Sound Source S tops fricatives and affricates produced in vocal tract as the sound source For voiced stops fricatives and affricates there are two sound sources P eriodic laryngeal source combined ID: 514627
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Slide1
Stop/PlosivesSlide2
Sound Source
S
tops
, fricatives and affricates
produced in vocal
tract as the sound
source
For voiced
stops, fricatives and
affricates,
there are two sound
sources
P
eriodic
laryngeal source combined
with
A
periodic
vocal tract sound
source
Aperiodic
sound is produced by two different
manners:
Sudden release of air
pressure (burst/transient) behind closure
Stop/plosives
Turbulence
as air
rushes through a narrow
constriction
fricativesSlide3
Manner of Production
S
tops produced with -
complete closure
within the oral cavity,
build
up of pressure behind
the
closure and
rapid release
of
closure with air rapidly expelledSlide4
Acoustic Events
D
ivided
into five components
Occlusion
Transient
Frication
Aspiration
Transition
In practice, it is difficult to differentiate the transient from the frication, thus, this complex is generally referred to as the
burst
.Slide5
Acoustic Events
occlusion
is the period during which there is a stoppage of the airflow during which the pressure increases. It is
characterised
by silence or the absence of energy. Voiced stops may have low frequency (0 - 500Hz) periodic energy during this phase.
transient
corresponds to the release of the closure. It is
characterised
by a spike on the spectrogram of intense energy with a duration of about 10msec.
frication
component is the result of the combination of high intra-oral pressure being released through a narrow opening at the point of
release.
aspiration
phase is the result of the vocal tract opening even further with turbulence through the glottis rather than the oral constriction. Formants are often present during this phase.
transition
is the component where formants are present and the oral tract is moving to the position for the following vowel target.
In practice it is difficult to differentiate the transient from the frication so this complex is generally referred to as the
burst
.Slide6
ACOUSTIC CUES TO THE VOICED/VOICELESS DISTINCTION
VOT
F1 of vowel following stop/plosive
Preceding vowel duration
Other cuesSlide7
1. Voice Onset Time (VOT)
Voiced and voiceless stops differ in the coordination between
supralaryngeal
and laryngeal events
Difference is referred to as differences in
Voice Onset Time (VOT)
Voice onset time is the time that voicing begins relative to consonant release
In English, voiceless stops have large VOT values and voiced stops have small or negative VOT values.
Negative VOT occurs when periodicity begins before stop release i.e. during closure
English speakers hear a consonant as voiceless if VOT is over 25msec for bilabials, over 35
msec
for
alveolars
and over 40
msec
for velars
VOT values separating voiced from voiceless stops are language specificSlide8
1. Voice Onset Time (VOT)
Spanish
and French make use of
prevoiced
stops (negative VOT) and contrast these with positive VOT stops. English does not
recognise
a difference between
prevoiced
and voiceless
unaspirated
Thai speakers make a three way distinction for bilabials and
alveolars
. Voiced, voiceless
unaspirated
, voiceless
aspirated
Values also change in
context
VOT separation decreases for stops produced in sentences compared with initial stops produced in isolated
words
Stressed voiceless are produced with greater VOT values than
unstressed
VOT increases when stops occur in Stop Approximant sequences
VOT for
unaspirated
stops (/
sC
/ clusters) is close to VOT for voiced stops in CV
syllablesSlide9
2. F1 for Following Vowel
F1
provides important acoustic information
about
voicing
characteristics
F1 is very low during complete closure.
For voiced
stops--
F1
rises very quickly from
burst to
vowel target formant
position
R
ise steepest
in open vowels
(high F1),
and flattest in close vowels (low F1
)
For voiceless
stops
Periodicity (voicing)
occurs at least 30
msec
later than voiced stops so less of the formant will be pulse
excited
By
the time pulse excitation begins,
F1
has almost reached the vowel
target
On spectrograms, voiced stops
characterised
by a voiced, rising F1 transition which is
NOT present
in voiceless stops due
to
pulse
excitation begins later in the transition for voiceless
stops
aspiration requires open glottis
which (due to the large resonating sub laryngeal chamber) causes an attenuation of
F1
For VC
syllables—
F1
should fall sharply into the closure for voiced
stops
O
ffset frequency should
be higher for voiced than voiceless
stopsSlide10
3. Preceding Vowel Duration
Duration of vowels before voiceless stops is shorter than before voiced stops.
52-69% shorter vowel duration before voiceless than voiced
stops
Examples: Pop vs. BobSlide11
4. Other
Voiced stops have voicing/periodicity during closure when in intervocalic or postvocalic
position
Duration of
intervocalic closure provides an additional cue to
voicing
Closure
greater
for voiceless than voiced e.g. rapid
vs.
rabid
O
nset
frequency of
Fo
higher
following voiceless than voiced stops.
