GR Research Open Baffle Dipole Subwoofers

[KeithL]

Flipping the wires will result in a phase shift of exactly 180 degrees at all frequencies.
I would expect subs that have a physical "0 / 180 switch" to be doing the same thing electronically.

But I wouldn't expect anything anywhere near that consistent from any sort of "variable control".
A fancy DSP-based network could probably come close - depending on exactly how it's done - but still isn't going to be perfectly linear.
(But the simple analog "all-pass shift networks" you find on most subs are not going to be even close...)

And, MarcL, in your case, I would expect the servo to add a whole bunch of complications as well.
The signal from the feedback coil may be phase shifted from the main signal... and there is going to be stability compensation in the servo itself.
(The servo's feedback MUST remain "negative" at all frequencies or the system will be instable and turn into a giant buzzer.) 

Sept 23, 2024 9:41:18 GMT -5 marcl said:

Sept 22, 2024 15:36:16 GMT -5 leonski said:

One thing about 'phase'. I suspect it would work best if Both the subs were acting 'in unison'.
After settling on your settings and recording the data?
Do an A/B polarity swap on ONE of the subs and listen. Swap back than turn the phase control to the opposite 'stop'.

I'm curious about what changes you may or may not hear.

Of course my goal with this build is to improve the sound for music and movies.  But also I really want to quantify a lot of things that have never been clearly documented about how dipole subs work in general and specifically how this design with this servo amp works.  So I'm optimizing along the way but also doing extra tests to quantify what each setting does.

Sure, listening is important.  But many, many people ONLY listen and never measure.  And I know from doing both, that I can listen on two different days or even morning and evening the same day and have it sound different ... and I changed NOTHING.  But what would someone say?  "Oh that's because the noise in your power line was lower at night ... or on Sunday night vs Monday morning".   No ... not a chance ... it's psychoacoustics and brain stress and sensory distraction ....

So I do not make random changes and try to decide if I hear a difference.  I make changes based on a hypothesis, measure the result, see if the result confirms the hypothesis ... and THEN I listen to see if the expected audible effect of the hypothesis is evident in the listening.  

Anyway, what are the most important things to do when integrating subs?

1. With more than one sub they must be time and level aligned to each other.  That means the same level at the MLP and with an impulse response measurement that starts playing at the same time.
2. When crossing between a main speaker and sub for Bass Management, the goal is to align the phase of the two speakers at the crossover point.  Phase changes with frequency so they will not be in phase at every frequency, but you try to align them at the crossover.

Phase, delay and polarity are terms that should be carefully and separately used, as is appropriate.

Right now if I measure my left front speaker and the left sub ... the polarity is the same with the phase control of the sub at 180.  The levels are matched.  Response is smooth across the 60Hz crossover point.  But it appears that the front speaker is playing 3ms ahead of the sub, and there's nothing I can do about that in my current setup.   

Today I have a few ideas for testing, and the first thing I'm going to do is see if switching the wires on the sub does exactly the same thing as turning the phase knob from 0 to 180.

[KeithL]

And things are going to get a lot more... interesting... when the sub is NOT in the middle of the room.  

Sept 13, 2024 11:29:09 GMT -5 marcl said:

Sept 13, 2024 10:57:25 GMT -5 KeithL said:

The only catch there is that, when you eliminate some of the room modes, you are left with just a few that "purely dominate" the situation.
And, arguably, "that's how an organ pipe works".
(One of the arguments in favor of using a bunch of small subs is that the room modes are all different... so they sort of blur together and average out.

And, for anyone considering this sort of project, one of the disadvantages of an open-baffle sub is that it will tend to have a 12 dB/octave rolloff as you go lower below the "baffle cutoff".
So they require active EQ to provide a 12 dB/octave boost...
And lots of big drivers, with a lot of surface area, and a lot of travel, to move all that air...
And enough power to do the job driving them...

You can design a reasonably small sealed or ported sub that can deliver a lot of output at rather low frequencies.
That's just not an option with an open-baffle sub.

Another interesting thought...

Another "design variation" would be to stack several sheets of felt inside the cabinet behind each of the woofers.
Basically stack sheets of felt behind the drivers... with some sort of grating to serve as a "back panel" to hold them in place.
If you do that then they sort of transition from "open baffle" to "infinite baffle" at low frequencies...
The felt provides resistive absorption for at least some of the back-wave energy...
But only "sort of" since you can't possibly absorb all of the back-wave.
But the situation, as seen by the driver, is more damped, so resulting resonances should at least have a lower Q.
(It might produce interesting results... and could be tried without major modifications...)

