Okay, we've covered. uh, omnidirectional microphone patterns because they're the easiest to do with. The next simplest pattern is the figure eight pattern. figure eight pattern.

Let's do it. What Mes use the figure eight pattern. Uh, actually, a surprisingly wide variety. Pretty much anything that's termed a noise cancelling mic will be a figure eight.

Okay, now, these conceptually are probably even easier than the omnidirectional microphone because we've got a wobbly diaphragm. wobbly diaphragm again. And basically it's sus. it's fastened within a uh, well, picture of that as being the microphone body Mhm.

So we've got a front aperture and we've got a rear aperture. and Le Just just pretend that you know we've got something in there that uh, can sense relative movement of that. Okay, this is a relatively bizarre thing. You get three different effects from this.

First of all, picture this. Let's uh, let's hang on. Let's call this one the front side and that one the back side. Back side.

Yep. Okay, let's say we've got sound coming at it this way. Slight angle? Yeah, yeah, yeah. well.

vaguely. front front on all right. Okay, the pressure on there comes from the front wave Mhm. but then a bit of a while later, we get pressure on the back.

We do if the incoming frequency is very, very low. so we've got long wavelengths. mm. Okay, the pressure on there is downn near zero because we're going with the pressure.

Yeah, positive and negative pressure. And that diaphragm can almost not move at all at all given a long enough wavelength. Yeah, right. Because the pressure at the front and the back of the diaphragm are almost identical.

I.E Sensitivity at that frequency is zero. Yeah, go higher frequency. Okay, and Etc Etc and the pressure at the front Mhm arrives there. Well, how do you put it physically sooner than the wave at the back? So the frequency response winds up looking like that.

Okay, get get to a well. Actually, in principle, it just keeps on going right. But we get that uh one one on F Type frequency response simply due to the physical size of the thing. Got it? Okay, uh, we get a second effect.

Okay, so that in okay, you can edit out the can't you I can, but I won't Okay I Thought this was supposed to be free zone. Um, all of this. Okay, I've drawn those way front with some curvature, but we're actually looking at distant sound sources, right? giving rise to what's effectively a one onf frequency response. Let's have a look at more proximate okay sound sources and which is what these are typically used.

Yes, for they used close up on your lips, right? Okay, so these would be what studio or M or no, um, uh, aircraft? Oh, of course. of course, cuz noisy environment, inherently noise cancelling, and telep call centers. Ah yes, then why do I hear every time I get a call center? I'm hearing all the bloody chatter in the background. There's probably a couple of good reasons for that and I'll tell you all about that too.

Uh, let's say we've got a proximate noise Source That's right there. Yep. Okay, so like an inch away. sorry I'm Imperial 25 25.4 mm away? Yeah, precisely.

Okay, Well guess what? The pressure entering here? Okay, that that's a certain distance from there and that's a much much relatively longer distance Totally. Square Law: Attenuation over distance applies. It's that's all there is to it is a square LW of yeah, proximity. And oh no, no, there is a third.

Factor But this means that proximate Sources: Yep, Okay, have this. Let's say that's uh, 20 mm. Let's say that's 30 mm. The attenuation here is going to be Uh 1 on 20 squ.

Sorry, the yeah. the pressure level. The pressure level due to that one is one on 30 squ. MH So you're talking about a four part to 9 part difference in sound pressure.

Now what that does too is this is uh, frequency independent. M So although at far distances, we get this one onf response. Yep, what we get close up is in fact more that Mhm. So this is also called the Uh Proximity boost effect, right? which occurs with a number of microphones where once you get close up onto them all of a sudden, you get bottom end appearing because of that effect there.

What a stupid question. What happens if you extend this out even further as a two and physically make that path much longer? Oh it. It actually gets, uh, both better and worse at its noise cancelling job. so there's an optimal size optional.