Burst intensity
of voiceless stops
greater
than
voiced stop.Slide12
CHARACTERISTICS OF ENGLISH STOPS IN
CONTEXTSlide13
Aspiration
When /
p,t,k
/
followed
by /
r,l,w,j
/
aspiration
manifests itself in the devoicing of the
approximants
"please", "try", "clean", "pew"
In final position and in unstressed syllables aspiration is
weak
When /s/ precedes /
p,t,k
/ initially , there is no
aspirationSlide14
Closure
/
b,d,g
/
only
fully voiced during closure
when occurring
intervocalicallySlide15
Release
Generally, stops have a release stage in the form of aspiration or as a following vowel. However, there are instances where the release does not
occur
No audible release in final position
: e.g. rope/robe
No audible release in stop clusters
: e.g. dropped, locked, good boy
Glottal reinforcement of final voiceless stops
:
Nasal
release
: If a stop is followed by a homorganic nasal in the following syllable, the release of air is usually via the nasal cavity. e.g. topmost, submerge, cotton, not now, red
nose
Lateral release:
When the homorganic stops /
t,d
/ occur before /l/ they are released laterally. The tip remains in contact with the alveolar ridge but one or both of the sides is lowered allowing the air to escape. e.g. cattle, medal,
atlasSlide16
Place of Production
Place of articulation for stops
determined by
burst
transitionsSlide17
Burst
Burst
is
combination
of
transient
and frication
phase
Provide information
for place of
production
F
requency
spectrum
for
alveolars
and velars results from resonance
of
cavity in front of
tongue constriction
Alveolars
--front
cavity is small and place of
production
doesn't alter greatly
under influence
of different
vowels
V
elars
,
front
cavity shape varies greatly with different
vowels
Three
important parameters of
burst
that allow
one to differentiate
the place of
production
of stops:
Energy level
Spectral centre of gravity (frequency location
of
main energy concentration)
Spectral variance (whether the spectrum lacks peaks or has multiple peaks)Slide18
1. Energy Level
Alveolar stops have the most intense
bursts
B
ilabials
have
weakest bursts
Due to
lack of resonance for bilabials as no front cavity to amplify the
sound
Little
difference between
alveolar
and
velarSlide19
2. Center of Gravity
Bilabials lack any main resonance in the 0-10kHz range as there is no front cavity
so
characterised
by
gradually
falling distribution of energy
throughout
frequency
range
Alveolars
- broad distribution of energy in the burst
characterised
by prominence about 1.8 kHz and another rise between 2.5 -4.5
kHz
Velar - compact concentration of energy
in middle
of
spectrum
which varies according to F2 and F3 of
following vowel
F
requency
position of
energy
for velars derives from the cavity in front of
tongue constriction
Prevelar
(before front vowels (/kip/, /
gis
/), compact
energy
distributed around
center
frequency of about 3
kHz
Postvelar
(before back vowels(/
ko:t
/, /
go:d
/) compact energy
distributed
around
center
frequency of about 1
kHz
High
frequency bursts = alveolar 3kHz to 4kHz
Low
frequency bursts = bilabial 350Hz (but higher for front vowels)
Bursts
with energy slightly above the F2 for the following vowel = velar
e.g
back vowels = low F2 :700Hz, front vowels high F2:
3kHzSlide20
3. Formant Locus & Transitions
The
locus theory
proposes that the place of
articulatory
closure for each of the three places of articulation is relatively fixed regardless of following vowel and that this
articulatory
invariance has its acoustic correlate in the starting frequency of the second formant. Even though the formants may not reach the actual locus position they will still point to it.
Once we know the locus frequency we should be able to predict the slope of the second formant transition if we know the following vowel formant frequencies.
Therefore: The locus for /b/ is low (720Hz) and most vowels would have an F2 value greater than that then the transition will be rising in /
bV
/ syllables.
The locus for /d/ being at 1800Hz means that for central and back vowels F2 will fall in /
dV
/ syllables but will be level or slightly rising in /
di,dI,de
/.
Only the
alveolars
can be considered to have a relatively stable locus at around 1800 Hz. Cassidy and Harrington (1994) found that the variability in F2 onset frequency is least for /d/ followed by /b/ then /g
/.Slide21
3. Formant Locus & Transitions
F
or
bilabials and
velars,
there is not an invariant locus value as modifying
following
vowel will produce large changes in
formant
frequency
values
For instance, for bilabials F2 and F3 will have rising transitions before front vowels but F2 will be falling before
back
vowels
When F3 information is
included, better
picture of how the stops
cluster
F2/F3
plots show
tendency of
three clusters corresponding to bilabial, alveolar and
velar
However
there are examples of bilabials which are potentially confusable with velars preceding back
vowels
If we examine the change in F2 relative to the change in F3 (the difference between the formant value at onset and the value at the vowel target) then these bilabials are well
separated
C
annot
separate place of articulation on just one dimension such as F2 locus
.
Several
variables are required to give the whole
picture
L
ocus
is not invariant as
it changes
substantially as a result of
coarticulation