Remember that most drivers have a rather high mechanical Q and, with an open baffle, there is really nothing to modify that.
And a servo that works by using feedback from a second coil is not as tightly coupled as a "purely electronic servo".
(The "in-box resonance" of your driver is pretty much the same as "its free air resonance" and most modern drivers do want/expect a bit of mechanical damping.)

I don't see where absorbing some of the back wave is a benefit.  You actually want the low frequencies below 120Hz to cancel at the top, bottom and sides so that those modes are greatly reduced and they don't act on their own or in combination with the length mode to disrupt the response.  My tests in the small untreated room demonstrated how the modes are mostly only present in the direction of the figure 8 radiation and additional modes do not combine.  Response was surprisingly controlled ... especially considering it was just one sub in the middle of the room.  Of course when I bring it into the treated listening room and compare directly to the response of the two conventional subs ... we shall see.

Yes I actually measured the rolloff of the dipole and even characterized the compensation required using my miniDSP ... all as expected.   But I don't see where the compensation filters have any downside.  The resulting response is 3db down at 16Hz.    Impulse response is excellent, and distortion (as measured in the ground plane test) was <2% above 30Hz and rising over 10% only below 20Hz.

But yes, you have to do all those things .... compensate, add drivers, enough power ... and the servo (which of course you can do in a sealed or ported sub too) helps to control the lighter cones with the net result being a very fast impulse response.

In the end what is hoped is that these subs provide a similar experience to the planar dipole Magnepan experience, with more dynamic range and clarity and less room interaction.

Distortion, Impulse and Phase for Hi, Med, Lo servo (light, middle, dark)

View AttachmentView AttachmentView Attachment

[marcl]

Sept 23, 2024 10:05:13 GMT -5 KeithL said:

Flipping the wires will result in a phase shift of exactly 180 degrees at all frequencies.
I would expect subs that have a physical "0 / 180 switch" to be doing the same thing electronically.

But I wouldn't expect anything anywhere near that consistent from any sort of "variable control".
A fancy DSP-based network could probably come close - depending on exactly how it's done - but still isn't going to be perfectly linear.
(But the simple analog "all-pass shift networks" you find on most subs are not going to be even close...)

And, MarcL, in your case, I would expect the servo to add a whole bunch of complications as well.
The signal from the feedback coil may be phase shifted from the main signal... and there is going to be stability compensation in the servo itself.
(The servo's feedback MUST remain "negative" at all frequencies or the system will be instable and turn into a giant buzzer.) 

Sept 23, 2024 9:41:18 GMT -5 marcl said:

Of course my goal with this build is to improve the sound for music and movies.  But also I really want to quantify a lot of things that have never been clearly documented about how dipole subs work in general and specifically how this design with this servo amp works.  So I'm optimizing along the way but also doing extra tests to quantify what each setting does.

Sure, listening is important.  But many, many people ONLY listen and never measure.  And I know from doing both, that I can listen on two different days or even morning and evening the same day and have it sound different ... and I changed NOTHING.  But what would someone say?  "Oh that's because the noise in your power line was lower at night ... or on Sunday night vs Monday morning".   No ... not a chance ... it's psychoacoustics and brain stress and sensory distraction ....

So I do not make random changes and try to decide if I hear a difference.  I make changes based on a hypothesis, measure the result, see if the result confirms the hypothesis ... and THEN I listen to see if the expected audible effect of the hypothesis is evident in the listening.  

Anyway, what are the most important things to do when integrating subs?

1. With more than one sub they must be time and level aligned to each other.  That means the same level at the MLP and with an impulse response measurement that starts playing at the same time.
2. When crossing between a main speaker and sub for Bass Management, the goal is to align the phase of the two speakers at the crossover point.  Phase changes with frequency so they will not be in phase at every frequency, but you try to align them at the crossover.

Phase, delay and polarity are terms that should be carefully and separately used, as is appropriate.

Right now if I measure my left front speaker and the left sub ... the polarity is the same with the phase control of the sub at 180.  The levels are matched.  Response is smooth across the 60Hz crossover point.  But it appears that the front speaker is playing 3ms ahead of the sub, and there's nothing I can do about that in my current setup.   

Today I have a few ideas for testing, and the first thing I'm going to do is see if switching the wires on the sub does exactly the same thing as turning the phase knob from 0 to 180.

Yeah so that's why I'm doing extra tests to see the effects.   Right now the subs are where I expect to keep them ... around 7ft from the front wall and 3ft from the side wall.  All of this testing is with the 14Hz extension filter, 10Hz rumble filter, and damping on Hi.

