Speech physiology, acoustics, and perception

1. Speech physiology, acoustics, and perception

2. What is at stake/why this is important • The Echelon listening project: http://www.aclu.org/echelonwatch/index.html • Spectra are what specialized neural receptors “see” – spectra are what account for the fact that when I make a certain complex speech wave/tone with a certain spectral shape, you here, e.g., an [i], a [g], and so on and so forth. • We call these spectral shapes that seem to be correlated with the perception of one or another speech sound acoustic correlates.

3. The physiology of speech: “Source-filter” theory of speech production • What is physiology as opposed to anatomy? • The source (for vowels, and voiced consonants): the laryngeal sound wave – vibrating vocal folds • The filter: everything above the larynx – basically, the shape you contort your SLVT into using your “articulators” such as your tongue and lips.

4. The physiology of speech that you already know… • Linguistic function of the lungs – flow of speech is segmented into sentence-length units. A complete sentence is usally produced on one expiration. • The larynx is a gizmo we all have in our throats that can convert the stead flow of air out from the lungs into phonation, a periodic sequence of puffs of air. This periodicity allows for some unique and important physics that are the basis of much speech production (but not all). • We are so far in the domain of voicing . . .

5. Source-filter theory of speech production • The uniqueness of the phonated laryngeal source – a special kind of sound wave that is composed of a fundamental frequency and harmonics. • Metaphors – stained glass, child lying on sprinkler; even better – taking a hose and squishing it with your fingers. • Key thing is manipulation or “filtering” of the source by changing the shape of the concert hall – the mouth (or, more specifically, the SLVT) • Live demo using my lips and my tongue.

6. Complex tones dude … • A soundwave is said to be “complex” if it is in reality the sum total of a number of simultaneous vibrations. • There are two sorts of complex tones in speech: periodic and aperiodic. Periodic simply means that the waveform repeats itself after a certain interval of time or frequency. • Speech uses both kinds (aperiodic and periodic). The aperiodic are associated with certain types of consonant sounds that involve turbulence (i.e. your fricatives), the periodic with vowels. (see picture on p. 133 Encyclopedia) • Vowels are really where the action is at – they are psychoacoustically very robust.

7. Complex tones dude … • Anytime you have steady phonation – and this is the case, e.g., with all the vowels in American English, then you have a complex periodic soundwave coming out of the mouth. • This soundwave enters the ear (hopefully) of the listener and is decoded by special acoustic neural receptors in the brain. • What these neural receptors do is “see” the overall spectrum of the sound, where by “spectrum” refers simply to an acoustic analysis of a complex wave whereby this wave is broken down into its primary components: a fundamental frequency, its harmonics, and the “filter” shape that is imposed on this fundamental frequency and harmonics. This produces a “shape” – a spectrum. And depending on what the shape is is whatever vowel is perceived (or consonant). Pretty amazing!

8. The physiology of speech: Focus on vowels • The fundamental frequency of phonation is the rate at which the puffs of air exit the larynx. This determines the “pitch” of a speaker’s voice. • This fundamental frequency is very special – it has a unique and wonderful physics that go with it – it produces harmonics – frequencies that are multiples of the fundamental frequency. Acoustic energy is present only at these “harmonics.” • Crying versus phonating; “You got me phonating in my beer” • “Superimposed” on these harmonics is some sort of filter shape – depending on the shape of your mouth (e.g. making an [i] as opposed to an [u]).

9. The acoustics of the laryngeal source

10. And now the filter: formant frequencies • Formant frequencies are the frequencies at which the supralaryngeal filter (the mouth and upper part of the throat) would let maximum acoustic energy through. • The brain has neural acoustic receptors which – like spectrographic software – “decodes” the vibes coming into the ears and “sees” different formant frequency patterns. What it “sees” (perceives) are different, e.g., vowels. • Note that there may not be a harmonic (acoustic energy from phonation) present exactly at that formant, but typically it is close enough so that you can pull off a vowel!

14. Acoustic correlates: vowels • The acoustic correlates of vowels are the distributions of formant frequency patterns associated with that vowel. For example, [i] has two very high frequency 2nd and 3rd formants. [a] has the first two formants squished together and a third higher. And so on and so forth. See formant frequency patterns of AmE vowels in next slide.

15. Acoustic correlates: nasal sounds • The acoustic correlates of nasal sounds are the selective dampening of sound at certain frequencies.

16. Acoustic correlates: stops • The acoustic correlates of stops have to do with the shape of the onset of the formant frequency (energy concentrations) associated with the “burst” – the acoustic reflex of the turbulent airflow leaving the blocked occlusion at, e.g., your lips in the case of a [b], [p], or [m]. • Also something very interesting about VOT, which we may or may not have time to discuss in class.

17. Acoustic correlates: fricatives • The acoustic correlates of fricatives has to do with overall “noise” level – remember, frication (turbulence) produces noise and the presence of formant frequencies at certain places.

18. Acoustic correlates: liquids & glides • The acoustic correlates of liquids and glides, interestingly enough, seem to have to do with the duration of formant patterns, as well as shape. In a way, liquids and glides are, acoustically speaking, long, drawn-out stop consonants. In fact, if you artificially monkey with the duration of the formant frequencies of liquids and glides and make them shorter, then subjects will perceive them as stops.