At puberty, the growth of the larynx is larger in boys than that of girls. The growing voice box in boys can be seen as a protruding part of the throat called Adam’s apple, so that the voice becomes deep and harsh.
This is caused mainly by male hormone (regulatory chemicals) during adolescence. As a result of this, muscles (chords) attached to the cartilage get loosened and thickened.
When air passes through these loosened and thickened chords a hoarse sound is produced.
In girls larynx is hardly visible from outside because of its small size and the voice becomes high pitched.