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Cardioid mic with diffraction sphere
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king2070lplaya
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15th January 2012
Old 15th January 2012
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Cardioid mic with diffraction sphere

Obviously these aren't intended to be used in this way, but has anyone tried using diffraction spheres to turn a cardioid mic into an omnidirectional microphone?

It is my understanding that many small diaphragm mics of the same series (neumann km series, akg ck series) share identical capsules and diaphragms. Thus the only difference between an omnidirectional and cardioid microphone is the assembly between the capsule and amplifier.

So as long as the diffraction sphere in question covers the side vents on the microphone and attenuates the rear incoming signal, the pattern would become omnidirectional, that's for sure.

Where it seems like you get in to trouble in practice is that the plate behind a cardioid capsule has tiny air vents drilled into it, and an omni does not. It seems that, if the sphere covering the side vents doesn't completely attenuate the incoming signal to the rear you may get some interesting phasing equalization.

Comments/Thoughts/Ideas?
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15th January 2012
Old 15th January 2012
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Don't forget that the diaphragm is tensioned differently in an omni compared to a directional mic..

Also an omni has a pressure equalisation hole (pin-prick size) so that a change in atmospheric pressure doesn't bow the diaphragm.
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15th January 2012
Old 15th January 2012
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Take a cardioid mic, wrap some duct tape around the side vents..has it now become an omni ? When a rock singer onstage grasps their SM58 high up on the 'ice cream cone' just below the pop filter basket and covers the lower end of the basket with their fingers, the feedback howl suggests this is indeed what's happened !
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15th January 2012
Old 15th January 2012
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0VU
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Quote:
Originally Posted by king2070lplaya View Post
Where it seems like you get in to trouble in practice is that the plate behind a cardioid capsule has tiny air vents drilled into it, and an omni does not. It seems that, if the sphere covering the side vents doesn't completely attenuate the incoming signal to the rear you may get some interesting phasing equalization.

Comments/Thoughts/Ideas?
Since the sphere covering the side vents doesn't cover the holes in the back of the capsule (or adjust the diaphragm tensioning and pressure equalisation that John mentions) what you get is a wrongly tensioned diaphragm in front of a resonant chamber (or chambers) between the rear of the diaphragm/capsule and the inside edge of the sphere covering the external vents. You're left with an omni right enough but with plenty of the "interesting phasing equalization" you mention resulting in a nasty sounding, unpredictable, uneven sounding omni with massive frequency dependent response, polar pattern and sensitivity variations. It might be an interesting 'effect' but a useful omni mic it isn't.
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15th January 2012
Old 15th January 2012
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I'd say: Yes, it will become more omni... And more 'interesting' as well: All kinds of resonance & such.
Read this, you'll get it! it's quite well explained, in the beginning of the book.

P
king2070lplaya
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15th January 2012
Old 15th January 2012
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Thanks everyone, this is exactly what i was looking for. I've known the "don't cover the back of your stage mic" rule for a while, but wasn't sure how it affected the other pickup characteristics of the microphone beyond the pattern.... Now I know!

Quote:
Originally Posted by 0VU View Post
Since the sphere covering the side vents doesn't cover the holes in the back of the capsule (or adjust the diaphragm tensioning and pressure equalisation that John mentions) what you get is a wrongly tensioned diaphragm in front of a resonant chamber (or chambers) between the rear of the diaphragm/capsule and the inside edge of the sphere covering the external vents. You're left with an omni right enough but with plenty of the "interesting phasing equalization" you mention resulting in a nasty sounding, unpredictable, uneven sounding omni with massive frequency dependent response, polar pattern and sensitivity variations. It might be an interesting 'effect' but a useful omni mic it isn't.
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