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Author Topic: shielding effectiveness of brass or steel mesh  (Read 1700 times)

Piedpiper

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shielding effectiveness of brass or steel mesh
« on: March 04, 2012, 12:00:29 am »

In the context of head basket mesh, which shields hum fields better, brass or steel?
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Kai

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Re: shielding effectiveness of brass or steel mesh
« Reply #1 on: March 04, 2012, 10:39:33 am »

It doesn't matter as long as they are electrically connected to the mic housing / cable shield / inhouse ground.

Hum introduced into a condenser capsule is electrostatic, not magnetic.
For eletrostatic shielding you need a farrady cage.
So any grill made of electrically conductive material will do.

Inside the mic head there is no coil, not even a closed loop wire connection, therefore no magnetic pickup of hum can occour.

What I have seen on an early AKG C414EB is insufficient shielding of the switches, with the result that the mic was sensitive to hum.

After shielding the parts with copper foil the problem was gone.

Another problem that can arise: the whole recording system is not connected to ground potential, where ground really means the physical earth.
I had to face this with a Sony PCM2000 recorder with non-original, 2-pin switching power supply that had originated from a computer.
Measured relative to ground the device and the connected mic's were on 110V AC (with our german 220V line voltage) potential and delivered a huge hum from the condenser mics.

Regards
Kai
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Piedpiper

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Re: shielding effectiveness of brass or steel mesh
« Reply #2 on: March 05, 2012, 11:45:43 am »

Thanks for the clarification, Kai. I have two scenarios prompting this query. I have a Peluso VTB that I had done some modifications to, including removing the inner fine mesh in the head basket. Although I liked the sonic effect, it ultimately developed an intermittent low level hum that eventually became constant. Curious about the intermittent aspect of this, I checked all relevant connections to ground and experimented with mic position. The only thing that seems to effect it is placing a screen on the back of the head basket. Interestingly, placing it on the front has little to no effect, regardless of mic position. I have resorted to replacing the fine mesh screen back into the rear of the head basket.

The other scenario was a Sony C37P that I thought might be "improved" for my purposes by replacing the head basket with something less opaque.

In any case, you've answered my question. Thanks again...
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