People produce sounds, including speech, through the vibration of a pair of membranes called the vocal folds that are stretched across the larynx, or voice box, which is an organ present in the neck of mammals. Air from the lungs is pushed through the vocal folds, causing vibration at specific frequencies that produce audible sound. The expulsion of air through these membranes is controlled in such a way that a variety of different sounds can be produced. The precise traits of these membranes varies significantly based on sex, age, and a variety of other factors. Men, for instance, tend to have lower voices than women because, among other characteristic differences, their vocal folds are larger.
Physically, vocal folds are located at the top of the trachea, or windpipe, in the larynx, or voice box. They contain few muscles and are white in color because little blood circulates through them. They are flat and have generally triangular shapes. The membranes are referred to as "folds" because they are, to an extent, folded inward rather than simply stretched tight.
The vocal folds are not only involved in the production of sound — they also serve a valuable role in moderating airflow and keeping food and water from entering the lungs. The vocal folds are open during inhalation and exhalation to allow air in and out. They are almost closed during singing, speech, and other sound production, as the air forced through must cause the membranes to vibrate. When one swallows food or holds one's breath, the membranes are tightly closed. This allows one to hold air in and to protect the larynx from exposure to food debris.
Located slightly above true vocal folds are the "false vocal cords," also known as the vestibular or ventricular folds. They exist in order to protect the true folds and are somewhat thicker. Though they play only a limited role in sound production, they are used in the production of very deep sounds and in musical styles that involve screaming or growling, such as death metal and "screamo."
The vocal folds develop and change as people age. The folds of newborns have only a single layer while the folds of adults have three. The immature membranes of a child tend to produce sounds much higher in pitch than those in adults. Male infants, for instance, produce sounds at a higher pitch than adult females, who generally produce higher-pitched sounds than adult males. During puberty, the membranes grow and mature in response to many different hormone signals, generally causing a change to a deeper, more mature-sounding voice.