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Mic modify cardioid>>omni Boundary Microphones
Old 31st December 2015
  #1
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Mic modify cardioid>>omni

I'm posting this question partly to quiz myself.

If you were to cover the vents of a cardioid mic, would that make it omni, or is there something else in play electronically?

Let's say, on a small diaphragm condenser.

Please, for the purposes of this question, let's put aside the issue of damaging the mic, etc etc. Let's say that with no harm to the mic, the vents are as sealed as they would be if the capsule was omni.

If I understand right, this is the only difference between a cardioid and an omni cap, in design? Am I missing something?
Old 31st December 2015
  #2
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I'm not 100% sure. It will certainly change the pattern, but it may also change the frequency and transient response. You've just made an enclosed, "springy" mass of air behind the diaphragm, where before it was open.
Old 31st December 2015
  #3
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Quote:
Originally Posted by Matt Nolan View Post
I'm not 100% sure. It will certainly change the pattern, but it may also change the frequency and transient response. You've just made an enclosed, "springy" mass of air behind the diaphragm, where before it was open.
Yeah it seems it's not so straightforward.

I'm partly wondering, for example, how the CAD e100 (both the current and prior models) managed a "supercardioid" or hypercardioid pattern.... with the capsule just sort of floating there in space. It has something to do with how the voltage is applied to the cap, but I can't find out how.
Old 31st December 2015
  #4
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Quote:
Originally Posted by szegedin View Post
Yeah it seems it's not so straightforward.

I'm partly wondering, for example, how the CAD e100 (both the current and prior models) managed a "supercardioid" or hypercardioid pattern.... with the capsule just sort of floating there in space. It has something to do with how the voltage is applied to the cap, but I can't find out how.
The capsule is an electret, so there is no voltage applied to it externally, it has a built-in polarisation due to a fixed charge separation.

I'm pretty sure it is supercardioid just using controlled venting / labyrinth to the rear of the diaphragm. This is all built into the capsule itself, in much the same way that a multi-pattern dual diaphragm condenser is effectively made up of two cardioid pattern capsules back to back. Each half has a combination of through and blind holes and, sometimes, some other chambers and holes which control the pattern and frequency response. In this latter case, the actual pick-up pattern is controlled by how much of each capsule you take and in which polarity, which is in most cases controlled by polarisation voltages.

Last edited by Matt Nolan; 31st December 2015 at 03:22 PM.. Reason: typo, can't spell separation
Old 31st December 2015
  #5
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Yes, converting the back-side vents would transform the capsule into omnidirectional. However, it would likely also change the frequency-response characteristics.

That is the simplified, first-order explanation. In the Real World, things are more complex and subtle than that.

Indeed, the spaces behind the capsule (or ahead of the capsule) are the major factors in the design of the directionality of the microphone. And, as Mr. Nolan observed, many condenser (non-electret) microphones can be adjusted by the user for various polar patterys by use of two (or more) capsules and changing their sensitivity and polarity of the charging voltage.
Old 31st December 2015
  #6
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Thanks both, yes I was getting those two-capsule "multi pattern" mics mixed up with the venting issue.
Old 31st December 2015
  #7
Les
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An omni condenser needs to be stiffness controlled (high tuned) for flat response. A cardioid condenser needs to be resistance controlled (mid tuned)
for flat response. Covering up the latters rear ports would completely spoil the frequency response.

About the back vent...what is it?

In conjunction with the capsule internal volume it's an acoustic low pass filter.
At lower frequencies it has fairly constant group delay (meaning time delay).
This gives the right properties to make cardioid or other patterns.

At high frequencies the back vent doesn't pass sound anymore, and the cardioid becomes an omni-type pressure microphone. It remains unidirectional though, due to acoustic shadowing.

Les
Old 10th February 2016
  #8
Old 10th February 2016
  #9
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Quote:
Originally Posted by szegedin View Post
OK. Simply citing a review from 2003 doesn't seem particularly notable.

Yes, certainly there have been microphones designed to be user-selectable for either omnidirectional or directional polar response pattern. But the key word there is DESIGNED.

Taking a microphone that designed to be directional and blindly covering the back-vents may or may not achieve an omnidirectional microphone of usable performance.
Old 10th February 2016
  #10
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I have a pair of classic Sony C-37a tube condenser microphones. They have a mechanical "shutter" on the back of the capsule which can be opened to produce a cardioid pattern, or closed to make an omnidirectional pattern. They are one of the few mics with published polar and frequency response graphs that look like real measurements vs those fake things drawn by the marketing department. Note the significant difference in frequency response between the cardioid pattern vs the omni pattern.



Old 10th February 2016
  #11
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Richard Crowley's Avatar
 

I have a pair of classic Sony C-37a tube condenser microphones. They have a mechanical "shutter" on the back of the capsule which can be opened to produce a cardioid pattern, or closed to make an omnidirectional pattern. They are one of the few mics with published polar and frequency response graphs that look like real measurements vs those fake things drawn by the marketing department. Note the significant difference in frequency response between the cardioid pattern vs the omni pattern.



Old 10th February 2016
  #12
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Richard Crowley's Avatar
 

I have a pair of classic Sony C-37a tube condenser microphones. They have a mechanical "shutter" on the back of the capsule which can be opened to produce a cardioid pattern, or closed to make an omnidirectional pattern. They are one of the few mics with published polar and frequency response graphs that look like real measurements vs those fake things drawn by the marketing department. Note the significant difference in frequency response between the cardioid pattern vs the omni pattern.



Old 10th February 2016
  #13
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Quote:
Originally Posted by Richard Crowley View Post
I have a pair of classic Sony C-37a tube condenser microphones. They have a mechanical "shutter" on the back of the capsule which can be opened to produce a cardioid pattern, or closed to make an omnidirectional pattern. They are one of the few mics with published polar and frequency response graphs that look like real measurements vs those fake things drawn by the marketing department. Note the significant difference in frequency response between the cardioid pattern vs the omni pattern.



That 'shutter' design is brilliant, and I particularly appreciate it as a photographer. Thanks your post is useful. The difference in frequency response between the omni and cadioid on that chart is almost undetectable. I'm seeing 1-2db here and there. Obviously the freq resp will always be different betwn omni & cardioid because you're not getting as much hf information from the rear, and low end performs differently.

The issue seems pretty well sorted to me. It's probably mostly a matter of getting a tight seal.

I get what you and others are saying. That it's not as simple as putting gaff tape over the vents of a cardioid in terms of getting normal performance specs. But it's also not something hard to engineer, and in some cases maybe worth tinkering.
Old 2 weeks ago
  #14
sto
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Hello,

How about dynamic microphones, can I also change from supercardioid to omni or at least to cardioid?
These are cheap for a test: the t.bone MB 60 SET
Thank you
Old 2 weeks ago
  #15
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The principle is the same whether dynamic or condenser.
The issue is whether you can actually access all of the rear vent holes to plug them.
Old 1 week ago
  #16
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Schoeps made a capsule for their 221 microphone that switched between omni and cardiod by means of mechanically blocking the rear ports. It worked quite well on my 221's. I dont remember the details as its been a long time since I've used them mine were stolen (SN#12 & 15) in the 90's.
But I digress.
The blocking material is metal and it's also a rigid material for the C37. So masking tape is not really going to work that well (it's more compliant), but might be interesting to try? Again remember that the Schoeps & Sony (and maybe some others) were designed for mechanical switching.
Note: this is not a good idea for dual capsule mics create their polar patterns differently.
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