box fans are usually 20-24" squares and so are furnace filters. you can get real high MERV filters, it doesn't matter how slowly the air flows if you're mostly

box fans are usually 20-24" squares and so are furnace filters. you can get real high MERV filters, it doesn't matter how slowly the air flows if you're mostly using it for noise, the filter stops your fan from constantly just blowing dust around. If you don't wanna do 120VAC you can do 12V engine room fans in 1sqft boxes[1] with carbon cloth as the filter material. It really depends on the timbre or tone you want the noise to have. 12V vent fans are higher pitched, box fans a little lower, "squirrel-cage" blower fans are lower yet - if they're the HVAC type - if they're the carpet drying type it's higher pitched because there's a grille over input and output that perturbs the air. I got tired of replacing $60 sets of HEPA filters after moving in to a forest. I actually bought K&N 20" furnace filters because they're washable. i wash them every 3 months or if they're very obviously grey approaching black, or, during pine mating season, if they're yellow approaching black. I use mean green or simple green, whichever i have handy, and the hot tap in the shower to just wash them off with soap and water. I have to replace the ~$20 box fans every 3-5 years. They just wear out, probably by design. They break in the startup, if you can get them spinning it'll keep spinning. [1] shot with on-board mic but you can see the idea. The metal inside is one of those "hold your kitchen utensils" metal baskets, it keeps the form of the carbon filters inside, and stops stuff from getting sucked through. The airflow is in to the hole in the box, through the filter, out the fan. https://www.youtube.com/shorts/zmzs9H4NUCQ?feature=share https://www.youtube.com/shorts/zmzs9H4NUCQ?feature=share