# Impulse responses explained



## Gubbe (Jul 4, 2011)

Hello,

Should I adjust the startpoint of my impulse responses?
When using loop back as timing reference the impulse response window always start before the peak of the impulse. What does this mean for the rest of the graphs? Does it affect group delay and phase graphs?

If I want to measure several speakers by them self and then look at there contribution to the overall frequency and impulse response of the room by adding them together, how do I do this? And what effect does the impulse response window have? Should I just leave it so that the signals sum together, or is there any advantages to adjusting the impulse response so that the window starts at the first peak?

Is there any command so taht the window automatically adjusts itself so that it starts at the impulse response first peak?


----------



## JohnM (Apr 11, 2006)

The key factor in the phase and group delay derived from the impulse response is the point at which the time axis value is zero, not the time at which the window starts.

Traces can be summed using the Trace Arithmetic controls on the "All SPL" graph.


----------



## Gubbe (Jul 4, 2011)

Thank you for your answer John.

In the documentation for REW all the pictures of the impulse response window, shows a window that starts at the peak of the impulse. My default position for the impulse window always starts before the peak of the impulse. Why is this happening? Is it related to the timing reference inputs and outputs? What effect will this have on the rest of the graphs? Will it have no effect as long as the timing axis has its zero at the peak? Is this set automatically?

I think I have read most of the documentation for REW, do you have any links to more information about impulse response, acoustic delay influences on phase and group delay, and window position?


----------



## JohnM (Apr 11, 2006)

Gubbe said:


> In the documentation for REW all the pictures of the impulse response window, shows a window that starts at the peak of the impulse.


I may be misunderstanding your meaning, but that is not the case - look at the graph image in the Impulse Response Windows help topic, for example.











> My default position for the impulse window always starts before the peak of the impulse. Why is this happening?


If you are referring to the start time of the left window, this needs to be before the peak of the impulse otherwise the window would be cutting off part of the impulse - the impulse starts before its peak, not at it.



> Is it related to the timing reference inputs and outputs?


No



> What effect will this have on the rest of the graphs? Will it have no effect as long as the timing axis has its zero at the peak? Is this set automatically?


The window is used to select the part of the impulse that is analysed, you can see exactly what is being analysed in the "Windowed" trace. The time value assigned to the location of the impulse response peak depends on the Analysis Preferences settings, details are in the help.



> I think I have read most of the documentation for REW, do you have any links to more information about impulse response, acoustic delay influences on phase and group delay, and window position?


Try Spectral Audio Signal Processing.


----------

