proper way to measure noise floor?

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pucho812

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Oct 4, 2004
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third stone from the sun
tyrying to do the math here... But recently been helping my buddy do some sound proofing on what will become a control room.

After putting up soundboard and so forth. He took a measurment today.

Using an omni mic and no one in the room recorded room tone/noise. In order for the mic to get any sort of noise in the mic, had to have the mic pre up to around 60DB and ended up with he says was around 30Dm nois floor. Now I know 30 DB is bad real bad. But what is it really if the Mic pre is up that high or is he doing it all wrong.

My ears tell me the room is fine for keeping noise out.
suggestions?
 
I would say just record from the engineer's position at a preset gain and subject the room to your noise.

Then perhaps figure in the preamp gain VS the DB level noise floor for a full-range FFT analysis of said preset gain and compare the preamp gain setting with the recorded noise floor to determine the actual DB level of the noise floor as emitted from the source.

You might want to know your actual equipment noise floor before starting, but I'm guessing that your soundproofing isn't going to go below -110dB for any decent source.

Did that make sense? Or should I just go to bed?
 
First: roll-off the bass and see the difference. My concert hall approaches -5dB SPL above 2KHz, but exceeds +70dB SPL below 50Hz. We've complained, but it is not unworkable. And this bass-rise (though not so steep) is VERY normal. A spectrogram may help.

Then: get an SM-57/58 and a 300 ohm resistor on an XLR plug. Crank-up the resistor noise until you can read it on a meter. Then replace the resistor with the SM-58 in the room, and see how much the level rises. If "hardly at all" (discounting bass rumble), you are doing very well.
 

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