Mic testing / poor-man's anarchoic chamber

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I've had to replace a FET in one of a once matched pair of phantom  
powered mics. We're not too worried about any difference in gain as that  
can be balanced off on the mixer. But is there more noise compared to  
its fellow?
The only mixer I have around with 48V and balanced feed mic inputs has a  
fan in it. However I wrap the mics I hear/measure the fan noise. Long  
lead and I get mains hash/hum pick up. Looks as though I'll have to go  
inside the mixer and temporarily cut the fan. Any other ideas or what to  
place the mics in that is as acoustically dead/mechanically  
non-transmissive ,that  I can conveniently find laying around

Re: Mic testing / poor-man's anarchoic chamber
Four foot by, oops ;;;

A piece of closed cell foam 1.2 meters by about 0.3 meters about 6 cm thick
. Wrap it round the thing and staple or something the ends shut. And of cou
rse move it as far away from the mixing board as possible. If that ain't go
od enough I dunno. Msybe put it inside a sealed system speaker cabinet and  
screw wood to the opening for the woofer.

Re: Mic testing / poor-man's anarchoic chamber
  snipped-for-privacy@gmail.com wrote:

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Or put the package in a box in a box in a box?

I wonder if it is possible to isolate the microphone element to  
eliminate audio pickup?  If you could replace each element with a  
capacitor, the capacitor would shunt any stray pickup and block the  
phantom power.

Fred

Re: Mic testing / poor-man's anarchoic chamber

"Fred McKenzie"
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** That is by far the easiest way to do it - open the mic and substitute the  
capsule connection with a cap ( ideally polystyrene type ) with the same  
capacitance value as the capsule.

This ought to give the same noise level outcome as placing the mic in a  
vacuum chamber.

BTW:

Few anechoic chambers are really quiet enough to tests a mic's self noise  
level (anechoic = echo free, not silent) and a simple solution is to use a  
small, airtight enclosure for the mic itself.

One I know of  ( used by Rode Microphones in Sydney ) was in the form of a  
cast iron sphere that opened to allow the mic to go inside and plug into a  
multipin socket on the wall. When closed up, the sphere was airtight and as  
long as it was suspended in a fairly quite room ( checked with a SPL meter )  
it was dead silent inside.


....  Phil


  



Re: Mic testing / poor-man's anarchoic chamber
On 08/01/2014 06:54 PM, Phil Allison wrote:
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What he said.


Re: Mic testing / poor-man's anarchoic chamber
On 01/08/2014 22:57, Fred McKenzie wrote:
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I tried that and was picking up a bit of mains hum, I'll try again with  
some shielding over the 100pF cap, I'll even try some mu-metal.
I was wondering how much mechanical noise came up through the mic cable,  
I'll try clamping some lead sheet around its middle, might deaden any  
through vibration

Re: Mic testing / poor-man's anarchoic chamber

"Nutcase Kook"
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<< Fred Makenzie"
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** No shit, Sherlock......


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** Wot an utter moron.

Funny how many condenser mics have steel mesh all around the capsule and are  
completely hum free.

A wacky idea invented by some guy called Faraday in 1836.


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** Lots, but only if you have rat knawing on it  ....



....  Phil





Re: Mic testing / poor-man's anarchoic chamber

"Trevor Wilson"
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** Hey look  -  we have Dumb & Dumber here !!!!!!!!

The reason mu-metal is absurd is because is it *magnetic* shielding while a  
condenser mic capsule is insensitive to magnetics.

Duhhhhhh...............


....   Phil






Re: Mic testing / poor-man's anarchoic chamber
Laid the mixer on thick low-density rubber slabs over concrete floor and  
a sleeping bag loosely wrapped over the fan outlet. mic pushed into a  
large block of foam rubber and wrapped inside another sleeping bag.
Had to wait until no cars moving within 100 yards and no planes,  
including what I didn't expect , them taxiing at the airport 3 miles away.
In simplistic DVM RMS voltage terms the replacement FET one was 19%  
lower noise minimum for the same amp gain settings than the original.
Setting each over a speaker with a more normal sound level, the old one  
had 22% more gain for the same amp settings.
Hopefully that means no extra noise intrusion with the replacement FET  
and just balancing off the gains at the desk. The FET was not made for  
low noise purposes but was 4 pin in a shielded metal can. I'll leave at  
that rather than move onto frequency curve plots and let the owner see  
what he makes of them in normal studio use.

Re: Mic testing / poor-man's anarchoic chamber
On 08/01/2014 06:51 AM, N_Cook wrote:
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Place I worked has a 1 cu foot birdhouse with a speaker inside inside.  
Can be configured for silence or for a tone when an IR photocell in the  
middle of the chamber is tripped. For testing the noise you describe I'd  
put a fixed capacitor in place of the diaphragm and measure noise, on  
both mics.

Re: Mic testing / poor-man's anarchoic chamber

"Nutcase Kook"


**  " .... / poor- man's anarchoic chamber "

Is that where they put yobbo, political ratbags to cool off  ???




....  Phil




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