What's more important, flat response or phase/timing of subwoofer? - AVS Forum
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post #1 of 14 Old 03-25-2013, 06:02 PM - Thread Starter
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So I've been playing around with my subwoofer phase and distance setting, trying to find the correct/optimal/best setting.  You can see my previous thread with more details at this link, http://www.avsforum.com/t/1464406/setting-subwoofer-phase-properly-with-omnimic

 

Looking at the image below, all traces are sub plus left speaker with 80hz crossover

black = with Audyssey set distance, so no tweaks from Audyssey

blue = same as black but increased sub level 3db

orange = added 3.5ft to sub distance and adjusted phase from 0 to 180deg (same sub level as black trace, so -3db from blue)

 

 

 

 

What bugs me is they sound much different (comparing orange to blue as black obviously has less bass).  So I'm wondering how I choose the "correct" setting.  The blue trace is more what I have been listening to previously and the bass seems to hit hard and be timed properly, at least according to my ears.  The orange trace seems best from the charts, saves me 3db subwoofer headroom and makes the subs really disappear but hard to tell if that is how it should be or if it is smearing things.

 

Which would you choose and why?

 

One more piece of information, with the Audyssey set distance (blue and black traces) the time alignment seems correct with the center channel as well based on plots.  If I go with the orange trace settings but center channel is out of phase with the sub and I get a nasty dip of -15db at 80hz.

 

Looking at the charts I would think orange and blue would sound nearly identical for music (which is what I have been listening to for comparison), but they don't.  This almost appears to be one of those things where even though the measure the same the sound different (though I'm not usually in that camp).  So looking for input and feedback from others.

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post #2 of 14 Old 03-25-2013, 06:08 PM
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Originally Posted by primetimeguy View Post

Which would you choose and why?
The one that sounds the best, because that's what matters.

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post #3 of 14 Old 03-25-2013, 06:31 PM - Thread Starter
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Originally Posted by Bill Fitzmaurice View Post


The one that sounds the best, because that's what matters.

They both sound good, neither is best, just different....there has to be one that is more accurate right?

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post #4 of 14 Old 03-25-2013, 07:28 PM - Thread Starter
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This is an interesting read...

 

http://www.silcom.com/~aludwig/Phase_audibility.htm

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post #5 of 14 Old 03-25-2013, 07:28 PM
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Originally Posted by primetimeguy View Post

They both sound good, neither is best, just different....there has to be one that is more accurate right?
Technically yes, but it's very difficult to hear accuracy below 100Hz., as opposed to say 500Hz, where it's very easy to hear even the very slightest inaccuracy. If you can't tell the difference go with the one that measures better, if only so you don't kvetch about it.
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post #6 of 14 Old 03-25-2013, 11:22 PM
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Quote:
Originally Posted by primetimeguy View Post

Looking at the charts I would think orange and blue would sound nearly identical for music (which is what I have been listening to for comparison), but they don't.  This almost appears to be one of those things where even though the measure the same the sound different (though I'm not usually in that camp).  So looking for input and feedback from others.

Going with what sounds best to you is the golden rule but obviously there's room to train your ears to appreciate an accurate representation of the source. That said, bass test tones can really only help identify the nulls and peaks of your room's frequency response. You can have a flabby sub that could generate an admirable response calibrated with sine sweeps but sound terrible with music.

Adjust your phase (delay / distance setting) so that you get the loudest reading at the crossover point without going a wavelength beyond (+/- 14 feet at 80hz). You shouldn't adjust phase beyond this, unless all you do is listen to sine sweeps. Having a good response (as you do) without mucking up the timing (by introducing too much / little delay), you're doing it right.

I wouldn't choose orange as that gives a -15db drop at the crossover during multichannel playback, especially if your other trace sounds different but not worse.
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post #7 of 14 Old 03-26-2013, 08:13 AM - Thread Starter
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Thanks for the feedback so far guys.  

