microphone pattern q: 1/2 of figure 8 VS. cardioid

What's the significant difference between half of a figure 8 pickup pattern and a cardioid pattern? I'm thinking about building an M/S mic with 3 of the side mount MXL 990 capsules. I'm guessing two cardioids facing away from one another would work fine, but maybe not?

Brad,
Perhaps the difference is in the damping? Thats what a mic tech told me when i asked him why my Neumann U48 sounded so different on Cardiod compared to a U47. I believe that the oposing diaphragm is "held" at the same voltage as the backplate, instead of floating around in space . . . Please someone correct me if I'm wrong!


Andy P

Brad, if you put two cardioid pattern mics back-to-back, you get a figure-8 response. The back parts of the cardioid response effectively cancel each other out, just leaving the 'circular' response each side. This is a purely acoustic phenomenon - although obviously you could alter the characteristic of it slightly by playing with the capsules. That's how any dual-diaphragm mic achieves a figure-8 - it's two cardioid responses added together with some cancellation in the middle.

There's an SOS article about this here.

[quote author="bradzatitagain"]What's the significant difference between half of a figure 8 pickup pattern and a cardioid pattern? I'm thinking about building an M/S mic with 3 of the side mount MXL 990 capsules. I'm guessing two cardioids facing away from one another would work fine, but maybe not?[/quote]

Yes, it will work. Place "fig8" capsules on top of each other (i.e. side on side vs. back to back), and reverse their polarity. You will need to play with their acoustical arrangement, though (remember 603 vs. 990 sound difference).

Perhaps the difference is in the damping? Thats what a mic tech told me when i asked him why my Neumann U48 sounded so different on Cardiod compared to a U47. I believe that the oposing diaphragm is "held" at the same voltage as the backplate, instead of floating around in space . . . Please someone correct me if I'm wrong!

Click to expand...

Andy,

The acoustical damping of both diaphragms is the same, and this effect is due to electrical cancellations. Besides, they sound different because of pattern diffrences.

Thank you Marik, I saw that arrangement in my mind's eye.

Hey Steve, that's a fantastic link, thank you. I'll try it both ways.

Yes, that will work.
You will probably have to cut down the capsule housings with a tubing cutter to get the best back-to-back spacing for figure of 8, i.e. null at 90° off axis.
I was talking to Jiulong Ma at 797 about using the SP C4 capsules this way and he thinks there should be a backing like the conical plastic piece you see when you remove the capsule of an MXL603, behind both capsules.
A while back though, there was an AKG C33 stereo mic in the Stephen Paul lab that had felt (homonym) behind the opposing 451 capsules. The felt appeared to be there for the purposes of damping.
To answer your question: One half of a figure of 8 does not exist on its own. Its occurrance is entirely due to 180° opposite polarity between the two optimally-spaced back to back cardioids. The opposite polarity is due to the voltage potential of the two backplates being 2x above and below, respectively, that of the two diaphragms.


Brent Casey


[quote author="bradzatitagain"]What's the significant difference between half of a figure 8 pickup pattern and a cardioid pattern? I'm thinking about building an M/S mic with 3 of the side mount MXL 990 capsules. I'm guessing two cardioids facing away from one another would work fine, but maybe not?[/quote]

There are two "pure" types of mic transducer, pressure, and pressure gradient. They are also known as Omni and figure eight. If you stack one on top of the other, and add the signals together, the out of phase lobe of the f8 will cancel the omni, while the in phase side will reinforce. Instant cardioid! MS can be done easily by multing and phase flipping the f8 and panning the two opposite. you now have 180deg opposing cardioids that cancel perfectly in mono to an omni (mid). It's kind of tough to use a Jecklin disk though. :green:
Josephson uses this method for pattern in his series seven mic, and he's a pretty sharp feller. Actually, one of my first AESs I was passing his booth and spent an hour or so hanging around just within earshot listening to him talk. He made mention about the omni and f8 being the only phase flat capsules, and adding them like that results in a phase flat cardioid, unlike the standard method. Apparently the tuning of the backplates affects this and a standard capsule polarized to omni or f8 isn't quite the same.
There is some good stuff on his site http://www.josephson.com

There are two "pure" types of mic transducer, pressure, and pressure gradient. They are also known as Omni and figure eight. If you stack one on top of the other, and add the signals together, the out of phase lobe of the f8 will cancel the omni, while the in phase side will reinforce. Instant cardioid!

