Hi welcome to Fundamentals Friday Got something special for you today. Microphones 101 Pretty Niche field and well, we need someone who knows all about it. Where is he? T it's a computer popup Doug Ford from Doug Ford Analog Design hey Doug than for Jo us. If you've been watching the blog, you know Doug and Doug is the former head designer at Road Microne and at Jans Electronics Yep, and uh.

I've spent my time also designing telep headsets for call centers. so you're one of the microphone gurus on the No. I've just been forced to gunpoint to find out about microphones. Excellent.

So we're going to learn all about it. Microphones 101 multiple Uh videos for this one I Don't think we can do it all in one? Probably not. No. let's go.

Pretty much every microphone that I've ever come across, with one exception relies on every vibration hitting a wobbly diaphragm. and then we do something. wobbly diagram: Wobbly diaphragm. How's it? Go Wobbly diaphragm.

Every microphone. Yeah, yeah, pretty much and they all then boil down to sensing what's happening with the movement of the diaphragm. Yep, go back to the earliest days and we had a diaphragm. Quite often, cone-shaped sound would impinge on that, and vibration on that would press against a capsule full of carbon granules, turn at the back, turn at the front as it pushed on the granules.

It would change the resistance of the granules, right? What? What about size of the granules? That would make it? That would be all the difference, Wouldn't it? It does? That would be the secret. Source Well, to a pretty fair degree. some variants didn't use granules. They actually used stacked discs of graphite under a small amount of pressure, just enough to hold them together.

Yep, as they pressed together, uh, their resistance varied and they were used in, uh, all telephone systems going pretty much from Alexander Graham, Bell or uh. the other Italian guy who apparently beat him to the punch whose name I can't remember uh, right through to about the oh, 1960s, even 1970s. There you go. One of the interesting things about those, incidentally, oh, that'll be the phone Ridge and action action.

We've changed our shot. We've changed our shot. This is better, All right. Okay, okay, uh.

carbon granular stuff I Won't dwell on it or anything, but uh, the old telephone beasties that you used to have M the front plate, the back plate uh, and the Satchel of carbon granules in there. After a period of time, the carbon granules would all settle down here and stick to each other and get noisy and insensitive so you'd pick up the handset and bang it on the table a few times to loosen things up. Got it? Uh, Now, after that? uh, they. they were pretty much the First microphones and nobody makes those anymore in finish applications.

If you're scraped around on eBay, you'd probably find something you right? Okay, so there's no application for those anymore these days. No right, No, except in maybe Legacy phone systems in Upper Slovia or something. Who knows. Uh, what's much more frequently done these days is you tie that to a coil and suspend that in a magnetic field that looks familiar.

Uh, yeah, uh. magnetize the hell out of that. So so that you get a field across there as the coil moves within the field, you can generate an EMF dynamic microphone Dynamic and a speaker. y.

It's a speaker. Speakers work as mic speakers work beautifully As microphones. they're pretty are beautifully beautifully. You get two identical speakers you know, maybe 3in rubbish that you scor out of photocopier or PC speakers, right? You connect them with a run of wire and you listen to one and get somebody to talk into the other.

You will hear clear as a bell. There you go. They are surprisingly effective. Frequency response as a microphone is absolute rubbish.

rubbish. and they're probably insensitive as hell as well. Uh, they're actually relatively sensitive, despite the fact that they're a low impedance device. Yep, um, how you put it there? Uh, their voltage sensitivity.

Yeah, you wouldn't write home about, but their ability to convert sound pressure into electrical energy is probably better than a actual microphone. Right there you go. Just that. the speaker's got fairly low impedance wires and, uh, a microphone has way more turns of much, much finer wire.

Yep, so it's better at converting it into a voltage output. Got it? There you go? Okay, um, and these are these are still used. They still frequently used. Uh, most Uh stage microphones these days are Dynamic Shore are probably the the recognized leaders in dynamic microphones.

Their Sm58 is a iconic model been around for I Think they just celebrated their 50th birthday or something like that, right? So they're fairly frequently used because you can scream into you got to have them right in your Gob right? Yes, And you got to scream in. You can scream in a huge dynamic range is that thing? Because there's no fundamental limitation on how loud you take these. The fundamental limitation, in fact, is moving the cone so far that the corrugations around the edge lose linearity. Got it? There's no other fundamental limitation and that's probably not going to happen until about 160, 170 or 180 DB SPL And by then you're deaf or jet engines.

just G By Got it? Incidentally, there's an interesting variation on that where the diaphragm itself is also the conductor that moves in the field and what they are is a uh, an Al this in 3D uh picture a sheet of aluminium typically uh which is actually corrugated so the entire length of it is free to flex in and out right. and then you put around here a dirty grate horseshoe magnet. So if you're looking down from the top, this would be a diaphragm and you've got this huge magnet around here as that flexes is backwards and forwards in the magnetic field, you're developing a voltage from the bottom to the top. They're considered by a lot of the studio fraternity to be the Ducks gut, right? But there's an issue.

