An
allophone is one of several similar speech sounds: an allophone can be thought of as a variant of a phoneme. Each allophone is the contextually specific implementation of phoneme, and phoneme is the (language dependent) smallest distinguishable unit of sound.
In a particular context an habitual approximation of the phonemic ideal usually becomes so familiar as to be conventional.
A phoneme itself, however, is really too abstract and context variant to have a simple frequency decomposition. A phoneme as one of the abstract signals of the phonetic system of a language
corresponds to a set of similar speech sounds which are perceived by speakers of the language to be a single distinctive sound in that language.
See
Phonology,
Phonetics, and
voice production.