The heavy mecanism is used mainly by men, it can produce sounds between 80 Hz and 400 Hz. In this mode of operation, arytenoid cartilages, which are at one end of the vocal cords, move at the same time as these.
The light mechanism is more particularly used by women and children. In this mode, the vocal cords are more strained, and the arytenoid cartilages do not move at all. The frequencies produced this way are approximately between 300 Hz and 1500 Hz.
These mechanisms are not restricted to one sex or another. Men can use the light mechanism and women the heavy mecanism. The most recent studies show that there is no other mode of production, it uses either one or the other, and there is no smooth transition between the two.
When a man start from a low-pitched note rising gradually to a high-pitched note (and the opposite for women, from high-pitched to low-pitched), one can hear a discontinuity at the time the singer change from a mechanism to the other.
It is possible to train for that passage to becomes inaudible, then we can produce a continuous note like a warning device.
The transition is not done at the same frequency for everybody. For non trained voices this transition is done generaly between 290 and 350 Hz for the men, and between 300 and 310 Hz for the women.
One speak of "head voice" for the voice produced by the light mecanism by a woman. When it is by a man, one speak of "falsetto voice" (from an Italian word).
For the heavy mecanism used by men, one speak of "chest voice".
These terms were created at a time we did not know yet the exact mechanism of the production of the voice.
There are two other mechanisms less used than the two main, they are the "fry" and the "whistle".
The fry is a kind of moan produced in the very low pitch and made by relaxing the vocal cords with a low air pression. It is more easyly made by men.
The whistle is more easily produced by women in the very high pitch. The air passes forcefully in a narrow passage between the vocal cords which pratically do not vibrate. It is the turbulence of air that produces the sound.