The UMIK is automatically calibrated by REW for sound level (this information is in the calibration file). But you need to set your system to generate a suitable signal level. Click on the Signal Generator button and set the parameters like illustrated below. Position your microphone at the listening position and turn the volume of your system down. Then click on the Play button (green triangle), and turn the volume up until the test signal is at a comfortable level. Now open the REW SPL Meter. Click on the red button in the lower right corner to turn it on, and adjust your system volume until the meter reads about 75 dB.

Click on the Measure button near the top left of the main REW Screen. Check that the level is set to -12 dB, the output and input channels are properly set and finally click on the Start Measuring button. REW will make a “whoo-oop” sound through your speakers. A short time later, you should see your first in-room measurement and present the measured frequency response in the window.