# sub position and group delay



## rtrt (Aug 30, 2014)

Been thinking and reading a bit more and concluded that i need to spend more time locating my sub relative to my mains before using REW to look at the combined response in room.

For example the sub i have, has significantly more group delay than my mains. I take that to mean that my sub needs to be closer to the listening position than my mains. Thats not something i've read anywhere before so maybe i've misunderstood something. 

Typically i read that the sub would be in line with the mains or even behind - meaning that the sub sound will arrive later than from the mains. 

I'm assuming thats something to avoid. Can anyone shine some light for me here?

Is there a rough rule of thumb along the lines of 'compare the group delay at say 40Hz and for every 10msec difference you need to pull the sub forward 1 foot' to compensate for the group delay difference.

That would allow me to look at my room to see if there are any acceptable locations for the sub.


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## jtalden (Mar 12, 2009)

Group delay and phase rotation are different measures of the same. The amount change in GD of a SW depends on several design choices. The most important being the order of the box design and the order of any filter being applied. SW location and the room acoustics will not change this characteristic. There is a lot of misplaced discussion on the importance of GD of a SW. 

The more important characteristic is the timing/distance setting between the SW and the mains. We want the sound of the SW and the mains at the XO frequency to arrive at LP at the same time. Adjustments to the distance setting in the AVR or to the delay timing in a MiniDSP for example changes this timing. By adjusting the distance/delay settings properly the SW can be located at any distance from the mains and still provide the proper handoff.

So:
> Locate the SW wherever it results in the smoothest frequency response at the LP.
> Set the distance/timing such that there is a smooth SPL handoff at the XO frequency.

There are of course several options that can be chosen in the distance/timing that can satisfy the general guideline above and several ways to measure and confirm the efficacy of the settings. Some rooms can be very problematic due to room modes occurring in the XO range.

Let the REW measurements be your guide rather than any rule of thumb.


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## rtrt (Aug 30, 2014)

jtalden said:


> The more important characteristic is the timing/distance setting between the SW and the mains. We want the sound of the SW and the mains at the XO frequency to arrive at LP at the same time.


Thanks jtalden, this is what I was getting at. 



jtalden said:


> Adjustments to the distance setting in the AVR or to the delay timing in a MiniDSP for example changes this timing. By adjusting the distance/delay settings properly the SW can be located at any distance from the mains and still provide the proper handoff.


Unfortunately as i have a hifi setup of laptop/dac/active speakers, then I have no means of setting additional delay on the mains to match the sub. The only adjustment i have (apart from physical movement) is a variable phase control on the sub. As far as I can tell this will only serve to delay the sub further i.e. going the opposite to what I need.



jtalden said:


> Let the REW measurements be your guide rather than any rule of thumb.


Ok but given the above lack of delay on mains available to me, wouldn't i be best to identify the correct distance needed between mains and sub, then using REW to select the best specific location, from a smooth frequency response perspective, that matches this distance?

For example I was thinking that once I identified the distance in question, it might be possible to move the sub from in between the mains, put it on one of the side walls at the correct distance to the listening position.


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## jtalden (Mar 12, 2009)

rtrt said:


> Unfortunately as i have a hifi setup of laptop/dac/active speakers, then I have no means of setting additional delay on the mains to match the sub. The only adjustment i have (apart from physical movement) is a variable phase control on the sub. As far as I can tell this will only serve to delay the sub further i.e. going the opposite to what I need.


I would think that some music servers would have that option available through plugins. That may be an option for you.

Also maybe *Equalizer APO* provides this capability.

I guess it depends on where/how the XO is implemented if these are viable options.

Someone with more experience than I have can probably make a specific suggestion for the music server you use and where the XO is.



> Ok but given the above lack of delay on mains available to me, wouldn't i be best to identify the correct distance needed between mains and sub, then using REW to select the best specific location, from a smooth frequency response perspective, that matches this distance?


