Ngày tải lên :
22/01/2014, 12:20
... m
1
and m
2
, the (nonlinear) springs s
1
and s
2
, the coupling spring k
c
, and the nonlinear
c
1999 by CRC Press LLC
FIGURE 44.1: Display of speech signal: (a)waveform, (b) spectrogram, and ... speakers and speaking styles. Examples
of speech sounds that are difficult to parametrize in this way are nasal and mixed-excitation sounds
(i.e., sounds with an added fr icative component) and “simple” ... NasalCoupling
Nasalsoundsareproducedbyopeningthevelumandtherebycouplingthenasalcavitytothevocal
tract.Innasalconsonants,thevocaltractitselfisclosedatsomepointbetweenthevelumandthe
lips,andalltheairflowisdivertedintothenostrils.Innasalvowelsthevocaltractremainsopen.
(NasalvowelsarecommoninFrenchandseveralotherlanguages.Theyarenotnominallyphonemes
ofEnglish.However,somenasalizationofvowelscommonlyoccursinEnglishspeech.)
Intermsofchainmatrices,thenasalcouplingcanbehandledwithouttoomuchadditionaleffort.
Asfarasitsacousticalpropertiesareconcerned,thenasalcavitycanbetreatedexactlylikethevocal
tract,withtheaddedsimplificationthatitsshapemayberegardedasfixed.Thecommonassumption
isthatthenostrilsaresymmetric,inwhichcasethecross-sectionalareasofthetwonostrilscanbe
addedandthenosereplacedbyasingle,fixed,variable-areatube.
Thedescriptionofthecomputationsiseasiertofollowwiththeaidoftheblockdiagramshown
inFig.44.5.Fromaknowledgeoftheareafunctionsandlossesforthevocalandnasaltractsthree
chainmatricesK
gv
,K
vt
,andK
vn
arefirstcomputed.Theserepresent,respectively,thematricesfrom
glottistovelum,velumtotractclosure(orvelumtolips,incaseofanasalvowel),andvelumto
nostrils.
FromK
vn
withsomeassumedimpedanceterminationatthenostrils,theinputimpedanceof
thenostrilsatthevelummaybecomputedasindicatedinEq.(44.16b).Similarly,K
vt
givesthe
inputimpedanceatthevelum,ofthevocaltractlookingtowardthelips.Atthevelum,thesetwo
impedancesarecombinedinparalleltogiveatotalimpedance,sayZ
v
.Withthisastermination,the
velocitytovelocitytransferfunction,T
gv
,fromglottistovelumcanbecomputedfromK
gv
asshown
c
1999byCRCPressLLC
FIGURE44.5:Chainmatricesforsynthesizingnasalsounds.
inEq.(44.16b).Foragivenvolumevelocityattheglottis,U
g
,thevolumevelocityatthevelumis
U
v
=T
gv
U
g
,andthepressureatthevelumisP
v
=Z
v
U
v
.OnceP
v
andU
v
areknown,thevolume
velocityand/orpressureatthenostrilsandlipscanbecomputedbyinvertingthematricesK
vn
and
K
vt
.
44.4 SourcesofExcitation
Asmentionedearlier,speechsoundsmaybeclassifiedbytypeofexcitation:periodic,turbulent,or
transient.Allofthesetypesofexcitationarecreatedbyconvertingthepotentialenergystoredinthe
lungsduetoexcesspressureintosoundenergyintheaudiblefrequencyrangeof20Hzto20kHz.
Thelungsofayoungadultmalemayhaveamaximumusablevolume(“vitalcapacity”)ofabout5
l.Whilereadingaloudthepressureinthelungsistypicallyintherangeof6to15cmofwater(6000
to15000Pa).Vocalcordvibrationscanbesustainedwithapressureaslowas.2cmofwater.Atthe
otherextreme,apressureashighas195cmofwaterhasbeenrecordedforatrumpetplayer.Typical
averageairflowfornormalspeechisabout0.1l/s.Itmaypeakashighas5l/sduringrapidinhalesin
singing.
Periodicexcitationoriginatesmainlyatthevibratingvocalfolds,turbulentexcitationoriginates
primarilydownstreamofthenarrowestconstrictioninthevocaltract,andtransientexcitations
occurwheneveracompleteclosureofthevocalpathwayissuddenlyreleased.Inthefollowing,we
willexplorethesethreetypesofexcitationinsomedetail.Theinterestedreaderisreferredto[18]
formoreinformation.
44.4.1...