Pantyhose popfilters and COVID-19: not so good

Keeping our studio safe

Can you protect your microphones, and the people who use them, with a filter in front of the pop shield? The answer is maybe, but maybe not. Maybe very not.

As technologists, we create danger if we recommend protection measures which seem OK, but aren’t. In these times, we can’t risk creating a false sense of security. Zero tolerance.

Here’s some science to help you think about and explain why care is needed with pop filters.

Virus protection and microphones

Earlier tonight I saw a suggestion that studio hygiene could be helped if a pantyhose-style shield was put in front of a mic, with the pantyhose material being changed and/or washed regularly. That seems like a really good and practical suggestion, and it’s certainly well-meant. But does it do what you want? Engineering analysis suggests that it’s actually a really BAAAAD idea if what you want to do is stop a potential COVID-19 vector.

It’s all about the holes

Simply, the pantyhose shield idea puts a barrier between a mouth and the microphone, and relies on the barrier to stop germs from getting through. Can it work?

To protect the mic from exhaled breath and viruses, a washable pantyhose filter (or any intermediate filter, reusable or disposable) **MUST** have a pore size smaller than the virus. If the pore size is bigger than the virus, then a virus which hits the filter can go through. That’s sort of stating the obvious, but it’s the starting point and very important.

The soak test

Try this. Take a pantyhose-style pop filter. Hold your hand on the far side of the pantyhose and spray water from the near side. Pretend the water spray is your mouth and you’re talking realistically, so you’ll need to spray the water at plosive or sneeze velocity. Spray-spray. Does your hand get damp? If it does: the filter has allowed water droplets through and those droplets could carry hundreds of thousands of viruses.

Because pantyhose is relatively coarse, breath droplets possibly containing a virus could relatively easily squeeze through the pantyhose filter and hit the foam filter that covers the mic. They stay there. When you change or replace the pantyhose filter, and the next person breathes in, there’s a finite possibility that the virus gets sucked off the foam and back through the pantyhose filter.

It could work

Definitely. A filter *can* do the job of stopping bug transmission: the filter pore size just has to be smaller than the virus you want to block. Another way: if the hole is too small for the thingy, it wedges in and can’t pass.

Back-of-envelope maths

How small does the pore have to be to stop a virus? The coronavirus is given as about 120-125nm in diameter (by the way, that’s big for a virus. There are some MUCH smaller, down to 5nm). A filter pore size of 90nm would be prudent and would do the job.

It’s not easy to find info on pop-shield pore sizes – somewhat esoteric information, but one patent application reviewed for a porous solid pop-shield (as good as they get) had a median pore size of 10-60 microns, which is really small, and smaller than foam. To get a reasonable frequency response, the pore size needs to be more like 100 microns

But consider that 120nm, the approximate size of the SARS-CoV-2 coronavirus, converts to 0.12 microns. In other words, really small pop shield pores are, at smallest, still one hundred times bigger than the virus and a practical shield that sounds OK would be ONE THOUSAND TIMES BIGGER than the virus. A lot of stuff could get through the pop shield holes. Quick maths check: a sphere that fits through a 100 micron hole could contain 640,000 coronaviruses. That’s a 100 micron water drop which is 0.1mm. Not all of the critters will get through but, statistically, it’s possible that some will. This is a numbers game in which the best defence is to reduce the statistical likelihood of inhaling the bug.

Pantyhosed

What’s the pore size for pantyhose? Huge. This type of filter has really, REALLY big holes; elephantine by comparison with bugs and water drops. To be blunt: pantyhose stops pretty much nothing that you need to stop, except plosives. The bottom line is that a pantyhose-style pop filter sounds good and is very effective in the intended role of stopping pops, but potentially does nothing useful as a virus barrier.

And if you’re making other connections, filter pore-size analysis does at least at least tally with the advice from those medical experts who say that facemasks don’t help the healthy.