Why does the difference between F1 and F2 become smaller in the vowel quadrilateral?

The difference between F1 and F2 becoming smaller in the vowel quadrilateral is attributed to the position of the tongue in relation to the oral cavity. When the tongue moves lower in the oral cavity, it primarily affects the frequency of the first formant (F1), which corresponds to vowel height. A lower tongue position results in a higher F1 frequency because the pharyngeal cavity becomes longer.

At the same time, when the tongue lowers, depending on the specific vowel being produced, the second formant (F2) can also shift, though the degree and direction of that shift vary based on the specific vowel's characteristics such as backness or frontness and roundedness. As the tongue moves to various positions, the changes in these formants can lead to smaller differences between F1 and F2, particularly in the more central vowel areas of the vowel quadrilateral.

This relationship is crucial in understanding how vowel sounds are produced and recognized based on tongue position, thereby fitting into the overall acoustic properties of human speech.