Help intrepreting Phase - Home Theater Forum and Systems - HomeTheaterShack.com

Old 06-15-10, 10:46 AM Thread Starter
Senior Shackster
Bill

Join Date: Nov 2008
Location: Southeast MI
Posts: 117
Help intrepreting Phase

I need help interpreting Phase information.

Here is my sub alone.

Here is my sub and mains to 200 hz

Thanks
t6902wf is offline

Old 06-15-10, 11:26 AM
Senior Shackster
Bill

Join Date: Sep 2009
Location: Austin, TX
Posts: 676
Re: Help intrepreting Phase

Quote:
t6902wf wrote: View Post
I need help interpreting Phase information. ...
There's not a lot to be learned from these two graphs, so I'm not sure what question in particular you are trying to answer. But I can give you a few observations.

The rise in phase at the low end, below 20Hz, is normal. (Please don't ask me to explain whether it is just from the longer wavelengths or the presence of the crossover.)

At the higher end, you see changing behavior in the phase at the same points, 110Hz in the sub alone and 130Hz in the sub+main, where you see the large changes in level. It makes sense that, where one or more reflections are canceling the direct sound, the phase of the combined signal would change.

Perhaps you are asking why, at the point where you have the cursor set, the phase value is different in the two graphs. If you don't use the left (or other) channel for timing, REW interprets the impulse as starting at time zero, and calculates the phase from that point. With the sub alone, the impulse curve is just from the sub. When you add the main, it becomes the predominant part of the impulse with its higher frequencies, and it determines the instant interpreted as time zero. As the zero point has moved, the calculation of the value of the phase at any point changes. You'll notice that the shape of the curve in that range, where the main is not contributing, is unchanged; it is just translated downwards ~20 degrees.

I would think the more important question is why the level of the main appears so low compared to the sub. Is this just in the range to 200Hz? If you measure sub+main to a higher level, e.g., 400Hz, 1kHz or 2kHz, does the level of the main come back even with the sub? Or, if not even, within about 3dB if you are using both mains together?

Bill
laser188139 is offline
Old 06-15-10, 02:43 PM Thread Starter
Senior Shackster
Bill

Join Date: Nov 2008
Location: Southeast MI
Posts: 117
Re: Help intrepreting Phase

Quote:
laser188139 wrote: View Post
There's not a lot to be learned from these two graphs, so I'm not sure what question in particular you are trying to answer. But I can give you a few observations.

The rise in phase at the low end, below 20Hz, is normal. (Please don't ask me to explain whether it is just from the longer wavelengths or the presence of the crossover.)

At the higher end, you see changing behavior in the phase at the same points, 110Hz in the sub alone and 130Hz in the sub+main, where you see the large changes in level. It makes sense that, where one or more reflections are canceling the direct sound, the phase of the combined signal would change.

Perhaps you are asking why, at the point where you have the cursor set, the phase value is different in the two graphs. If you don't use the left (or other) channel for timing, REW interprets the impulse as starting at time zero, and calculates the phase from that point. With the sub alone, the impulse curve is just from the sub. When you add the main, it becomes the predominant part of the impulse with its higher frequencies, and it determines the instant interpreted as time zero. As the zero point has moved, the calculation of the value of the phase at any point changes. You'll notice that the shape of the curve in that range, where the main is not contributing, is unchanged; it is just translated downwards ~20 degrees.

I would think the more important question is why the level of the main appears so low compared to the sub. Is this just in the range to 200Hz? If you measure sub+main to a higher level, e.g., 400Hz, 1kHz or 2kHz, does the level of the main come back even with the sub? Or, if not even, within about 3dB if you are using both mains together?

Bill
Thanks for the reply Bill. This was my first run with the new REW so I had not seen phase before. I guess I was looking for smooth (for no particular reason) and was confused by the radical movement of the line.

I am pretty sure the level change is an anomaly. I would hear that radical a change and it is not there.

I'll run it higher next time I pull out the gear.
t6902wf is offline
Old 06-20-10, 12:01 PM
Senior Shackster
Bill

Join Date: Sep 2009
Location: Austin, TX
Posts: 676
Re: Help intrepreting Phase

Quote:
laser188139 wrote: View Post
... At the higher end, you see changing behavior in the phase at the same points, 110Hz in the sub alone and 130Hz in the sub+main, where you see the large changes in level. It makes sense that, where one or more reflections are canceling the direct sound, the phase of the combined signal would change. ...
Something I forgot to mention before: Where you are seeing the phase alternate between large positive and negative values, the visual representation is making it look more unstable than it is. If you go into the SPL & Phase controls and select Unwrap Phase, you may see the phase values relatively stable, just oscillating around -180°. This is a natural consequence if the soundcard is inverting the polarity on output or on input. If this is what you see, in the Impulse graph Controls, you can select Invert Impulse. The phase in the SPL & Phase graph will then oscillate around zero.

Bill

Last edited by laser188139; 06-20-10 at 12:22 PM.
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