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Discussion Starter · #1 ·
Making some experiments in my room with REW (5.01 beta 21) I found a strange behaviour.
In the All SPL window, the Trace Arithmetic functions seem to give wrong results.
See for instance the plot here below.
wrong average.jpg
The green and blue lines are two measurements while the red line is supposed to be the average (A+B)/2 between them. But it's fully nonsense: from 1 to 5 kHz it's well below both lines while it should be somewhere in the middle. I also tried the sum with same result, just 6 dB above (well, this seems to be correct at least!).
If I sum 5-10 measurements, for some frequencies I might even get a result which is LESS than each single plot, which is wrong too.

I have a small suspect, that the sum is not performed on the frequency plot but on the acquired measurement scan and, due to some small time mismatch, what I see is something like a comb filter.
In that case, it would be nice to have the function to just average the spectra.
Or maybe I just forgot to set-up something: please check and let me know.

Premium Member
1,562 Posts
Welcome to HTS!

The trace arithmetic functions work properly IN REW. It processes the 2 IRs given their relative position to each other using complex math; the phase relationship of the responses are considered. Since your result is not what you measured that indicates the 2 input IRs are not aligned/spaced/timed as they actually are.

By default, REW changes the position of the IRs to place them near 0ms. This is helpful for normal analysis and charting, but not for arithmetic operations. REW allows for "Loopback Timing" to be used with a loopback cable, provided your setup supports it. With that feature selected the relative positions of each measured IR is retained and any arithmetic operations will result in the correct trace.

A USB mic will not support Loopback timing in REW. All XLR mics connected to a 2 channel audio interface will support a loopback cable on the 2nd channel and Loopback timing can be activated.

See REW "help" for more information.

If SPL of two or more measurements is needed without regard to the impact of the phase/timing, then a simple average can be done using the "Average the Responses" button on the "All SPL" tab. In that case the phase relation is ignored.
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