It felt like the foam did improve audio quality but I think the only thing that measurably reduced noise from outside was solid core door. Pricey though! Agreed that a good microphone is a huge help. I currently use Elgato Wave:3 which is not as fancy as one can go.

Thanks for that post, quite the journey! If you're looking for a new mic, I can recommend the Rode Procaster, it's a a dynamic mic which hardly does pick up any ambient noise whatsoever. I am very happy with it, it should be quite close to your mouth though (not everyone likes that), and I had to get a pre-amplifier (CloudLifter) for some extra gain. Without it, volume was a bit low. Here's a write-up of my set-up touching on the mic and some other stuff: https://www.morling.dev/blog/whats-on-my-desk/.

Thanks for the recs! It's so hard to decide when/whether to upgrade my mic. Would Rode Procaster be better than the Elgato Wave:3 (what I have now)? They seem to cost similarly.

I have never used the Elgato, so I can't speak to its qualities. When I was on the market for a mic though, I've found the Procaster to be on a completely different level than any USB mic I had tried (the Procaster connects via XLR, i.e. an audio interface is needed). It just gives you this nice soothing "professional audio" sound. No affiliations with the vendor btw, just really happy with the mic :)

I liked your post, btw. I just think a pricey headset would have freed some money to attempt a proper ventilation.

CO2 levels num your mind. I have a bigger walk-in and sometimes I take calls there. After 15 min. I cant stand the air quality.

Noise cancelling headsets only help isolate yourself from the outside world and won't help to cancel noise for your audience. Good microphones are built to pick up every tiny sound around them including all the background noise - that's what they're meant for. And buying a crappy "noise cancelling" microphone is just going to ruin the good quality sound that you're looking for from a standalone mic - you might as well just add a noise gate (which also sounds a bit jarring and isn't ideal). One option is to instead buy a shotgun mic but that's also not an ideal option for a live stream where your head is probably not going to be in one location for long periods of time.

Basically, no amount of hardware or software can fix a bad sound environment.

Hence the boom microphone. Preferably with acoustic cancelling.

The boom places the microphone close to your mouth so it's gain is very low. Having two mics on the boom setup to constructively add only the speaker's voice does extra cancelling.

This is how call centers work

EDIT: by boom mic I mean headsets with a boom attached. High end ones are specifically designed to eliminate ambient noise.

Noise-cancelling boom mics are great in my experience. Even loud external noise just disappears. I did experiments with office noise, planes, trucks, weedeaters, doors slamming, etc., and only the door slams got picked up.

Best part is they work without needing a CPU intensive algorithm guessing what part of the signal is noise and introducing lag.

Do you have any specific recs? Happy to research mic upgrades.

No. I'm just interested in the physics/electronics due to my academic background. I have, maybe, two meetings a week in a basement all to myself

I disagree because the context here is that they're a streamer. It sounds like there were two goals with the acoustic treatment: (1) to improve sound quality from within the closet which I take to mean echos and reverb and (2) to avoid picking up their partner's voice during meetings. I think it especially makes sense given the closet is in the office where their partner is taking meetings.

Your suggestion of noise canceling head-set only helps prevent her from hearing her partner, not the stream's audience.

Hence the boom mic.