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post #8 of 14 Old 03-26-2013, 08:42 AM
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First, I would run the sweep again at a loufer level(90 dBs) and adjust the scale to have 5 dBs in between, you have 15 dBs and far from flat. Flat is usually +/- 3 dBs so your graph would have to be within a 6 dB range, yours is +/- 7.5 dBs or a 15 dB range. There will be a huge difference in sound once really flat.

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post #9 of 14 Old 03-26-2013, 09:35 AM - Thread Starter
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Originally Posted by MKtheater View Post

First, I would run the sweep again at a loufer level(90 dBs) and adjust the scale to have 5 dBs in between, you have 15 dBs and far from flat. Flat is usually +/- 3 dBs so your graph would have to be within a 6 dB range, yours is +/- 7.5 dBs or a 15 dB range. There will be a huge difference in sound once really flat.

I have done plots at higher level in the past, no major differences until extremely loud (near reference).  I think you looked at the chart wrong, grid lines are 5db now so my orange trace is +/-3db now, granted that is 1/6 octave smoothed.

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post #10 of 14 Old 03-26-2013, 10:03 AM
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Originally Posted by primetimeguy View Post

I have done plots at higher level in the past, no major differences until extremely loud (near reference).  I think you looked at the chart wrong, grid lines are 5db now so my orange trace is +/-3db now, granted that is 1/6 octave smoothed.

Oh OK, I would use no smoothing but otherwise just use what sounds best. I like getting really low and a house curve with a rise below 20hz. Your graph only goes to 20hz so I would think for movies the blue trace would sound best and for music the orange line.

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post #11 of 14 Old 03-26-2013, 10:29 AM - Thread Starter
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Originally Posted by MKtheater View Post


Oh OK, I would use no smoothing but otherwise just use what sounds best. I like getting really low and a house curve with a rise below 20hz. Your graph only goes to 20hz so I would think for movies the blue trace would sound best and for music the orange line.

Thanks for the input.  I think I will try some measurements at louder levels just to see.  Last time I think I did just the main speakers to see when they really started to give up.

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post #12 of 14 Old 04-05-2013, 04:23 PM
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interesting question.

with an 80hz crossover, 1/4 wavelength or about 90 degrees of phase is 3.5 feet. so sub -3.5 rolls it back in time 1/4 wavelength.

then you flip the phase to -180 degrees, which rolls it back 1/2 wavelength for -3/4 wavelength total.

normally, the suboofer is a little slower, so it should actually be moved forward in time to align with the mains.

my guess is that you started out with a sub that was about 1/4 wavelength behind in time and then rolled it back another 3/4, so you are matching the sine wave cycle but are now one full cycle behind where you should be.

try moving the mains back in time 3.5 ms or 3.5 distance feet electronically and set your subwoofer for zero. ideally, that will align your sub with your mains.

this is the same concept as the backward sloping baffle to time align drivers, but with 3.5 feet of virtual angle because the frequency is so low.

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post #13 of 14 Old 04-05-2013, 06:23 PM - Thread Starter
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Originally Posted by LTD02 View Post

interesting question.

with an 80hz crossover, 1/4 wavelength or about 90 degrees of phase is 3.5 feet. so sub -3.5 rolls it back in time 1/4 wavelength.

then you flip the phase to -180 degrees, which rolls it back 1/2 wavelength for -3/4 wavelength total.

normally, the suboofer is a little slower, so it should actually be moved forward in time to align with the mains.

my guess is that you started out with a sub that was about 1/4 wavelength behind in time and then rolled it back another 3/4, so you are matching the sine wave cycle but are now one full cycle behind where you should be.

try moving the mains back in time 3.5 ms or 3.5 distance feet electronically and set your subwoofer for zero. ideally, that will align your sub with your mains.

this is the same concept as the backward sloping baffle to time align drivers, but with 3.5 feet of virtual angle because the frequency is so low.

