I've recently started listening to acapella groups that all have very good bass singers( pentatonix,home free,voiceplay) and I think it sounds amazing when one of the bass singers sings a super low note. I can already sing pretty low (d2) but I really want to sing very low bassnotes or notes in the first octave. I can kinda make a croak or a growl sound but it never really comes out right. I also really want to learn how to control the notes so I'm not just singing something random and I want to be able to make actual words rather then noises.
Subharmonic tones are theoretical and aren’t actually present in a given wave form; they don’t exist.
The “croak” sounds you’re currently making are called “vocal fry”. It’s really not good for your voice and you can’t count them as part of your actual vocal range. Your vocal range includes notes you can produce with a clear coordinated tone.
Vocal warmups everyday; find a teacher if you can (one with conservatory training); this will take years and years to develop and cannot be gained quickly.
There is at least one style of singing which emphasizes these very deep tones. The Kargyraa style of Tuvan Throat Singing inflects overtones above a deep fundamental which is produced by Vocal Fry or false-flap vibration.
I don't disagree with any of the points made by jjmusicnotes' answer, but it may be possible to gain some technical ability in this area without damaging your instrument.
Despite what other answers say, there actually is a technique called "subharmonic singing". (Whether this is the "correct" name for what's actually produced, I can't tell.)
As far as I understood the physical process, you let your vocal folds vibrate with different frequencies in certain ratios: e.g., to produce a sound an octave below the actual sung tone, you let them resonate with a ratio of 3:2 (fundamental + a fifth above)
Example: you sing an A at 110 Hz, resonate an E a fifth above that at 165 Hz, then the resulting sound is an A at 55 Hz.
For what I have read (no guarantee I got this correctly), this may be due to some psychoacoustic effect called missing fundamental, where the brain recognizes the correct fundamental even if it's only hearing the overtones of that fundamental.
There are YouTubers that did some Videos about this technique and how to learn it; for example check out David Larson's Playlist "How to Sing Lower/Subharmonic Tutorials/Examples" (https://m.youtube.com/playlist?list=PL_FobEjB9289dSfskESk9IJwM0gxZaMvm).