To begin a vocal process, energy from increased air pressure in the lungs is necessary. In normal inhalation and exhalation of airflow in breathing, pressures within the lung cavities rise above and below outside atmospheric air pressure. In inhalation, contraction and relaxation of rib and diaphragmatic muscles enlarge the lung cavity, which drops the air pressure below that of the outside atmosphere. Air is forced in from the outside to equalize the pressure. In exhalation, contraction of muscles in the abdomen and resiliency of stretched membranes decreases the cavity size that causes the air pressure to be greater than that of the outside. The compressed air forces its way back through the open passageway to the outside of the body, thus creating the energy to vibrate the vocal cords.
Airflow from the lungs passes through the throat where the vocal cords, also called vocal folds, are located. When the vocal cords are closed around the cords through muscular contractions, the air pressure from the lungs forces the folds to open. The resiliency of the folds then forces them to close. The air pressure forces them to open again and the fold resiliency closes them again. Vibrations are then established. These vibrations are very fast. For example, when a soprano sings an A above middle C, the vocal cords vibrate approximately 440 times per second. Since the amplitude of vibrations is very small, resonance chambers are needed for amplification before any sound to the outside can be heard.
Vibrations of the vocal cords travel along bones, cartilages and muscles into cavities of the head. They are also reflected back into chest cavities. Air within the cavities vibrates sympathetically, as resonance chambers of the vocal cords. Therefore, these chambers become secondary vibrators, and are essential to producing audible sound waves as they exit the body. Singers develop the control of which the resonant chamber is dominant. In the low register of singing, vibrations are echoed in the chest cavity; in the middle and upper registers, vibrations are echoed in the nasal and oral cavities in the head.
Articulation is the final phase of voice production. It involves changing the shape and dimension of the oral cavity in the mouth, which produces sounds in speech or singing. Properly pronounced syllables, formed by consonants and vowels, automatically make these changes. Teachers generally use syllables to get students to form good mouth positions before going into details of placement, such as consciously moving the jaw up and down, raising and lowering the tongue or moving the corners of the lips away and toward the center of the mouth to produce desired results.