Conversion of omni capsule to cardioid
« on: February 16, 2019, 03:02:24 PM »
Generally a cardioid capsule works by means of the venting to the rear of the capsule. Often it's the same capsule that is used in an omni mic, but in a housing that has that venting. I guess the Schoeps MK5 and the Shure KSM141 are examples where the polar pattern can be switched just by mechanically closing or opening that venting.

But do all omni capsules have the same characteristics to the back of the capsule that allow this?

I'm wondering whether this omni capsule can be housed such that it is cardioid:

Any thoughts?


Re: Conversion of omni capsule to cardioid
« Reply #1 on: February 16, 2019, 04:51:18 PM »
i fear you are over-simplifying.

A big difference between all pressure transducers and pressure gradient transducers is the membrane tension. Real pressure transducers tend to have higher membrane  tension than pressure gradient transducers. (was explained to me like that by J. Wuttke, former head of engineering at Schoeps).

Also one of the reasons why you can not simply close the rear entry ports of a cardioid and make it omni - it will lose some/most of its directivity, but it will not become a "good omni" and will show "interesting" frequency and phase response.

Thats why Schoeps made quite an effort in their mechanically switchable capsules to re-adjust tension as they switch patterns. And still they claimed that it was only a "compromise" for higher flexibility, with "cardiod only " and "omni only" performing better.

Cardioid or omni, the whole capsule system - membrane tension, dampening system, acoustic resistance et al. must be adjusted so that the system in itself works. Changing just one parameter without tackling the others will most probably kick it out of balance.

And that's why we spend lots of money to buy omni and cardioid microphones. If it was that simple, nobody would invest money in omnis and just shut the rear entry ports closed.


Also Shure writes quite similary: http://www.shure.com/americas/support/find-an-answer/sm81-and-r104a-omni-capsule




Re: Conversion of omni capsule to cardioid
« Reply #2 on: February 16, 2019, 05:13:00 PM »
Great -- thanks very much for that detailed explanation.
You're right -- I am over-simplifying in an attempt to game a good microphone, but then so are these manufacturers, and to very good effect with the MK5, in my view. It may be a compromise from optimal Schoeps capsules, but it's very good, and I would prefer it to a lesser omni and a lesser cardioid, for the price. That's the point.

I find this interesting that "Real pressure transducers tend to have higher membrane  tension." Are there other performance impacts (other than polar pattern) from this higher tension? I expect that there are, and that this is part of the explanation for why the best omnis are better than the best cardioids. Am I on track there?

I'm posting to this group because I'm more interested in experimental techniques and exploration -- less interested in towing the line of conventional professionalism.

Re: Conversion of omni capsule to cardioid
« Reply #3 on: February 16, 2019, 05:28:13 PM »
in daily operation there are some effects  easily noticeable :

 Handling noise and sensitivity to wind/pop noises.
The harder the membrane can be tensioned, the less sensitive it will be to handling and wind noises.

ref: https://www.dpamicrophones.com/mic-university/directional-vs-omnidirectional-microphones 

There is also an interesting long article by Jörg Wuttke, however available only in german, p11 and following.

https://schoeps.de/fileadmin/user_upload/user_upload/Downloads/Vortraege_Aufsaetze/Mikrofonaufsaetze/Mikrofonbuch_komplett.pdf
« Last Edit: February 16, 2019, 05:36:45 PM by tonzauber »

Re: Conversion of omni capsule to cardioid
« Reply #4 on: February 16, 2019, 05:51:48 PM »
Awesome. Interesting.

I'm thinking of trying the DPA Acoustic Pressure Equalization ball.
https://www.dpamicrophones.com/accessories/acoustic-pressure-equalizer-ball-for-4006a

For $57 I think it is worth experimentation because reduction of poor-sounding room reverberation is exactly what I'm interested in -- without sacrificing the sound of that omni. Any thoughts.

Re: Conversion of omni capsule to cardioid
« Reply #5 on: February 16, 2019, 06:24:47 PM »
That is a good idea, sucessfully used in Neumann M50, TLM50, M150, and subsequently the reason why the decca tree works so well ;)

However I am not sure if your mics diameter matches the hole of the DPA Balls.

If you have access to a good bench drill, consider buying a few wood balls (can be found in craft and art shops) and drill a hole with the matching diameter in it. Or make it a bit wider and line the hole with something flexible to not scratch your mic's surface.

That should cost you just a few euros, and you are free to experiment with different sizes which determine the corner frequency from where directivity will start to rise.

Marik

Re: Conversion of omni capsule to cardioid
« Reply #6 on: February 16, 2019, 06:41:21 PM »

A big difference between all pressure transducers and pressure gradient transducers is the membrane tension. Real pressure transducers tend to have higher membrane  tension than pressure gradient transducers.

This is correct, esp. for measurement capsules. However, there are details. In the capsule we have two limiting factors--noise and output (which can be correlated, but not always). In the real life, while the smaller diameter the higher the resonant frequency, for an extended top response and fair S/N ratio pressure capsule still have a fair amount of resistance component, which is common for both cardioid and omni capsule. However, for optimal work that amount is quite different for either one--that's why the capsules with mechanical switching (like Schoeps, Shure, Sony) will always have compromise whether it is in frequency, polar, or output responses.

Best, M
« Last Edit: February 16, 2019, 06:45:31 PM by Marik »
Samar Audio & Microphone Design

www.samaraudiodesign.com

The Art of Ribbon Microphones

Re: Conversion of omni capsule to cardioid
« Reply #7 on: February 16, 2019, 06:42:18 PM »
Cool thanks, I will try that. I'm looking for sound samples of what effect those things have on reflections. Does anyone know where to get pre-drilled wooden balls about 5cm/2"?

The mic is another story -- I am going to use the B&K (prepolarized  electret) capsule on some preamp body, haven't figured out which one yet.  ;D
« Last Edit: February 16, 2019, 06:48:41 PM by szegedin »


 

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