I have to keep reminding friends that this sub does NOT have any DSP.

[marcl]

Sept 23, 2024 10:05:13 GMT -5 KeithL said:

Flipping the wires will result in a phase shift of exactly 180 degrees at all frequencies.
I would expect subs that have a physical "0 / 180 switch" to be doing the same thing electronically.

But I wouldn't expect anything anywhere near that consistent from any sort of "variable control".
A fancy DSP-based network could probably come close - depending on exactly how it's done - but still isn't going to be perfectly linear.
(But the simple analog "all-pass shift networks" you find on most subs are not going to be even close...)

And, MarcL, in your case, I would expect the servo to add a whole bunch of complications as well.
The signal from the feedback coil may be phase shifted from the main signal... and there is going to be stability compensation in the servo itself.
(The servo's feedback MUST remain "negative" at all frequencies or the system will be instable and turn into a giant buzzer.) 

Sept 23, 2024 9:41:18 GMT -5 marcl said:

Of course my goal with this build is to improve the sound for music and movies.  But also I really want to quantify a lot of things that have never been clearly documented about how dipole subs work in general and specifically how this design with this servo amp works.  So I'm optimizing along the way but also doing extra tests to quantify what each setting does.

Sure, listening is important.  But many, many people ONLY listen and never measure.  And I know from doing both, that I can listen on two different days or even morning and evening the same day and have it sound different ... and I changed NOTHING.  But what would someone say?  "Oh that's because the noise in your power line was lower at night ... or on Sunday night vs Monday morning".   No ... not a chance ... it's psychoacoustics and brain stress and sensory distraction ....

So I do not make random changes and try to decide if I hear a difference.  I make changes based on a hypothesis, measure the result, see if the result confirms the hypothesis ... and THEN I listen to see if the expected audible effect of the hypothesis is evident in the listening.  

Anyway, what are the most important things to do when integrating subs?

1. With more than one sub they must be time and level aligned to each other.  That means the same level at the MLP and with an impulse response measurement that starts playing at the same time.
2. When crossing between a main speaker and sub for Bass Management, the goal is to align the phase of the two speakers at the crossover point.  Phase changes with frequency so they will not be in phase at every frequency, but you try to align them at the crossover.

Phase, delay and polarity are terms that should be carefully and separately used, as is appropriate.

Right now if I measure my left front speaker and the left sub ... the polarity is the same with the phase control of the sub at 180.  The levels are matched.  Response is smooth across the 60Hz crossover point.  But it appears that the front speaker is playing 3ms ahead of the sub, and there's nothing I can do about that in my current setup.   

Today I have a few ideas for testing, and the first thing I'm going to do is see if switching the wires on the sub does exactly the same thing as turning the phase knob from 0 to 180.

Okay now laugh at me Keith .... I forgot that last bit when I switched the drive wires I forgot to also flip the servo wires ... yep, a fortunately brief and not destructive GIANT BUZZER!

But here is the answer ... what is the difference between turning the phase knob from 0 to 180 vs just switching the +/- wires?   Switching the +/- wires results in a symmetrical flip of polarity .... turning the Phase from 0 to 180 does not.  I'm not surprised.


Now the question is .... which way is correct, with respect to getting the sub better aligned with the left speaker at the crossover?

[ttocs]

Sept 23, 2024 9:41:18 GMT -5 marcl said:

Anyway, what are the most important things to do when integrating subs?

1. With more than one sub they must be time and level aligned to each other.  That means the same level at the MLP and with an impulse response measurement that starts playing at the same time.

Time alignment is paramount!

Gain level being the same for all subs is where I differ with many. When using subs in multiple locations I use each location's strength and try to minimize each location's weakness when possible. To do this sometimes means to adjust a specific subwoofer's gain level to help the frequency response when this sub is joined with others in a group.

I've tried so many scenarios I've gotten kinda used to what to expect to some degree when tweaking in one way or another. So I do realize there are some limits in adjustments which have a point beyond which things go backwards or haywire. So some degree of finesse is very helpful.

With my two stacks of 4 subs each, the bottom two are set to the same level in one stack, but to a different level in the other stack. This is due to the location of each stack, one stack having much better very low frequency response - so I make good use of this to compensate for the other stack being weak in the same range. Then the top pair of subs is set to a higher gain level than the bottom pair of subs in the same stack, but not the same setting in each stack. There is a setting on my subs that came in handy, and it's called 20-30Hz adjustment. It simply boosts or cuts the range of 20-30Hz by as much as 10dB either way. So on the bottom two subs in each stack this adjustment is boosted way up, and the overall gain level of each sub is turned down but not as much as the 20-30Hz range is boosted. The overall effect is that the range from 20-30Hz is a few dB higher than the top subs after factoring in the volume level of the subs. This way, I get four subs playing 20-30Hz louder, plus, the other four subs cover this range in a less prominent way so they can be a bit better at the upper range of bass.

