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The points of zero crossing and the points of zero slope of the oscillograms of speech sounds are considered to contain the essential information for intelligibility. The intervals between zero crossings θ0, and the intervals between zero slopes θm, are plotted as points in rectangular coordinates. The ordinate of the dot is a function of θ (θ0 or θm), and the abscissa is a function of the time of occurrence t of the particular θ. The resulting intervalgram gives a half-tone picture (consisting of dots) of speech sounds. The patterns may be proportioned to show either a detailed or general representation of the variation of the interval distribution. One type of pattern portrayed at the speech rate on a cathode-ray oscilloscope with a screen of long persistence has been found quite similar in certain respects to the patterns obtained using the sound spectrograph as described in the book Visible Speech by Potter, Kopp, and Green. The equipment involved in obtaining the intervalgram, however, is much simpler.
Chang et al. (Thu,) studied this question.