The voltage coming out of those is so low that you have to put those into a Transformer quite often with a couple of turns there and a couple of quillion turns over here to get any useful voltage out of them. Got it? The impedance of that is way under an oh, there you go. They're also reputed to be relatively fragile if you go at them. That corrugated sheet are very, very thin.

Aluminium just goes. And and the so. these are ribbon mics. They ribbon microphones.

ribbon mic. And so the audio fools still use them. Oh, they still swe yeah, they love them. and uh, well, hey, they're quite a good microphone.

They know, realistically, they're know better or worse than any other good quality microphones. But some people just swear by them for certain purposes. Yep, uh, well. that's the thing.

A lot of uh, you'll get a lot of musicians or whatever swearing by a certain mic. CU It has the certain sound. It's got their sound. It's got their sound.

And they might have. uh, a microphone that they love using on high hats. Another couple of microphones that they love using on vocals. this one for guitar cabinets, this one for kick drums.

Uh, this one for recording outdoor explosions F effects And like that. How much difference is there In terms of the Distortion CU we're talking about, the sound differences between mic effectively comes down to a distortion component, Does it? No, uh, it's a combination of things. First of all, the raw microphone frequency response M which can vary between uh, you something that might look like that yep, or something that's got quite nasty Basse Peak or whatever you feel like that, uh, some of them are simply ruler flat measurement microphones are an example of course. Yes, uh.

so the frequency response is one thing, but that's the Co frequency response. You also get micro structure to the frequency response, which is shaped by the fact that the microphone pickup element is surrounded by stuff y the supports uh, the you know, whatever kind of mesh housing it's got the microphone body itself. All of which adds kind of very small deviations in there, which are little type of resonant, mechanical, resonant type. Some of it's mechanical, some of it's acoustic, some of it's proximity based.

If you've got a microphone there that's sitting in a uh microphone housing, then it's going to receive both the direct sound and anything reflected from the structure. Yep, all of which adds to uh, what a lot of people call this microphone uh, Micr structure, right? And some people claim it's audible, Some people people don't. I'm bowing out of that particular argument. Directional characteristics have a whole lot to do with it.

We'll come to that one. Okay, H overload characteristics and linearity. Uh, and oddly enough, sometimes, even just the noise. the kind of noise floor of the microphone got it.

How easy is it to characterize and measure something like that? even with a $50,000 audio precision and professional uh, level mics and stuff like that? Happ Which aspect? Well, the the the mic getting the frequency response of the micro structure actually being able to see the difference? That's actually a lot easier. Now with the Advent of Fft based Uh measurement systems. Uh Once Upon a Time with swept sign measurement systems really difficult to see, but Fft based stuff. uh, it just pulls it out of the yes, pulls the data out of the mind you.

sometimes you can't tell what's Micr structure, what's Fft measurement system noise, and Randomness and what's uh, room reflection and structure around the actual measurement system itself. Yeah, they will Sharp differences, Got it? Yeah, But uh, the whole issue about linearity and SPL capacity has a lot to do with it. And as we'll see, the um, fality of the thing, y has a whole lot to do with it too, right? Okay, yes, the Patn. Are we going to get into Patn here? We will shortly shortly next week, people.

All right. other types of microphones? Uh, well, no. Actually, patents will get into this time. Okay, sorry you can edit that out, can't you probably? Yeah, we're not doing a one pass recording, are we? Yes, we are all right.

Let's go all right. Uh, other kinds of microphones? Uh, uh. There's the class one that's come up recently, which is a fiber optic cable with a cut end and sound pressure impinging on the cut end of the fiber optic cable can be sensed by an interferometer. Yep, a laser interferometer down at the receive end.

Guess what? Still a variation on a wobbly diaphragm, right? Uh, Everywhere you look you, they're all going to be based on wobbly diaphragms. Uh, the only one that's not is this laser based system which apparently can sense the movement of, uh, a given air volume right by using again laser endomet techniques. To me, it's all smoke and mirrors. but no, uh, no pun intended, no pun absolutely intended.

Uh, the big one. The big technology that's in use these days though, is the condenser mic or electric Mark right? An electric Mark is a variation on a condenser. Mark Where effectively the same, you have your diaphragm. Yep, notice I've drawn the uh, the corrugated edges so that it can actually Flex although that might not necessarily be there near a parallel plate.

