Acoustic center of ob servo sub driver?

TRADERXFAN · « **on:** 2 Jul 2023, 04:53 pm »

I am about to rebuild my ob servo sub and think i am just going to make them forward firing and adjust the design for my own needs.

If they all face forward then I'm thinking to have the acoustic centers align with the midpoint of the depth instead of the baffle...

Is that a good idea? So the rear wave is equidistant to the front wave?

What is that depth? I was guessing 1/3rd of the 161mm in the drawing for the total depth behind mounting ring... about 2 inches?

So i would put the center baffle 2 inches forward of the midpoint.

Thoughts?

TRADERXFAN · « **Reply #1 on:** 2 Jul 2023, 05:36 pm »

Nevermind, i will just mount them push pull and take advantage of that....

Tyson · « **Reply #2 on:** 2 Jul 2023, 07:32 pm »

Quote from: TRADERXFAN on 2 Jul 2023, 05:36 pm

Nevermind, i will just mount them push pull and take advantage of that....

That’s how mine are on my Super 7’s.

HAL · « **Reply #3 on:** 2 Jul 2023, 08:21 pm »

If you make the center baffle symmetrically placed in the middle of the h-frame you can mount both drivers facing forward.

That is how the 3x12's I took to LSAF2023 were setup in the modular H-Frame OB boxes. Works like a charm. I tried both orientations and not a difference I can hear at those frequencies and I go up to 190Hz.

TRADERXFAN · « **Reply #4 on:** 3 Jul 2023, 05:28 am »

Thanks guys.

I had the dual obs in the W version push- pull I just took them out. I need to re-do it because the bottom of them got damaged and thinking to add a 3rd one.

If I go to 3 drivers per side, then a push/pull/push won't be truly balanced to fully obtain those theoretical cancellation benefits anyway.

Maybe with everything else going on in a room, this is just not big enough to be a noticeable difference, like you found Rich.

Tyson · « **Reply #5 on:** 3 Jul 2023, 05:44 am »

I also have NX Oticas with triple stacked subs in my 2nd system and the subs all face forward and it’s fine.

HAL · « **Reply #6 on:** 3 Jul 2023, 10:33 am »

The 3x12 servo sub stack with the A370XLR3 servo amp setup I use.

mlundy57 · « **Reply #7 on:** 3 Jul 2023, 04:38 pm »

Facing the drivers all the same way also makes wiring them up easier. If a driver is mounted in reverse, both the driver and servo coils have to be wired out of phase with the forward facing drivers. It can be done and there are wiring diagrams available for reverse mounting. I just find it easier not to have to deal with it.

Vince in TX · « **Reply #8 on:** 6 Jul 2023, 04:07 pm »

Mine are facing forward. They sound amazing!

ajaarn · « **Reply #9 on:** 7 Jul 2023, 12:27 am »

What is the effect if you change the shape of the H-frame, but maintain volume from the original calculation / standard design? Side view examples in attached pics.

TRADERXFAN · « **Reply #10 on:** 7 Jul 2023, 01:51 pm »

I definitely think the middle case is a bad idea because you are shortening the path for cancelation, which means less output at certain freqencies.

The 3rd one I am not sure. Changing the symmetry of the front to back wave likely changes the shape of your response output in some ways.

See this for a quick explanation

Quote from: mlundy57 on 26 Jun 2017, 03:59 am

...
They do sound very good. The drivers sound a lot better in this cabinet than they did in the smaller H-Frame that I used under the Wedgies. That's because the wells for each driver were 2" deeper on these cabinets. The deeper wells (8" vs 6") allow these speakers to play lower and louder than they did before. Danny designed the 6" depth for use with the Wedgies which crossed at 200Hz. The higher the crossover point, the shallower the driver cavity need to be. These speakers are being paired with Maggie 3.6's and will be crossed over is in the 60-80Hz region. This meant the driver wells could be deeper which allowed for the improved low end performance.
...

