Aperiodic, continuous sounds

For aperiodic sounds there is no repetition, but rather random noise. This kind of sound is called aperiodic. Figure 3.5 shows 0.1 s of the voiceless fricative [s] sound. If you compare this with Figure 3.4, you will see that it looks very different: [s] has no repeating waveform, and the amplitude varies apparently randomly.

Friction noise is generated when the airflow between two articulators is turbulent. The correlate of this in a waveform is a very much more irregular, random pattern than we find for periodic sounds; it lacks the regular ups and downs of a periodic waveform.

In Figure 3.3, the aperiodic portion lacks the clear formant structure and the vertical striations we saw for periodic portions. However, the pattern of the frequencies does change. As the lips close to form the [p] sound, the [s] sound changes, and sounds as though it gets lower in pitch: this can be seen in the end of the segment marked ‘aperiodic’.

A combination of voicing (periodic) and friction (aperiodic) is also possible.