What is resonance?
Almost everyone, at some point in their artistic career, has picked up a bottle and blown across the opening, making it sound like a small foghorn (this seems to be a particularly popular activity with beer bottles, for some reason). The pitch of the foghorn changes as the level of liquid goes down or, more importantly, as the amount of airspace inside the bottle increases. This is a simple example of a resonator, and of its natural resonant frequency changing as the resonant space is altered.
John Backus, in ‘The Acoustical Foundations of Music’ (7), gave a precise definition of the phenomenon:
“Whenever a system that can vibrate with a certain frequency is acted upon from the outside by a periodic disturbance that has the same frequency, vibrations of large amplitude can be produced in the system. This situation is called resonance.”
What do these observations have to do with singing?
The human voice consists of a sound source (or periodic disturbance), and a resonator. The sound source is the vibration of the vocal cords as air flows between them, inside the larynx. This could be considered roughly analogous to playing the reed and mouthpiece of a saxophone (and sounds equally horrible if heard in isolation from the rest of the instrument).
The rest of the vocal tract above the vocal cords (the upper part of the larynx, the pharynx (throat), the mouth and perhaps the nose) constitutes a complex resonator. The size and shape of this resonator can be altered by various means (movement of the jaw, lips, tongue, soft palate, or the larynx itself) in order to change the frequencies at which the space will naturally resonate.
Why is resonance important?
Resonance is responsible for the accuracy of vowels, and for the overall tone quality of the vocal sound. (This section may be difficult for those with slight knowledge of musical acoustics).
A well-produced periodic disturbance (vocal sound originating in the larynx) contains not only the fundamental frequency (the pitch being sung), but also a series of harmonic overtones above it. In other words, a number of different frequencies are present in the sound, although the fundamental pitch is the one chiefly recognized by the human ear. The complex shape of the vocal tract resonator causes some of these frequencies to be amplified, because the vocal tract has been adjusted to resonate at those frequencies (like a complicated beer bottle). Other frequencies are reduced in volume due to lack of resonance. The sound which we hear outside has been filtered in this way so that some frequencies are stronger than others, and whether we hear an ‘oo’ or an ‘ee’ depends upon which frequencies have been become dominant.
If the frequencies which are being amplified, or resonated, are reasonable balanced between high and low frequencies, the sound will have a combination of dark and bright qualities (the Italian chiaroscuro), and be pleasing to the ear. An imbalance will cause the sound to be either too dark or too bright. Thus resonance determines the overall quality of the voice sound, as well as the vowel sounds.
Resonance also affects sound production. If the resonator is correctly adjusted for the vowel, pitch and dynamic (volume), then producing the sound is relatively easy. The vowel sounds correct, and a rich, beautiful tone can be obtained. If the resonator is adjusted for some other set of frequencies, the vowel sound will be incorrect, and producing the sound will be hard work, or perhaps even impossible. Having to work too hard to produce a sound results in a forced, shouty tone, and is stressful on the instrument. Continued singing in this manner may eventually cause damage.
How do I teach resonance?
By example, feedback, and correction. A resonant sound is louder, richer, easier to produce and altogether more satisfying to the singer (and the teacher), so there is never any absence of motivation. As the principles are understood and the singer improves the control of the resonator, optimal coordination between support, phonation and resonance can be approached. The mirror, the video camera, and the recording of lessons are indispensible.
Most vocal texts discuss the practical aspects of resonance. Barbara Doscher’s account in “The Functional Unity of the Singing Voice” (1) is a good example. For the more ambitious, a very good grounding in the scientific principles can be obtained from “The Science of the Singing Voice”, by Johan Sundberg, (8) and/or “Principles of Voice Production”, by Ingo Titze. (9)