Best method to cancel human voices at work or public space?

Ultimately, technology is limited. What you need is a culture of "library voices". To wit: "Don't raise your voice unless you absolutely have to. Better, don't use your voice, period, if you can just do a chat session with the person. If you must use your voice -- just keep the volume down, and and the duration minimal. If you must have a full-volume conversation for any length of time -- get a room."

Basically, people in office environments get used to "yapping" for a whole lot of reasons not related to any actual need to exchange information (to vent and shoot the shit, basically) and to do so in rollicking, loud "party" voices without any regard to the downsides. Meanwhile, all it takes is a bit of introspection to realize that about 80% of this noise is just that. And a little bit of discipline to institute a culture of (relative) quiet and solitude -- even in an open plan office.

What, you say -- no time for introspection? No interest in discipline? No way to even bring up the idea of "library voices" in your culture?

Then your problems are much bigger than can what be helped by any advanced technology.

Thanks for your reply. I agree with your "library voices" concept and I'm very respectful in regards to keeping volume down and minimal duration if there was a need for a chat. The issue is when others are not and you have no control over it, making it difficult to focus on the task at hand.

I think I have used them in the past but did not have a great experience. Maybe I didn't rolled/tested them properly. Will try again. Thanks for the link.

I wasn't planning to add noise, but it may be a good option. I'll try this as well. Thanks

Voices, especially high ones, are kryptonite for NC, but the very latest models do better with them than I've ever heard before. My Bose QC35s and QC20s both do a great job of making a voice six feet away sound at least 20 feet away, and voices over 10-15 feet away not sound at all. I think the QC25s and QC30s will be similarly good. The new Sony MDR-1000X are also supposedly showing next-gen NC performance, according to early reviews.

A set of one of those will probably do the job of lessening voices to something acceptable--they do to the point that my very, very distractable self can work in an open office, whereas older NC headphones did not--but they won't completely remove it.

If you really want that, I'd suggest a set of 34dB+ reduction earplugs. If that doesn't work, put them underneath NC over-ear headphones. If that doesn't work, play white noise on the headphones. I'll be surprised if you heard anything external after that.