Apparently, I’m really into old-fashioned microphone technologies at the moment, and really, I’m just fine with that. I’ve had this boundary-microphone idea in my head for a while – I even ordered a bunch of parts to build it – and that idea and crystal microphone technologies go together!
Okay, first, crystal microphones were an actual thing. Popular from the mid-1930s through the early 1950s, they were used on-air and in music recording. They’re still in use in certain applications, much like carbon microphones are, but more widely – if you’ve heard of a “piezo buzzer” or “piezo instrument pickup”, that’s exactly the same technology, only applied to a different goal.
The underlying physics: there are crystals which, when flexed, will produce electricity. The charge is positive or negative, depending upon how the flexing is done. Sound waves are enough to do it, which means bing! Microphone technology! This is Neat. And, yes, I have a sample mp3 below.
Hey, that 60s and 70s Skiffy fascination with crystals had to come from somewhere.
(Speak clearly into the pinky ring, Zed.)
It works the other direction, too – current one way will flex the crystal one way, current the other way will flex it back. You can make speakers out of this, and that’s been done. This is also how piezo buzzers work – cycling AC power through a piezo-effect crystal.
I’ve built a couple of piezo-based pickups before, using the Cortado kits, so that seemed like a good place to start. I’m not bothering with a second board-construction write-up; the first one is here, if you’re curious.
But building the carbon microphone driver circuit as an external box made me realise that I should build this mic using an external driver circuit as well, so I can experiment without taking apart the box every time. So I used the housing from a dead laptop power supply I’d recycled a while ago.
The best part was that the AC mains connector slot was almost exactly the right size to hold the XLR connector. I just had to file away a bit at the narrowest points. And, of course, I had to line the whole thing in conductive metal tape, for RF shielding, and ground it.
The Now-Modular Cortado. Piezo lead on the right, XLR to board on the left.
Looks almost professional!
TRS: Tip is outer disc, ring is crystal disc, sleeve is shield ground
Standard balanced XLR mic-level output
This lets me plug in anything crystal or crystal-signal-level-like and use this amplifier on it, just as with the carbon microphone, but for carbon-technology elements. In this case, I’ll be plugging in a piezo disc. But since that’s just the crystal, the real question becomes, what resonates it? What vibrates in the presense of the sound, causing the crystal to flex?
My initial idea for materials involved a lightweight, rigid plastic. I’d also thought briefly about metal, but decided that would be too heavy, and I was right about that. The bad news is, that also turned out to be true for the plastic – it takes too much energy to make it move, so it doesn’t move very much just from soundwaves, and the signal levels were really low.
This is the best I got, using the lightest of the “heavy” plastics. That recording was made talking into a small, clear rigid plastic sheet – I think it’s some sort of acrylic, but I don’t know. It came from Boeing! But does not fly.
I love distant-shortwave-sound of this recording, but that hiss isn’t an added effect – it’s amplifier noise from boosting the signal high enough to hear properly. So, obviously, that won’t work as planned – unless I need exactly this effect, of course.
Still, I’m thinking I could put it in front of a guitar amp or something else VERY LOUD. It’s modular, so there’s no huge reason not to keep it, and I have like 50 of these piezo discs. It also works as a gigantic contact microphone/pickup.
So I started working my way down material weights until I found something too lightweight.
THE HEAVY HEAVY DELRIN SOUND
The thin and tinny base of a styrofoam cup
FAILURES, ALL OF YOU! GET OUT OF MY SIGHT!
The lighter I got, the more response to sound I got, and the more signal – to a point, of course.
It turns out that the best weight is way closer to the styrofoam cup bottom than to any of the plastics I’d hoped would work out. A pair of thin foam dinner plates did actually rather well – I’d thought it was just one thicker plate, but no, it was two plates! – and I’ll try that again with a better (by which I mean actually shielded) test harness pickup, and plates that don’t have divided food sections.
And also, that styrofoam plate with the last 15mm or so of the “cup” still attached worked pretty darned well, without the echoy effect of a “cup” microphone. Some people want that; I am not one of those people. (But again, modular! And I have 50 of these piezo discs, I could make one anyway.)
This gets closer to the original construction materials used in the original crystal microphones, so really, I have no business being surprised here. I was just hoping that with improvements in crystal technologies that a heavier plate would work. But it’s just not generating enough signal output.
And that’s really kind of putting the kibosh on my whole boundary-microphone idea – at least, using this technology. Nothing strong enough to deal with the requirements of a boundary microphone – they’re quite large – is going to react enough to sound to give a decent amount of signal. Unless there’s some unexpectedly light and strong foam.
At least, not with these discs.
Maybe NASA has something I could, you know, appropriate. And I wonder if I can find that crystal material in, oh, one big giant sheet, and stick that to something strong enough. It has to come from somewhere…
This is part of a collection of posts on building microphones and microphone-related kit, such as mic pre-amps.</p>