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Please help me interpret Avia sub readings

Discussion in 'Archived Threads 2001-2004' started by Barry BB, Dec 22, 2001.

  1. Barry BB

    Barry BB Stunt Coordinator

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    I took a couple of SPL measurements using Avia warble sweep and LFE sweep. According to Avia the phase switch should be set where the SPL meter maintains it's highest reading. On the warble sweep with the phase switch at zero my meter dips down to about 68hz at two different points during the warble sweep, once at about 78hz and once at about 67hz. With the phase switch at 180 the dip stays higher (aroud 72hz). I get a similar 4db difference in the LFE sweep.

    My sub is in a corner on the same side of the room as my mains. My understanding is that the sub phase switch normally is set to zero if it is placed in the same part of the room as the mains. But from my SPL readings it looks like 180 is where I should set mine. Am I interpreting these results correctly?
     
  2. David Judah

    David Judah Screenwriter

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    It also says with the least amount of variation, so from the numbers you mentioned it seems like having it set at 180 degrees provided that.

    DJ
     
  3. Barry BB

    Barry BB Stunt Coordinator

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    Thanks for the quick reply. I got confused with this because I've had my phase switch at zero since I got my system 3 months ago. I didn't think that it would be "out of phase" with the mains since they are in the same area.

    I wonder what is accounting for this phase discrepancy? Looking forward to reading other responses tomorrow.
     
  4. Barry BB

    Barry BB Stunt Coordinator

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    I'm interested in more opinions so bumping to the top.
     
  5. Guy Kuo

    Guy Kuo Supporting Actor

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    Barry, the crossover itself can shift the signal phase.

    David Ranada designed the phase warbles to one could use the old and time honored method of watching needle wobble to compare response of frequencies near each other as the warble waveres up and down in frequencies during an overall gradual sweep across the crossover frequency range. If response is flat, you see less needle movement. The idea with that method is to adjust phase so the overall amount of cancellation is minimized when averaged across the crossover frequency range. I don't find that very easy to do.

    The audible phased noise test is a band of noise centered across the subwoofer crossover freq range. The two parts of the test are identical in content, but flipped 180 degrees in phase. If phase is on average correct for the crossover range, the in-phase portion of the test will be louder or sound wider bandwidth due to less cancellation. That's easiest to check if someone else adjusts the phase while you listen. Since I usually don't have a willing assistant to patiently adjust the phase while I listen, I take advantage of the SPL meter.

    Placing the SPL meter at the seating position exactly at the position where my head would be allows the meter to measure the resultant sound level. I then adjust subwoofer phase to maximize the SPL meter reading during the in-phase portion of the subwoofer phased noise for either the left main or right main speaker. Since the phased noise covers the entire crossover range, the overall summation of cancellation and reinforcement for those frequencies is reflected by the SPL meter reading. Since the left, center, and right speakers should already be in phase if distances are correctly set choosing either left or right main works.

    You should also do a search in this forum about performaing accurate phase adjustments of the mains speakers using your SPL meter.
     
  6. Barry BB

    Barry BB Stunt Coordinator

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    *** Barry, the crossover itself can shift the signal phase. ***

    Guy, I'm not sure what you mean by this. My receiver's crossover is set at 80hz and that is where I am getting my first significant SPL dip. But at a phase of 180 on the sub the dip stays 4db higher than the zero phase. So does that mean that cancelation is occuring less drastically at the 180 setting? I've searched the forum as you suggested but I guess I'm not getting the point.
     
  7. Guy Kuo

    Guy Kuo Supporting Actor

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    If that is the direction at which the overall crossover band is in phase, then it will read higher. Problem is, you can't judges it at a single frequency but have to pay attention to stuff one octave above and below the crossover point. That's why I prefer using the phase noise instead of the sweeps and warbles. The phase noise lets you simultaneously sample multiple frequencies.
    What I mean by the crossover itself altering phase is that the crossover circuitry can shift the phase of the signal making the results differ from what you'd expect without doing any testing and measurement.
    The hyper-accurate phase setting thread was here http://www.hometheaterforum.com/htfo...e+avia+speaker
    It's about the main speakers, not the subwoofer, but if you follow the information you could quite possibly attain much better imaging.
     
