Fricatives

April 1, 2010

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.

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.

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)

Whispering Example

whispered “had” voiced

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

Turbulence• Turbulence is more likely with:

• a higher volume velocity

• less channel area

• All fricatives therefore require:

• a narrow constriction

• high airflow

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

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

Quantal Vowels

stable (quantal) region

less stable region

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

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”:

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

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:

Noise Spectrograms

“White” noise

“Pink” noise

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”

[h] in different vowel contexts

“heed” “had”

[h] in different vowel contexts

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

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”

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.

vs.

“shy” “thigh”

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.

vs.

“sigh” “shy”

[s]

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.

Even Further Back• Examples from Hebrew:

uvular pharyngeal

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)

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

vs.

[si]

[zi]

“Voiced” /h/• In English, /h/ often surfaces as breathy voiced when it appears between two vowels.

“ahead”

“head”

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…

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…

Huh?• The two most confusable consonants in the English language are [f] and .

• (Interdentals also lack a resonating filter)

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…

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.

The Sub-lingual Cavity

•Let’s check the videotape...

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.

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.

Behind the Constriction

[s]

• Let’s check the ultrasound…

Secondary Articulations• What effect might lowering the center of the tongue have on formant values?

• (think: perturbation theory)

• Check it out in Praat.