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Vocal resonance may be defined as "the process by which the basic product of phonation is enhanced in timbre and/or intensity by the air-filled cavities through which it passes on its way to the outside air."McKinney, James (1994) The Diagnosis and Correction of Vocal Faults, Nashville, TN: Genovex Music Group. Throughout the vocal literature, various terms related to resonation are used, including: amplification, filtering, enrichment, enlargement, improvement, intensification, and prolongation. Acoustic authorities would question many of these terms from a strictly scientific perspective. However, the main point to be drawn from these terms by a singer or speaker is that the result of resonation is to make a better sound, or at least suitable to a certain esthetical and practical domain.
In a more symbolic/perceptual way, rather than physical, the various terms applied can represent vocal "colors" in a continuous scale: from dark ( chest) resonance to bright ( head-nasal) resonance. We may call this spectrum a resonance track. In the lower range, the chest/dark color predominates; in the middle range, the mouth-nasal resonance is dominant; and in the higher range, the head-nasal resonance (bright color) predominates. The objective of using such images by several teachers and coaches is to achieve command of all the "colors of the spectrum". That, ultimately, may allow a greater scope of emotional expression. The emotional content of the lyric or phrase suggests the color and volume of the tone and is the personal choice of the artist.
There are some singers who are recognized by their pronounced nasal quality; whereas others are noted for their deep, dark, and chesty sound; and still others are noted for their breathy or heady sound; and so on. In part, such individuality depends on the structure of the singer's vocal instrument, that is, the inherent shape and size of the vocal cords and of the vocal tract.
The quality or color of a voice also depends on the singer's ability to develop and use various resonances by controlling the shape and size of the chambers through which the sound flows. It has been demonstrated electrographically in the form of "voice-prints" that, like fingerprints, no two voices are exactly alike.
There are two kinds of resonance: sympathetic resonance (or free resonance) and forced resonance (or conductive resonance)McCoy, Scott (2004) Your Voice: An Inside View. Princeton, NJ: Inside View Press. The essential difference between both types is what causes the resonator to start vibrating. In sympathetic resonance, there is no need of a direct physical contact between the two bodies. The resonator starts functioning because it receives vibrations through the air and responds to them sympathetically, as long as the resonator's natural frequencies of vibration coincide with the exciting oscillations. In forced resonance, the resonator starts vibrating because it is in physical contact with a vibrating body, which "forces" the resonator to replicate its oscillations.
Both types of resonance are at work in the human voice during speaking and singing. Much of the vibration felt by singers while singing is a result of forced resonance. The waves originated by the airflow modulated by the vibrating vocal folds travel along the bones, cartilages, and muscles of the neck, head, and upper chest, causing them to vibrate by forced resonance. There is little evidence that these vibrations, sensed by tactile nerves, make any significant contribution to the external sound.
These same forced vibrations, however, may serve as sensation guides for the singer, regardless of their effect on the external sound. These sensations may provide evidence to the singer that their vocal folds are forming strong primary vibrations which are being carried from them to the head and chest. Thus these vibratory sensations can supply sensory feedback about the efficiency of the whole phonatory process to the singer.
In contrast, the sound a person hears from a singer is a product of sympathetic resonance. Air vibrations generated at the level of the vocal folds in the larynx propagate through the vocal tract (e.g., the ducts and cavities of the airways). In other words, the voice's resultant glottal wave is filtered by the vocal tract: a phenomenon of sympathetic resonance.Sundberg, Johan(1989). The Science of the Singing Voice, Northern Illinois University Press, The vocal resonator is not a sounding board comparable with stringed instruments. Rather, it's a column of air traveling through the vocal tract, with a shape that is not only complex, but highly variable. Vennard says:
In general, the larger a resonator is, the lower the frequency it will respond to; the greater the volume of air, the lower its pitch. But the pitch also will be affected by the shape of resonator and by the size of opening and amount of lip or neck the resonator has.
A conical shaped resonator, such as a megaphone, tends to amplify all pitches indiscriminately. A cylindrical shaped resonator is affected primarily by the length of the tube through which the sound wave travels. A spherical resonator will be affected by the amount of opening it has and by whether or not that opening has a lip.
Three factors relating to the walls of a resonator will affect how it functions: the material it is made of, the thickness of its walls, and the type of surface it has. The resonance characteristics of a musical instrument obviously will vary with different materials and the amount of material used will have some effect.
Of special importance to singing is the relationship of the surface of a resonator to its tonal characteristics. Resonators can be highly selective, meaning that they will respond to only one frequency (or multiples of it), or they can be universal, meaning that they can respond to a broad range of frequencies. In general, the harder the surface of the resonator, the more selective it will be, and the softer the surface, the more universal it will become. "A hard resonator will respond only when the vibrator contains an overtone that is exactly in tune with the resonator, while a soft resonator permits a wide range of fundamentals to pass through un-dampened but adds its own frequency as on overtone, harmonic or inharmonic as the case may be."
Hardness carried to the extreme will result in a penetrating tone with a few very strong high partials. Softness carried to the extreme will result in a mushy, non-directional tone of little character. Between these two extremes lies a whole gamut of tonal possibilities.
The final factor to be mentioned is the effect of joining two or more resonators together. In general, the effect of joining two or more resonators is that the resonant frequency of each is lowered in different proportions according to their capacities, their orifices, and so forth. The rules governing combined resonators apply to the human voice: for the throat, mouth and sometimes the nose all function in this manner.
"The supraglottic resonators being in the main muscular and moveable structures must be voluntarily controlled to produce conditions of optimal resonance either by varying degrees of tension in their walls, or by alterations in the size of their orifices and cavities during the articulatory movements."
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