What factor is primarily responsible for the variation in fundamental frequency in speech sounds?

The variation in fundamental frequency during speech sounds can be attributed to multiple factors, including the speaker's age, vocal effort, and gender. Each of these factors plays a significant role in determining an individual's pitch and how they produce sound.

When considering the speaker's age, younger individuals often have higher fundamental frequencies due to physiological differences in their vocal folds. As people age, their vocal folds may become longer and thicker, typically resulting in lower fundamental frequencies.

Vocal effort also influences fundamental frequency. When a speaker exerts more effort or intensity in their speech, it can lead to changes in pitch. Increased tension in the vocal folds during phonation can raise the fundamental frequency, making the voice sound higher.

Gender is another key factor. Generally, males have larger vocal folds than females, which contributes to lower fundamental frequencies in men's voices compared to women's. This biological difference results in distinct average pitch ranges associated with different genders.

Since all these elements—age, vocal effort, and gender—combine to affect the variation in fundamental frequency, the most comprehensive choice is that all of the above factors are responsible for this variation. This highlights the complexity of voice and speech production and illustrates how diverse influences come into play in the acoustic properties of speech sounds.