So one question I may get is, are both stacks the same SPL at the MLP? I don't know, and I don't care. In reality it's probably really close, but it's a mixed bag with multiple ranges of frequencies being output at various levels throughout the eight subs. So in the end, I guess I could measure each stack's FR and compare. Maybe someday I might just do that.

To put this into an easy to digest way, here's a graphic showing the Volume Level of each sub (yes, this is the wording used for this brand of sub). This is an accurate depiction of the front wall of the room, so that's why the system is offset to the right.

Sept 23, 2024 9:41:18 GMT -5 marcl said:

When crossing between a main speaker and sub for Bass Management, the goal is to align the phase of the two speakers at the crossover point.  Phase changes with frequency so they will not be in phase at every frequency, but you try to align them at the crossover.

This is exactly what I did when I had a different setup from what I have now. Each stack of two subs each were next to each front speaker and aligned to that speaker only. Then, I combined both front stacks with a third stack in the rear of the room and separately aligned the three stacks to each other using a different method so the six subs would provide BM for all the Small channels, and LFE. And while I really liked this setup the best until this current setup, it couldn't handle summed bass very well and required a lot of effort to mostly overcome this shortcoming.

My current setup far exceeds all other attempts, even though it's not perfect either. I'm in that last tiny percentage of possible improvement with the components I currently have at my disposal, so I'm very happy!

[marcl]

Sept 23, 2024 11:47:23 GMT -5 ttocs said:

Sept 23, 2024 9:41:18 GMT -5 marcl said:

Anyway, what are the most important things to do when integrating subs?

1. With more than one sub they must be time and level aligned to each other.  That means the same level at the MLP and with an impulse response measurement that starts playing at the same time.

Time alignment is paramount!

Gain level being the same for all subs is where I differ with many. When using subs in multiple locations I use each location's strength and try to minimize each location's weakness when possible. To do this sometimes means to adjust a specific subwoofer's gain level to help the frequency response when this sub is joined with others in a group.

I've tried so many scenarios I've gotten kinda used to what to expect to some degree when tweaking in one way or another. So I do realize there are some limits in adjustments which have a point beyond which things go backwards or haywire. So some degree of finesse is very helpful.

With my two stacks of 4 subs each, the bottom two are set to the same level in one stack, but to a different level in the other stack. This is due to the location of each stack, one stack having much better very low frequency response - so I make good use of this to compensate for the other stack being weak in the same range. Then the top pair of subs is set to a higher gain level than the bottom pair of subs in the same stack, but not the same setting in each stack. There is a setting on my subs that came in handy, and it's called 20-30Hz adjustment. It simply boosts or cuts the range of 20-30Hz by as much as 10dB either way. So on the bottom two subs in each stack this adjustment is boosted way up, and the overall gain level of each sub is turned down but not as much as the 20-30Hz range is boosted. The overall effect is that the range from 20-30Hz is a few dB higher than the top subs after factoring in the volume level of the subs. This way, I get four subs playing 20-30Hz louder, plus, the other four subs cover this range in a less prominent way so they can be a bit better at the upper range of bass.

So one question I may get is, are both stacks the same SPL at the MLP? I don't know, and I don't care. In reality it's probably really close, but it's a mixed bag with multiple ranges of frequencies being output at various levels throughout the eight subs. So in the end, I guess I could measure each stack's FR and compare. Maybe someday I might just do that.

To put this into an easy to digest way, here's a graphic showing the Volume Level of each sub (yes, this is the wording used for this brand of sub). This is an accurate depiction of the front wall of the room, so that's why the system is offset to the right.
View Attachment

Sept 23, 2024 9:41:18 GMT -5 marcl said:

When crossing between a main speaker and sub for Bass Management, the goal is to align the phase of the two speakers at the crossover point.  Phase changes with frequency so they will not be in phase at every frequency, but you try to align them at the crossover.

This is exactly what I did when I had a different setup from what I have now. Each stack of two subs each were next to each front speaker and aligned to that speaker only. Then, I combined both front stacks with a third stack in the rear of the room and separately aligned the three stacks to each other using a different method so the six subs would provide BM for all the Small channels, and LFE. And while I really liked this setup the best until this current setup, it couldn't handle summed bass very well and required a lot of effort to mostly overcome this shortcoming.

