Graph of SPL difference between two setups?

[Max Rockbin]

I know a lot of people make small changes to their setup and then compare the graphs in REW to see if it made things better.

I'd like to know if there is a way in REW to see a graph of just the difference between two SPL graphs. I think that'd be a really useful quick way to see what actually changed.
For example, here are graphs of before (green) and after (red) moving the location of one of my bass traps and moving my sub (just have one) close to the trap.
It'd be great if, like the generate average button, there was a generate difference button.

For reference - this is of a Genelec 8030c right speaker + sub. Mic in main listening position 4 1/2' from speaker. The bass trap is 2' x 4' x 17" of rockwool positioned in the corner of the room.
old position - flat against the back wall, oriented horizontally - long side on the floor. New position, diagonally in the back left corner, creating a triangular cavity behind the trap - about 16" on a side (top of the triangle & trap covered with wood.) I had read that by creating a cavity like that, you greatly increase the effective thickness of the trap, making it much better at absorbing low bass.

1/24 smoothing:

 

[sm52]

There is such a button. ALL SPL tab, Controls, Trace Arithmetic, A: Graph 1, B: Graph 2, at the bottom select A / B, and Generate.

 

[Max Rockbin]

Thanks! One thing that confuses me...
Can you tell me why I want A/B instead of A-B? It seems to work. A - B gives a graph that looks sort of like one of the original graph with values between about 60 - 90db. A/B gives values mostly +/- 10dB, which is more like what I'd expect for A-B.

I'm going to guess it's because dB are logarithmic, so subtraction is really division? So A/B refers to the absolute (not logarithmic) sound pressure ratio and the result is expressed in dB which is logarithmic. But just intuitively I would have thought A - B of two graphs expressed in dB would be the dB difference, so I'm not sure that's right. Purple is A/B. Blue is A-B for the graphs in my previous post.

 

[sam_adams]

See my question to @John Mulcahy over here.

See my post over here on AVS.

 

[Max Rockbin]

I found the answer to my exact question here
Where

All trace arithmetic is done on the linearly scaled values, with conversion back to dB afterwards. Multiplication is analogous to cascading transfer functions - for example, multiplying a drive unit measurement by a filter response will show how the response would look with the filter applied. Addition is analogous to combining sources, as Matthew notes - for example adding response measurements from a sub and a main speaker to see the combined result.

Which is what I kinda guessed. It's not intuitive (to me), but people seem to find it more useful for making filters and such.

 

[thothsong]

I'm still confused about this. Here's a chart of two measurements that are fairly uniformly ~12 dB apart above ~120 Hz, but A/B is showing larger differences above 2 KHz, for example as shown at 9 KHz, 19 dB versus 11.7 dB. Why the difference? (REW 5.20.1)

 

[John Mulcahy]

Different phase responses.

 

[thothsong]

Different phase responses.

Ah, OK. Can you point to things I should read to better understand that? And in the meantime, to get the difference ignoring phase, is the right thing to do to a) select each measurement individually in turn, b) click Average the Responses to get a magnitude, c) smooth the two resulting magnitudes as desired, and then d) generate A/B using those magnitudes?

 

[John Mulcahy]

Intuitive Arithmetic With Complex Numbers – BetterExplained

betterexplained.com

The process you set out is fine for getting magnitude-only results.

 

[thothsong]

OK, thanks much.