Assimilation is a process of alteration of speech sounds as a result of which one of the sounds becomes fully or partially similar to the adjoining sound. The word "assimilation" is an example of this phenomenon. Types of assimilation can be distinguished according to: (1) direction; (2) degree of completeness; (3) degree of stability. Direction of Assimilation The influence of the neighboring sounds in English can act in a progressive, regressive or reciprocal (double) direction. When same articulatory features of the following sound are changed under the influence of the preceding sound, which remains unchanged, assimilation is called progressive. For example: (1) The pronunciation of the plural suffix -s of nouns depends on the quality of the preceding consonant: it is pronounced as [z] after voiced consonants, eg. pens [penz], calls [kɔ:lz] and as [s] after voiceless consonants, eg. desks [desks] , books [bʊks]. (2) Within the words sandwich, grandmother, etc under the influence of [n] the consonant [d] changed into [n] and then disappeared, eg. sandwich ['sændwıtʃ-ꞌsænwıdʒ]. When the following sound influences the articulation of the preceding one assimilation is called regressive. For example, within the word width and in the word combination in them, the alveolar [d] and [n] become dental, before the interdental [ð] and [Ɵ]. Reciprocal or double assimilation means complex mutual influence of the adjacent sounds. For example, within the word tree [tri:] the sonorant [r] is partly devoiced under the influence of the voiceless [t] and the alveolar [t] becomes post-alveolar before the post-alveolar [r].
Degree of Completeness According to its degree, assimilation can be complete and incomplete. Assimilation is called complete in the case the two adjoining sounds become alike or merge into one. It always takes place when the two sounds differ only in one articulatory feature. We find cases of complete assimilation within words, eg. cupboard ['kʌpbәd -'kʌbәd]; and al the word junction in fluent speech, eg.less shy ['les ʃai - 'leʃʃai]. Assimilation is called incomplete when the likeness of the adjoining sounds is partial as the assimilated sound retains its major articulatory features. For example, the sonorants [w, l, r] are partly devoiced when preceded by the voiceless fortis[p,t, k, s, f, Ɵ] within words: sweet [swi:t], place [pleis], try [trai].
Degree of Stability Such changes which have taken place over a period of time within words are called historical, eg. orchard (ort + yard) – [ꞌɔ:tjәd - ꞌɔ:tʃәd]. In modem language obligatory assimilations are special allophonic variants characteristic of the natives' speech. The use ofthe wrong allophone, though a non-phonemic mistake, amounts to mispronunciation and may be one of the causes of a foreign accent making understanding difficult. For example, a dental allophone of the alveolar [t] should be used when it is followed by (inter)dental [Ɵ] or [ð] as in eighth [eitƟ]. Besides there are a lot of widely spread but non-obligatory cases of assimilation which can be traced mainly at word boundaries, eg. ten minutes ['ten 'mınıts - 'tem'mınıts] ten girls ['ten gз:lz - ꞌteŋꞌgз:lz]. Non-obligatory assimilations are characteristic of fluent or careless speech and should he avoided by public speakers (lecturers, teachers, etc).
