Final answer:
The intensity or volume of a vocal sound is controlled by the airflow from the lungs, which affects the amplitude of the sound pressure wave. More forceful air expulsion results in a louder voice, with the intensity of sound being proportional to the amplitude squared.
Step-by-step explanation:
The intensity or volume of a vocal sound is controlled primarily by the amount of air flow from the lungs, which increases the amplitude of the sound pressure wave, thus making the voice louder. To control the loudness of vocal sounds, one must adjust the air flow; it's not about lowering the frequency or volume of speech directly, but rather about managing how forcefully air is expelled through the vocal cords.
The loudness of a sound, sensed by our ears, is affected by both the amplitude and the frequency where the ear has varying sensitivity. The intensity of a sound wave is scientifically described as proportional to the square of its amplitude, and thus as the amplitude increases, so does the perceived volume. When speaking of open-pipe and closed-pipe resonators, which relate to musical instruments and the human voice, the intensity and volume are subtly manipulated through the shape and positioning of the throat and mouth, along with tongue placement, affecting both the fundamental frequency and the overtones produced.