There is kind of sort of because of this third effect. Okay, which is okay. let's get rid of some of the extraneous. Okay, okay, what happens as we start moving our Sound Source from there to there, to there, to there y to there Etc If our Sound Source is there, it's going to react.

particularly if we're close up and personal. MH If we're over here, close up and personal y Once again, it's going to react because that is speaking onto that side much louder than it's speaking on that side. Are they birir? Are they symmetrical in that effect? So all figure eight mics a symmetrical yes, right? What happens when we start yelling at it from that direction? Mhm? Doesn't matter how loud we scream, the path lengths are identical. yep, and a good noise cancelling figure of eight microphone will completely cancel any noise coming at it from sideways.

yep, and with a lesser effect as it gets out towards. Which leads us to Polar responses. Got it? Okay, all of that was a very long hand way of saying yes. Let's just say okay.

this is a polar diagram of the pickup. Yep. and here see: I Can see where our Figure Eight's going to come from already and this is our microphone. That's the front Direction Say that's the back Direction Guess what? An omnidirectional microphone at Mid mid frequencies before we get high enough in frequency to get durational effects.

it's got a polar pattern that's that way. Doesn't matter what direction the sound's coming from, that microphone is equally sensitive. A figure8 microphone has zero. at at 90 off angle, it's got zero.

It's got maximum maximal response for and half. and its polar pattern looks like that that, hence the name. That's right now. An interesting thing.

let's talk about phase. Okay, okay, Omni microphone doesn't matter what direction you're coming from. That entire uh Circle has if you like positive phase, right? I.E A positive pressure wave produces a, say, positive output from the diaphragm. Mhm.

Not so for this one. This one has a positive output for pressure that way. But if you push, put, push the pressure in that way. Negative opposite.

That's right. So that's the figure Eight response. And these noise cancelling microphones have some magnificent properties, including the fact that uh, how you put this is difficult to describe without lots of hand waving in three dimensions. But if I'm if I'm pointing a microphone at you, then it'll hear you.

It'll hear me and it's got a cone of silence and in fact, a ring of Silence extending all around that way. Okay, whereas an Omn microphone, it doesn't matter why we pointing at the he everything. Okay, now the other thing with the figure eight is The Closer I Get Not only the louder it is, but also the more bottom end. It's picking up because right, because the distance from here to here yep is so much less than the distance from here around to that side of it.

So that's the proximity effect, got? It makes sense. Yeah, and indeed, if you were to increase the the length of that, Yep, that does a couple of things. First of all, it changes. Uh, how do you put it? the distance at which the proximity effect starts right? If I've got something, that's where's where's the pen cap.

Yeah, okay. If I've got something that's that big stay, I'm pivoting here, then the ratio distance from there to there is not very far. That one's going to work much better up close. like that.

Pick a physically larger item and I'm going to get that same ratio with that position much further away. So the longer it is physically y, the greater the distance at which it exhibits proximity effect Mhm. Unfortunately, the worse it gets as a noise cancelling microphone because it also means that distant sounds around there. I'm not talking about the immediately off, but distance sounds over there still get to see a reasonable low frequency response out of it.

Are we getting into the shotgun effect here? Uh, and shotgun mics? Not quite, but the same principles are involved. Okay, it's all about physical size and proximity and all of that. Yep, Okay, everybody's been wanting know. Cardioid microphones? Haven't they? They have.

M Yes, let's go. Okay, let's just imagine that side by side, we position an omnidirectional microphone and kind of superimpose on top of it. I'm losing my pens again. Oh, wcky thump lad.

That's why that's why this is here. Yeah, and side by side, Like, right on top of it, Will'll position a figure8 microphone, right? We'll connect their outputs together. Interesting. Okay, let's see what happens to the directional response.

Okay, over here. we've got that response plus this response. So we get okay. Polar diagram: We get an output of, say, two units.

Okay, plus a unit from that one. Yep, and plus one unit from that one one, right? Okay, let's work our way around over to halfway on. we get plus one unit from the Omni and none from the figure eight. So that'll give us a plus one and a plus one.

Let's now move over to here. and we get a plus one from the Omni and a minus one. Yep, Guess what? It plummets to zero Zero. So we get a figure that looks like that.

That's our CID pattern. Yeah, and why is it called cardioid? Yep, Because it's heart-shaped. heart-shaped. That's it.

Simple as that. Easy makes sense. And they're all constructed like that physically using two elements. No, no, all right.

and this is where the black magic comes in. All right, let's go. Oh, incidentally, some of the properties of a cardioid microphone. It's halfway between the properties of your figure8 and your Omni seen it.

