Surprisingly, both girls and boys are affected by voice changes during their teenage years.
In girls, the change is hardly noticeable - their voices only go down by a couple of tones. Boys however experience quite a dramatic change in tone. Their voices might drop by as much as a whole octave.
The voice usually begins to deepen shortly after the penis has finished its phase of rapid growth. It is not clear why this change occurs. The males of other species develop deep voices to attract women and to intimidate other men. It is possible the human male voice developed for the same reasons.
The voice box
Sound is made by the mouth as air is pushed out of the lungs, through the voice box. The voice box is a structure at the top of the windpipe that is made of cartilage. Stretched across it are two vocal cords, which are a bit like elastic bands. As air is expelled from the lungs it passes between the vocal cords, making them vibrate. This produces the sound of the voice.
Several changes that occur in teenage boys cause their voices to deepen. The cartilage in the voice box is very sensitive to testosterone. As this cartilage receives testosterone, sent from the testicles, it grows larger and thicker.
At the same time, the vocal cords grow 60% longer and become thicker. Now when they vibrate, they do so at a lower frequency than before. A boy's vocal cords might go from vibrating 200 times per second to just 130 times a second.
While this change takes place, the voice box tilts to a different angle in the neck. It can start to stick out as a prominent 'Adam's Apple'.
Another change also occurs. Boys' faces change quite considerably during puberty because the bones in them are sensitive to testosterone. As the facial bones grow, they create bigger spaces within the face. Larger cavities in the sinuses, nose and back of the throat give the voice more room to resonate in, and this deepens it further.
Croaking and warbling
Changes to the voice take place gradually over a period of time. In some boys however, there can be a dramatic period of change. This is described as their voice 'breaking'. Boys at this time can struggle to control their voice and often start to warble or croak. This can be embarrassing, but usually doesn't last very long.