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The structure of the acoustic signal picked up during swallowing and displayed as a waveform appears, at first viewing, to be complex and random in nature. After viewing many such waveforms, however, one is struck by the relatively high frequency of occurrence of similar patterns within and surrounding the course of different swallows. This is most clear in the rhythmic suckle feeding of infants, where a large number of similar, successive swallows can be observed and compared. However, similar patterns have been observed across the age span of infants to adults. For example, we have observed brief phonatory-like sounds and/or initial and final discrete sounds (Vice et al, 1990) preceding and following the swallows of infants and adults. Similarly, patterns of "slow waves" or high frequency "squeeze" sounds have been observed during the swallows of both infants and adults.
From these and other observations, it is hypothesized that swallow sounds consist of assemblies of components, sometimes overlapping in time, which correspond to specific physiological events. If this hypothesis is correct, it will be important to find ways to decompose the swallow sounds into separate components so that their relative timing and, perhaps, amplitudes might be compared. In this way it may be possible to obtain more quantitative information concerning the nature and normality of the swallow.
At present, acoustic analysis is ahead of our understanding of the relation between the acoustic signal and physiology. By separating swallow sounds into their components, it may become easier to identify the physiological events that give rise to specific parts of the acoustic signal. This paper will illustrate the observations made above and will discuss several methods for decomposing the acoustic signal into its component parts.
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