The buzz created by the vocal folds resonates (vibrates) the air column and this in turn causes the structures above and around the larynx to vibrate/resonate as well.
Remember from Phonation that the vocal folds vibrate and chop the air stream into little puffs of air. The rate or frequency at which these puffs come out is very fast - from 32 cycles per second or Hertz (Hz) for a very low bass to 3136 Hz for a very high soprano. We speak somewhere between 98 Hz and 262 Hz. This rate is called the fundamental frequency.
Almost anything can vibrate, and will vibrate at a natural frequency, that is at a rate that is most conducive to its vibrating. A good example of natural frequency can be seen on a backyard swing: no matter how hard you push the swing, the rate of frequency of swinging is the always same, no matter how far you travel each time. Another familiar example is the tuning fork. It vibrates most powerfully and the longest at its natural frequency. In the same way, the various structures and tissues of the body, resonate more easily at certain pitches.
- You can feel this by tilting your head back and saying, in a low voice, "Awwwww". Put a hand on your chest and feel it rumble. Now tilt your head forward, place a hand on your head and say, "Heeeeeeeeeee" in a high voice. Feel the vibrations in your skull?
The parts of the body that can vibrate in harmony with the voice are often called Resonators.
The Air Column
Extending from your larynx to your lips, the air column vibrates at a natural frequency - in much the same way that the pipes of an organ do. As you shorten the organ pipe, and thus the air column, the pitch gets higher.
- Try blowing across a bottle and then adding water and blowing again - observe how the pitch rises.
In speech, the rate of vibration of the vocal folds creates the fundamental frequency, or pitch of the the sound. The frequency or frequencies at which the air column vibrates determines the quality of the tone.
In fact, the air column from the larynx to the lips functions very much like a uniform tube, without much variation in width. In an average adult, the tube is approximately 17.5 cm long in adult males and 14.7 cm long in adult females. Apparently the approx. 90° bend seems to make no difference acoustically.
The sound created by the vocal folds isn't a pure tone - it is complex. It is made up of the fundamental frequency (the rate at which the folds vibrate) and a number of partials, which are harmonics of the fundamental frequency, vibrating 2 times, 3 times, 4 times, etc... as fast as the fundamental. Our voices are made up of a spectrum of the fundamental and these "overtones".
Physics tell us that our tube, which functions as if it is closed at one end (since the larynx closes it off at the bottom), naturally resonates at the odd numbered multiples of the fundamental. That is, if the fundamental is 100 Hz, the partials will resonates at 300Hz, 500Hz, 700Hz, etc... These "standing-waves" of sound are also known as formants, and are labelled F0 for the fundamental frequency, F1 for the first formant, F2 for the second formant, F3, etc. In shaping speech, the first three formants are most important. In a way, it is as if each vowel is a "chord" -- like playing three notes together on the piano, where the bottom note stays the same and the notes above change.
Remember, we saw that :
"the rate of vibration of the vocal folds creates the fundamental frequency, or pitch of the the sound. The frequency or frequencies at which the air column vibrates determines the quality of the tone."
What we recognize as "vowels" are actually changes in the quality of the tone. Our tongues allow us to change the shape of the "tube", specifically changing the cross-sectional "width" by sliding forward or back.
- Move your tongue forward into an [i] /"EE" (as in "reed") position: feel how your mouth is narrowed by your tongue just behind your teeth?
- Try shortening it by pulling back into an [u]/"oo" (as in "mood"): the tongue is pulled back here. (Notice how the degree of lip-rounding also affects the sound?)
- Try going back and forth from "EE" to "oo", keeping the same pitch throughout. As you tongue slides back towards the "oo" you may hear how part of the sound drops in pitch, even though your F0, fundamental frequency remained the same.
Lip-rounding essentially lengthens the tube, modify the first, F1 formant. (The middle of our tongues primarily shape the F2 formant, and our tongue tips shape the F3 formant.
- You can hear you that by saying "uh" and "er", lifting and lowering your tongue tip.
The use of harmonics can be emphasized and focussed to make a beautiful form of singing, called Overtone Singing. There are many styles of this, including the chanting of Tibetan Monks, Throat Singing & Khoomei and a Western-ized style. Exploring these vocal style can really expand a performer's range in terms of the sound qualities available.
The following areas vibrate "sympathetically" with the air column. The way in which each area vibrates acts as a cue to the quality of the sound created, giving tactile motor feedback to the brain, which works in conjunction with the hearing mechanisms.
What is perceived or described by voice teachers as "placement" of the voice, generally describes how the vibration of the air column interacts with these structures to accentuate or diminish the size of the second and third formants ( F2 and F3 ). The term "placement" indicates where one feels the augmented vibration due to the change the relationship of the formants to areas of the body.
The following is a list of Resonators and the pitches or vocal qualities that most easily activate them
- Chest and Lower Body - low pitches and open sounds ("AW")
- Throat - mid-range, easy speaking tone
- Mouth - upper mid-range, mid-vowels
- Nasal - close, front vowels, especially when followed by a nasal
- Facial - high range, front vowels
- Sinus - given that there are so many sinuses, many different quality
sounds activate them
- Bones of the skull - falsetto, very high range, closed vowels.
More on Resonation:
Acoustics on the Threshold of the 21st Century
Johan Sundberg, the expert on music accoustics puts it all in perspective in this page base on a talk given in Braunschweig, in December 1996. Excellent images and examples, if a bit technical. Also explains the singer's formant.
Steven Sklar documents overtone singing, especially the Khoomei style of Tuva on his site. Check out the sound samples and spectrographs!