  8. Barry BB

    Barry BB Stunt Coordinator

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    Guy, thanks for the follow up. I'll read the thread that in your link and I'll come back with more questions if I have to.
     
  9. Barry BB

    Barry BB Stunt Coordinator

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    Guy

    If I understood your response from the other day correctly you said that you find it hard to use the phase warbles and prefer the audible phased noise test. Is that the phased filtered pink noise test in the subwoofer section or are you talkng about another test? I used the the phased filtered pink noise test with a fast response setting on the meter. My system is calibrated at 75db. With a zero phase setting the meter swings as low as 72db. With a 180 setting the lowest meter swing is about 74db. So if I'm doing this test right I still come out with a 180 phase setting being better. Am I doing things right here or am I just not understanding things.

    Again, thanks for taking the time to explain.
     
  10. Guy Kuo

    Guy Kuo Supporting Actor

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    Yes, you are doing things right. It is the phase filtered pink noise that I am referring to. From the response not being much different between the two phase settings, you may have the sub in a position which is intermediate between the phases (about 90 degrees so neither makes a huge difference). Or it could simply be the speakers not having a lot of overlap (hope that's not the case).

    Try moving the sub temporarily about three or four feet closer or farther to intentionally shift the phase relative to the main speakers. Recheck to see if the two phases then differ more in SPL reading. If they do, then that would tend to suggest that the sub is positioned at a place which is intermediate between being optimal at 0 or 180 degrees. If that is the case, playing with the subwoofer distance/delay setting in the receiver may help.

    If nothing seems to alter the 0 and 180 degree difference check again that the sub's own crossover is bypassed or at least turned to maximum frequency so there isn't a hole in coverage.

    Finally, it is possible for a sub or main speaker to be in a position which creates a null at the crossover frequencies or very different acoustics for the speakers so the two never get a chance to reinforce/cancel each other. That would be tougher to correct and I'd probably not worry too much unless music reproduction were critical.
     
  11. Barry BB

    Barry BB Stunt Coordinator

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    Guy, thank you for continuing to follow up on this with me. Hopefully other people are following this thread and learning something. I'm very new to Home Theater so forgive me if I'm slow to be catching on but I'm understanding more with each explanation you give. I have more questions on what you have said.

    *** From the response not being much different between the two phase settings, you may have the sub in a position which is intermediate between the phases (about 90 degrees so neither makes a huge difference).***

    Question - With proper positioning of the sub can you give me any general guideline for how much of a difference I should see in the meter reading between the 2 phase settings?

    *** Or it could simply be the speakers not having a lot of overlap (hope that's not the case).***

    Question - Not sure what you mean by this so I'll tell you what I have for speakers and you can tell me if you think this is an issue. I have Paradigm studio 20's for mains, studio center, and an HSU VTF-2 sub.

    *** Try moving the sub temporarily about three or four feet closer or farther to intentionally shift the phase relative to the main speakers. Recheck to see if the two phases then differ more in SPL reading. If they do, then that would tend to suggest that the sub is positioned at a place which is intermediate between being optimal at 0 or 180 degrees. If that is the case, playing with the subwoofer distance/delay setting in the receiver may help. ***

    Question - I think I understand what you are saying here. If changing my sub position gives me a difference between phases then I should increase the number of feet distance that I have set on my receiver for the sub.

    *** If nothing seems to alter the 0 and 180 degree difference check again that the sub's own crossover is bypassed or at least turned to maximum frequency so there isn't a hole in coverage. ***

    Question - I definitely have the bypass switch on so I know this is not an issue.

    *** Finally, it is possible for a sub or main speaker to be in a position which creates a null at the crossover frequencies or very different acoustics for the speakers so the two never get a chance to reinforce/cancel each other. ***

    Question - I've seen you mention reinforcement and cancelation but I don't quite understand what this means. Could you give an example of this?
     
  12. Guy Kuo

    Guy Kuo Supporting Actor

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    Very simply, reinforcement and cancellation. Imagine the signals below as coming from a main speaker and a subwoofer. I'm using plus and minus signs just for ASCII illustration purposes.

