I have one that cost me $30 all in and you can't hear it at all on the lowest setting (but still decent airflow when you put your hand down). Honestly the biggest benefit in my home environment is how much it has killed the fungus gnats living in my plants. I checked the filter after running it for a few weeks and it still looked relatively clean, but probably a thousand dead flies were in the bottom of it too. I have taken other measures against these flies with not much success (vinegar traps, sticky paper, watering from the bottom, mosquito bits designed to kill larvae, I tried everything I could find one the internet to no help).

I have box fan filters, and a small room Honeywell filter. On the lowest settings, they're about the same, on the highest, the box fan is definitely a little louder (but moves a lot more air).

Turn them down when you're in a room with them - best advice I ever read.

Yeah, I think these DIY filter plans are great and I've built them myself, but I switched to commercial purifiers because of the sound. I don't have measurements but the box fan is substantially louder than either the Blue Air 211 or the Levoit Core 200 that I have.

On the other hand a loud fan is great to help shut out the neighbors annoying dog barking.