Spoken language relies on human physical ability to produce sound, which is a longitudinal wave propagated through the air at a frequency capable of vibrating the ear drum. This ability depends on the physiology of the human speech organs. These organs consist of the lungs, the voice box (larynx), and the upper vocal tract the throat, the mouth, and the nose. By controlling the different parts of the speech apparatus, the airstream can be manipulated to produce different speech sounds.
The sound of speech can be analyzed into a combination of segmental and suprasegmental elements. The segmental elements are those that follow each other in sequences, which are usually represented by distinct letters in alphabetic scripts, such as the Roman script. In free flowing speech, there are no clear boundaries between one segment and the next, nor usually are there any audible pauses between words. Segments therefore are distinguished by their distinct sounds which are a result of their different articulations, and they can be either vowels or consonants. Suprasegmental phenomena encompass such elements as stress, phonation type, voice timbre, and prosody or intonation, all of which may have effects across multiple segments.
Consonants and vowel segments combine to form syllables, which in turn combine to form utterances; these can be distinguished phonetically as the space between two inhalations. Acoustically, these different segments are characterized by different formant structures, that are visible in a spectrogram of the recorded sound wave (See illustration of Spectrogram of the formant structures of three English vowels). Formants are the amplitude peaks in the frequency spectrum of a specific sound.
Vowels are those sounds that have no audible friction caused by the narrowing or obstruction of some part of the upper vocal tract. They vary in quality according to the degree of lip aperture and the placement of the tongue within the oral cavity. Vowels are called close when the lips are relatively closed, as in the pronunciation of the vowel [i] (English "ee"), or open when the lips are relatively open, as in the vowel [a] (English "ah"). If the tongue is located towards the back of the mouth, the quality changes, creating vowels such as [u] (English "oo"). The quality also changes depending on whether the lips are rounded as opposed to unrounded, creating distinctions such as that between [i] (unrounded front vowel such as English "ee") and [y] (rounded front vowel such as German "").
Consonants are those sounds that have audible friction or closure at some point within the upper vocal tract. Consonant sounds vary by place of articulation, i.e. the place in the vocal tract where the airflow is obstructed, commonly at the lips, teeth, alveolar ridge, palate, velum, uvula, or glottis. Each place of articulation produces a different set of consonant sounds, which are further distinguished by manner of articulation, or the kind of friction, whether full closure, in which case the consonant is called occlusive or stop, or different degrees of aperture creating fricatives and approximants. Consonants can also be either voiced or unvoiced, depending on whether the vocal cords are set in vibration by airflow during the production of the sound. Voicing is what separates English [s] in bus (unvoiced sibilant) from [z] in buzz (voiced sibilant).
Some speech sounds, both vowels and consonants, involve release of air flow through the nasal cavity, and these are called nasals or nasalized sounds. Other sounds are defined by the way the tongue moves within the mouth: such as the l-sounds (called laterals, because the air flows along both sides of the tongue), and the r-sounds (called rhotics) that are characterized by how the tongue is positioned relative to the air stream.
By using these speech organs, humans can produce hundreds of distinct sounds: some appear very often in the world's languages, whereas others are much more common in certain language families, language areas, or even specific to a single language.