As I mentioned in an earlier post, Tim Storms is the current world record holder for lowest vocal note. I found a clip of him singing “Amazing Grace” with the group “Rescue”.
The lowest note he sings in that clip is the lowest C on a piano (C1), approximately 32Hz. That’s over an octave below textbook bass range. According to the record, his lowest recorded note was 8Hz, which is two octaves lower yet, and well below the range of human hearing.