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Fricatives
April 1, 2010
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To Begin With…• Perception homeworks to turn in…
• Remember: static palatography demo in 441 later this afternoon
• Professional Faculties 114
• 3:30 pm.
• Some new readings have been posted.
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Nasometer Wrap• The following patterns (from Steph’s paper) were pretty ubiquitous:
• high vowels had more nasalance than low
• Initiation of nasality begin sooner in closed syllables than in open syllables.
• …except for maybe Silke.
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Source/Filter (again)• So far, we’ve considered the following source/filter configuration:
• source: voicing at the vocal folds
• filter: the resonating vocal tract
• Q: What would happen if we changed the source by:
• Opening the glottis (i.e., not voicing)
• And increasing airflow so that…
• there is some audible turbulence as the air passes through the vocal folds?
• A: We’d get something called whispering (technical term)
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Whispering Example
whispered “had” voiced
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Turbulence• The sound “source” of whispering is the turbulence that airflow creates as it passes through the vocal folds.
• Some handy technical terms:
• laminar flow: a fluid flowing in parallel layers, with no disruption between the layers.
• turbulent flow: a fluid flowing with chaotic property changes, including rapid variation in pressure and velocity in both space and time
• Whether or not airflow is turbulent depends on:
• the volume velocity of the fluid
• the area of the channel through which it flows
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Turbulence• Turbulence is more likely with:
• a higher volume velocity
• less channel area
• All fricatives therefore require:
• a narrow constriction
• high airflow
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Fricative Specs• Fricatives require great articulatory precision.
• Some data for [s] (Subtelny et al., 1972):
• alveolar constriction 1 mm
• incisor constriction 2-3 mm
• Larger constrictions result in -like sounds.
• Generally, fricatives have a cross-sectional area between 6 and 12 mm2.
• Cross-sectional areas greater than 20 mm2 result in laminar flow.
• Airflow = 330 cm3/sec for voiceless fricatives
• …and 240 cm3/sec for voiced fricatives
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Quantal Theory• Stevens (1972) observed: “When a particular articulatory dimension is manipulated through a range of values, there is a nonlinear relation between this dimension and its acoustic consequences.”
• This is the essential idea of the quantal theory of speech production.
• Areas of articulatory-acoustic stability (hypothetically) provide the foundation for distinctive features.
vowel stop
fricative
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Quantal Vowels
stable (quantal) region
less stable region
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The Next Plateau• Evolutionary analogy: consistency of replicability
• Another example of articulatory-acoustic discontinuities:
• glottal frication “glottal trilling” glottal closure
An alternative discontinuity:
Turbulence
intensity
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Turbulence Acoustics• Aerodynamic turbulence provides the “source” of all fricative sounds.
• Turbulent waveforms are aperiodic.
• Their pressure values vary randomly over time
• = generally perceived as “noise”
• Here’s a waveform snippet of aperiodic “white noise”:
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White Noise Spectrum• Recall: white light is what you get when you combine all visible frequencies of the electromagnetic spectrum
• White noise is so called because it has an unlimited range of frequency components
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Fricative Noise• Fricative noise has some inherent spectral shaping
• …like “spectral tilt”
• Note: this is a source characteristic
• This resembles what is known as pink noise:
• Compare with white noise:
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Noise Spectrograms
“White” noise
“Pink” noise
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Fricative Filtering• The sound source of fricatives resembles pink noise.
• …and this aperiodic noise may be filtered by the vocal tract in the same way that voiced vowels are.
• Ex: [h] tends to take on the spectral characteristics of its surrounding vowels
• [h] just replaces the voicing source with an aperiodic sound source.
• = coarticulation
• The “filter” of both sounds is the same vocal tract shapes that we find in vowels.
• In a sense, [h] is a “voiceless vowel”
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[h] in different vowel contexts
“heed” “had”
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[h] in different vowel contexts
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Turbulence Sources• For fricatives, turbulence is generated by forcing a stream of air at high velocity through either a narrow channel in the vocal tract or against an obstacle in the vocal tract.
• Channel turbulence
• produced when airflow escapes from a narrow channel and hits inert outside air
• Obstacle turbulence
• produced when airflow hits an obstacle in its path
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Channel vs. Obstacle• Almost all fricatives involve an obstacle of some sort.
• General rule of thumb: obstacle turbulence is much noisier than channel turbulence
• [f] vs.
