Window Ref Time setting for room response

[Stecco70]

Hello. First of all I would really thank you for your work! REW is powerful and very user friendly.

In order to measure room response (especially first modes so from 20Hz to 500Hz) I set the IR Window to:
Left Window (ms): 0,0
Window Ref Time (ms): 0,0
Right Window (ms): 1742

To find the Right window value I started from the default and increased until I didn't notice any difference in the SPL response anymore.
For the Left window I didn't notice any difference in the SPL if I increased it so I left it to 0.
Is this a correct approach (not in general but for the described Use Case)?

For the Window Ref Time the SPL changes a lot changing for example from 0 to 40ms

When I make a measure (in a different position of the room or moving the sub) the Window Ref time is set automatically to a value different then 0.
Resetting it to 0 changes its SPL.
What should I do to compare the different results in order to find the best position for the sub?
Set the Window Ref Time of all graphs to 0 or keep the value set by the system?
Or should I check the time response graph and set the Ref Time to the point where the impulse starts?

In the Help I read: If the window settings are subsequently changed Generate Minimum Phase should be used again to reflect the new settings.
Should I do it (in this Use Case)?

Thanks a lot,
Stephan

 

[John Mulcahy]

The default window settings are fine for room response measurement. You should certainly leave the left window at the default of 125 ms, making that zero is the cause of the variations in response with small shifts in the reference time. The default 500ms for the right window is also fine, increasing it provides a little more low frequency resolution if the noise floor is low enough but for room response measurement 500 ms is sufficient. If you want to use a longer window set it in the Analysis preferences, uncheck the option to set IR windows automatically and enter the value you want to use.