The comfortable midpoint in one's range is called the resonance tonal expression optimum pitch

The comfortable midpoint in one's range is called the optimum pitch.

Explanation:

Our voices can be used at several different levels. Some will certainly feel more comfortable than others depending on each person. We all have a habitual level, the pitch in which we normally talk. However, the habitual pitch isn't necessarily the best one. The optimum pitch is the level in which it feels the most comfortable to speak, the level in which our voices sound natural. When using the optimum pitch, we do not get tired from speaking, that is, we do not strain our vocal tract.

some sort of sound device i believe.