1. Stop/Plosives

2. Sound Source
Stops, fricatives and affricates produced in vocal tract as the sound source
For voiced stops, fricatives and affricates, there are two sound sources
Periodic laryngeal source combined with
Aperiodic 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
fricatives

3. Manner of Production Stops produced with -
complete closure within the oral cavity,
build up of pressure behind the closure and
rapid release of closure with air rapidly expelled

4. Acoustic Events Divided 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.

5. 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 ( Hz) 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.

6. ACOUSTIC CUES TO THE VOICED/VOICELESS DISTINCTION
VOT
F1 of vowel following stop/plosive
Preceding vowel duration
Other cues

7. 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 specific

8. 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 syllables

9. 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
Rise 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
Offset frequency should be higher for voiced than voiceless stops

10. 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. Bob

11. 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
Onset frequency of Fo higher following voiceless than voiced stops.
Burst intensity of voiceless stops greater than voiced stop.

12. CHARACTERISTICS OF ENGLISH STOPS IN CONTEXT

13. 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 aspiration

14. Closure
/b,d,g/ only fully voiced during closure when occurring intervocalically

15. 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, atlas

16. Place of Production Place of articulation for stops determined by
burst
transitions

17. Burst Burst is combination of transient and frication phase
Provide information for place of production
Frequency 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
Velars, 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)

18. 1. Energy Level Alveolar stops have the most intense bursts
Bilabials have weakest bursts
Due to lack of resonance for bilabials as no front cavity to amplify the sound
Little difference between alveolar and velar

19. 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 kHz
Velar - compact concentration of energy in middle of spectrum which varies according to F2 and F3 of following vowel
Frequency 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: 3kHz

20. 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/.

21. 3. Formant Locus & Transitions
For 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
Cannot separate place of articulation on just one dimension such as F2 locus.
Several variables are required to give the whole picture
Locus is not invariant as it changes substantially as a result of coarticulation