The propagation of sound waves in air can be modeled by an equation of motion (conservation of momentum) and an equation of continuity (conservation of mass). With some simplifications, in particular constant density, they can be given as follows:
where is the acoustic pressure and is the acoustic fluid velocity vector, is the vector of spatial coordinates x,y,z, t is the time, ρ0 is the static density of air and c is the speed of sound in air.
Acoustic speech output in humans and many nonhuman species is commonly considered to result from a combination of a source of sound energy (e.g.
In this model the source of acoustic energy is at the larynx – the supralaryngeal vocal tract serves as a variable acoustic filter whose shape determines the phonetic quality of the sound (Fant, 1960).
Formants are those frequencies at which local energy maxima are sustained by the supralaryngeal vocal tract and are determined, in part, by the overall shape, length and volume of the vocal tract.
For a given body of theory to be considered part of established knowledge, it is usually necessary for the theory to characterize a critical experiment, that is, an experimental result which cannot be predicted by any established theory.
Unfortunately, the usage of the term is muddled by cases such as string theory and "theories of everything," each probably better characterized at present as a bundle of competing hypotheses for a protoscience.
In the humanities, theory is often used as an abbreviation for critical theory or literary theory, referring to continental philosophy's aesthetics or its attempts to understand the structure of society and to conceptualize alternatives.
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