# Need help interpreting phase graph



## NixPix (Jul 11, 2010)

Hello. I am new to REW. I have made several measures of my subwoofer and have achieved a fairly good frequency response with the help of this excellent program. So now I wanted to try and measure my front speakers but I really need help interpreting the phase graph of my measurement. This is how it looks:










I understand how to read the frequency response curve but I have no idea how to read and understand the phase curve. Is it really normal for the phase to go up and down like crazy as it does? If its not normal what could be wrong?


----------



## JohnM (Apr 11, 2006)

Yes, it is normal. Phase is cyclic, such that 180 degrees is actually the same as -180 degrees. It is represented on graphs by "wrapping" it at the 180/-180 points so that, for example, as the value decreases -178, -179, -180 the next value is shown as 179, then 178 rather than -181, -182, with a line from -180 to 180 at the wrap point. You can see the phase "unwrapped" as a continuous curve by using the button in the graph controls, but the total span is then typically unusable.

The other issue with phase is that it is affected by time delays, any time delay (such as the time sound takes to get from the speaker to the mic) adds a phase shift that is proportional to the frequency, so to see the underlying phase response you need to remove any delays. You can use the "Estimate IR Delay" button in the graph controls to have REW do that for you.


----------



## NixPix (Jul 11, 2010)

JohnM said:


> Yes, it is normal. Phase is cyclic, such that 180 degrees is actually the same as -180 degrees. It is represented on graphs by "wrapping" it at the 180/-180 points so that, for example, as the value decreases -178, -179, -180 the next value is shown as 179, then 178 rather than -181, -182, with a line from -180 to 180 at the wrap point. You can see the phase "unwrapped" as a continuous curve by using the button in the graph controls, but the total span is then typically unusable.
> 
> The other issue with phase is that it is affected by time delays, any time delay (such as the time sound takes to get from the speaker to the mic) adds a phase shift that is proportional to the frequency, so to see the underlying phase response you need to remove any delays. You can use the "Estimate IR Delay" button in the graph controls to have REW do that for you.


Thank you very much for clarifying that  I tend to overanalyze and it had me a bit worried for a while hehe. I have also found when measuring my subwoofer, playing with the subs built in filter settings (low pass filter, EQ for boosting +-6dB @ 35,40,45 or 50Hz + Q value, crossover etc etc) will affect the phase and introduce/remove such sudden drops (phase cycles). 

This is my current measurement of my subwoofer, would you care to comment on it and perhaps give some advice? It digs way deep on movies and such, sounds really good with heavy metal kinda music but lacks a bit of the last OOMPH in trance and techno songs.


----------



## JohnM (Apr 11, 2006)

Looks great to me, though I suppose you could try pulling the 47Hz bump down a little. Some people find a gradual rise towards the lowest frequencies sounds better to them than flat - look for posts on "house curve".


----------



## NixPix (Jul 11, 2010)

JohnM said:


> Looks great to me, though I suppose you could try pulling the 47Hz bump down a little. Some people find a gradual rise towards the lowest frequencies sounds better to them than flat - look for posts on "house curve".


Thank you John. The 47Hz peak is about as low as I can get it I'm afraid :| I have tried just about anything and everything to lower it further (including different positions and various settings in the sub filter + eq in reciever), but without a parametric eq, like somekinda behringer or likewise it probably wont go down more.

The settings I'm using now are the following (please let me know if something seems off/odd:

Subwoofer ASP (filter): Crossover 60Hz - Slope 48dB, EQ [email protected] Q2, Phase 0
Reciever: EQ - Subwoofer +1dB @ 63Hz
Sweetspot to Sub 2,7m (real distance)

Distance setting in reciever to sub: 1,2m (This one I originally had at 2,7m but lowering it and therefore setting a delay lowered the 47Hz peak by roughly 2dB) <-------- This one worries me a bit though, I am not exactly sure how it affects the rest of the performance and/or worse integration with the front speakers.


----------



## laser188139 (Sep 19, 2009)

You can swap the x-axis to log mode with the Freq. Axis button at the upper right; the display will then give you a better reflection of what is happening, as every octave will be the same width. 

If you expand the frequency range to go beyond the crossover, perhaps to twice the crossover, you might gain more insight from the phase curve as you will see how it changes from sub to main. By changing the sub distance, you may be reducing the peak at 47Hz by putting it out of phase with the main, but also be creating dips higher up. By reducing the sub distance 1,5 m, this may be enough for you to hear the sub be slightly delayed after the corresponding sound in the mains. (The Limits button in the upper right lets you control the frequency range directly.)


----------

