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Make a "dummy mic" by connecting a 150 ohm resistor between pins 2 and 3
of an XLR plug. Plug it into the interface, turn the gain up full, and
record the noise. You don't actually need to record, just read the
record level on the meter of your DAW program.

Most room noise is low frequency rumble which will over power the "white noise" on the meters.

The resistor will not have any room rumble obviously.

It would be better to compare the two noise signals with an RTA and then you can ignore the low frequency room rumble.

If you can't use an RTA and are stuck with meters, maybe an appropriate High pass filter will help.

m