Pitch and fundamental frequency

The pitch of speech is related to the rate of vibration of the vocal folds: grossly speaking, the higher the rate of vocal fold vibration, the higher the pitch. This is not a straightforward relationship because of the way our hearing mechanism works, and as we have seen, the relationship between air pressure, airflow and vocal fold vibration is not quite simple. We use the term ‘pitch’ to refer to a percept rather than a physical event. The rate of vibration of the vocal folds is often called the fundamental frequency, because it is the lowest component frequency of speech. Fundamental frequency is often abbreviated as f0.

The relation between pitch and fundamental frequency is not a linear one, but is more logarithmic in nature. Linear relationships are where an absolute difference of a certain number of units always has the same effect: for example, if the f0 : pitch relation were linear, then the difference between 100 Hz and 200 Hz would sound like the same difference as that between 200 Hz and 300 Hz: a difference of +100 Hz in each case.

For logarithmic relations, the important factor is the proportionality. For example, the difference between 100 Hz and 200 Hz sounds the same as the difference between 200 Hz and 400 Hz, because in each case the second figure is twice the first one: a proportion of 1 : 2. The difference between 200 Hz and 300 Hz is not in the proportion 1 : 2, but 1 : 1.5.

Figures 4.2 and 4.3 show a pitch trace for a production of ‘oh thank you for calling’ by a female speaker. The figures are scaled according to this speaker’s range: her lowest pitch is 80 Hz, and her highest pitch is 585 Hz. Her average pitch is 220, marked on the right. Figure 4.2 shows a linear pitch trace: the steps 200–300–400–500 Hz are equal on the y-axis. The speaker’s average pitch seems rather low in her range on this graph, and certainly lower than half way through her range.

Figure 4.3 shows the same thing on a logarithmic scale. Here, the distance between 100 Hz and 200 Hz is the same as that between 200 Hz and 400 Hz, because the proportion 100 : 200 is the same as 200 : 400, that is 1 : 2 – the second number is twice the value of the first. On this representation, the higher frequencies appear squashed together; and the speaker’s average pitch is more in the centre of the graph. This is closer to what we perceive.