Phonaesthetics (from the Greek: φωνή, phōnē, "voice-sound"; and αἰσθητική, aisthētikē, "aesthetics") is the study of the inherent pleasantness (euphony) or unpleasantness (cacophony) of the sound of certain words, phrases, and sentences. Poetry is considered euphonic, as is well-crafted literary prose. Important phonaesthetic devices of poetry are rhyme, assonance and alliteration. Closely related to euphony and cacophony is the concept of consonance and dissonance. The term was introduced by J. R. Firth in 1930 "The phonæsthetic habits [...] are of general importance in speech." Firth defined a phonaestheme as "a phoneme or cluster of phonemes shared by a group of words which also have in common some element of meaning or function, though the words may be etymologically unrelated."
The closely related but different concept of phonaesthesia should be distinguished from this meaning. Phonaesthesia does not refer directly to aesthetic attributes of sound, but to phonetic elements that are inherently associated with a semantic meaning.
Sub-phonematic euphony 
In most languages, phonetic combinations which are difficult to pronounce will be adapted to allow more flowing speech, for reasons of ease of pronunciation rather than aesthetics. These adaptations will be sub-phonematic at first, but over several generations will lead to phonematically relevant sound changes. Most of the euphony or mellifluous design of a formal language is pure coincidence, yet phonaesthetics relations with meaning can arise to frequent use and may even become cliché.
See also