Grab Bag! (Palatography, Fricatives +

Perception, part 2)

April 10, 2012

Almost There…• Categorical Perception homeworks are due!

• (Stay on target…)

• Two oral presentations on Thursday: Christine + Jonathan

• (Stay on target…)

• Then we’ll wrap up auditory perception + maybe do some synthetic speech

• (I can’t hold him! He’s coming in too fast!)

• If people are interested, we can do a review session on Monday.

• (WHAHWHAHAWBOOOSH!)

Electro-palatography

Therapeutic Applications

Glamour Shots!

thaw saw shop

More Photographic Flattery

top law

Important things to keep in mind:

Don’t forget your camera!

Keep a record of everything that your consultant produces.

Finally, Fricatives• The last type of sound we need to consider in speech acoustics is an aperiodic, continuous noise.

• Ideally:

• Q: What would the spectrum of this waveform look like?

White Noise Spectrum• Technical term: White noise

• has an unlimited range of frequency components

• Analogy: white light is what you get when you combine all visible frequencies of the electromagnetic spectrum

Turbulence• We can create aperiodic noise in speech by taking advantage of the phenomenon of turbulence.

• 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

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 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:

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]

Sampling Rates Revisited• Remember: Digital representations of speech can only capture frequency components up to half the sampling rate

• the Nyquist frequency

• Speech should be sampled at at least 44100 Hz

(although there is little frequency information in speech above 10,000 Hz)

• [s] has higher acoustic energy from about 3500 - 10000 Hz

• Note: telephones sample at 8000 Hz

• 44100 Hz

• 8000 Hz

Further Back

[xoma]

palatal vs. velar

• In more anterior fricatives, turbulence noise is generally shaped like a vowel made at the same place of articulation.

Even Further Back• Examples from Hebrew:

At the Tail End• [h] exhibits a lot of coarticulation

• [h] is not really a “fricative”;

• it’s more like a whispered or breathy voiced vowel.

“heed” “had”

Aspirated Fricatives• Like stops, fricatives can be aspirated.

• [h] follows the supraglottal frication in the vocal tract.

• Examples from Chinese:

[tsa] [tsha]

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)

• Note: labio-dentals are more intense than bilabial fricatives

• (channel vs. obstacle turbulence)

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…

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...

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.

And now for something completely different…

• Q: What’s a category?

• A classical answer:

• A category is defined by properties.

• All members of the category exhibit the same properties.

• No non-members of the category exhibit all of those properties.

The properties of any member of the category may be split into:

• Definitive properties

• Incidental properties

Classical Example• A rectangle (in Euclidean geometry) may be defined as

having the following properties:

1. Four-sided, two-dimensional figure (quadrilateral)

2. Four right angles

This is a rectangle.

Classical Example• Adding a third property gives the figure a different

category classification:

1. Four-sided, two-dimensional figure (quadrilateral)

2. Four right angles

3. Four equally long sides

This is a square.

Classical Example• Altering other properties does not change the category

classification:

1. Four-sided, two-dimensional figure (quadrilateral)

2. Four right angles

3. Four equally long sides

This is still a square.

A. Is red.

definitive properties

incidental property

Classical Linguistic Categories• Formal phonology traditionally defined all possible speech sounds in terms of a limited number of properties, known as “distinctive features”. (Chomsky + Halle, 1968)

[d] = [CORONAL, +voice, -continuant, -nasal, etc.]

[n] = [CORONAL, +voice, -continuant, +nasal, etc.]

…

• Similar approaches have been applied in syntactic analysis. (Chomsky, 1974)

Adjectives = [+N, +V]

Prepositions = [-N, -V]

Prototypes• The psychological reality of classical categories was

called into question by a series of studies conducted by Eleanor Rosch in the 1970s.

• Rosch claimed that categories were organized around privileged category members, known as prototypes.

• (instead of being defined by properties)

• Evidence for this theory initially came from linguistic tasks:

1. Semantic verification (Rosch, 1975)

• Is a robin a bird?

• Is a penguin a bird?

2. Category member naming.

Prototype Category Example: “Bird”

Exemplar Categories• Cognitive psychologists in the late ‘70s (e.g., Medin & Schaffer, 1978) questioned the need for prototypes.

• Phenomena explained by prototype theory could be explained without recourse to a category prototype.

• The basic idea:

• Categories are defined by extension.

• Neither prototypes nor properties are necessary.

• Categorization works by comparing new tokens to all exemplars in memory.

• Generalization happens on the fly.

A Category, Exemplar-style

“square”

Back to Perception• When people used to talk about categorical perception, they meant perception of classical categories.

• A stop is either a [b] or a [g]

• (no in between)

• Remember: in classical categories, there are:

• definitive properties

• incidental properties

• Q: What are the properties that define a stop category?

• The definitive properties must be invariant.

• (shared by all category members)

• So…what are the invariant properties of stop categories?