phase vs. distance? - Page 4 - AVS Forum
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post #91 of 95 Old 07-18-2013, 03:21 PM
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Originally Posted by Larry L View Post

Since the Bass FR at a given listening position is influenced by the sub phase relationship with all speakers in the room, it seems the goal would be to flatten out the FR around the XO freq with all speakers fed a single source tone during a frequency sweep. What I have observed doing tis is that I was able to flatten out the respons of the entire system by just shifting the sub distance one foot. from where the peak output was obtained at the XO freq.

However, I do agree that the phasing with the CC is most important if I am mostly watching movies. So, now I am thinking I should remeasure and re-tweak with just the CC for best FR. I believe that gave me a difference of 6 feet from what I have now.

My understanding is that dicrete audio channels that are getting different content need to be looked at individually. If they were all getting the same signal all the time i would think calibrating with a mono signal of all speakers would be the way to go. Acoustic measurement standards for stereo listening is a good resource.

http://www.acousticfrontiers.com/whats-new/2011/10/13/acoustic-measurement-standards-for-stereo-listening-rooms-pu.html
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post #92 of 95 Old 07-18-2013, 04:16 PM
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regardless of the specific content, each speaker stays in the same place, so its delay from acoustic transimission through the air stays the same, and their native phase (non)linearity stays the same at any given frequency. IOW the splice at 80 Hz is the same regardless of what content contains 80 Hz information for any given speaker. I think. Of course when there are different crossovers for different speakers, the talsk of maximizing the response in what is now multiple crossover frequency regions becomes more complex.
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post #93 of 95 Old 07-18-2013, 07:14 PM
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I might be wrong here, but I think that would be the case outdoors as the only boundary is the ground and all same distance from it, but I think when we add boundaries and differing distances to them and there influence in the modal region and the longer wavelengths and the acoustic summation it changes things. So if speakers same distance from mlp, probably not same distance from front wall. And I think this would be happening at 80hz. Also besides different crossovers, what if you don't have three matching fronts? Unfortunately I don't have an acoustic transparent screen and have to use a compromised horizontal center at this time. I also have right side of room open to basement but left speaker is within 1/4 wavelength of 80hz. Do have 4in treatment with 4in air gap though to try and balance room. Here are some quotes from earlier in the thread.

Bill-
"That would be it. Keep in mind that every response dip and peak is caused by time/phase interactions, and that not only does every source introduce time/phase issues, but so does every boundary within the room, each acting differently with every source, and with every listening position. The more sources you have the more they tend to even each other out, but you still have to make compromises in trying to get the best overall result."

Mark-
"Subwoofer distance is a very fuzzy setting with such long wavelengths and the interaction with the main speakers of various types. The selected crossover frequency can often shift the optimal setting. Just remember that at 80Hz, ~3.5' is only 1/4 wavelength. Impulse measurements from a main speaker make sharp spikes with the first sound arrival. With a subwoofer an impulse is measured as a ramp/mound of energy as high frequencies are required to make any sharp shapes. The peak of that mound is not always the best setting, especially when the bass from the main speakers always lags the high frequencies they use to set the distances."

My question is what frequency point does the impulse stop having a sharp spike with the first sound arrival and transition into a ramp/mound of energy and does this relate to the schroeder frequency? If the distance were set by 80hz arrival time instead of the high frequencies, would the distances readings between high frequency and 80hz be different if speaker set to differing boundary distances but still same distance from mlp?
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post #94 of 95 Old 07-18-2013, 08:23 PM
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Quote:
Originally Posted by Luke Kamp View Post

I might be wrong here, but I think that would be the case outdoors as the only boundary is the ground and all same distance from it, but I think when we add boundaries and differing distances to them and there influence in the modal region and the longer wavelengths and the acoustic summation it changes things.
You're not wrong at all, especially with subs. Since they radiate omni-directionally even the wall behind them is a boundary that has its own influence. When software tries to calculate the sub distance from the mic it doesn't have one source to consider, in the average room it has at least seven. It's not a wonder that Audyssey doesn't always get it right, it's a wonder that it ever does.

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post #95 of 95 Old 11-10-2013, 04:07 PM
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Just remember using a tape measure and doing the math to set the delay on a sub will never be correct. It can get you close with mains and rears. Time delay should be used to match the phase between the sub and mains at the crossover frequency.

This is fairly simple to accomplish. If you do not have an RTA you can get close with your ears.

To simplify this procedure I will discuss how to set the mains and sub. As in a 2 way setup with a sub added.

With ears:
-Set your crossover point on both the sub and mains, lets say 50 Hz.
-Put either your mains, or sub 180 deg out of phase. This can be done either physically or mechanically. If reversing the mains (L/R), reverse + and - on both speakers, so they are both out of phase with sub, but in phase with each other.
-Play a sign wave that is the same as the x-over frequency. In this case, 50 hZ
-Sit in the main seating position and have an assistant slowly adjust the time delay on the sub (or do it yourself via remote). You want to listen carefully to 50 Hz tone. You are trying to find the point at which the tone is at its lowest dB level. You may find that once it hits its lowest level a few more ms of delay will not change anything, but after those few ms the volume will increase. Mark down where the first ms number and last ms number right before and right after this lowest point. Set the ms delay between these two settings.

With RTA:
--Set your crossover point on both the sub and mains, lets say 50 Hz.
-Put either your mains, or sub 180 deg out of phase. This can be done either physically or mechanically. If reversing the mains (L/R), reverse + and - on both speakers, so they are both out of phase with sub, but in phase with each other.
-Play a sign wave that is the same as the x-over frequency. In this case, 50 hZ
-Place your mic in the main seating position.
-change the display resolution on TrueRTA so it is only showing 20 Hz for the low frequency limit and 100 Hz for the high frequency limit. You can and should also limit the upper and lower dB limits so you can magnify the peak of the 50 Hz frequency. Change the speed trade off to 40 Hz (med), and the sampling to 2. Turn of the bar graph so that you are only seeing the line graph.
-Wait for the peak to settle, then hit stop and save the graph (Alt-1). Now hit Go again.
-Turn the delay until the peak reaches its lowest point. The peak will lower in amplitude on the display. If the peak gets to its lowest point and stays there for a few more clicks of delay find the middle ms number and leave it there.
-Put the speakers back in phase.
-You are done.

You may find that this ms number is way off from one that would be obtained through measuring with a tape and subtracting the difference, but it will put the mains and sub in perfect phase at the given seated position where the mic was placed.

Putting one set of speakers out of phase when doing this increases the magnitude of the change (than if they were in phase) and is easier to hear and/or see on the RTA program.
I've been using this method for about 25 years and it works every time.
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