If you have a custom calibration file for a USB mic like the UMIK-1, that includes a sensitivity figure, it will automatically provide the necessary calibration for REW’s internal SPL meter. In that case, no outboard meter would be needed at all:
No SPL Calibration routine is required, as mentioned in the first section of the Help File’s Calibrating the SPL Reading section.
REW’s SPL meter could be used when calibrating your speakers to 75 dB.
The SPL calibration figures on the left side of frequency response graphs will be accurate, not merely relevant.
It is a USB mic Anthems ARC
I was told it could not be used as a measurement mic with REW. However, the guy I was going to buy it from, he downloaded REW and it shows up in Preferences and Anthem ARC.
However, pretty sure it is probably not calibrate, right ?
A calibration file is needed to compensate for any deviations from flat response that the mic might exhibit. Without it, any frequency response measurements would be a combination of in-room response plus response of the mic itself.
That said, if all you’re interested in is time domain readings such as ETC, IR, etc., an uncalibrated mic can be used.