Click to expand...

This is also, I my understanding, how you acheive a cardoid pattern in a dual-membrane capsule - the gradual "addition" of the two basic pattern types is performed by the so-called "acoustic resistance" or "phase network" internally, between the two capsule halves...

Jakob E.

[quote author="aurt"]There are two "pure" types of mic transducer, pressure, and pressure gradient.
[/quote]

Cardiod is also a pressure gradient type of transducer. This pattern could be done whether acoustically--such as one diaphragm transducesrs with phase network, two diaphragm Braunmuhl-Weber type, or electirically--combination of omni and fig8.
To avoid any further confusion let us distinguish "pure" types of transducers as:
1) true pressure type as true omni, and
2) true pressure gradient type as "true" fig8 (such as fig 8 ribbon mics or some exotic transducers like one in Senheiser MHK30).
The arrangement Josephson uses in his series 7 by no means could be called "phase flat" as it uses a Braunmuhl-Weber type as a fig 8, which is by definition is a phase cancellation type.

[quote author="aurt"]There are two "pure" types of mic transducer, pressure, and pressure gradient.[/quote]
I agree with Marik - it would be far more accurate to say that there are two basic ways in which transducers can respond to pressure changes caused by acoustic events. Any diaphragm which is only directly exposed on one side to the acoustic wave can only respond to direct pressure changes - albeit modified by the nature of the cavity at the rear. Any diaphragm that is exposed on both sides to the wavefront and is aligned with it will respond to the gradient of the wave (ie the rate at which it is changing) rather than the absolute pressure.

It follows, of course, that it is only a device that responds to the absolute pressure that can have a response down to the acoustic equivalent of DC - which is why all acoustic pressure transducers that are not intended to measure this have a small pressure-equalising hole at the rear or in the side. Otherwise, we'd have even more weather-dependent mics!

By the way,

Combination of true fig8 and true omni cannot be called "phase flat" due to the fact that there is a distance between two capsules. At least on high frequencies, there will always be a phase shift, as a result of this distance translated to a wavelength.

Steve, Marik,

True, thanks for the clarification.
What sort of capsule exacly does the mkh30 use? I suppose if you inverted the normal capsule construction, putting the membrane between two well perforated, oppositely charged backplates, you could get an electrostatic fig. 8.

Jakob,

Yes, in mine as well,but I believe the acoustic addition in the backplate results in less than perfect phase response compared to, say, a rbbon and an omni added electrically.

[quote author="aurt"]
What sort of capsule exacly does the mkh30 use? I suppose if you inverted the normal capsule construction, putting the membrane between two well perforated, oppositely charged backplates, you could get an electrostatic fig. 8.
[/quote]
To tell you the truth, I wasn't sure what was inside an MKH30... But the membrane between the backplates trick has been done very ingeniously inside the Schoeps Mks 5&6, which use mechanical means to vary the response patterns. There is a description of one of them here, but it's only a diagram of the inside that really does them justice, and I couldn't find one that I could link to. Anyway, a little more investigating suggests that most of the MKH series uses the same trick - but how they alter the patterns, I'm not sure, because they use their RF technique on them, and this rather precludes altering the sensitivity by polarisation changes. Does anybody know how they arrange the pickup from this system to arrive at a cardioid response? I know that they are not averse to doing some nifty things around the RF section, but that's as far as it goes.

Whatever they do, it must work pretty well - these mics have something of a reputation - and a price tag to go with it. I always thought that it might make quite a good DIY project to have a go at a RF-style condensor mic. Any takers?

For some more info on MKH30 look here
It is VERY different from MK6 though--two completely different animals. MK6 uses one backplate, and all the rest is done by means of very clever arrangement of acoustical and mechanical parameters of the system.

[quote author="Marik"]
It is VERY different from MK6 though--two completely different animals. MK6 uses one backplate, and all the rest is done by means of very clever arrangement of acoustical and mechanical parameters of the system.[/quote]
According to Schoeps:

• In addition to its circuit design its switchable three-pattern transducer was a special feature of the CMT 20. It worked according to the push-pull principle, with a single diaphragm mounted between two back electrodes. The output stage of this microphone had no transformer. Later it was built without coupling capacitors as the CMT 200. This circuit topology is found in Schoeps microphones up to the present day.