Actually, since I increased the sub distance it will now fire sooner than the other channels, so I moved it up 1/4 wavelength of 80hz, then added 1/2 wavelength delay for a net 1/4 delay.  And sure enough if I just subtract 3.5ft from sub distance (so adding 1/4 wavelength delay) I get pretty much the exact same response.  And to your point, are these two settings really the same or is one a full cycle off?  And keep in mind that my mains are dual 8" woofers and ported so I they go fairly low but at those low frequencies I will be hearing the port which will be 180deg out of phase.  So my issue is keep the Audyssey set distance which is in phase will center and surrounds, or tweak the distance to make it in phase with low freq of mains but out of phase with other channels.

 

And those distances really only apply at 80hz.  Odd thing is there is much difference at 80hz in any of the traces.  the big difference is in the 40-60hz so nearly twice the wave length

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Quote:
Originally Posted by primetimeguy View Post
And keep in mind that my mains are dual 8" woofers and ported so I they go fairly low but at those low frequencies I will be hearing the port which will be 180deg out of phase.
Not sure what you mean by 'those frequencies'.

A ported enclosure actually has a combined response and only gradually transitions to primarily port-only response that reaches its maximum exactly at port tuning (while the driver is almost motionless), and at that point the port is actually ~360 degrees (delayed by one full cycle) not 180 degrees.

Unless I am just having a brain fart here, your phase at 'port resonance' should be virtually identical to your phase at 'driver resonance' in the frequency domain because 360 degrees is audibly indistinguishable from 0 degrees with a simple steady-state periodic signal such as a sine wave. Where you start having issues with out-of-phase in the steady state is below resonance and your crossover should substantially nix that along with the relatively abrupt port rolloff below resonance.

To make things even more complicated, the transient response (here I am actually thinking of a tone burst) at port tuning starts off like a sealed enclosure with the woofer providing all the output, sort of, because the port contributes almost nothing at all, not even much additional pressurization on the backside of the woofer over that of sealed enclosure, until one half cycle has happened and the port starts fighting the woofer on its backside. The port actually applies 180 degree out of phase wave to the backside of the driver, not to the room, under steady-state periodic waveform at port resonance.

If anything, the port will contribute to the first cycle more like an open back, with a greatly attenuated and somewhat delayed back-wave response to the initial cycle of the burst that will detract (a little) from the initial 'sealed' response to the first cycle, because the woofer has to actually 'prime' the resonator with energy before it starts to resonate and add constructively. Each successive cycle takes energy from the driver and adds it to the port output on the next cycle until the system reaches steady-state equilibrium several cycles down the road!

So ports (any resonators actually) add a time-varying character to the response that cannot be completely described as a 'phase' when working in the time domain. The system is actually gaining energy as it builds towards steady-state, and that energy will also gradually dissipate after the input is abruptly removed (even if removed at a zero crossing) in the form of continued resonant output, not a gradual release of pressure nor a simple phase delay.

Fortunately, these resonant effects are happening all around us in a room anyway and our hearing is quite forgiving, so ports tend to sound natural even though what comes out of them bears little resemblance to what went in, in the time domain.

When off port tuning, the picture gets even more complicated because the system starts to have a steady-state output with significant contributions from port and driver simultaneously. Since the port is operating off resonance its magnitude is attenuated and its phase also varies. That means it adds or subtracts from the total system response to varying degrees as a function of frequency and it also similarly fights the backside of the driver to varying degrees as frequency changes.

All the while, the somewhat aperiodic signals of audio are adding energy to the resonator and letting that energy die out as the signal changes. It really becomes quite a complicated mess.

People tend to think of these things in black/white terms. We like to use frequency response and phase to put things into simple terms we can visually comprehend but that is not what is really happening in the physical world. We use phase and frequency to mathematically describe specific useful physical system characteristics but the frequency response graphs do not paint a picture of what the time domain looks like, they only tell us what the steady-state looks like.

In this case I believe you got the port phase at resonance reversed in your mind. Either that or I just completely misinterpreted something. Even I get confused.
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