Friday, August 28, 2020

Making KN95 masks better

I have found (using a small single blind test) that disposable masks have better audio quality for teaching: cloth muffles the voice, especially I think an already somewhat muddy male voice like mine. Surgical-style masks have a lot of leaks around the edges, so I went with cheap ebay KN95 masks.

I reuse them, "disinfected" by airing for seven days (as recommended for N95 masks by the N95's inventor), leaving them hanging on a little wooden rack with nails in it. (I had some fun with my CNC router on it as you can see.)
The cheap KN95s have some air leaking around the edges and top, and occasionally I've had an earloop break off. So, here are three mods I've made. You can do the first two without special equipment, but the third requires a 3D printer. (But if you're someone in my social circle at Baylor, I could 3D print some for you.)

First: add a bit of Shoe Goo under, around and over where the earloops meet the mask. This seems to greatly increase the strength of the connection. No broken off earloops since.

Second: I added some rubber bands joining the earloops. The main point was to make the fit more snug, reducing air leaking. Sadly, it puts more pressure on the ears. (There is probably a sweet spot in rubber band length where it reduces pressure on the ears, but I'm putting safety over comfort.)

Third: I replaced the flimsy metal nosepiece with a hefty 3D printed one. It took me five prototypes until I got the size and shape right, but then it worked great in teaching. It's 2mm thick, printed in PLA, and glued on with Shoe Goo. (The metal strips came off very easily from at least one of the brands of cheap KN95s.) No more fiddling with fitting the nosepiece, and no more feeling of air going up and out along the nose bridge, so I expect it increased the protection for others from exhalation. I don't normally need to wear glasses with a mask, but I tested with my sunglasses while walking home from class and found no fogging. I still fiddle with and adjust the mask, but a nice bonus is that I mainly need to touch the plastic strip, which is probably cleaner than the filtering surface.

I don't know that the strip increased the protection for me as significantly, because the KN95s already would seal around the face when breathing in. (There have been a lot of claims made that masks protect others more than they protect the wearer. I am somewhat skeptical of this in the case of KN95s and well-fitted cloth masks, because my experience is that when you breathe in, fitted masks pull to the face and seal much more tightly than when breathing out.)

My 3D printing files are here. Unless I have an identical twin I don't know about, you'll need to edit the OpenSCAD files and tweak the Bezier parameters to make them work for you. Mine I did mainly by trial and error with five prototypes, but when I made one for my son, I had him press a wire around the bridge of his nose, and then scanned the wire along with a ruler for size, and traced Beziers over the wire (if you're in my Baylor social circle, I can do this for you, on the basis of a good photo of a bent wire and a ruler or other calibrating object).


Alexander R Pruss said...

For a further mod, see:

John Kennedy said...
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