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

[Guest]

[quote author="SmG"]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. [/quote]
Oh yes, von Braunmuehl s principle.
I have Braumuehl/Weber s einfuehrung in die angewandte akustik
(Leipzig, 1936) on the table. Nice book Also stereophony is
inside.
But if we can make cardioid from two membrane microphone
via making back membrane without voltage,
this back membrane makes dummy capacity load and decrease
microphone noise figure by 6 dB.
For figure 8 we must have minus polarization voltage source.
Why is not better to do separate amplifier to each
half of Braunmuehl s transducer and possible operation do
in the mix pult. And would not be that two amps outputs
some kind of one point stereo ??
Possible effect: 6 dB lower noise, spare of DC/DC converter.

That are questions which Braunmuehl can not respond for me?
Can do it someone of yours ?

xvlk

 

[Marik]

[quote author="xvlk"]
Why is not better to do separate amplifier to each
half of Braunmuehl s transducer and possible operation do
in the mix pult. And would not be that two amps outputs
some kind of one point stereo ??
Possible effect: 6 dB lower noise, spare of DC/DC converter.

That are questions which Braunmuehl can not respond for me?
Can do it someone of yours ?

[/quote]

It is possible--very possible. In fact, I have an old Russian tube microphone, with exactly the same arrangement. I remember Keith talking about the same principle in AKG. The idea was to make a noiseless pattern switch.

I am not sure how usefull will be this one point stereo for a recording, but it might be usefull for MS stereo, where you could eliminate matrix--just use it as a Side, pan it to L and R, and mix with a cardioid (or whatever you use) in a center. I'd expect the pattern to be different from "regular" MS, with Fig8 Mid, though.

The cons are--too expensive--you need two sets of head amplifiers, beefier PSU (if tube). Both amplifiers should be very well matched.

 

[Guest]

[quote author="Marik"]
The cons are--too expensive--you need two sets of head amplifiers, beefier PSU (if tube). Both amplifiers should be very well matched.[/quote]
One jfet = $1 , Matched or monolythic jfet pair $1.50
It can not be much expensive in solid state form.

xvlk

 

[SmG]

[quote author="bradzatitagain"][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]
Sorry for the delay - all sorts of things happened. Here is a scan of the page:

.

xvlk's idea is also implemented in Samson's cheapo C03 mic - the one with the dodgy electret capsules. This uses two impedance converters and some switching to achieve pattern changing. I wouldn't have thought that this was going to be ridiculously expensive, or even that hard to achieve - it's not the entire mic electronics that you have to double up, is it?

 

[bradzatitagain]

Has anyone taken an MK 6 apart? What is the large grey mass in front of the front of the capsule? It appears as though there's a reasonably conventional drilled "backing plate" on both side of the diaphragm, is that close to right? It also appears that those "backing plates" are chargeable when switching through the pattern selection. And REREADING the whole thread, that's obviously the case.

Interesting, a little far afield of using two 990 capsules for fig8, but interesting.

So far I register perhaps three different responses to the original question, all affirmatives: Marik's answer-- "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;" Steve's answer-- "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;" and Brent's answer-- "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."

I'll make a minute and post a picture of the 991/990 capsules here along with the acoustic network (have I used that term correctly?) on the 991, i.e., the delrin cone and the dimension of the slots cut in the mic body. Marik's scheme is probably the simplest to execute, physically. Back to back w/out two cones may be a trick, the electrode is longish. Back to back with cones would require some machine work and turning some plastic on the lathe. All approaches are interesting, this will be an interesting project. Pics in a minute (day) or two.

 

[Marik]

Brad,

Basically, I and Brent were talking exactly the same thing. He's just spelled out my sentence: " You will need to play with their acoustical arrangement, though (remember 603 vs. 990 sound difference)". Just don't forget to reverse polarity on one of the capsules, otherwise you'll get omni.

Has anyone taken an MK 6 apart?

At the price of almost $1400 :shock: I wouldn't want to do it... well probably I would... just for educational purposes...

The "grey mass" in front is a guy, which makes an acoustical balance for fig8 pattern. Look at pic. C. You can see that sound hits the diaphragm symmetrically in this pattern setting. Besides, it adds acoustical resistance.

Only backplate on the right is charged during operation. All patterns are changed mechanically, and unlike Braunmuhl--Weber type, there are no intermittent ones, and the charge is fixed.

 

[bradzatitagain]

[quote author="Marik"]Brad,
The "grey mass" in front is a guy, [/quote]

And, predictably, my next question is, what's a guy and what's it made of?

 

[SmG]

In Eargle's economical text that accompanies the figure, he says of the figure-8 response:

• In figure 5-17C, both inner and outer moving assemblies have been positioned to the right, and a new rear opening, symmetrical with the front opening, has been exposed. This front-back symmetry will produce a figure-8 pattern. The left portion of the design is nonfunctional mechanically; essentially, it provides a matching acoustical boundary condition on the front side of the microphone, which matches that on the back side when the microphones is in the figure-8 mode.

I don't have a Mk6 microphone, so I have no idea what the block is actually made of. But in an acoustical sense, all it has to do is match the housing at the rear when it's configured in the fig-8 mode. Any light but solid object of the correct shape would do that.

To give some indication of the scale of the microphone, Eargle also says that this whole assembly fits in a cylindrical housing of 20mm diameter.

