As we have just seen on the page about sounds, one of the main characteristics of speech sounds is that they are made up of many different frequencies. In order to understand how the resonances at these different frequencies do in fact arise, we need to think about how exactly speech sounds are produced.
The organs that produce the initial vibration – if there is any present – are the vocal folds/cords, situated in the larynx, which in turn is located in the trachea or windpipe. The space that may open up between the vocal folds if they’re pulled apart is called the glottis. But of course, the vocal cords on their own could not actually vibrate unless there was some energy driving them, just like there would be nor ripples on the water unless we threw a stone in, or the wind or tides caused a movement. In most speech sounds, this energy is provided by the lungs, which push air up through the windpipe and set the vocal cords in motion.
To test and see – or rather feel – whether there’s any vocal cord vibration present, you can either touch your larynx while you produce a sound or cover your ears and feel the vibrations inside your head. The second test may actually work better for women because, biologically, they tend to have smaller larynges than men do, and which will thus be more difficult to locate.
The opening and closing of the vocal cords then provides the initial vibrations at relatively regular intervals, but as the air keeps moving up through the vocal tract (comprised of the oral and nasal tracts), it is modified further by the shape of the(se) tract(s), as well as the positions of the (active and passive) articulators. The configuration of the articulators at any given time is responsible for the shapes of the complex waveforms that are transmitted from speaker to hearer. Of course, it is not only sounds with vocal cord vibration that get filtered in this way, but also those that are not accompanied by any vibration.
A theoretical model that attempts to characterise the production of sounds in this way is called the source-filter theory, where the source of the sound is seen as the being provided by the airstream passing through the glottis, potentially setting it in motion, which is then filtered by the vocal tract configuration. The vocal tract can here be compared to a tube or pipe that has a basic resonance frequency depending on its length and diameter.
The filtering mechanism works by lengthening/shortening, opening/closing or blocking/opening the vocal tract cavities, so that the resonance frequencies in these cavities can be manipulated in order to produce a variety of different sounds. We’ll try to explore manipulating the organs inside the oral tract as much as possible on this course, so that you will develop a better feeling for how to produce and identify these sounds effectively and reliably.
To get an impression of how changing the size and shape of a cavity changes the frequencies produced when making air move inside it, fold your hands so that you leave a cavity in between your palms and then blow in between your thumbs, slightly opening and closing your hands as you do so.
Clark, J. & Yallop, C. (1995). An Introduction to Phonetics and Phonology (2nd ed.). Oxford: Blackwell.
Johnson, K. (1997). Acoustic & Auditory Phonetics. Oxford: Blackwell.
Kent, R.D. & Read, C. (1992). The Acoustic Analysis of Speech. San Diego, California: Singular Publishing Group.