When acquiring language, children learn to pronounce sounds by listening to the pronunciation of their parents or other people. The acoustic signal seems to be sufficient to find the articulation which is needed to realize the sound. Acoustically, speech is just a series of changes in air pressure, quickly following each other. A spectrogram is a “graph with frequency on the vertical axis and time on the horizontal axis, with the darkness of the graph at any point representing the intensity of the sound” (Trask, 1996, p. 328).
In this section we present the use of spectrograms for finding segment distances. Segment distances can also be found on the basis of phonological or phonetic feature systems. However, we prefer the use of acoustic representations since they are based on physical measurements. In Potter, Kopp and Green’s (1947) Visible Speech, spectrograms are shown for all common English sounds (see pp. 54-56). Looking at the spectrograms we already see which sounds are similar and which are not. We assume that visible (dis)similarity between spectrograms reflects perceptual (dis)similarity between segments to some extent. In Figure 3 the spectrograms of some sounds are shown as pronounced by John Wells on the audiotape The Sounds of the International Phonetic Alphabet (Wells and House, 1995). The spectrograms are made with the computer program P
RAAT14.
Figure 3. Spectrograms of some sounds pronounced by John Wells. Upper the [i] (left) and the [e] (right) are shown, and lower the [p] (left) and the [s] (right) are visualized.
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