My current setup far exceeds all other attempts, even though it's not perfect either. I'm in that last tiny percentage of possible improvement with the components I currently have at my disposal, so I'm very happy!

Of course I only ever had two subs, but I did tweak the levels for best overall response after I initially got them equal.  And of course when I had the two subs playing through a miniDSP I time-aligned them.  Then I let Dirac align them to the rest of the system.

But now the two subs are equidistant from and in front of the MLP.  They're 8'4" away and symmetrically placed so they are aligned to each other.  They also sit a good bit more forward than the L/R speakers, even given my Rooze arrangement.  But the wave from the L/R still arrives first by 2.5-3ms.   I don't have a way to get the L/R exactly aligned with their subs, but it's real close.

Now that I switched the wires to get flipped polarity vs 180 degree phase shift ... demonstrating that they are not the same thing ... I feel like flipped polarity is the right way to go.

[ttocs]

Sept 23, 2024 12:06:45 GMT -5 marcl said:

Sept 23, 2024 11:47:23 GMT -5 ttocs said:

Time alignment is paramount!

Gain level being the same for all subs is where I differ with many. When using subs in multiple locations I use each location's strength and try to minimize each location's weakness when possible. To do this sometimes means to adjust a specific subwoofer's gain level to help the frequency response when this sub is joined with others in a group.

I've tried so many scenarios I've gotten kinda used to what to expect to some degree when tweaking in one way or another. So I do realize there are some limits in adjustments which have a point beyond which things go backwards or haywire. So some degree of finesse is very helpful.

With my two stacks of 4 subs each, the bottom two are set to the same level in one stack, but to a different level in the other stack. This is due to the location of each stack, one stack having much better very low frequency response - so I make good use of this to compensate for the other stack being weak in the same range. Then the top pair of subs is set to a higher gain level than the bottom pair of subs in the same stack, but not the same setting in each stack. There is a setting on my subs that came in handy, and it's called 20-30Hz adjustment. It simply boosts or cuts the range of 20-30Hz by as much as 10dB either way. So on the bottom two subs in each stack this adjustment is boosted way up, and the overall gain level of each sub is turned down but not as much as the 20-30Hz range is boosted. The overall effect is that the range from 20-30Hz is a few dB higher than the top subs after factoring in the volume level of the subs. This way, I get four subs playing 20-30Hz louder, plus, the other four subs cover this range in a less prominent way so they can be a bit better at the upper range of bass.

So one question I may get is, are both stacks the same SPL at the MLP? I don't know, and I don't care. In reality it's probably really close, but it's a mixed bag with multiple ranges of frequencies being output at various levels throughout the eight subs. So in the end, I guess I could measure each stack's FR and compare. Maybe someday I might just do that.

To put this into an easy to digest way, here's a graphic showing the Volume Level of each sub (yes, this is the wording used for this brand of sub). This is an accurate depiction of the front wall of the room, so that's why the system is offset to the right.
View Attachment



This is exactly what I did when I had a different setup from what I have now. Each stack of two subs each were next to each front speaker and aligned to that speaker only. Then, I combined both front stacks with a third stack in the rear of the room and separately aligned the three stacks to each other using a different method so the six subs would provide BM for all the Small channels, and LFE. And while I really liked this setup the best until this current setup, it couldn't handle summed bass very well and required a lot of effort to mostly overcome this shortcoming.

My current setup far exceeds all other attempts, even though it's not perfect either. I'm in that last tiny percentage of possible improvement with the components I currently have at my disposal, so I'm very happy!

Of course I only ever had two subs, but I did tweak the levels for best overall response after I initially got them equal.  And of course when I had the two subs playing through a miniDSP I time-aligned them.  Then I let Dirac align them to the rest of the system.

But now the two subs are equidistant from and in front of the MLP.  They're 8'4" away and symmetrically placed so they are aligned to each other.  They also sit a good bit more forward than the L/R speakers, even given my Rooze arrangement.  But the wave from the L/R still arrives first by 2.5-3ms.   I don't have a way to get the L/R exactly aligned with their subs, but it's real close.

Now that I switched the wires to get flipped polarity vs 180 degree phase shift ... demonstrating that they are not the same thing ... I feel like flipped polarity is the right way to go.

It would be nice to see how all these impulses trail off. But to my eye the Sub Phase 180 looks cleaner and with less delay. So that's why I say the way the impulse trails off might be better for the Switched Wires, but it's not in the plot.

