perception and production in l2 acquisition week 1: a brief introduction to acoustic phonetics tom...
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Perception and Production in L2 Acquisition
Week 1: A Brief Introduction to Acoustic Phonetics
Tom Lentz
(slides Ivana Brasileiro)
Lab 1 12 november 2008.ppt
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Acoustic Phonetics
• Physics of the speech signal
• Relationship between activity in the speaker's vocal tract and the resulting sounds
• Contrast: articulatory phonetics
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What we will do…
• Key concepts: frequency, formants, and acoustic cues
• Measuring speech sound
• Vowels and formants
• (Acoustic cues: VOT)
• (Problems in Language Acquisition)
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Frequency
Cycles of vibration per second
Measured in Hertz (Hz)
E.g. 100 Hz = 100 repetition per second
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Frequency
Figure 1: Two periodic signals with frequencies of 200Hz and 400Hz
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Sine Waves vs. Complex Sound Waves
• Sine waves: sounds formed by one frequency only
• Complex sound waves: all sounds which are not sine waves
• All complex sound waves can be described on the basis of the sine waves
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Sine Waves vs. Complex Sound Waves
Figure 2: three sine waves (left) and resulting complex sound wave (right)
a
b
c
d
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Question
• If in figure 2, (a) has frequency A; (b) has frequency B and (c) has frequency C, what is the frequency of picture (d)?
• Answer: A
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Speech Sounds
• Speech sounds are always complex waves • Two sources of sounds
– Vocal folds – Oral cavity
• Sounds produced with the vocal folds are periodic: usually perceived as voiced
• Sounds produced with the oral cavity are aperiodic: usually perceived as voiceless
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Speech Sounds
Periodic Aperiodic
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Measuring Sounds
• Oscillogram
• Spectrum
• Spectrogram
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Oscillogram
• Amplitude x time
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Spectrum
• Shows all frequencies present in the signal at a given point in time
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Spectrogram• Combines properties of oscillogram and spectrum • Measure three dimensions: time, frequency, and
amplitude
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PRAAT practice
Task: find vocal fold vibration frequency
• Record your voice: which phonemes ?
• Analyse: how?
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Formants (1)
• The shape of the vocal tract increases some frequencies and decreases others
• The increased frequencies can be seen in the spectrum as peaks; and in spectrograms as darker spots
• These frequency peaks are the formants
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Formants (2)
• Formants are commonly used to describe vowels
• The first 3 formants (F1, F2 and F3) are important for the vowel quality
• Other formants (F4 and F5) are important for the naturalness of the speech sound
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Vowels Acoustic Space
• Vowels are usually plotted in a F1 x F2 graph, since they play a prominent role in the quality of the vowel
• F1 in the y-axis and F2 in the x-axis
• This graphic represents the acoustic space of vowels, the so called vowel triangle (see handout monday)
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Articulatory correlates
• F1 correlates with vowel height– higher F1, lower vowel
• F2 correlates with degree of frontness– higher F2, ‘fronter’ vowel
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Dutch Vowels
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Acoustic Cues
• Refers to any piece of signal that has been found by experiment to have an effect on percept (Lieberman 1996)
• Examples of cues are: VOT, bursts, transitions, duration and formants
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Voice Onset Time (VOT)
• Refers to the time of voicing in relation to the consonant articulation / release burst
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Acoustic Cues
• No one-to-one relationship between ‘cues’ and a single percept (see handout)
• Voicing contrast in Dutch has been shown to have about 6 acoustic correlates
• Cues differ in how important they are• “Cue reliance” refers to how strongly listeners rely
on a specific cue to identify a contrast• “Cue weighting” refers to how much each cue
counts
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Problems in Language Acquisition
• Segmentation problem– Coarticulation– Cue ambiguity
• Mapping problem
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Coarticulation• Information about more than one sound is
often encoded in the same portion of the acoustic signal
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Cue ambiguity
• The interpretation of acoustic cues depends on their position in the signal (handout Kager)
• Example: VOT in English ‘pin’ ‘spin’ ‘bin’ • ‘pin’: aspirated, long lag VOT (30-35ms) • ‘spin’: plain, voiceless unaspirated, short
lag VOT • ‘bin’: voiced, short lag VOT or prevoiced
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Mapping problem
• How do phonetic categories relate to phonological categories?
• Allophonic variation
• Contextual variation
• Normalization