I may have misinterpreted your original question, given the other answers received.
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"Can an in-room measurement predict what sound is direct vs reflected?"
I don't have anechoic measures with which to compare or predict.
If I measure the in-room, I'll just find ways in REW to "look" at what is direct vs reflected.
Using the output of the preamp as an "anechoic ideal" with which to compare is something I'm looking at.
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You can easily separate the direct vs reflected when you shout into a large reflective enclosusre "HELLO....
HELLO....
HELLO...."
REW can assist looking at much smaller enclosures.
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I'm more of a practical or experimental than theoretical amateur audio scientist (if I may be so bold as to call myself one)
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Example:
Preamp output vs in-room. I didn't expect the curve for the preamp, but I suppose that shows some limits for what could be seen with a speaker measured anechoically - like comparing the drive signal to the in-room result.
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