Do you believe it might be possible that the frequency band of human speech is not determined by such factors at all but more of a function of height? kids have higher voices adults have deeper voices. Similar to stringed instruments: viola high pitched and bass low pitched.
I'm no expert in these matters just speculating...
It's not height, but vocal cord length and thickness. Longer vocal cords (induced by testosterone during puberty) vibrate more slowly, with a lower frequency/pitch.