The Physiology Of Speech
Speech requires movement of sound waves through the air. Speech itself is air that is moved from the lungs through a series of anatomic structures that mold sound waves into intelligible speech. This capacity can be accomplished in any volume from a soft whisper to a loud shout by varying the force and volume of air expelled from the lungs. All languages are spoken by the same mechanism, though the words are different and require different usages of the anatomy.
To expel air from the lungs the diaphragm at the floor of the thorax is relaxed. This allows the diaphragm to return to its resting position which is domed into the thorax, expelling air from the lungs. Also, the muscles of the chest tighten, reducing the size of the interior thorax to push more air from the lungs. The air travels up the windpipe (trachea) and passes through the larynx.
The larynx is comprised of a number of cartilages. The largest is the cricoid cartilage which is joined to the top of the trachea. It is structurally different from the rings that form the trachea. The cricoid is a complete cartilaginous ring, while the tracheal rings are horseshoe shaped (open in the back). The back of the cricoid is a large, solid plate. The front slopes down sharply and forms a V angle. Atop the cricoid lies the thyroid cartilage, which is more elongated front to back in males. The cartilage forms an angle of about 90° in males. In females the cartilage is flatter, forming an angle of 120°. Thus the male cartilage protrudes farther forward and often is evident as a knob in front of the throat (known as the Adam's apple).
The two cartilages form a hard cartilagineous box that initiates sound by means of the vocal cords that lie at the upper end of the box. The glottis, entrance to the larynx at the upper end, is protected by a flap called the epiglottis. The flap is open during the process of breathing but closes over the glottis when food is swallowed. Both air and food traverse the same area in the throat, the pharynx, and the epiglottis prevents food from entering the trachea and directs air into the lungs. Infection of the epiglottis can occur when a child has a sore throat. The resulting inflammation can progress rapidly, cause complications in respiration, and may be fatal if not treated promptly because the inflamed epiglottis can close off the laryngeal opening.
If an individual is simply breathing and not talking, the vocal cords lie relaxed and open to allow free passage of air. A series of muscles in and around the larynx pulls the vocal cords taut when speech is required. The degree of stress on the cords dictates the tone of voice. Singing requires especially fine control of the laryngeal mechanism. Word emphasis and emotional stress originate here. Air puffs moving through the larynx place the vocal cords or vocal folds in a state of complex vibration. Starting from a closed configuration the vocal folds open first at the bottom. The opening progresses upward toward the top of the fold. Before the opening reaches the top of the vocal cord the bottom has closed again. Thus the folds are open at the bottom and middle, open at the middle and closed on each end, open at the middle and top, and then only at the top. This sequence is repeated in fine detail during speech.
Once the sound leaves the vocal cords it is shaped into words by other structures called articulators. These are the movable structures such as the tongue and lips that can be configured to form a given sound.
Above the larynx lies the pharynx through which the sound moves on its way to the mouth. The mouth is the final mechanism by which sound is tailored into words. The soft palate at the back of the mouth, the hard or bony palate in the front, the teeth, the tongue, and the lips come into play during speech. The nose also provides an alternate means of issuing sound and is part of the production of speech. Movement of the entire lower jaw can alter the size of the mouth cavern and influence the tone and volume of the speech. Speech is a complex series of events that takes place with little or no conscious control from the speaker other than selection of the words to be spoken and the tone and volume at which to deliver them. The speech center in the brain coordinates movement of the anatomic structures to make the selected words become reality. Speaking in louder tones is accomplished by greater force on the air expelled from the lungs. Normal speech is accompanied by normal levels of respiration. Whispering involves a reduction in the air volume passing through the vocal cords.
The tongue is the most agile of these articulators. Its musculature allows it to assume a number of configurations—flat, convex, curled, etc.—and to move front and back to contact the palate, teeth, or gums. The front of the tongue may move upward to contact the hard palate while the back of the tongue is depressed. Essentially these movements open or obstruct the passage of air through the mouth. During speech, the tongue moves rapidly and changes shapes constantly to form partial or complete occlusions of the vocal tract necessary to manufacture words. The vocal tract is open for formation of the vowels, moderately open to produce the R or L sounds, tightly constricted to S or F, and completely occluded for P and G.
In addition to the formation of words, speech entails rhythm. This rhythm can be seen by the motions made by the speaker as he or she talks. He or she may chop his or her hand or move his or her head in time to the stresses of speech, marking its rhythm. Rhythm is essentially the grouping of words and sounds in a time period. Rhythm often is most emphatic in children's taunts: "Thom-as is a teach-er's pet." In more complex speech the rhythm is not as exact but listeners are disposed to placing a rhythmic pattern on what they hear even though the speaker may not stress any such rhythm.