It exhibits proximity boost. Yep, Okay, it's It has a better low frequency response up close than it does far away. It does. It's got this hole at the back so that uh, uh, let's say that's the front and that's the back.

Okay, it's going to receive me at full strength and full frequency response there. Yep, as I've turned it away from me. Okay, I'm 6db down. Remember I've gone from two units to one unit as I turn it fully around.

it should bloody near cancel right now. One of the measures, in fact of a good cardioid microphone, are if we draw its natural frequency response which might look a bit like that and that's front on. Yep, what is the response at the rear? Now, if it's not a very good cardioid Mhm, which is actually rather typical of your little Electrc, they don't make very good cardioids like the one you're wearing at the moment, which claims to be cardioid. But how good is it? It might have a response which is pretty much the the response in that path there Mhm.

It's probably got no differentiation at low frequencies. Up at the mid frequencies, it might get to, you know, 15 DB down, 10 DB down. And then it's got no directional characteristics. Yep, Okay, a good Studio Oh okay, that's say 10 DB If on a good day, it'll be 10 DB Okay, uh, a good studio microphone.

Apart from having a frequency response that goes down much much further. Uh, it will have. First of all, it will depart from there much earlier, much earlier. Yep, be much lower in the past band.

It'll come back a lot later. And on a good one, you might get as much as 30 DB rejection. That's a lot different. Yeah yeah.

and this is why in a uh, a public address system. So for a rock group, you want a really, really good cardioid microphone because you've got a foldback wedge about a meter and a half away there. and these things are loud. Explain FB wedge Uh F back wedgees So the singer can hear what they're singing in the presence of guitars and drums behind them.

Big PA Stack there. They actually want to hear what they're doing so they can keep keep in tune. Vaguely so. And they actually work in a massive Rock venue that's pumping that's bleed, You've got blood coming out everyone's ears, and they can hear themselves from the yeah you can be talking uh, anywhere between 100 and 115 DB sound pressure level on stage? No problem.

So first of all, how how do they get adequate signal and noise ratio? First of all, they're up on the microphone. That's why they're swallowing on the damn thing. Yep, so that wh bottle in one hand and I've got stories about that too. Okay, complete nonse.

Im images of Nara 1984 come back to mind. Yes. Jimmy Barns did offer Awig for his from his bottle of vodka. Uh, anyway, they're right up on there, so they good get good signal to noise ratio, signal being lips to microphone noise being guitar Stacks to microphone and in particular, fold back wedge to microphone.

Now if they've got that null behind the microphone, Mhm. if they've got that null in the frequency response point into the fback wedge. Yep, they can stand right on top of the foldback wedge and not get feedback. Yep, mind you, as soon as they go off angle a bit, then they're into that region there and they're bugging.

So that's a combination of a good 30 DB rejection here, plus the fact across a wide band across a wide band, plus the fact that you're right up in it. if you, if you had it 6 in away, you might be screwed if you had the system gain turned up so high that you could actually get usable working at that kind of distance. Uh, you've got problems because your signal to noise ratio is just not good. and a lot of uh, beginner singers.

uh, they need to be taught that compromise between not so close that they get that screaming into the microphone proximity effect. They need to get natural sound, but if they work with it too far away, they're going to get into the Realms of feedback. Or what they're doing these days is using in monitors. Yes, you you always see them.

They they have their Inm monitor so they don't these days. Yeah, yeah, got it. But uh H yeah. back in the 70s so we pretty much covered uh.

directional pickup patency. Oh no. Shot shy, No, no hypercardioid. Oh, you get a hypercardioid simply by changing the balance of of, uh, if you like, uh, figure eight to Omni got it in the mix.

For example, if you reduce a little bit, the amount of omni what you wind up getting is instead of the cancellation occurring at that angle there, you can get the cancellation to occur at that angle. There your the pattern that you get out of that might look like uh, yep, that green one. There is a hypercardioid basically by tuning the relative amounts of figure eight and Omni instead of having that, uh, cone of silence immediately behind the microphone, you've actually turned it literally into a cone of silence instead of a spot of Silence. It's still going to have a bit of pick up in your direction, but you've made a cone of silence that way and also made the forward lobe just a little bit more directional.

