USB-C Microphone Parts List

TL;DR a 20 mesh is absolutely fine, but you can go as low as 50 (and probably less) without serious issues.

To a degree this is a constructor's choice since this is a Faraday cage (the sort of thing that allows you to watch your dinner cooking in a microwave without getting cooked yourself. No really, that's what the grid or mesh in the window of your microwave does.

Sound is a pressure wave (vs. electromagnetic) so it just goes right through the holes provided it's not obstructed by a surface. EM waves are a bit different and get "trapped" in the metal shield provided there are no huge holes in it.

The rule of thumb is the holes in the screen are no larger than 1/10th the length of the wavelength of the signals you're trying to block.

About as clear as mud if physics isn't your thing. 

But if you're worried consider a typical pair of speakers or even the pop screen that many speakers and singers use to prevent the plosive ("P" sound) in speech, which is easy to remember because you just remember "explosive" and you're there.

The meshes on those screens are so tight you generally can't see through them unless they are held up to a light - and yet sound passes right through.

If he doesn't believe us, get some "hose" (you know the stuff robbers put over their heads in old movies and comics)... [cough, splutter, choke)] hold it taught and try taking through it. You'll see that even with a very fine nylon net that sound is largely unencumbered.

This is the same with a mic.

A design I've been testing for Matt (@DIYPerks) uses a MEMS (micro-electronic mechanical system) microphone by Infineon. The admittance (hole) for the sound to get to the on the board is 0.8mm across - and the hole on the device itself is half that and it's sealed at the back! Sound pressure has to go in and out of that tiny hole to make the mic operate.

Auris has marginally better performance than the JLI2555 and it's technically "omnidirectional" but right now this is an experimental design due to the long lead times in getting the part. These things aren't something you'd want to try at assembling at home - for scale in the picture below, the disc is 26mm across at the device sits on those three little pads.

The price is very competitive so I'm going to add a new design to my stable that will give the requisite 40dB gain to drive the digitiser which might be useful as a kit option in future, but we have to order 5+ at a time to comply with minimum order quantities.

@marcdraco Thank you so much for all the great info and taking the time for your detailed response!

You're very welcome. The latest and greatest designs are about 50% through production now and if they work as predicted (and I have evicted the gremlins) they promise great performance: even (shhh) stereo!

Well they're back and so far the MEMs units perform at least as well as expected (and better in some cases). There are some gremlins in the BME-800 conversion kit but they were half-expected anyway.

I have done a couple of "weird" designs (more of this later) to use a THAT1512 similarly to Matt's original but with a 12-20V supply. I don't think the THAT Corp like me that much. Either THAT (sic) or these beggars don't like a single supply. I've done it using level shifting but trying to get through a bunch of boards when you have to nick parts from the last one to test the current one is not really the way to do it. 🙂

I included a version for "traditional" electret capsules that come with their own FETs but I haven't found any suitable ones to try yet. 

The main power and headphone board needs more testing (turns out a Baxendall volume control gets really snippy when you try to run it without some feedback). But assuming that works, the board looks to be working well.

The top row, Josh, Elizabeth and Nicole all work as does Phil. The others require assembly (at my end) as I've been working on some of the other MEMs designs and the USB headphone (live monitor) section. The MEMs versions are all omnidirectional so are less useful for pure voice work (a cardioid pattern is better here), but they are exceptionally lightweight and can be fixed to the end of a cheap torch; and powered by batteries. You'll just need to cut a hole for the audio cables, but the level is such that it's unlikely to require screening. A bright emerald LED indicates operation and regulates the low voltage so they can be powered from 3-6v, so USB power is a cinch and they will drive a line-level digitiser input making them potentially very compact and versatile.

I'll post some actual pictures and sound files in a few days when I have everything wired together. I'm chasing a couple of gremlins in the BME-800 adaptors, you gotta love single, 5V power for giving audio designers a huge pain in the neck.