Nuclear Accent Shape and the Perception of Syllable Pitch

Rachael-Anne Knightrak34@cam.ac.uk

LAGB16 April 2003

Outline

Factors affecting perception of pitchContour shapeExperiment

Design Results

Theoretical and psychoacoustic explanations

Conclusions and implications

The Perception of Pitch

The perception of the pitch of a complex sound is related to the fundamental frequency

Sounds that have a higher fundamental frequency sound higher in pitch And in speech are associated with greater

prominence

Syllable Position In an utterance the

position of a syllable also affects the perception of pitch

If two syllables have identical F0, the one later in the utterance will sound higher in pitch Explained as the listener

‘normalising for declination’

Freq

uenc

y

Time

Contour Shape

Real contours are not stylised peaks and troughs

The majority of falling nuclear accents are realised as more of a flat stretch of contour

Peak Plateau

Definition of the Plateau

Plateaux are defined as being 4% down from any absolute peak in F0

4% is the range of perceptual equalityPeak

4% rangePlateau

Segmental and Prosodic Effects on the Production of the Plateau

Plateaux take up more of syllables that have sonorant onsets and codas

Plateaux are aligned later in the syllable in polysyllabic than monosyllabic feet

Some speakers align the end of the plateau earlier in the syllable before a word boundary

The End of the Plateau

The end of the plateau is stably aligned within the syllable regardless of pitch span

More errors made with incorrect EP alignment in a true/false judgment task

The end of the plateau seems to be the real target (rather than the peak)

Experiment

How does a plateau affect the perception of pitch?

3 possible hypotheses No effect A longer plateau makes a syllable sound higher

in pitch A longer plateau makes a syllable sound lower

in pitch

Stimuli

Test sentence “…came with Manny” taken from “Anna came

with Manny”

Resynthesised nuclear accent (12 versions) Frequency of contour

o 160, 180, 200, 210 Hz Shape of contour

o Peak, 50ms or 100 ms plateau

Examples of Different Contours

Anna Manny

Procedure

7 subjects heard pairs of sentences ‘Manny’ differed only in shape never in pitch

Question: “In which version is ‘Manny’ higher in pitch?”

Responded by pressing 1 or 2 on a keyboard

160 Hz plateau, peak 210 Hz peak, plateau

Results

Percentage of times the longer stretch of contour sounds higher in pitch than the shorter stretch

Overall 50ms vs. peak 100ms vs. peak 100ms vs. 50ms

By frequency 160Hz 180Hz 200Hz 210Hz

By Position Longer stretch in utt. 1 Longer stretch in utt. 2

By shape 1 50ms vs. peak 100ms vs. peak

By shape 2 100ms vs. 50ms

Results Overall

The shape of the contour does affect the perception of pitch

73% of responses were for “longer stretch of contour sounds higher”

0102030405060708090

100

% r

espo

nses

Longer = LowerLonger = Higher

Results by Frequency

The significant result holds at each of the 4 frequencies

160 Hz 67% 180 Hz 72% 200 Hz83% 210 Hz67%

0102030405060708090

100

% r

espo

nses

160 180 200 210Hz

Longer = LowerLonger = Higher

Results by Position

The result is not significant when the plateau occurs in the first token

The result is significant when the plateau occurs in the second position

0102030405060708090

100

% r

espo

nses

1st Token 2nd Token

Longer = LowerLonger = Higher

Results by Shape 1

Both lengths of plateau are perceived as longer when compared to a peak

50ms 77% 100ms 77%

0102030405060708090

100

% r

espo

nses

50 100ms

Longer = LowerLonger= Higher

Results by Shape 2

There is no significant difference between the two lengths of plateau

0102030405060708090

100

% r

espo

nses

Longer = LowerLonger = Higher

Results Summary

Overall plateaux are perceived as higher in pitch than peaks Regardless of the frequency

This result is only significant when the plateau is in second position Suggesting an interaction with position

There is no significant difference between the perception of the 2 plateau lengths Suggesting a categorical rather than gradient difference

Possible Explanations

There are two possible explanations for why longer stretches of contour may sound higher in pitch

Integration

Temporal Smoothing

Integration

The listener may be integrating the area under the curve There is a larger area under the curve of a

plateau

Smoothing

The listener may be smoothing the curve So peaks will sound lower than they actually

are

Predictions

Integration As the plateau gets longer pitch will be

perceived as increasingly higher

Smoothing There will be a cut-off point at which a longer

plateau no longer sounds higher in pitch

Psychoacoustic Explanation

“Stability-sensitive weighting” In pitch perception less weight is given to

portions of the signal where frequency is changing rapidly

Due to the sluggishness of the auditory system

Pitch Perception

There are 2 mechanisms for extracting pitch from a signal

Place mechanism Perceived pitch corresponds to the place of

maximum excitation on the basilar membraneTemporal mechanism

Pitch is derived from the frequency of nerve firings

Phase Locking

The Inner Ear

The Cochlea

Cochlea Cross Section

The Organ of Corti

Phase Locking

Neural firings firings occur at the same phase of the waveform each time

The intervals between firings will be integral multiples of the period of the wave

Sluggishness

If the pitch is changing too rapidly The auditory system does not have enough time

to phase lock Pitch may not be perceived accurately

Conclusions

The shape of the contour does affect perceptions of pitch Longer stretches of pitch sound higher than single

turning points There is a categorical difference between peaks

and plateau Suggesting an explanation based on smoothing

This effect can be explained as the sluggishness of the phase locking mechanism

Implications

My PhD Plateaux may occur to enhance the prominence of the

nucleus Models of pitch span

May need to take account of contour shape Intonational phonology

The situation is more complicated than peaks and troughs in the contour

A more perceptual approach is necessary