ETC might show some things also, as you well know and taught me!

[marcl]

Sept 23, 2024 12:13:43 GMT -5 ttocs said:

Sept 23, 2024 12:06:45 GMT -5 marcl said:

Of course I only ever had two subs, but I did tweak the levels for best overall response after I initially got them equal.  And of course when I had the two subs playing through a miniDSP I time-aligned them.  Then I let Dirac align them to the rest of the system.

But now the two subs are equidistant from and in front of the MLP.  They're 8'4" away and symmetrically placed so they are aligned to each other.  They also sit a good bit more forward than the L/R speakers, even given my Rooze arrangement.  But the wave from the L/R still arrives first by 2.5-3ms.   I don't have a way to get the L/R exactly aligned with their subs, but it's real close.

Now that I switched the wires to get flipped polarity vs 180 degree phase shift ... demonstrating that they are not the same thing ... I feel like flipped polarity is the right way to go.

It would be nice to see how all these impulses trail off. But to my eye the Sub Phase 180 looks cleaner and with less delay. So that's why I say the way the impulse trails off might be better for the Switched Wires, but it's not in the plot.

ETC might show some things also, as you well know and taught me!

Flipped polarity in orange, 180 degree phase knob in pink.

ETC

Phase

[marcl]

Timely video from Matthew Poes

[marcl]

The case for stereo subs ... with qualifications ...

Yes I have ranted there is no such thing as stereo bass. But I have a hypothesis.

Aside from the fact that vinyl records have only mono bass below 200Hz and a digital source made from a vinyl master would also have mono bass … the assertion has been that humans can’t discern the direction of low frequencies below 80Hz. That’s the premise. Well I propose that this could be for a couple of reasons (assuming there IS actual bass content in stereo below 80Hz … which these days, there could be) …

First, most rooms are not treated with bass traps and modal resonances dominate … especially below 100Hz, even if there are bass traps. There are so many reflections from every surface of the room coming from every direction, how could someone distinguish the source from an early reflection? And this is exacerbated by the fact that conventional subs – ported or sealed – radiate an omnidirectional spherical wave that bounces off of every surface. Despite discussions of whether a sub sounds better “firing” at the wall or into the room … that’s not what’s going on. Pointing a sub at the wall or into the room just moves the acoustic center of the driver closer or farther from the wall, which changes how the bass is reinforcing the modal resonances.  It's still omnidirectional.

Given that … an OB dipole sub is not omnidirectional … a spherical wave of opposite polarity propagates from the front and back, while the sides and top and bottom radiate very little due to dipole cancelation … thus engaging possibly only 1/3 of the room resonances. I just walked around one of my subs with a mic and SPL meter and the level several inches away on the side at 40Hz was -25db below the level in front of the drivers. And also my particular room has a lot of bass traps. So … I wonder if it IS possible to distinguish which sub is playing bass below 80Hz.

I’ll do this double blind sometime when I have a suitable subject. But for today I sat on the couch with the REW signal generator on the TV and I played sine waves. I checked first with the RTA that there were not any harmonics … just the fundamental. And I didn’t play the tones real loud … just maybe 70db. I found conclusively that I could distinguish left and right at 60 and 50. I believe also at 40. Not sure about 30 … and at 20 there is some harmonic distortion coming into play.

I think that given the musical content between 40 and 60 it’s worth leaving my subs stereo. And note that bass management and LFE are still mono.


p.s. this does not contradict my long ranted premise that if you have two subs they should be placed as Welti said … center of the front and back of the room. The context of that was valid for sealed or ported subs with the goal of canceling some of the modal resonances. OB subs have to go away from the walls, and and their best spot is the 25% point. Actually … that’s also the best place for sealed and ported subs … but nobody will do that 😊

p.p.s.  Note that the two Outlaw subs still shown in the diagram front and back of the room are no longer connected.

[marcl]

Puzzled about the difference between setting the phase control to 180 to fix inverted polarity in the subs, vs switching the +/- wires on the speakers and setting the phase control to 0 ... I asked the guy at Rythmik about it and here's what he said:

"180 degrees phase shift via phase knob is not same as invert the polarity. There is no analog circuit that can do that. The phase control is to adjust the phase of the upper end for crossover purpose."