 

[bradzatitagain]

So delrin would probably suffice. Is the axis parallel or perpendicular to the MK6 capsule?

 

[SmG]

My impression of this is that the front backplate (if you see what I mean!) doesn't have holes in the centre of it, but acts as a mounting position for the block of whatever in front of it. Since the diaphragm is round, then the block is also round, and mounted perpendicularly on the front of it - leaving a space all around the edge for soundwaves to enter. So if you spin any of those drawings around so that the LHS is towards you, you would see a 15mm or so circular block of material bolted to the plate in front of the diaphragm - This is a cross-section through an essentially circular object.

 

[bradzatitagain]

I'm sorry, Steve, I meant the polar pattern; is the pattern axis parallel or perpendicular to the mic body?

 

[SmG]

Oh, in that case, perpendicular. You don't actually need to have a mic diphragm exposed directly to the source of pressure changes for it to work - round the side is fine.

Since I've told you what Eargle said about the figure-8 response, I might as well tell you what he says about the other two responses as well - this might help to clarify things:

• Omnidirectional response. In fig 5-17A, both inner and outer moving assemblies are in their far left positions, effectively closing off the back openings to the diaphragm and resulting in an omnidirectional response.
  
  Cardioid response. In fig 5-17B, the inner moving assembly has been positioned to the right, exposing the rear opening. The delay path through the rear of the microphone is equal to the path around the microphone, so sounds arriving from the rear of the microphone cancel at the diaphragm.

So the variable responses of this microphone (other than omni) are arrived at by pressure cancellation - which requires no more than the diaphragm being in the same pressure system. But since the cancellation is effectively taking place at the diaphragm, then the responses other than the omnidirectional one will be perpendicular to it - just as they are with any other pressure-operated mic.

You might wonder how they get away with this at higher frequencies, where sound becomes rather more directional. It's quite simple. To make an effective acoustic shadow, an object has to have a diameter that's greater than a whole wavelength at the frequency that's required to be shadowed. The wavelength of sound at 20kHz is (in metric) 330/20,000 = 0.0165m = 16.5 mm. That block's diameter is smaller than that!

 

[Guest]

[quote author="SmG"]

.
This looks like 1970 acoustic disertation. Only academic and no else interest, To use lumped network in condenser mic.
Telefone transducer is example of it, Helmholtz resonator,
is "flat" to 3 kHz.
If you scaling telephone transducer 1:5, you can obtain
15 kHz ... with noise increased with quadrat of it
(10*log 25 = + 13 dB !!!)
Reisz (Teacher of Georg Neumann) figured microphone as
(to plane wave) system with distributed parameters.
Neumanns are all distributed, and Braunmuehl (as newer
design than Neumann) is distributed also.
Why not make mechanically controled distributed microphone
with variable dirrection pattern on acoustic way. It is possible and
simple:

It may be some 1930 patent, I can not remember number,
I am not so old. Maybe someone here?, or at Schoeps?
Are Mr. (T.?) Schoeps from Karlsruhe alive? I don t know...
And if is he dead, why Schoeps is not Harman ... ?
xvlk

 

[Marik]

[quote author="xvlk"]
This looks like 1970 acoustic disertation. Only academic and no else interest, To use lumped network in condenser mic.
Telefone transducer is example of it.....
[/quote]

Agree! I've never been a fan of resonators in microphones. If a concept needs it, then there is something wrong with the concept.

Why not make mechanically controled distributed microphone
with variable dirrection pattern on acoustic way. It is possible and
simple:
http://mujweb.cz/www/xvlkxvlk/varbm.gif

I don't see how it'd work for fig8 without significant low frequency losses, besides, front and back waves would not hit the diaphragm symetrically.

The simplest and most elegant solution for mechanical pattern control is Olson's "interrupting aperture" system.

 

[SmG]

[quote author="xvlk"]
This looks like 1970 acoustic disertation. Only academic and no else interest, [/quote]
So why is it still on sale all over the place, and for a lot of money?

Incidentally, I can't open your gif file - it says it is corrupted. Any chance of a jpg?

 

[Marik]

[quote author="SmG"][quote author="xvlk"]
This looks like 1970 acoustic disertation. Only academic and no else interest, [/quote]
So why is it still on sale all over the place, and for a lot of money?
[/quote]

'Cause it Schoeps :grin:

Incidentally, I can't open your gif file - it says it is corrupted. Any chance of a jpg?

Copy this link

http://mujweb.cz/www/xvlkxvlk/varbm.gif

and paste it into browser.

 

[SmG]

[quote author="Marik"]

Copy this link

http://mujweb.cz/www/xvlkxvlk/varbm.gif

and paste it into browser.[/quote]
Tried that - even downloaded the file, saved it and opened it in other apps. PSP says that it's corrupt. Photoshop says it is too. how big is the file supposed to be?

 

[Marik]

[quote author="SmG"][quote author="Marik"]

Copy this link

http://mujweb.cz/www/xvlkxvlk/varbm.gif

and paste it into browser.[/quote]
Tried that - even downloaded the file, saved it and opened it in other apps. PSP says that it's corrupt. Photoshop says it is too. how big is the file supposed to be?[/quote]

OK, here it is:

 

[SmG]

Cheers! :thumb:

 

[Tim Campbell]

There's a lot of great info in this thread. Are any of you planning on building one of these type capsules?

Though the idea is good, this design could be a nightmare to get to function properly in a real capsule.