how to calculate signal to noise ratio for a sound recorded by microphone?

i am trying to make a noise detection tool to detect weather the room is very noisy, noisy or quite .. so how can i calculate the signal to noise ratio for recorded sound for 200 mSec. ?

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to tell if a place is noisy, pros usually measure

dBA: on a microphone with known reception diagram, weighted air pressure signal power

weighted, you have to ponder the audio frequencies to correct the human hearing that is not flat over audible frequencies.

MATLAB already has an Audio Toolbox that, once the hardware correctly set up, allows to measure both audio pressure power weighted and unweighted.

There is already a standard dBA scale: for instance 30dBA is quiet, 120dBA is ~30m from jet take-off, there is a standard once you get to catch dBA in order to tell how noisy is a place.

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how to calculate signal to noise ratio for a sound recorded by microphone?

how to calculate signal to noise ratio for a sound recorded by microphone?