierences inair pressure which diuses in alldirections.

An acoustic waveform is a record ofsound5producing pressure :uctuationover time.


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Types of waves

Transverse ;ave3

e.g. the me*ican wave.

ongitudinal ;ave3

The motion of the individual particlesis parallel to the motion of the wave.

e.g. sound waves.

(n a transverse wave the motion ofthe individual particles is

perpendicular to the motion of thewave.


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Sound wave consist of air pressure

variations.

The speed at which these airpressure variations spread through a

space is called the speed of sound. The speed of sound depends on the

density and the elasticity of the

medium. The speed of sound is around 1


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Types of sound

). Periodic sound3 periodic sounds havea pattern that repeats at regularintervals.

0. Aperiodic sound3 aperiodic sound donot have a regularly repeating

pattern> they have either a randomwaveform or a pattern that doesn?trepeat.


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Periodic Sounds

).Simple periodic sounds.

0.Comple* periodicsounds.


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Simple periodic waves

Simple periodic waves are also calledsine waves.

The name comes from a wave of thisshape graphs the geometric sinfunction of an angle as it moves from6@ to 16@ +one cycle/. A cosine wave

has the same shape as a sine wave8but begins at the ma*imum value +)/rather than 6.


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Any wave that


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Any wave thathas the shapeof a sine wave8

regardless ofdierences inphase8 is calleda sinusoidal

wave.

They resultfrom simpleharmonicmotion.


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%epresentation where the sound

pressure is plotted vertically againsta horiBontal time a*is8 is called anoscillogram orwaveform.


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(n order to dene a sine wave8 we need to

know three principal dimensions+properties/3

1.Frequency

2.Amplitude

3.Phase


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;hat is freDuencyE

The number of times the sinusoidalpattern repeats per unit.

"ach repetition of the pattern iscalled a cycle.

The duration of a cycle is its period.

#reDuency is e*pressed as cycle persecond8 which by convention is called$ertB +$B/


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Oneycle

!a"imum

!inimu

m


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$ow do we get the freDuency of asine wave in $ertB.

&ivide one second by the period+theduration of one cycle/

f# 1$%& where % is theperiod in seconds


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Amplitude

The displacement of the vibratingmedium from its rest position.

The ma*imal displacement from the

Bero line is known as amplitude. (t shows the vertical range of the

waveform.


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The distance between a ma*imum

and the ne*t minimum is calledpea'(to(pea' amplitude.

The higher the peak5to5peak the

dierence between the air pressurema*ima and the air pressure minimais larger.

This means that the acoustic signal isperceived as being louder.

Amplitude is measured in terms ofdecibel)d*+.


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&amping3 The gradual loss of

energy+and amplitude/ from cycle tocycle is known as damping.


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Phase

Phase3 The e*act position of aspecic point in a waveform.

(t is measured in terms of degrees.


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;hy freDuency8 amplitude importantfor acoustic phoneticsE

Any oscillating system whose periodand velocity have the inverserelationship dened above and

captured by the sine waves aresimple harmonic motion.

Systems that oscillate in Simple

harmonic motion produce a simpletone.


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The mathematics of sinusoidal motionsare well understood.

Sinusoidal waves can be described interms of their freDuency8 the amplitudeand the phase.

Phase is not usually that important forspeech analysis.

So8 if we know the freDuency andamplitude of a sinusoid we knoweverything important there is to knowabout anything vibrating in simpleharmonic motion.


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#rench mathematician Fean ,aptisteFoseph #ourier proved in )G6H that

every kind of vibration+including allcomple* speech sound/ can bedescribed as the sum of a set of

simple sinusoid of varyingfreDuencies and amplitude.

An understanding of sinusoidal

motion dened by freDuency andamplitude is the key tounderstanding all speech sounds.


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Comple* periodic waves

The result of adding sinusoids or simpleperiodic waves is a comple* wave.

Comple* waves are not sinusoidal

itself8 but it is periodic. As comple* waves is made up of some

numbers of component freDuencies8

the basic freDuency8 the rate at whichthe whole patterns repeats8 is calledfundamental freDuency+#6/


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#6 determines the pitchof a soundwave.

The loudness of the sound dependson both freDuency and amplitude.

-iven a #68 greater the overall

amplitude8 the louder the sound.

The component freDuencies arecalled harmonics.

The dierent freDuencies andamplitudes of the componentharmonics give the sound its Duality.


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The fundamental freDuency is alwayseDual to the greatest common factorof the comple* freDuency.

Component waves of 46$B8 )46$B8and 046$B will have a #6 of 46$B.


