Thx Peter, the idea of aligning first arrivals via phase trace is getting more and more clear.
Phase alignment is what i have been trying to do all along, but i also want to understand how delay finder works, and what the terms such group delay, minimum phase, etc, all mean.
I can grasp that group delay is the derivative with respect to frequency, i was just have trouble picturing 'derivative at what frequency or frequency interval'.
Here's a pretty good link i found on that ...http://na.support.keysight.com/pxi/help/latest/Tutorials/Group_Delay6_5.htm
If you or others have further reading you've found that helps...much appreciated !!!!
Here is an example of the phase and group delay using a reflex speaker plot.
Have a look a the phase plot at 100Hz - draw a line tangent to it (red line) - its slope will be the derivative.
Group delay is the -ve first derivative of phase.Measure the slope/gradient using the points where the line intercepts 0 degrees and 180 degrees
This gives you a gradient of .... 180 degrees / 280Hz - 18Hz = 180 degrees /262Hz
Now one cycle = 360 degrees so the equation becomes 180 degrees /(262 *360 degrees)/sec = 0.0019s or 1.9ms
i.e. the group delay @ 100Hz = 1.9ms ... Refer group delay plot at 100Hz = 1.9ms.
Note: be careful once you have phase wrap http://www.cjs-labs.com/sitebuildercontent/sitebuilderfiles/GroupDelay.pdf
