Psychology 2115A/B Lecture Notes - Acoustic Phonetics, Voice-Onset Time, Vocal Tract

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Published on 21 Apr 2013
School
Western University
Department
Psychology
Course
Psychology 2115A/B
Lecture 13 Review: Speech Perception
By the end of this section, you should know:
The acoustic signal of speech
How variability affects speech perception
Categorical perception
Multimodal issues in speech
Word segmentation
Brain areas involved in speech
What is Speech?
Can be understood up to a rate of 50 units/second normal speech is 12 units/second
We can compare our rapid perception of speech to non-speech sounds in which our perception is
limited to 0.67 units/second
Our ability to perceive speech sounds persists even when the sounds are distorted in various ways
like cell phones filtering the voice sound considerably we lose a lot of resolution but are still able to
perceive the speech sounds and make sense of what they mean
An acoustic signal or stimulus
o Air pushed up through our lungs and through our vocal chords into the vocal tract and created by
the vibration of the vocal chords in the throat
o Articulators change the shape of the vocal tract in some way narrow to create higher pitch,
widens for a lower pitch
A collection of ordered sounds to which meaning is attached
o Phoneme sound, vowel sound, high or low pitch
Spectrograms: graphs of the amount of energy at various frequencies over time
o Darkness reflects how intense the sound is
o 4 dark bands/regions called formant, referring to bands of energy in the spectrogram
o We number our formants lowest is 1, etc.
o The first 2 formants we pay the most attention to they tell us about vowels and the difference
between 1 and 2 tells us about which vowel is being sounded
Vowels
Produced by vibrating the vocal folds as air moves out of the lungs through the open mouth
o If you produce any vowel, your mouth is fairly open and the sound we produce depends on the
various positions of the articulators in your mouth
Sound produced depends on positions of structures of the vocal tract
o Tongue position is a critical part of producing vowel sounds
o Vowel sounds largely produced in conjunction with vocal chords moving in and out and the
articulators
Consonants
Produced by closing or constricting the vocal tract
Place of articulation: at what point is the vocal chord restricted?
Manner of articulation: how is the vocal chord constricted?
Voiced vs. unvoiced: reflects how the air is being pushed through the opening itself
o Related to both place and manner
o Voiced: constricting the airflow
o Unvoiced: do not begin their vibrations in the vocal chord until a long time after constriction
The Phoneme
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How we measure units of speech
Phoneme: smallest unit of speech
Contains sound energy at a number of different frequencies, creating an acoustic signal
In English, we have 47 phonemes and our vocal tracts can produce 100-120 different sounds
The Variability Problem Context
Context changes the relationship between the acoustic signal and individual phoneme
Problems of variability no 1 to 1 simple correspondence between acoustic speech signal and the
actual phoneme
If we looked at an acoustic speech signal, could we pick out the phonemes in the signal?
Why is that a problem? Because the speech context changes the way the speech signal and the
phoneme react to one another or the relationship between them
The context of phonemes in speech don’t always sound the same
This is a problem because of co-articulation: the overlap between articulation of 1 phoneme and a
neighboring phoneme in a speech sound/word
Boot/bat articulation of the b-lips are rounded differently… the 2 b’s from a signal point of view are
different from one another but from a perceptual point of view, we hear them both as the same sound
but really they are variations
The Variability Problem Speaker Variation
Variability from different speakers creates a problem in terms of constancy of sounds
E.g. all our voices sound different but we can still make sense of a sentence said by different people
Can still make sense of what someone is saying if the pitch, speed, or pronunciation is changed even
though each vary from person to person
Categorical Perception
How do we solve the variability problem?
o One solution is the fact that we can categorically perceive certain sounds
o By categorizing certain sounds into one category or another, this constrains the variability that
exists in the signal
Categorical perception occurs when a wide range of acoustic cues results in the perception of a
limited number of sound categories
o Associate colour labels to colours
o Turns out consonants are categorically perceived
Voice Onset Time
Manipulate VOT
o Voice onset time: the time delay between when the sound begins and when we start voicing that
sound
Phonemic boundaries
o You can shift the phonetic boundary by giving you exposure to different phonetic categories
Is Speech Special?
Its a fact that we can hear and perceive phonemic rapidly and have categorical perception that we
don’t have for other sounds in the environment
This suggests that we have a special place in our perception for speech, but we do not
o Phonemic boundaries are perceived by non-human species
Boundaries are perceived by non-humans who do not have language/speech (e.g. hamsters
not unique to humans)
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Document Summary

By the end of this section, you should know: Can be understood up to a rate of 50 units/second normal speech is 12 units/second. We can compare our rapid perception of speech to non-speech sounds in which our perception is limited to 0. 67 units/second. A collection of ordered sounds to which meaning is attached: phoneme sound, vowel sound, high or low pitch. Produced by vibrating the vocal folds as air moves out of the lungs through the open mouth. If you produce any vowel, your mouth is fairly open and the sound we produce depends on the various positions of the articulators in your mouth. Sound produced depends on positions of structures of the vocal tract: tongue position is a critical part of producing vowel sounds, vowel sounds largely produced in conjunction with vocal chords moving in and out and the articulators. Produced by closing or constricting the vocal tract.

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