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Aperiodic waves

The moment5to5moment pressurevariation are random8 there are norepeating pattern.

A special category of aperiodic soundis transient.

Transient sounds are instantaneous8

there is a momentary disturbance8not drawn out or repeated.

e.g. knock on the table8 slamming of

a door


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%esonance

The reinforcement or prolongation ofsound by re:ection from a surface orby the synchronous vibration of a

neighboring obIect. !atural resonant freDuency 3 "very

obIect has a basic freDuency8 or a set

of freDuencies at which it willnaturally oscillate when energy isapplied.


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(f an input freDuency is synchroniBedwith the natural freDuency of any obIect8

the two system are in resonance. ;hen energy is applied in resonance

with a natural freDuency8 the amplitudeof movement at that freDuency isincreased8 because the two forces areacting together.

;hen energy is applied that is not in

resonance with a natural freDuency8 thatenergy is Duickly dissipated because theforces are cancelling each other out8 andamplitude at that freDuency dies out.


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ObIects do not vibrate freely> they

are tunedto resonate only to anarrow freDuency band.

(f the freDuency of the sound from asource happens to match the naturalresonant freDuency of the obIect8 theobIect will vibrate in resonance withthe sound8 passing along the pattern

of vibration at a high amplitude>otherwise the sound energydissipates and the vibration dies out.


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The resonating body thus acts as a,lter8 allowing only somefreDuencies to get through3 resonant

freDuencies are amplied and theother freDuencies are lost.


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The 9ocal tract as a sound producingdevice3 source5lter theory

The vocal tract is a resonatingsystem.

(n a vowel sound the vibrating vocal

folds provides the driving force8which induces resonance in the airtrapped in the vocal tract.

The energy is output as sound. This is known as the source(,lter

theory of speech sounds.


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9ocal tract sound source may beperiodic or aperiodic.

The vibrating vocal folds provide aperiodic source8 which dominates insonorants.

An aperiodic source is mostimportant for Obstruents.

The turbulence created by a fricative8

aspiration is sustained aperiodicnoise.

The release burst of a stop is a

transiant.


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-ource

-iven the right amount of tension andthe right amount of egressive airstream8the vocal folds vibrate.

The opening and closing of the vocalfolds in the air column providesrepeated burst of air pressure.

The comple* vibration of the vocal foldsprovide rich source and generatewaveforms composed of multipleharmonic freDuencies.


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The comple* movements of the vocalfolds leads to comple* signals8 whichcarries freDuencies far above the

fundamental freDuency. This Jrichness of the source signal

allows us to produce many dierent

speech sounds from the same sourcesignal by ltering it with the vocaltract.


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9ocal Tract #ilter

The vocal tract can be appro*imatedby a cylindrical tube8 which is openat one side+the pips/ and virtually

closed at the glottis. The length and width of the tube

determines the acoustic properties of

the tube. The bending of the tube has little

eect on the acoustic properties


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The resonance freDuencies of thevocal tracts are very important and are

called the formant frequencies. The formant freDuencies are numbered

and are named #)8 #08 #18 etc.

The numbering of the formantfreDuencies have nothing to do withthe fundamental freDuency.

#6 is the property of the vocal foldvibration +the voice source/ and theformant freDuencies +#)8 #08 #18K/ areproperties of the vocal tract+the lter/


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Formants

#ormants are the property of the vocaltract itself8 independent of whether alaryngeal source signal is present or not.

The shape of the vocal tract determinesthe formants8 whether there is a sourcesignal or not.

#ormant freDuencies do not alwayscorresponds to the harmonics of thelaryngeal signal.

The position of the articulators


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pdetermine the location of the formants.

Since8 the formant freDuencies of the

depend on the vocal tract8 it is possibleto formulate some general rules abouthow the position of the articulatorsin:uences the formant freDuencies onthe basis of perturbation theory)hiba and a/iyama& 101+.

Perturbation theory is a way to compute

+e*plain/ whether the resonancefreDuency for an arbitrarily constrictedtube are higher or lower than those forunconstricted cylindrical tube.

As a rule of thumb low vowels in the


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As a rule of thumb8 low vowels in thevowel Duadrilateral have a high #) andhigh vowels have a low.

Similarly8 front vowels have a high #0and bac' vowels have a low #0.

The terms low& high& front andbac'

refer to positions in the vowelDuadrilateral that re:ect idealiBedtongue positions8 i.e. an articulatorydescription.

#ormant freDuency values can serve asa basis for a rough classication ofdierent vowels.


