Curious Human voice

Voice frequency - Wikipedia

The voiced speech of a typical adult male will have a fundamental frequency from 85 to 180 Hz, and that of a typical adult female from 165 to 255 Hz

So nowhere near 20Hz and even less below that.

A human speaking very very quietly will still produce frequencies within that range and not (way) below it.

Even worse: whispered voice seems to have a much reduced Bass content than same spoken at normal level.

Not surprised you found nothing (I mean speech information) when exploring the very low frequency band
 

Hearinspace

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Paid Member
2008-06-03 5:18 am
. . . . . If a person talks extremely quietly without even opening their mouth and no one else can hear it. . . . . . .

It brings to mind the term subvocalization, used as an important part of plots in sci-fi novels by David Brin and William Gibson . Read them a long time ago but I think they used microphones implanted in the jaw or molars.
Google searches bring up a lot of interesting stuff, including a NASA page on it.