Aha!  So since I had switched the wires and had the phase set to 0, I measured the L/R fronts and the L/R subs and compared phase at the 60Hz crossover point ... off by 30-40degrees.  So I dinked the phase control of each sub up a bit until I had both subs and both fronts aligned at 60Hz.  I could not do that until I had the polarity fixed.


p.s.  The Rythmik guy rightly suggested that I find out why the polarity was reversed.  Well ... the speakers were wired up right from their connector to the drivers, because I tested them with a non-servo amp and they were correct polarity.  I had double checked the wiring on the connectors from the amps, and they were wired correctly, and both the same (so I didn't make a mistake on one and not the other).  So I think it had to be an internal wiring error in the amps.  No harm ... I fixed it.

[KeithL]

Physical phase on drivers is easy to test... connect a plain old D-battery to the terminals.
When you connect the + on the battery to the + on the speaker the cone should move OUT.
(The voltage on a D-cell is low enough that it won't push enough current to bother the speaker.. but I would only connect it for a second.)
(Of course this only works with cone drivers that show visible motion.)

With analog circuitry what you're looking at is some sort of "all-pass network"...
It's basically a filter that produces phase shift without affecting the frequency response...
They're tricky to design for a fixed phase shift over the entire audio band...
And it gets more interesting if you want to make it variable...
(To do it right you need to vary several matched resistors... and it still won't be perfect.)

Here's a lovely explanation of how to design one...
chrome-extension://efaidnbmnnnibpcajpcglclefindmkaj/https://www.analog.com/media/en/training-seminars/tutorials/MT-202.pdf

And here's a simple one:
sound-au.com/project103.htm

Sept 24, 2024 15:19:56 GMT -5 marcl said:

Puzzled about the difference between setting the phase control to 180 to fix inverted polarity in the subs, vs switching the +/- wires on the speakers and setting the phase control to 0 ... I asked the guy at Rythmik about it and here's what he said:

"180 degrees phase shift via phase knob is not same as invert the polarity. There is no analog circuit that can do that. The phase control is to adjust the phase of the upper end for crossover purpose."

Aha!  So since I had switched the wires and had the phase set to 0, I measured the L/R fronts and the L/R subs and compared phase at the 60Hz crossover point ... off by 30-40degrees.  So I dinked the phase control of each sub up a bit until I had both subs and both fronts aligned at 60Hz.  I could not do that until I had the polarity fixed.
View Attachment

p.s.  The Rythmik guy rightly suggested that I find out why the polarity was reversed.  Well ... the speakers were wired up right from their connector to the drivers, because I tested them with a non-servo amp and they were correct polarity.  I had double checked the wiring on the connectors from the amps, and they were wired correctly, and both the same (so I didn't make a mistake on one and not the other).  So I think it had to be an internal wiring error in the amps.  No harm ... I fixed it.

[marcl]

Stable and operational ... the Open Baffle Dipole Subwoofer project is fully implemented and it sounds .... 🤪😁😲

It's inevitable that there may be some tweaks, but nothing in the plan right now. Aesthetically, I'm waiting for some non-reflective black foil to cover the magnets on the center drivers ... and possibly the sides of the cabinets to reduce reflection from the TV ... if it works.


Flat to 16Hz, -3db at 14Hz    Distortion under 2%, generally under 1%, all the way down to 20Hz

[ttocs]

Sept 25, 2024 7:16:16 GMT -5 marcl said:

Stable and operational ... the Open Baffle Dipole Subwoofer project is fully implemented and it sounds .... 🤪😁😲

It's inevitable that there may be some tweaks, but nothing in the plan right now. Aesthetically, I'm waiting for some non-reflective black foil to cover the magnets on the center drivers ... and possibly the sides of the cabinets to reduce reflection from the TV ... if it works.

Impressive Impulse and Waterfall !

[marcl]

Tuesday evening (after some psychiatric counseling from ttocs ) I completed a Dirac calibration and determined that the OB subs are well-integrated and the system sounds and measures great.   

Now ... I'll quantify some differences between the OB Dipole subs and the two Outlaw ported subs.  And I'll qualify my observations with the obvious fact that a different pair of ported or sealed "conventional" subs might test and sound similar in some ways.  But there are some unique characteristics of OB Dipole subs that are just not possible with conventional sealed or ported subs.

The specific setup for the OB subs has them 7ft from the front wall and 3ft from the side wall, next to their respective Magnepan 3.7.  The Outlaw Ultra X12 12" sub was center of the front wall, and the Outlaw LFM-1 Compact 10" (nearly) center of the back wall.  With the OB subs, when they play left and right channels, <60Hz goes to the L/R subs respectively.  When they play LFE, both OB subs play together.