• Also: obstacle turbulence is louder, the more perpendicular the obstacle is to the airflow
• [s] vs. [x]
• [x] is a “wall fricative”
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Sibilants• Alveolar, dental and post-alveolar fricatives form a special class (the sibilants) because their obstacle is the back of the upper teeth.
• This yields high intensity turbulence at high frequencies.
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vs.
“shy” “thigh”
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Fricative Shaping• The turbulence spectrum may be filtered by the resonating tube in front of the fricative.
• (Due to narrowness of constriction, back cavity resonances don’t really show up.)
• As usual, resonance is determined by length of the tube in front of the constriction.
• The longer the tube, the lower the “cut-off” frequency.
• A basic example:
• [s] vs.
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vs.
“sigh” “shy”
[s]
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Further Back
[xoma]
palatal vs. velar
• In more posterior fricatives, turbulence noise is generally shaped like a vowel made at the same place of articulation.
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Even Further Back• Examples from Hebrew:
uvular pharyngeal
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Back at the Ranch• There is not much of a resonating filter in front of labial fricatives…
• so their spectrum is flat and diffuse
• (like bilabial stop release bursts)
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Voiced Fricatives• It turns out that voiced fricatives are particularly hard to produce…Why?
• Note: voiced fricatives have two sound sources.
• one at the glottis
• one at the fricative constriction
• In voicing, air rushes through the glottis in short, regular bursts
• Glottis is closed part of the time
• Difficult to maintain a steady stream of flowing air at the fricative constriction.
• Frication (second source) can be lost
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vs.
[si]
[zi]
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“Voiced” /h/• In English, /h/ often surfaces as breathy voiced when it appears between two vowels.
“ahead”
“head”
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Fricative Internal Cues• The articulatory precision required by fricatives means that they are less affected by context than stops.
• It’s easy for listeners to distinguish between the various fricative places on the basis of the frication noise alone.
• Result of both filter and source differences.
• Examples:
• There is, however, one exception to the rule…
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Spectral Moments• Jongman et al. (2000) looked at the possibility of distinguishing between fricatives on the basis of the statistical moments of their power spectra.
• Moment 1: mean (average)
• This measure discriminated best between [s] and .
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Spectral Moments• Moment 2: variance
• Sibilants have lower variance; non-sibilants have higher variance.
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Spectral Moments• Moment 3: skewness
• Post-alveolars have positive skew; the other places don’t.
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Spectral Moments• Moment 4: kurtosis
0 + -
• Alveolars had higher kurtosis than the other places of articulation.
• Note that there were no spectral moments that clearly distinguished between labio-dentals and interdentals…
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Huh?• The two most confusable consonants in the English language are [f] and .
• (Interdentals also lack a resonating filter)
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Helping Out• Transition cues may partially distinguish labio-dentals from interdentals.
• Normally, transitions for fricatives are similar to transitions for stops at the same place of articulation.
• Nonetheless, phonological confusions can emerge--
• Some dialects of English substitute [f] for .
• Visual cues may also play a role…
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Acoustic Enhancement
• E.g.: is post-alveolar and [s] is alveolar
• more space in vocal tract in front of
• including a “sub-lingual cavity”
• This “filter” of resonates at lower frequencies
• In English, this acoustic distinction is enhanced through lip rounding for
• this extends the vocal tract
• further lowers the resonant frequencies of
• another form of “adaptive dispersion”
• Fricative distinctions can be enhanced through secondary articulations.
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The Sub-lingual Cavity
•Let’s check the videotape...
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Fricative Stereotypes• Strand (1999) combined synthetic fricative noises along an acoustic continuum with visual recordings of male and female speakers saying “sod” and “shod”.
• The catch: one female and one male (visual) speaker were judged to be more stereotypical female and male exemplars.
• The other speakers were less stereotypical.
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Fricative Shift• When paired with male faces, the frequency boundary between fricatives was lower.
• The effect was stronger for the stereotypical faces.
• Evidence for…top-down influences of gender expectations on low-level phonetic perception.
• Also note: McGuire + Babel perception of attractiveness.
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Behind the Constriction
[s]
• Let’s check the ultrasound…
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Secondary Articulations• What effect might lowering the center of the tongue have on formant values?
• (think: perturbation theory)
• Check it out in Praat.