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

Acoustic Phonetics

• Physics of the speech signal

• Relationship between activity in the speaker's vocal tract and the resulting sounds

• Contrast: articulatory phonetics

What we will do…

• Key concepts: frequency, formants, and acoustic cues

• Measuring speech sound

• Vowels and formants

• (Acoustic cues: VOT)

• (Problems in Language Acquisition)

Frequency

Cycles of vibration per second

Measured in Hertz (Hz)

E.g. 100 Hz = 100 repetition per second

Frequency

Figure 1: Two periodic signals with frequencies of 200Hz and 400Hz

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

Sine Waves vs. Complex Sound Waves

Figure 2: three sine waves (left) and resulting complex sound wave (right)

a

b

c

d

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

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

Speech Sounds

Periodic Aperiodic

Measuring Sounds

• Oscillogram

• Spectrum

• Spectrogram

Oscillogram

• Amplitude x time

Spectrum

• Shows all frequencies present in the signal at a given point in time

Spectrogram• Combines properties of oscillogram and spectrum • Measure three dimensions: time, frequency, and

amplitude

PRAAT practice

Task: find vocal fold vibration frequency

• Record your voice: which phonemes ?

• Analyse: how?

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

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

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)

Articulatory correlates

• F1 correlates with vowel height– higher F1, lower vowel

• F2 correlates with degree of frontness– higher F2, ‘fronter’ vowel

Dutch Vowels

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

Voice Onset Time (VOT)

• Refers to the time of voicing in relation to the consonant articulation / release burst

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

Problems in Language Acquisition

• Segmentation problem– Coarticulation– Cue ambiguity

• Mapping problem

Coarticulation• Information about more than one sound is

often encoded in the same portion of the acoustic signal

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

Mapping problem

• How do phonetic categories relate to phonological categories?

• Allophonic variation

• Contextual variation

• Normalization