Yes, REW measurements can be used to determine the best settings. This requires the use of the "loopback timing" feature in REW to maintain the relative distance offsets when measuring the SW vs the Mains. A USB mic is not yet supported in REW for this job. Depending on your setup, it could also be problematic using a standard XLR mic. 



> For example I was thinking that once I identified the distance in question, it might be possible to move the sub from in between the mains, put it on one of the side walls at the correct distance to the listening position.


The correct SW distance is normally expected be very near main speakers measured distance. You could just place the SW at about the same distance from the LP as the mains are. Then measure SW, mains, and SW+Mains. The overlay of these 3 measurements will show amount of SPL support through the XO range. Some people like to invert the polarity on the SW and adjust the distance so that a deep null falls at the XO freq frequency. Just adjust the SW distance as needed. 

An option is to use the REW RTA feature to see the SPL impact in the XO range real-time as you move the SW distance.

A shortcoming of these methods is that if there is a room mode in the XO range it can disrupt SPL there an make it more difficult to determine the correct distance easily. 

If you have to move the SW very far from the initial distance that may indicate the SW polarity should be reversed. Inverting the SW polarity on a 80HZ XO will change the distance needed by about 2.2m.


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## AudiocRaver (Jun 6, 2012)

jtalden said:


> Group delay and phase rotation are different measures of the same. The amount change in GD of a SW depends on several design choices. The most important being the order of the box design and the order of any filter being applied. SW location and the room acoustics will not change this characteristic. There is a lot of misplaced discussion on the importance of GD of a SW.


jtalden: As always, thank you for your contributions and guidance. I hope HTS members and readers recognize that your posts as a rule are reference quality.

A point I have been thinking about that you might help illuminate: Is it not true that human perception of subwoofer frequencies is fairly sluggish? Thus leading to the points you have already made, that integration at crossover is the area to focus on, that the arrival time/delay of the lower frequencies is of lesser (not zero) importance? Thoughts?

Thanks.


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## jtalden (Mar 12, 2009)

Yes, The SPL uniformity through the XO range is far more important than the particular timing alignment chosen in my opinion. The few references to research on the audibility of group delay that I have run across do not even try to quantify how much GD it takes to become audible in the LF range. They do provide numbers in the MF and HF range. My own derived opinion based on various sources and my own experiments is not definitive.

I have come to think that from a theoretical perspective, close phase tracking throughout the XO range is the idealized objective for IIR XO's above the modal range (MF or HF). That creates a stable frontal lobe where the direct sound from the 2 drivers is always working together with similar timing to form a stable direct sound field. This is not as applicable to the lower XOs as in the modal range the field is not directional. I'm not sure this is worded correctly, but I think concept is directionally correct.

In my experimentation I sometimes think I can hear a difference between close phase tracking and crossing phase at the SW XO. It is not clear to me which sounds "better" however. I am not even convinced that I would be able to reliability identify that perceived difference in ABX testing. 

I choose to utilize close phase tracking through the XO because I have the necessary controls in my setup to do that. I also then remove the phase rotation in my setup by employing a FIR filter to removes the phase rotation. That way I have relatively linear phase from 20-20K when using my music server. Again, just because I can. If I remove the FIR filter I cannot identify a difference even using ABX testing with earphones. 

Summarizing for a SW IIR XO:
> If possible provide close phase tracking through the entire XO range.
> If not, adjust the timing to provide for a crossing phase at the XO frequency. It is better to have the SW pulled forward (less delay) compared to the mains rather than have the SW delayed further.
> The resulting SPL uniformity is more important than the timing so any setting that provide the smoothest response is preferable.

Obviously gross timing errors are clearly audible. SW timing errors within 1 WL of the ideal are probably not.


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## AudiocRaver (Jun 6, 2012)

Excellent. Thank you:T


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## FargateOne (Mar 3, 2015)

AudiocRaver said:


> Excellent. Thank you:T


A++


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