Speech propagates as a longitudinal wave in a medium, such as air or water. The speed of propagation depends on the density of the medium. It is common to plot the amplitude of air pressure variation corresponding to a speech signal as a function of time; this kind of plot is known as a speech pressure waveform or just a speech waveform. Here's an example taken from part of a vowel:

The plot shows the change in air pressure with time and we can see that in this example the pattern of changes is regular in that each up-down cycle has roughly the same shape. This corresponds to a periodic signal and these signals make up the voiced speech sounds such as vowels, and voiced consonants such as `b', `w' and `z'. Another kind of sound is irregular or aperiodic and so appears to be just random variations in air pressure. Examples are the hissing sound in `s' and `sh'.

The simplest form of sound that we can describe is called a sinusoid. This waveform corresponds to a pure tone and as we will see later forms the basis for all more complex sounds. Since a sinusoid has such a pure shape it is a useful starting point when describing the properties of sound waves.