What happens to vocal fold vibration when there is an increase in stiffness?

When there is an increase in stiffness of the vocal folds, the vibrations that occur during phonation become more intense. Stiffer vocal folds can produce higher pressure and a stronger resistance to airflow from the lungs, which allows for a more forceful vibration. This increased intensity contributes to a louder sound during speaking or singing.

Additionally, the relationship between stiffness and frequency of vibration is such that while increased stiffness can affect the pitch, it primarily enhances the intensity of the sound produced. Therefore, the correct answer highlights the direct effect of increased stiffness on vocal fold vibration, leading to more intense sounds.