Explain what trace Arithmetic is doing

[twelti76]

The peak of the result IR was outside the range and got chosen as the ref. The large artefacts outside the valid part of the result are caused by starting the measurements at a relatively high frequency (30 Hz) and ending the ref measurement at a very low frequency (300 Hz) that is also much lower than the span of the other measurements. Here is the relevant help section on how REW handles that:

The frequency span of the result of an arithmetic operation will be from the lowest start frequency to the highest end frequency of the traces operated on. Outside their frequency range traces are treated as being zero valued, with the exception of the divisor in a division operation which is treated as being unity outside its range. If the measurements actually have significant levels outside the measurement range the zero setting will generate oscillations in frequency and time domains, for best results use traces that span the full frequency range.

I'd suggest starting the measurements at zero and ending both reference and actual at the same frequency.

Yes that is a good idea. I wasn't thinking about using these measurements in this way when I made them. Anotherr way to get around this could be to band pass the IR. I have done this before. Of course, that won't fix the issue of the window being in the wrong place!

I wonder if there might be a way to gracefully return the numerator to zero outside the measurement bandwidth??

 

[John Mulcahy]

More could be done, especially with REW measurements since they span an octave above and below the chosen range when measured (within the bounds of the sample rate), albeit with a windowed rolloff outside the measurement range. However, trace arithmetic works with impulse responses from any source or SPL & phase data or SPL data only, treating REW measurements differently would mean notionally identical data sets could produce slightly different results, at least near the ends of the frequency span. I'll look at relaxing the constraints for REW measurements at some point though.