| Summary: | The study of sound change has been an important area of study in Linguistics throughout the modern era of the discipline. The approach taken to its examination, however, has varied considerably over the years, and it is only recently that phonetics has begun to fit seriously again into the equation. In this dissertation, an exploration of certain consonatantal sound changes observed in the Lower Cross languages of South-Eastern Nigeria is presented, taking as basic the assumption that a detailed knowledge of the phonetic characteristics of the languages in question and of general phonetic theory are essential in coming to grips with the phenomena of sound change. In order to achieve the desired aim of the dissertation is has been necessary first to provide an adequate phonetic description, since the Lower Cross languages are for the most part previously undescribed. This is done placing primary, but by no means exclusive, emphasis on Ibibio the largest language of the group. A variety of instrumental investigations (spectrography, aerometry, electropalatography, laryngography), as well as an impressionistic analysis, were conducted into the phonetics of this language, using a number of different speakers. Among the results of these investigations is a detailed description of the production of the labial-velar stop of Ibibio, to my knowledge the most detailed report on these stops to date, for any language. Description of the other languages of the group is primarily, but not exclusively, impressionistic in nature. To investigate the sound changes found in these languages, it has also been necessary to reconstruct the consonant system of their latest common ancestor, Proto-Lower Cross. For the most part, this reconstruction was arrived at with a high degree of confidence, however it is recognized that in the absence of an adequate reconstruction of the vowel system, certain aspects of the consonant reconstruction, at least, must be considered tentative. The description of the phonetics of the Lower Cross languages, and in particular the instrumental investigations of Ibibio, is then used to examine the observed consonantal sound changes. This is done using a phonetic approach to sound change based in the principles of signal detection theory, which suggests that the speech signal may be perturbed by 'noise' at various places during the transmission process, from encoding and production to perception and decoding. Emphasis in the present work is on exploring perturbations during production, which leads to consideration of an alternative theory of sound change, known as Articulatory Evolution. These two phonetically-based theories of sound change are then examined in light of the Lower Cross data, to determine to what extent they must be viewed as competing theories, or whether they in any way complement each other. This is done through considering the implications of the two for a number of long standing issues with regard to sound change, in particular the graduality controversy and the role of perception in sound change. It is concluded that although these two theories have been considered to be competing by their adherents, with regard to production they may to a large extent, be seen as complementary. With regard to the role of perception in sound change, the area of least common ground between the two, both are found wanting.
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