7.4.4 Interpreting Phones from a Waveform

Much can be learned from a visual inspection of a waveform. For example, vowels are pretty easy to spot. Recall that vowels are voiced; another property of vowels is that they tend to be long, and are relatively loud (as we can see in the intensity plot in Fig. 7.16). Length in time manifests itself directly on the x-axis, while loudness is related to (the square of) amplitude on the y-axis. We saw in the previous section that voicing is realized by regular peaks in amplitude of the kind we saw in Fig. 7.13, each major peak corresponding to an opening of the vocal folds. Fig. 7.17 shows the waveform of the short phrase ‘she just had a baby’. We have labeled this waveform with word and phone labels. Notice that each of the six vowels in Fig. 7.17, [iy], [ax], [ae], [ax], [ey], [iy], all have regular amplitude peaks indicating voicing.

For a stop consonant, which consists of a closure followed by a release, we can often see a period of silence or near silence followed by a slight burst of amplitude. We can see this for both of the [b]'s in baby in Fig. 7.17.

Another phone that is often quite recognizable in a waveform is a fricative. Recall that fricatives, especially very strident fricatives like [sh], are made when a narrow channel for airflow causes noisy, turbulent air. The resulting hissy sounds have a very noisy, irregular waveform. This can be seen somewhat in Fig. 7.17; it's even clearer in Fig. 7.18, where we've magnified just the first word she.

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Figure 7.17 A waveform of the sentence “She just had a baby” from the Switchboard corpus (conversation 4325). The speaker is female, was 20 years old in 1991, which is approximately when the recording was made, and speaks the South Midlands dialect of American English.

Figure 7.18 A more detailed view of the first word “she” extracted from the wavefile in Fig. 7.17. Notice the difference between the random noise of the fricative [sh] and the regular voicing of the vowel [iy].