Waveforms

Waveforms are a kind of graph. Graphs have an x-axis, which runs horizontally, and a y-axis, which runs vertically. In waveforms of speech, the x-axis represents time and is usually scaled in seconds or milliseconds, while the y-axis shows (to simplify a great deal) amplitude, a representation of loudness.

Figure 3.1 shows a waveform of a vowel. On the x-axis, time is marked at 0.1 second (or 100 ms) intervals. On the y-axis, there is a line marked 0 (the zero crossing) which goes through the waveform. The bigger the displacement from this line, the louder the sound is. The beginning and end of this waveform have no displacement from the zero crossing line, so the recording begins and ends with a period of silence. The sound starts just before 0.1 s into the recording, and is loudest around 0.2 s. From a little after 0.2 s to around 0.45 s, the sound gets quieter: or, a little more technically, the amplitude decreases. By about 0.45 s, the signal has died away.

With a little experience and practice, various other kinds of sound are also evident in waveforms. We will look at these after we have considered spectrograms.