# Guidelines for PHASE Graphs valutation ....



## actarusfleed (Nov 30, 2009)

Hi to all,
I'm new about REW.

I've hard problem to understand the Phase graph ...

Some one can explain me better (and easy) what it represent?

And more can you post some explanation graph of a very very good phase response? ... so I can compare it with mine?

please some info about this...

actarus


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## jtalden (Mar 12, 2009)

It is not possible to answer this question well without a little more detail. I don't think showing someone else's measurement will help.

For most users of REW that are finding the EQ settings needed for a SW located at about the same distance as the main speakers, it is not necessary to be concerned with the phase measurement. It is more helpful in determining the delays for active XO settings in a multiway speaker system. 

If you have a vague understanding of phase in general and want to understand why the graph can look so crazy for a measurement as a matter of interest that is understandable. If so and you want to attach an ".mdat" measurement file (and possibly ask a more detailed question) I will try to help you format the phase chart and interpret it.


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## actarusfleed (Nov 30, 2009)

jtalden said:


> If so and you want to attach an ".mdat" measurement file (and possibly ask a more detailed question) I will try to help you format the phase chart and interpret it.


Thank you for your feedback, in the next days I'll post my file!



jtalden said:


> It is more helpful in determining the delays for active XO settings in a multiway speaker system.


Very interesting...Two of my speakers are "2 way"...
Can you describe this method?

thank you,
actarus


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## jtalden (Mar 12, 2009)

actarusfleed said:


> ...
> Very interesting...Two of my speakers are "2 way"...
> Can you describe this method?
> actarus


There is general agreement that the phase does not need to be closely aligned at the XO, but it does help to provide a smoother SPL handoff in that area. 

If you have a passive XO there is no easy way to adjust the delays and align the phase other than just inverting the phase of TW to see which connection provides the smoothest SPL through the XO. Some builders offset the drivers with tilted baffle or use another physical offsets to get closer alignment. With active DSP XOs it is easy to make delay adjustments and fine tune the handoff if you like. 

There are various methods to adjust the delay. Some are pretty basic and relatively easy to implement. 

The method I use is more elaborate and summarized here:

Aligning Driver Phase REWv5 Example


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## fusseli (May 1, 2007)

A perfect phase response would be a straight slope without any curvature. A system with a perfectly linear phase plot would suffer no phase distortion.

As for the crossover region, I would agree that either phase response does not need to line up perfectly. However, it is better to have the SLOPE of either driver's phase align. This will ensure a better dispersion lobe, and will produce a better reverse null. If phase slopes don't match, this can be accomplished in passive crossovers by altering the filter order on either side of the Xover.


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## actarusfleed (Nov 30, 2009)

jtalden said:


> If so and you want to attach an ".mdat" measurement file...


here is my file ...

View attachment test4_FULLrange.mdat


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## jtalden (Mar 12, 2009)

actarus,
I reviewed your file and offer the following information. It may not be entirely technically correct, but it should serve to help in developing a working understanding of how to interpret phase results. [I’m not an expert.]

Your direct phase roughly follows the dotted line in:
Figure1:








All but 2 of the vertical lines in the chart are the result of multipath noise. The vertical lines at about 25 and 105 Hz indicate the direct phase has exceeded +180° (limit for the wrapped phase graph setting) and continues higher. In the case of all the other vertical lines REW is detecting delayed signal reflections from the room and plots their phase on the chart as opposed to the initial direct signal (even though the direct signal arrives first and is higher in magnitude!). If the mic is moved closer to the speaker the reflections would be much smaller in magnitude and would not influence the direct signal measurement nearly as much (less vertical lines). This would be a good exercise for you see and better understand the direct signal phase response of your speaker. 

So, how can we better see the direct signal phase measured from the LP? It’s relatively easy for the higher frequencies as shown in:
Figure2








The measurement data in this Figure is identical. The measurement data was just copied and then reformatted for this figure (and following ones). The IR was windowed as shown to isolate the initial 2.5 ms of the IR. This represents the initial arrival of the high frequency direct sound (800–10k Hz). The window is so short that the low frequencies are windowed out (eliminated from the data) so good data is only available above 800 Hz. The HF reflections are also windowed out so that they will not influence the direct phase calculation. 
The remaining minor variation is just the result of the freq response variation which results in a phase variation. 

So, how can we better see the direct signal phase in the middle frequencies? This is not as easy. We need to increase the right window setting. In this case I found that 10 ms let me see down to about 200 Hz without allowing the room to interfere until 800 Hz as shown in:
Figure3








Why does the phase drop off above 800 Hz? Well, I placed the cursor in the range of interest (at 250 Hz) to provide a reference point and then set the chart to “Unwrap Phase”. That allows the phase to drop down 360° each time it picks up a reflection instead of the direct signal. 

Below is what it would look like if “Wrap Phase” as still active and no smoothing was selected:
Figure 4








At each of the vertical lines in the circled area the phase will shift down 360° if unwrapped. This may be an accurate representation of impact of some of the reflections in the room, but is not indicative of the direct signal phase. The vertical line at 105 Hz is not the result of the reflection but indeed the phase of the low frequencies is more becoming more positive as indicated above. 

So, what about the low frequencies? Sometime these are easy and the chart is relatively clean. In this case it takes more experience to sort out. 

Looking back at Figure1 we see that there are vertical lines at 60, 70 and 80 Hz that I attributed to room reflections (or room modes at these frequencies) rather than the direct phase actually rotating beyond 180°. With REW I don’t have a clear prescription as to how to identifying which are which, but you can develop an eye that will do just that with experience. [I should have mentioned that it is usually easier to see the overall trend of phase as in Figure1 when no smoothing applied. If the IR is windowed such that the reflections are eliminated then smoothing is no issue.]


Some guesses relative to this particular measurement (just for fun):
> The XO from MR to TW is very smooth. There is a minor phase rise in the 3.5k to 4k Hz range that may be the result of an XO in this range, but it is may also be from other causes.
> There appears to be a SW XO in the 70-80 Hz range that is not well timed. The phase irregularity there may be due to room modes, but I think not. The frequencies below 100 Hz roll up in phase at what appears to me to be an excessive rate. It appears that the SW is delayed too much relative to the mains and there may also be a significant phase step at the XO. [I just now also looked at the SPL chart and indeed there is significant null or Room mode at 70 Hz. I would investigate this further to see if it is indeed a delay (distance setting) issue.]

I hope you don’t mine my guesses. They are just for fun. There is not really enough info here that I can be sure of anything. More test data would be needed.

Anyway I hope my efforts here provide at least a little better understanding of the phase chart. There is much more related information regarding minimum phase, excess phase and group delay that I felt best left out for this purpose. 

And again, I hope my comments are not too far off base from a more technical perspective.


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