Unfortunately, the REW Downloads page – and the video - neglects to mention the need for a proper calibration file when taking measurements with the mic in an upright position.
It’s a well-known fact that the mic’s off-axis response is depressed in the upper frequencies compared to its on-axis response. For reference, see the graph below of measurements for a Dayton EMM-6 mic as performed by Herb Singleton of Cross Spectrum Labs. As you can see, when the mic is oriented 90-degrees of-axis from the signal source, its response at 20 kHz is a full 8 dB down.
Thus, room measurements taken with the mic upright, without the proper 90-degree calibration file, will show a droop in response above ~2 kHz that will not accurately reflect the speaker’s response or what the user actually hears. We've seen it happen here many times over the years, REW users perplexed about their poor measured response in the upper frequencies, only to find out that they weren't using the proper calibration file when they measured with the mic upright.
Courtesy of HiFi ZineRegards,