Stop/Plosives

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

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

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.

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.

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

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

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

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

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

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.

CHARACTERISTICS OF ENGLISH STOPS IN CONTEXT

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

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

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

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

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)

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

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 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

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/.

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