So ... I'll have to do two or three posts due to the limitation of attachments, and I'll start with the time-related aspects of Impulse Response, Step Response and Energy Time Curve.  The design of the OB Dipole subs including servo controlled drivers and the Rythmik A370 amp should result in a more immediate response when a bass note is played, and a more damped and controlled decay ... stopping the drivers' motion when the note stops playing.   Here are three comparisons with the OB subs playing together on the left, and the front and rear Outlaw subs playing together on the right.

In the impulse response you see the left and right speakers (which cross over to each sub at 60Hz) compared to the LFE, which is both subs playing.  The OB subs respond instantly, while there is a significant lag with the Outlaw subs.  Step response shows instantaneous response from the OB subs and more uniform response after.  Similarly the Energy Time Curve reflects a fast rise and uniform decay.

Since each of the L/R speakers crosses to its OB sub at 60Hz, it's relevant to look at the step response of them too as the sub low frequencies will be trailing off after the the response of the Magnepan 3.7.  Here I'm showing just the left channel, and you can see the tweeter, midrange and bass panels of the 3.7 respond in order with a similar shape in both plots.  But once the subs kick in it's a very different story.  On the left the OB subs are well damped while the Outlaw subs are still bouncing along.


More in the next post ....

[marcl]

Now let's move on to distortion.

The first image shows the left and right (blue/pink) and LFE Total Harmonic Distortion.  The difference is quite dramatic, with THD rising quickly below 70Hz with the Outlaw subs. With the OB subs, it stays essentially below 2% and is especially low in the LFE all the way down below 30Hz.


Looking at just LFE, and the THD along with 2nd and 3rd harmonics, we see more detail.


Comparing distortion of just the left channel, with the Magnepan 3.7 crossing to the OB subs at 60Hz and the Outlaw subs at 50Hz, there is a dramatic difference in distortion below 70Hz with the Outlaws having considerable 3rd order harmonics contributing to the THD.  This is very significant because we are more sensitive to odd order harmonic distortion and it is subjectively more irritating.


And since I have some room here, let's look at Group Delay.  The plots show left, right and LFE ... but only the left and right have Group Delay around 40Hz, and it is significantly less with the OB Subs.  This is understandable because the OB subs are physically very close to their corresponding 3.7 speaker.  I can't get them exactly aligned without having a delay on the 3.7, but they are within 3ms.

[marcl]

And finally, some pretty colors ... Waterfalls and Wavelets!

These are interesting and I think I have an explanation for a particular difference.  First the LFE.  Well both waterfall plots are pretty good.  You might say with the Outlaws the decay below 60Hz and approaching 30Hz is a little slower.  The big difference I see is the long decay of the OB subs at 20Hz compared to the Outlaws.  Here's what I think is going on there.  The reason we put two conventional subs at opposite ends of the room is to cancel some of the modal resonances.  I think this accounts for the Outlaws canceling a lot of the 20Hz, which is the fundamental length axial resonance mode.  The OB subs do not excite most of the room resonances to the side, top and bottom ... but they do play right down the alley of the length mode, and with both at the same end of the room they don't cancel that mode like the Outlaws do


Looking at the right channel Waterfall though, you can see that the fact that the OB subs do not excite as many room resonances does result in improved response compared to the Outlaw.  Note that this is just the right OB sub playing with the 3.7 below 60Hz, not both subs.  In particular, the resonance at 40Hz is lower.


And finally the Wavelet.  This is a function option in the REW Spectrogram.   Red is high amplitude and blue is low, with time on the Y axis.  So even with both sub arrangements having a big peak at 20Hz, the OB subs settle down much more quickly.


p.s.   Someone might ask, why not put the 12" Outlaw sub in the back of the room and play it along with the OB subs so that it cancels some of the resonances.  Yeah I thought of that, then realized that since the Outlaw sub would be considerably further away and is inherently "slower" I would have to run all three through a miniDSP and slow down the OB subs to align with the Outlaw.  I'll live with some boom at 20Hz rather than compromise the rest.

[marcl]

One last comparison ... I put the Outlaw Ultra X-12 sub in the center of the 11x15x8ft den where I had built and tested the OB sub.  I measured both at 3ft.



I separated the traces for clarity, but you can see how dramatically the OB Sub does not energize the resonance modes of the room.

This is with no PEQ. The OB sub has servo on Hi, 14Hz extension filter, no rumble filter (green). Outlaw sub is in Max Extension mode with one port plugged (pink).

[leonski]

When you finally decided you wanted to go 'open baffle', what parameters did you look at for the woofers? Or what ratio of TS parameters makes for a good open baffle driver?

[adaboy]

Can post a video of those babies moving air?