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(t holds across dierent speakers8languages8 and dialects +Peterson L,arney8 )M40/


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Acoustics of vowels


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#) correlates with siBe of pharyngeal cavity anddegree of lip opening+when the tongue high8the pharyngeal cavity is larger8 as in Ni8resulting in lower #)/ 55 9owel openness or

height #0 correlates with the length of the oral cavity

55 frontness=backness+the longer the oral cavity5 due to the more retracted tongue 5 the lower

#0/ ip rounding protracts the oral cavity and thus

will decrease #0


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dentifying vowel qualitybased on formant frequencies

#) is inversely related to height

#0 is related to frontness=backness

+ip/ rounding lowers formantvalues +esp. #0/


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Acoustic property ofConsonants

Four acoustic properties of plosives


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Four acoustic properties of plosives

1. uration of stop gap 4 silentperiod in the closure phase

i.e. the closure duration of =p8 t8 k= arelonger than =b8 d8 g=

2. 6oicing bar 4 a dar' bar that isshown at the low frequencies and it7susually below 2889:

i.e. only for voiced plosives =b8 d8 g= 8 which

is a primary indicator of voicing in thespectrogram8 and all kinds of voicedsounds8 including vowels8 show this voicingbar at such low freDuencies


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3. ;elease burst 4 a strongvertical spi'e

i.e. (n general8 we observe a strongerspike for =p8 t8 k= than for =b8 d8 g=

. Aspiration 4 a short

frication noise before vowelformants begin and it is usuallyin 38ms

i.e. =p8 t8 k= of stressed syllable ininitial position e.g. =ph=in pin.

Aspiration is not the same as the release


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Aspiration is not the same as the releaseburst. The period of aspiration +which onlysome voiceless plosives have/ is much

longer than the very short release burst+which all released plosives have/.

$igh5intensity noise of = p = and =b = appears in the range of 18666548666$B


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6oiced stops identi,ed using formanttransitions


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6oiced stops identi,ed using formanttransitions


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#ricatives

#ricatives can be dividedintosibilantsversus non(sibilants.

Sibilants include Ns8R8 B8 . Sibilants

involve a turbulent airstream thatstrikes an obstacle8 such as the teeth.

non5sibilants involve turbulence at the

site of constriction sibilants tend to belouder than non5sibilants.

t f th i ti t


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ost of their acoustic energy occurs athigher freDuencies8 e.g. the bulk of theturbulence of both =s= and =B=occurs above1466$B8 and reaching as high as )68666$B8 and =R= has most of its acoustic energyfrom around 0666 $B up to )68666 $B.

9oiced fricatives show aspects of bothregular vocal fold vibrations and arandomly turbulent airstream. &ierentfrom their voiceless counterparts8 the

voiced fricatives have a substantial voicingbar occupying appro*imately the lower


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The typical properties of =f= include

high freDuency turbulenceconcentrated between 16665


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fricative = h =8 is voiceless. There is novoicing bar for =h=8 and its turbulence

appears to be strongest around 1888 9:.


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appro*imant

ike vowels8 appro*imants are

highly resonant

produced with a relatively open vocal tract characterised by identiable formant

structures

continuant sounds since there is no occlusion

or momentary stoppage of the airstream non5turbulent due to lack of constriction

oral sounds

They have faint formant structures


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They have faint formant structuresthat they all have a low #)+below

)666$B/ as they are voicedconsonants.

=w=8 a large downward transition of

#0 is characteristic due to the backtongue constriction.

ip rounding lowers the intensity of

all formants particularly #1. So =w= has #) +0465


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$/$8 the tongue is in the position for afront half close to close vowel

+depending on the degree ofopenness of the following sound/.

Therefore it has a similar formant

pattern to =i=. ips are neutral to spread but

rounded in anticipation of round

vowels. (t has a low #) +066 5 166$B/and a high #0 +)G46 5 0)66$B/ and#1 +006 5 1646$B/


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=r= is characteriBed by very low #1due to retro:e* articulation8 which is

usually below 0666$B8 sometimes8falling to as low as )466$B.

The freDuency of #) appears to be

related to lip rounding. i.e. low #) Ulip round

=r= normally has #) +1665146$B/8 #0

+)6665)066$B/ and #1+)665)H46$B/.


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#or $l$8 #) is low and there is nocontinuous transition at vowel

Iunctures. #) appro* 066 5


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!asals

The formants of all these three nasals


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The formants of all these three nasalsare not as dark as they are in vowels.

The freDuency of #) is very low +0665

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Acoustic phonetics.pdf