9. Vowels and Formants I: Resonance (with soy sauce bottle demonstration)

     We have already learned about fundamental frequency and harmonics or overtones. The next step to understanding vowels is to learn about formants 共振峰.

     We know that all voiced sounds have a fundamental frequency, that is, the number of times the vocal folds vibrate per second. And we know that in addition to this fundamental frequency are a theoretically infinite number of harmonics or overtones, or frequencies that are multiples of the fundamental frequency. These overtones decrease in amplitude the higher their frequency. (Loudness decreases by about 6 dB for each successive musical octave. We'll talk about decibels later.)

    However, when producing vowels and other voiced sounds, some of the overtone frequencies are louder or more prominent to our ears than others. It's as though the volume is turned up on frequencies within certain ranges. What causes this?

     Try blowing over an empty soda, soy sauce or other bottle. You will hear a specific note of the scale. This note will correspond to the natural frequency of the air inside the bottle. The bottle is a resonating chamber. If you add water, the air space in the bottle will become smaller, and you will hear a higher note when you blow over the bottle again. On the other hand, if you strike the bottle with a spoon, you will hear the pitch go down. Some related videos:

https://www.thenakedscientists.com/get-naked/experiments/blowing-bottles  

The Sci Guys: Science at Home - SE1 - EP9: Physics of Sound - Part 2: Bottle Flute - Helmoltz 

http://indianapublicmedia.org/amomentofscience/jugband-physics/

http://www.sciencekids.co.nz/gamesactivities/changingsounds.html

     You can imagine the vocal tract as a series of 'soda bottles', or cavities with different shapes and sizes, all connected together. Each cavity in the vocal tract will respond to vibrations (fundamental frequency and harmonics) coming from the vocal folds by vibrating at the cavity's own natural frequency. The vibrations from the vocal folds act like the airstream you blow over the bottle. The frequencies strengthened by resonances in the vocal tract sound louder than the ones without.

    This is enough for a start. Continue on to the next page when you're ready. You will see and hear an amazing demonstration of how resonances in the human vocal tract produce the familiar vowels [ɑ], [i], [e], [o] and [u] – and all the other vowels as well!


On to: Vowels and Formants II (with duck call demonstration)

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