An allophone is one of several similar speech sounds belonging tho a phoneme. Each allophone is the contextually specific implementation of a phoneme.
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.