    In phase the compression and troughs of air caused by the sub and main speaker arrive at your ear in step with each other.

    Peaks from one arrive simultaneous with peaks. Troughs arrive simultaneous with troughs. Peaks + Peaks = big peaks (reinforcement). Troughs + Troughs = deep troughs (reinforcement)

    Now if you flip the sub's output by 180 degrees, the troughs from one speaker arrive during the peak of the other. The resultant peak + trough = minimal signal (cancellation)

    It's like having two people pushing and pulling against you. If both work together, their efforts reinforce each other. If the one pushes while the other pulls, the results cancel.

    This all supposes that the two people are exactly in or out of phase. There are an infinite number of phase timings in between 0 and 180. If they are 90 degrees out of phase, the reinforcement or cancellation isn't complete.

    It's easiest to understand all this if you are willing to go plot some sine curves.

    y = sin(x) gives the signal from one speaker

    y = sin(x) + sin(x) gives the signal from speaker + sub if both are exactly in phase.

    y = sin(x) - sin(x) gives the signal from speaker + sub if the phase is flipped by the phase switch on the sub

    y = sin(x) + sin(x + pi/2) gives signal from speaker + sub if sub is 90 degrees ahead of the main speaker.

    y = sin(x) - sin(x + pi/2) gives signal from a speaker + sub if sub is 90 degrees ahead of the main speaker but the phase switch is flipped on sub.

    Notices that the result of flipping the phase switch 180 degrees yields similar amplitudes for signals which are originally 90 degrees out of phase.

    On a real speaker system, you aren't dealing with a simple sine wave of a single frequency but the effects over a band of frequencies and the effects of room acoustics. A signal reproduced on one speaker isn't exactly matched by that coming from the other speaker. Total cancellation and reinforcement need the signals to be identical so you never really see total reinforcement or cancellation. Real world, I'd expect to see a 6 dB or so difference on the phase filtered tests if the sub and main speaker are similar in acoustics and going from exactly in phase to out of phase. It would be easier to do things with an infinitely adjustable phase controls instead of a switch, but you work with what you have.
     
  13. Barry BB

    Barry BB Stunt Coordinator

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    Are you on here 24 hours a day? Good explanation, thanks.
    After reading your suggestion to temporarily move the sub I moved it about 3 feet forward from the front corner of the room. I still did not see a big difference in the 0 or 180 position. I then moved it to the back corner of the room right next to my seating area. So instead of a 9 foot distance I now have a 2 foot difference.
    In the phase filtered test I still am not getting more than a 2 db diference in the 0/180 position. Maybe I've got problems that I don't understand at this point. The only time that I'm getting a difference is on the low frequency sweep. When I had the sub in the front corner the 180 position gave me a 4 db higher reading than the zero position. With the sub in the back corner I get the opposite results. In the 180 position I get a 6db dip at around 85hz so the dip happens around my receiver's 80hz crossover. In the zero position the needle is maxed out throughout the test.
    Does this give you any additional insights as to what is going on? Should I keep my sub in the back of the room with a zero phase setting?
    Boston is a nice city to visit, I don't suppose you want to take a leisurely cross country flight over here to help me get this right.[​IMG]
     
  14. Guy Kuo

    Guy Kuo Supporting Actor

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    24 hours? Seems that way sometimes. Then again, I sometimes leave the forums for a few weeks.

    Probably back of the room with the 0 degree setting would be better out of the two.

    BTW, you have adjusted sub level to match the mains before doing the phase right? If not, then part of the low difference might be the sub not matching the mains in output level.
     
  15. Barry BB

    Barry BB Stunt Coordinator

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    Yes I matched the sub to the mains first, although I find that to be difficult to do too since the needle is moving all over the place. But I'm trying to average it out as best as I can.

    I've had my system for 3 months and when I first got it I did some quick calibrations to get going. Today is the first day that I really took some time to take some measurements and I was not understanding my results too well. Thanks for your help, I'll continue to play with some of the techniques that you've given and see if I can get some different results in the phase testing.

    Merry Christmas
     

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