And they don't indicate anywhere that they ever changed the method for later mics.

I've now found a set of pictures of sections through the Mk6, showing clearly how it works - you can see the twin backplate arrangement with the diaphragm between them clearly. It's fig 5-17 in John Eargle's Microphone Book. If you are interested, I can scan it and post it here if you want...

[quote author="SmG"]

According to Schoeps:

• In addition to its circuit design its switchable three-pattern transducer was a special feature of the CMT 20. It worked according to the push-pull principle, with a single diaphragm mounted between two back electrodes. The output stage of this microphone had no transformer. Later it was built without coupling capacitors as the CMT 200. This circuit topology is found in Schoeps microphones up to the present day.

[/quote]

Yes, I have the Eargle book and saw its croossection.
Thank you for clarification. Sorry, I was misinformed, and believed the second (front) electrode was a "dummy" one--the feature used in some capsules for acoustical reasons. I apologize for passing wrong information.

Still, acoustical arrangements in the MKH30 and MK6 are very different.

Oh, I can quite understand the confusion! Where Eargle says 'the left portion of the design is non-functional mechanically; etc, you could easily at a glance think that this might be electrically the case as well. I think that he could have made a slightly better explanation of how the design, which is undoubtedly very clever, electrically achieves the final patterns. This, he does not do, which is a shame.

Rather more significantly though, have a careful read from the bottom of page 158 (RF transmission principle) to page 160, especially the end of it. Nowhere is there an actual explanation of how, using the double-backplate principle, the response shape is varied. Any guesses?

[quote author="SmG"]
If you are interested, I can scan it and post it here if you want...[/quote]

That'd be fantastic, thank you.

[quote author="SmG"]Oh, I can quite understand the confusion! Where Eargle says 'the left portion of the design is non-functional mechanically; etc, you could easily at a glance think that this might be electrically the case as well. I think that he could have made a slightly better explanation of how the design, which is undoubtedly very clever, electrically achieves the final patterns. This, he does not do, which is a shame.[/quote]


SmG,

Now, I started thinking again about it and actually it seems that initially I was right. The passage from Schoeps website you posted :

http://www.posthorn.com/S_50hist.html

is about CMT20, which seems a RF design (and used PP capsules). The MK6 is a Colette series capsule and I don't see any way it could be a push-pull one, and the front electrode must be a "dummy". The fact there is an insulator on backplate and there is no on front one, as well as there is only one electrode (on the back), confirms it, as well.
The patterns in MK6 are achieved not electrically, but mechanically, and the Eargle book explains very well how it works.

Rather more significantly though, have a careful read from the bottom of page 158 (RF transmission principle) to page 160, especially the end of it. Nowhere is there an actual explanation of how, using the double-backplate principle, the response shape is varied. Any guesses?

Click to expand...

In these microphones the patterns are not variable, and MKH series (20, 30, and 40) had their patterns set for each capsule (not interchangable).
There is more info on this type in Glenn Ballou The New Audio Cyclopedia.

[quote author="Marik"]
is about CMT20, which seems a RF design (and used PP capsules). The MK6 is a Colette series capsule and I don't see any way it could be a push-pull one, and the front electrode must be a "dummy". The fact there is an insulator on backplate and there is no on front one, as well as there is only one electrode (on the back), confirms it, as well.
The patterns in MK6 are achieved not electrically, but mechanically, and the Eargle book explains very well how it works. [/quote]
I read it again a little more slowly, and yes, it appears that this is purely mechanical. Damned clever, all the same. I will scan it later and post it so that anybody who hasn't got the book can see it. But you made me concentrate on it a little more, which was worth it. Thank you.

In these microphones the patterns are not variable, and MKH series (20, 30, and 40) had their patterns set for each capsule (not interchangable).
There is more info on this type in Glenn Ballou The New Audio Cyclopedia.

Click to expand...

Now that book I haven't got here - but I know where a copy is, so I shall have a look at it during the week. I suspect that there may well be one more question about this, but I'll wait until I've seen it, because it may well be answered already.