Phonation is the term used for the process whereby breath expelled from the lungs is modulated by the larynx to create voiced sound.
Sound is generated in the larynx, which is a simple-looking organ in the neck involved in the protection of the trachea and in sound production. The larynx houses the vocal folds, and that is why it is commonly referred to as the “voice box”. The larynx is also where pitch and volume are manipulated. The strength of expiration from the lungs contributes to loudness, and is necessary for the vocal folds to produce speech.
To understand how sound is produced in the larynx; we must first understand the complex anatomy and physiology of the larynx.
Anatomy of the Larynx
The skeleton of the larynx is made up of cartilages with joints, which are held together by ligaments and operated by small muscles. A common air and food passage connects it above, and below it is the trachea. The larynx must me flexible and mobile for common movements of the head, neck and those of respiration. The movement of the laryngeal prominence, or Adam’s apple, while swallowing, is the evidence of its mobility. Therefore, the larynx is suspended and supported front and back, top and bottom by pairs of muscles.
To get a better picture of the skeleton of the larynx, imagine it is being built piece by piece. The cricoid cartilage, found on top of the trachea, forms the base of the larynx. It is a complete circle that takes the shape of a signet ring facing backwards. It is connected to the thyroid cartilage via a pair of joints called the crico-thyroid articulation and also by ligaments and muscles. This cartilage in turn is recognized as the “Adam’s apple” as it takes the shape of a shield and is concave posteriorly. Also, the epiglottis, which is a small elastic cartilage connected to the thyroid cartilage, does not aid in vocalization, but is important as it covers the entrance of the larynx during swallowing of food. The arytenoids, two smaller pyrimid-shaped cartilages, are also found on the top of the back of the cricoid, attached by ligaments and muscles. The joint structure of these two arytenoids cartilages gives them the ability to rotate and glide, thus moving the vocal folds. Lastly, the hyoid bone is shaped like a horseshoe and not only aids in providing attachment for the muscles of the tongue and floor of the mouth, it also suspends the larynx and thus is usually included in the discussion of it.
Abduction of the Vocal Folds
To create sound, first you must have breath. During breathing, the vocal folds must be abducted, or open, so that air can enter the trachea and into the lungs. This happens when the posterior crico-arytenoid muscles, which are located behind the cricoid cartilage and attached to the arytenoids cartilages, contract, thereby causing the vocal processes of the arytenoids to swing away from the mid-line, thus abducting the vocal folds and allowing air to enter the lungs.
Adduction of the Vocal Folds
After taking a breath, a voiced sound can be produced by adduction of the vocal folds. This involves two sets of muscles, the lateral crico-arytenoids and the inter-arytenoids. When the lateral crico-arytenoids contract, the vocal folds and vocal processes move toward each other. The inter-arytenoids also contract to bring the arytenoids cartilages together to complete the closure of the epiglottis. For sound produced to be clear, the vocal folds must meet evenly in the midline. If there is a gap in the folds due to the inadequate contraction of the inter-arytenoid muscles, a breathy sound is released.
Elongation of the Vocal Folds
As mentioned, the larynx can also alter the pitch and register of the voiced sounds by changing the length of the vocal folds. When the crico-thyroid muscles contract, the thyroid and cricoid cartilages pull away from each other. This is due to the fact that the crico-thyroid muscles are attached to the front of the cricoid cartilage and the inside and outside of the thyroid cartilage, causing the thyroid cartilage to move forward and the cricoid cartilage to move backwards when the muscles contract. This causes the vocal folds to stretch and thus raising the pitch of the sound created. Conversely, when the vocal folds shorten, the pitch will go down.
Shortening and Thickening of the Vocal Folds
A heavier sound, usually on low and medium pitches, can also be created by shortening and thickening of the vocal folds. The vocal folds are made up of the internal thyro-arythenoid, also known as the vocalis muscle, the vocal ligament and the vocal processes of the arytenoids cartilages. The vocalis muscle comes in a pair and forms the main body of the vocal fold. When it contracts, it causes the vocal folds to become thicker, and this creating a heavier sound.
From this essay, we now know that sound is produced through an interaction between the components of the larynx and their muscles. These muscles, like those in the rest of the body, are prone to fatigue, disease and emotional factors. Vocal abuse occurs when the voice is not given sufficient rest periods. It is thus important to warm up the muscles in the larynx as improper or insufficient warm up can possibly lead to vocal fatigue and inefficiency.