Changing the rate of vibration of the vocal folds

The rate of vibration of the vocal folds is affected by several things. First, more tension across the folds creates a faster rate of vibration. If the folds are tightened (adducted) by the arytenoid cartilages, then they will start to vibrate more quickly, and the pitch will rise. If on the other hand they are relaxed, and the tension is lowered, then they will vibrate more slowly. You can get a sense of this by singing a very high note. If you hold that note silently, you will feel quite a lot of tension in your larynx. You may also be raising your larynx: this facilitates the tension across the folds. Now drop from a high note to a low note quickly, and you will feel a change in tension and possibly also in larynx height.

Secondly, the more air pressure there is below the folds, the more quickly they will vibrate, other things being equal. Under certain conditions (stress being one of them), we typically breathe more quickly. As a result, the average air pressure below the folds increases, and with it both the loudness of our speech and its average pitch. On the other hand, if there is rather little air in the lungs, the air pressure below the folds will be low. Speech produced like this is more likely to sound ‘tired’ because it requires less energy to produce. But this can also be used as a more linguistic device: when coming to the end of a topic, one iconic device we can use to mark this in our speech is to talk quietly and with a low pitch.