10. Vowels and Formants II (with duck call demonstration)
On the previous page we learned about how a sound source can make a resonator vibrate and strengthen selected frequencies of the original sound. You can do another easy and fun experiment to show yourself how this works. Play a note on a piano or guitar string. After the sound has died out, sing this same note out loud. You will hear (and see, if you look) the piano or guitar string vibrating and producing a note, without your having touched it or struck a piano key at all. The vibrations from your singing move the air at just the natural frequency of the string, and set it into motion.
The cavities of your vocal tract change in shape and volume as you move your articulatory organs to speak. That means that their resonance frequencies will be constantly changing. These different resonant frequencies are called formants. Formants show up on a spectrogram as the thick black bands you see superimposed on the overtones of a speech sound. (Remember to use a narrow band spectrogram to see overtones clearly.) It is these formant patterns that create different vowel qualities.
This may be a little hard to visualize. But you can see how this works in actual practice with the following remarkable demonstration from the Exploratorium science museum of San Francisco. It starts with a sound source (really a duck call!) which is like your vocal folds vibrating to make a sound. Over this vibrating sound source you add plastic tubes, modeled after the vocal tract when it is making one of the five vowels: [ɑ], [i], [e], [o] and [u]. And you can really hear and recognize the five familiar vowels produced by this method!
1. duck call
(Don't forget the 'bonus sound' at the bottom of the page!)
You can use this method to synthesize your own vowels! Mark Huckvale of UCL shows you how on this page:
Make your own vowel resonators!
A guide to making tubes from simple household materials that can produce vowel-like sounds.
One phonetics II student found it hard to believe that our vocal tract really looks or is anything at all like the plastic tubes you see in the demonstration. Visit the MRI gallery of vocal tract imaging at the following site to see what a real human vocal tract (one male and one female) looks like when making the different vowels. (You might want to link first to this Web page for information on what MRI is.) Note that bending a tube does not affect the frequencies of a sound wave (think of how horns can be coiled up).
Isn't it all amazing?
A good reference on this topic is chapter five, "Resonance", of Ladefoged's Elements of Acoustic Phonetics.
More on vowels and formants to come.
Next: Getting ready to learn about decibels: a tutorial on logarithms