Mhm. Now bear in mind, neither cardioids nor hyper cardioids. You wouldn't call them a a fiercely directional microphone. Yep, uh.

a lot of people are under the impression that when you using a cardioid microphone, you really do have to be dead on Axis. You don't. That might be true for the very highest frequencies, but only for the same reason as an Omni because once you go off there the wavelength, it's just the wavelength effect across there. Mhm M their natural directivity at Mid mid frequencies.

not that high, right? The one thing that you do use a directional microphone for is to eliminate noise. So in a cardioid? Yep, if there's the noise point, the bum of the microphone at the noise? Yes, absolutely. If you've got a bunch of shops or something over there or traffic that's making noise over there and all you've got is one microphone, It's not a shotgun pick, maybe a hypercardioid so that it excludes that. that.

So that's one of the Practical applications, right? Yeah, Hypercardioid? Yeah. And figure eight, of course. Exceptional. For when you want H you've got bucket loads of ambient noise.

You don't know what direction the ambient noise is coming from. You want a microphone that's sitting here? It's up close. It's personal and it will adequately distinguish between that close Spe MH and noise coming in from a distance. Aviation Headsets and Telep Call centers.

And Telep call centers? Yes, Incidentally, I have an anecdote. A pattern related anecdote from the days of doing headset microphones. Now picture this. Okay, we've got a figure8 microphone capsule and it's in a headset Boom Mhm that looks a bit like that.

The internal structure of the of of the tip of the headset boom is that that bit of the electric microphone goes to an opening at the front there. Okay, and the back of this opened into a fairly large structure. There again, with an opening there, and that allowed the wires to kind of dribble down a yet another tube there. Okay, so just to clarify, we've got our figurate microphone there.

We've got a front Channel which is closer to your lips and this back channel in there which is supposed to allow room noise that's coming in both reach there Mhm. There's a problem though. whenever you have a tube and a cavity, you've just formed a low pass filter which might have a frequency response of something like that. Typically, typically it'll Peak mhm because it's ineffective.

You just made a whistle. Yeah, Now, if this whistle here is at a different frequency and in fact, I've drawn that poorly that that one would actually be at a higher frequency, because it's got a larger opening, you've got these two at two quite different frequencies. M Okay, from that frequency up to about that frequency, it'll cancel noise. Yep, what happens with ambient noise at frequencies above? There is.

first of all, look at the relative difference between the front and the back. The front of the microphone responds to all that ambient noise. So the aggregate response to ambient noise is can see it reasonably good cancellation, then does that, but then guess what happens round about there? If you're really lucky, you might get a huge Notch but then guess what at the frequencies above that that one becomes a whole lot louder than that one. So the aggregate frequency response does that.

So yes, right. Inadvertently created a microphone that had a response that far from being noise cancelling actually exceeded the response of either section at high frequencies. Was a commercial MK Yes, this is one that incidentally came in from Asia right and they didn't know what they were doing. They had no, we need an we need a front or office We need a back or office And it was so bad that the thing just howled around between the microphone and the earpiece and uncontrollably and oh great at the end of the day.

Uh, from memory the The Cure was to uh, either drill a larger hole in there because I think I've drawn these wrongly. uh I think that in, keep your head still. Yeah, we we we had to drill those out to hang on I I'll redraw that because that was actually quite a large and direct path. So it was the higher frequency of the two.

This one was quite a constricted orifice and was the lower of the two. We board it up to roughly match that simply by yeah, drilling it out, drilling it out. Nice. Yep, and you did that through experiment or did you? were you able to determine the whole size based on some Theory back it was Theory backed by picking the right drill out of the box.

Brilliant! I Was able to uh I made myself a little artificial voice which consisted of a just a diecast box with a crappy little loud speaker in it and a small hole right I then calibrated it by using a measurement microphone positioned here. Mhm when I drove that from the test system I then measured its response there, calculate the inverse drive to speaker with then the inverse response and you get a flat. You know, once you've removed your measurement microphone, you get a flat frequency response of drive at that point there at that point. Okay, Beauty so you can come along with your microphone and you can, uh, position it there and measure the response front on and then block off that hole with a lump of blue tack.

Block off that hole with a lump of blue tack. The individual responses here then stood out absolutely clearly and it was a pair of dogs balls. Yeah, yeah, yeah. Awesome.

Love your stories. Doug.