Notices by Don Romano (alt) (thor@noagendasocial.com), page 47

@awg There are certain scenarios where I imagine that a particularly quiet and sensitive microphone is handy, such as when recoding jazz or classical music. Getting a good recording of the opening to Ravel's "Bolero" or Grieg's "The Hall of The Mountain King" without much hiss can't be trivial. Then again, not many recording engineers actually make recordings like that. The more typical scenario is vocals, acoustic guitars, and rhythm and brass instruments.

@awg It can serve as a shortcut to that sound. Instead of having to work out what makes it sound special, you just use the thing itself. In your traditional studio situation with a whole band or string session present, and studio time coming at a premium, it's a sensible way of thinking.

@awg A smaller capsule is going to have more self-noise because the air molecules are bigger relative to the diaphragm. Same problem limits you when it comes to small CMOS sensors on smartphones, but it's not quite as severe, and with omnidirectional microphones, you can compensate by moving up close to the microphone. I also suspect you could use multiple capsules to get a larger effective diaphragm area.

@awg The only reason you don't use omnidirectional microphones everywhere is because a perfectly dead acoustic space is so difficult to accomplish, so even in a sound recording booth, a cardioid pattern will improve your recording.

I've heard some pretty good voice recordings made with mobile devices. Some of them have quite a full sound and a good SNR. I think the bias against small capsules is mostly historical and maybe a bit irrational.

@awg I'm pretty sure that if you record in an acoustically dead room, you could get pretty good voice recordings simply using a lapel microphone clipped to a microphone stand, since it would essentially behave like the bottom end of a large diaphragm microphone across the whole frequency range.

They say that the main reason that large-diaphragm condenser microphones sound warmer than small-diaphragm microphones is that they become more omnidirectional at lower frequencies.

I hadn't noticed before, but cigar mics are less sensitive at low frequencies. I guess they have to be, to achieve good directivity. That explains their cold sound.

I'm pretty sure you could emulate these properties with an array of MEMS microphones and some DSP.

I think that there are some programming languages you shouldn't learn, but merely admire from afar, because they'll ruin every other language for you.

@monerica From what I've heard, they're a mix of "smooth" (because they're mostly flat) and "airy" (because of the >10kHz peak). I wouldn't dismiss them for professional applications, especially not if you combine several of them to produce directional characteristics.

@monerica I would otherwise describe their sound quality as excellent. Their SNR isn't half bad. I know you can filter these things. I can make passive, active and digital filters. That's not the point.

The point is that I like to do a lot of research before I try something out. I more or less design things in my head long before I build them.

@monerica As I've pointed out elsewhere, you can't know how much DSP to apply without measuring each mic. The variation between capsules is rather large.

20 kHz outright isn't really audible to most people, no. 10-15 kHz? That's right in the "air" range and is definitely audible. I think I've heard unequalised MEMS microphones before, from people recording on mobile devices on YouTube. They are basically very sensitive to any kind of hiss, like rubbing your dry hands together.

Paper on acoustical design for MEMS microphones:

https://www.edn.com/Pdf/ViewPdf?contentItemId=4429422

It looks to me like almost no matter what you do to the opening to the front chamber, you get a resonance somewhere in the 10-20 kHz area.

Upon further investigation, it seems that the Helmholtz resonance of MEMS microphones is affected by the diameter, length and material of the opening to that chamber.

Interesting diagram from ST Microelectronics' tutorial for MEMS microphones. You get some pretty good MEMS microphones with a ~70 dB SNR (compare with ~80 dB for a Neumann U87) and a near-flat frequency response. Their problem is the Helmholtz resonance. It varies considerably from capsule to capsule, so if you wanted to flatten that with a filter, you'd need to calibrate each capsule. Why don't they reduce the cavity volume to move that resonance into the ultrasonic range?

@monerica If you wanted directionality for a range of frequencies, you'd presumably need frequency specific phase shifts / time delays. This makes me wonder how they even measure polar patterns for microphones. There's no way of making a microphone with a figure-eight pickup pattern at 20 Hz, for example. I'm guessing they just measure it at 1 kHz.

@monerica As far as I understand it, the higher the frequency, the better the directionality, both because shorter waves phase shift more when the source moves, and because smaller waves behave more like light and can't travel around corners.

@monerica Right, but if you have a panel of omni microphones, and you sum their inputs, does that in of itself create a directional pattern? How does that pattern change with frequency? Would a bigger panel be more directional for lower frequencies? Feels like you'd need some kind of simulation to get the full picture, since you have an awful lot of paths for the wave to take, including waves travelling around corners from behind the panel.

I'm still fascinated with MEMS microphones. I wish I knew more about DSP algorithms for beamforming.

All MEMS microphones have an upward bend on the high end of the spectrum. I found the explanation for this in a technical paper: It's not an issue with diaphragm resonance as I originally thought. It's the microphone cavity that forms a Heimholtz resonator. When you blow a bottle, that's a Helmholtz resonator. Speakers with bass ports are also Heimholtz resonators. It creates a peak at a particular frequency, determined by the shape of the opening and the volume of the cavity.

iPhones are usually considered pretty good for privacy, but I just found out about an Apple ad tracking system I didn't know about before:

https://blog.mozilla.org/blog/2019/04/15/the-bug-in-apples-latest-marketing-campaign/

It can be turned off in the settings, and the tracking ID can be reset, but as the blog entry from Mozilla points out, it would be better if it wasn't enabled to begin with, or if they at least asked you for permission during initial setup or after upgrades.