And what we're sensing is the capacitance between a usually a gold spotted film on there and a metal plate over here. Why does it need to be gold spotted? Uh, basically because you need a conductive surface. Oh, of course. yeah, uh.

some. But it doesn't have to be gold. Gold just sounds fancy in marketing marketing. W And it doesn't rot off.

Got it? Silver will. Oh okay, yes, you silver corrodes. Yes, it tarnishes and uh, incidentally, very very, very thin stainless steel is also used. Okay, interesting as the diaphragm.

Yep, but mostly it's gold spotted. Okay, yeah, and the reason for that is, uh, my thin sheet myar is a nice material to use for diaphragm because it's uh, flexible, but stiff if that makes sense. Does sputter some conductive material like gold? Uh, you could use aluminium. It's a bugger to connect to right, and even connecting to the gold spattering is a bit of an exercise.

Oh, okay, right. So it's not just in. China Somebody just solders it on with a he Have you ever tried soldering? Miler Your just go straight through it. So what is the point is that it is a point contact.

It's usually a pressure contact in fact. Uh, let's take that right to the edge. It's glued onto a ring right and it's either a conductive glue between there and that ring. y or so it'll be the entire circular ring that conducts through not just one point.

and just occasionally. And I've got an example here. Uh, you'll see they actually put a screw through the middle there. and uh, so so that actually forms a how do you put it? an annular shaped diaphragm.

because it's that bit that vibrates. The screw stays rigid Y, the ring stays rigid and that vibrates as a anulus, right? Okay, and they do. the electrical connection off that screw. Interesting.

There you go. But it all boils back down to wobbly diaphragms. Yep, okay. Microphones are used everywhere.

Telephones, uh, any recording device that you've ever come across, you've got one on your shirt right now. Yeah, look guys, just look. look around the room where you are and count how many appliances you've got that have got a microphone in them. Yep, and they're almost universally going to be an electric microphone.

and we're going to come to operation of electric microphone shortly. But uh, let's just have a look at the wobbly diaphragm and the Acoustics in involved. And how do we get different pickup patterns? Your one is a an electric condenser. Reason being that you don't have enough power down here to support an external polarizing voltage of 60 volts, 90 volts, 200 volts.

Whatever it takes. So it's assuredly going to be a condenser. Uh, sorry, an electric condenser as opposed to an externally polarized condenser. But it is a cardoid pattern I believe? Uhhuh that that may quite conceivably be.

So yes, even in such a small size, they can get a cardoid yeah, pattern. mhm Mhm. We'll get into that. Yeah, let's picture.

First of all. uh, the simplest microphone, which will be a wobbly diaphrag and We'll ignore how we're sensing its movement. We'll just take that as a given and we'll put that in a sealed can. All of a sudden, we' formed an omnidirectional microphone.

Doesn't matter whether the waves are coming at it that way. yep, or that way that way, or whatever, any pressure that impinges on that diaphragm will cause it to wobble. Mhm. Okay, the sound can come out from the back.

It's still going to cause pressure on that corresponding vibration. There is, of course, a caveat, and that simple frequency dependency. Uh, of course. let's okay.

we're starting to lose some arrays there. If the wavelength coming across, that is fairly long. Okay, let's just draw that there. And the size of that is small compared to a wavelength.

Yep, then it really doesn't matter what direction the waves are coming at it from, it's going to receive as soon as the diaphragm becomes physically large enough that, uh, as drawn here, you might get a pressure wave there and a null there pressure there. And a how physically big are we talking about? Uh, quarter wavelength? Quarter wavelength? Yeah. Okay, once you get to a a full half wavelength, that's if you like the danger mark, because when that becomes a half wavelength long and the sound's coming at it from the side, it means that I'll just redraw this for clarity. Oh dear.

We really are going to have to dress that shortly. No, we're losing the fight. Okay, so from the side, let's say we've got uh. Pressure Wave null Pressure Wave null Pressure Wave: Okay, as they move across the surface, we've got actually I should say uh.

positive pressure wave negative Peak Positive Peak Negative Peak Rather, Null. Got it? Okay, yes. Simultaneously, on the diaphragm, you're going to have a positive pressure Peak and a negative pressure Peak They're going to cancel Mhm. So the frequency response from the side is actually going to be damn near zero at that frequency, right? That's uh, how do you put it When the diaphragm comes to be equal to pretty much a half wavelength, That's when you're going to get a null.

How can it be though? Because audio is such a massive wavelength? Uh, not really. Uh, okay. Maths, it's Math's time. All right, and I'll have to go polarity here for the uh Imperials and the metrist.

But uh, okay. Propagation velocity equals frequency time. Wavelength: Mhm Okay, uh. for the Imperials We'll call that 345 m a second.

or I think it's about 1100. uh, feet per second. Feet? No, no, Yes, Yes, Yes, Yes, yes, you've got some Us viewers. They're still using feet and inches and and furong yeah and ruds and poles.

