Some sound changes occur as a result of the listener's misapprehension of the speech signal (see Ohala, 1985:453; 1990: 266 and 1992:322), and one of the most important types of sound change is assimilation which means the replacement of one sound by another under the influence of a neighbouring sound near to it in a word or sentence (Jones, 1976: 217f). So in rapid as well as normal speech, sounds tend to assimilate to their neighbouring sounds in order to maintain a continuous flow of speech. Assimilation is a universal phenomenon in the sense that all human beings have a tendency to economy. Anderson (1974: 11f), among other linguists, views assimilation as a universal phenomenon and asserts that "assimilation of one sound to a neighbouring sound... may be a language universal of change in as much as all languages undergo assimilatory processes". This does not mean that all languages have the same patterns of assimilation. In other words, what is easier for a speaker of one language may be difficult for a speaker of another language.