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.

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

"Fred McKenzie"

** 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

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

"Nutcase Kook"

"Trevor Wilson"

** 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

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.

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.

What he said.

"Nutcase Kook"

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

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

.... Phil