# Measurement arithmetic functions ?



## rha61 (Jun 21, 2011)

Hi all

how do i do to merge/add/divide measurements in REW ?
(i want to average 4 impulse responses )

thanks


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## JohnM (Apr 11, 2006)

Go to the All SPL graph and click the gear icon top right to show the controls panel. You can do arithmetic on pairs of measurements at a time, see ALL SPL help.


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## rha61 (Jun 21, 2011)

Thank you for this amazing software


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## Tchao (May 6, 2009)

REW can Add 2 measurements but what is the meaning of this adding ?
I mean, because of the non-minimal phase behavior of the room, add 2 IR (with 2 sweep log sine - 1 from speaker A, the other form the speaker B, in the same room) is possible mathematicaly :
but does the result represent a reality of the IR of the 2 signals ??

say it in another way:
A is the IR of the speaker SA in the room R (a non minimal phase system)
B is the IR of the speaker SB in the same room R
C is the IR of both speaker SA and SB in the same room R
same sample rate, distance, range of frequencies ....

Add A and B and find D, the IR of A+B
can we assume that D = C ??


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## JohnM (Apr 11, 2006)

Yes, the system is linear and superposition applies, but measurements must be made with a timing reference for accurate results. Per the notes on trace arithmetic in the help:

*Notes*


For meaningful results measurements that have impulse responses or phase data should be properly time aligned before they are combined. An exception is the Merge operation, for which REW will automatically align both magnitude and phase at the merge frequency, adjusting the trace B time delay as required for the phase match. The amounts of the adjustments are shown in the notes of the newly generated measurement.
The current impulse response window settings are used for each trace, the result uses the same window settings as trace A
The result is smoothed using whatever smoothing was being used for trace A


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## jtalden (Mar 12, 2009)

Tchao,

D=C if, and only if, the relative timing of the two signals arrival at the mic is preserved.

If we use REW's "Use loopback as timing reference" to preserve the relative timing then I would expect Trace Arithmetic "A + B" for 2 speakers measured individually to be equal to a measurement of both speakers together. Using this feature preserves the complex relationship of the 2 signals and thus preserves the appropriate SPL and phase response. This is particularly useful for combining the signal from 2 different subs or combining the signal from the SWs to a main speaker for the purpose of adjusting the distance setting (timing delay). I also use it to adjust the delay between 2 or more different drivers within a speaker channel such as Midwoofer + Tweeter (I use active XO's). I haven't run into a need to use it for combining 2 different main channels such as FL + FR, but it should work if we have a need to do this. I prefer to measure and EQ all main channels separately because when they are measured together we see comb filtering that can be very misleading.

If we want an SPL average of several mic positions for the purpose of EQ then we would want to use the "Average" button which takes a simple SPL average of any number of SPL measurements without regard to the impact of timing. All phase information is lost. This is useful if we want to EQ to the average within a measurement window surrounding an LP.


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## Tchao (May 6, 2009)

many thanks for your explanations, both of you, and tips for alignement.
I was not sure if a room would be a linear system (a LTI one to be more precise), or not, because some acoustical research explains that it's not (especialy speaker + room), but in first approximation it is.
the loopback method avoid a difference between internal path of the signal in the PC, but add a delay in both of the 2 IR (delay from output soundcard to input soundcard through speakers, microphone and room).

ps: thank you for all this years of hard work to obtain a so usefull tool, much more than a tool, indeed !:clap:


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