And yeah, no, that's the British. Um, okay, at a Kiltz say Okay equals one Kilz M times. Uh, 345 millim. So at a Kilz that's about a wavelength.

Okay, now a little microphone like that is way smaller than that. So at a KZ your typical little Omni mic that might be 10 mm on diameter, it's going to be very, very omnidirectional. Okay, let's go to 10 khz and the wavelength is down to 34.5 mm. Okay, down there.

Okay, yes, go to 20 khz and we're down to about 70 OD mm. And that's definitely the danger point for you. Typical 10 mm half inch microphone. Okay, bother it.

Thank you. Okay, that that's 17 mm. All right. All right.

Okay, so that's a caveat on omnidirectional microphones. They they really are omnidirectional Until the wavelength gets too small, right? And that means that the typical half in measurement microphone. And let's show a half in measurement microphone. Let's go get one.

These are two of my measurement microphones. They're omnidirectional and expensive. Uh, one of them. particularly.

Uh, this one from PCB Pizzat Tronics Ponics? Yeah, I Know them the it's my main measurement microphone and if I can remember how to open the damn thing. okay, it's a half in measurement microphone M half in diameter there? Uh, with nice simple BNC connection there BNC Straight out nice. Yeah, there's actually a reasonable amount of electronics in the body. The actual microphone is only just that tip.

Yep, And in fact, oh, now that's fairly tight. Oh, here we go there we go. That shows you. Possibly if we get the angle just right that there's actually not a lot down in there Mhm.

The microphone section itself is just the big couple of top millimeters there. Okay, there it is. Wow. You can see it's got a a film.

is that a clear down the bottom? You're probably going to be hard pressed to see what's going on there, but we'll draw a cross-section of that one maybe later and have a little look at that. that's the inside of a half in lab measurement microphone. Yep, Incidentally, these have got pretty much a rur of flat frequency response from uh, about I think 10 or 20 Hertz up to about 15 khz. and from 15 khz through to 40 khz the response is characterized.

It's not perfectly flat, though it deviates by about a DB DB and a half something like that that's still not too shabby. Yeah, it's pretty damn flat, right? So in in in its main band. How flat are we talking? Oh, 0.1 db2 DB right? Possibly flatter. Uh, in fact, I Think that any deviations more the result of the measurement system than the microphone got it.

So you rely on the inherent flatness of it as a measurement mic. or could you take into account the the full measured characterization of it? Well, that would be too much of a pain, wouldn't it h As long as we don't disturb the mechanical construction of that head. And why is that head got so many uh openings like that? Uh, basically it needs protection for the diaphragm, yet acoustic transparency so that the sound gets to it? Yep, of course. and with a large number of small, small, relatively small openings like that, we get better than uh, you know, 50% opening ratio Mhm and it will accept sound from pretty much any angle.

so it behaves pretty much like a bear diaphragm just with a bit of protection. Yep, so this is a multi thousand? Yeah, Uh, Omni measurement mic I Suspect these days they're about 225,000 Wow, Uh, cost me I think about 1,600 when I bought it and that was half a dozen years ago. And who can actually calibrate these? The manufacturer M can calibrate them for frequency response. We keep this honest because we've also got a 1 KZ spot calibrator Yep, which generates 94 DB sound pressure level plus or minus point2 DB yep and we use that for it spot checks just to make sure that its sensitivity is consistent.

How do you couple in that generator to it? Is it a set distance away or is it inside a cavity? Uh, it's inside a cavity it actually push fits into. It's a push fit, right? Yes. So it's sealed in there, Got it? And the calibrator itself has internal feedback and its own internal Servo system to maintain consistent pressure inside that cavity. Uh, secondary microphones are basically almost identical, except that these ones here, they really are awfully awfully similar.

Uh, it's identical. What are you talking about? You put a swifted oh well. actually damn near it. Yeah.

Spot the Difference pretty close. Uh, this one is from 797 radio Factory or uh I think they call themselves 797 Audio these days China And these cost about $250 $300 Way cheaper than these ones. And I Trust them almost as much because they come with a very nice cow certificate complete with you trust a Chinese cow certificate? Oh yeah, uh well. BR and Cur print BR and Cur print out.

Yep, Yep. Okay, I can trust that? Okay, yep, uh. which shows the uh uh, the free field and pressure response and you'll notice that that goes to 40 khz yep and deviates by well in this case. I think a DB and a half at 18 or khz with a positive peak of about what's that? Uh, a DB at 40 khz, it's pretty good.

Yeah for a couple under bucks. Yeah, So again, we can keep these microphones honest with the microphone calibrator with regard to sensitivity, and we we can reasonably assume that the frequency response is going to be flat within the frequency band of interest to us.