Mike

If you like math you can learn more on that here...
https://www.linkwitzlab.com/models.htm

mlundy57 · « **Reply #11 on:** 7 Jul 2023, 03:43 pm »

Keep in mind the dimensions I discussed that are referenced in TRADERXFAN's above post were regarding two projects both using 8" servo sub drivers in H-frames, being mated with different main speakers that had very different crossover points to the subs.

Triple 8" servo drivers in a shallower H-frame intended for a 200Hz crossover point

Triple 8" servo drivers in a deeper H-frame intended for a 60Hz crossover point

In addition to the H-frame design, there was a wedge shaped cabinet for the three 8" servo drivers designed to be more aesthetically pleasing with the Wedgie monitors

This wedge shaped design was adapted for use with two 12" servo sub drivers to work with the Wedgies

The wedge shaped design for the subwoofer enclosure worked well for it's intended purpose, blend seamlessly with the Wedgie monitor, but gave up some performance to the H-frame design.

TRADERXFAN · « **Reply #12 on:** 7 Jul 2023, 03:54 pm »

I have to say the wedgie design doesn't make sense to me, intuitively... I would be afraid of that one. But if it measures well, I just don't understand it lol.

Hobbsmeerkat · « **Reply #13 on:** 7 Jul 2023, 04:48 pm »

The idea behind the Wedgie was to have a very narrow baffle for improved imaging, essentially a much narrower version of the NX-Otica MTM using 4 of the original LGK drivers instead of the 2 NQ woofers, both of which used servo subs to take over the bottom end below the ~150Hz range.

Their ability produce both a very focused image and wide soundstage is really tough to compare to, but getting them properly crossed over to the subs is crucial for them to sound their best.

TRADERXFAN · « **Reply #14 on:** 7 Jul 2023, 04:52 pm »

But what is the movement/flow of the dipole woofer?
right side positive, left side negative?
are they both positive on each side?

Is the "D" distance from front wave to back wave the depth or the width?

I think they would both be positive and it is the depth - just not sure. If it works it works, just can't understand it right now. Seems like give up a lot in that design over the H or W frames.

Hobbsmeerkat · « **Reply #15 on:** 7 Jul 2023, 05:18 pm »

Quote from: TRADERXFAN on 7 Jul 2023, 04:52 pm

But what is the movement/flow of the dipole woofer?
right side positive, left side negative?
are they both positive on each side?

Is the "D" distance from front wave to back wave the depth or the width?

I think they would both be positive and it is the depth - just not sure. If it works it works, just can't understand it right now. Seems like give up a lot in that design over the H or W frames.

For the Wedgie-shaped subs, think of them as having a "cardioid" radiation pattern instead of a typical figure-8 of a more traditional di-pole subwoofer. Both woofers move "outwards" and inwards at the same time.

Something like this:

So the cancelation is to the back of the subs rather than the sides, it's a design that works well for smaller rooms, but tends loose output in large & open-concept rooms, as their output isn't nearly as "focused" as you would find in a traditional H-Frame sub.

TRADERXFAN · « **Reply #16 on:** 7 Jul 2023, 05:20 pm »

Ah ok. thanks for that.

While I have you - any comment on the acoustic center of the ob 12 drivers? I wanted to put that as the centerpoint in my design... [ the original post question I had here]

Hobbsmeerkat · « **Reply #17 on:** 7 Jul 2023, 05:41 pm »

The "acoustic center" will simply be the center point between 2 drivers playing the same frequency.
for the dual subs, that's the horizontal baffle that separates them, for the triple subs the acoustic center is the middle driver.

Unless you are referring to the "acoustic center" meaning the center of the voice coil?

If that's the case, considering the wavelengths you're talking about with subwoofers, shifting the inside baffle forwards or backwards a few inches won't make any tangible different in performance. Just like there's not any real performance difference between having both forward or opposing.

TRADERXFAN · « **Reply #18 on:** 7 Jul 2023, 05:58 pm »

Yes, asking about the voice coil of a driver. I was estimating 2inches in or so...

Hobbsmeerkat · « **Reply #19 on:** 7 Jul 2023, 07:16 pm »

The center of the voice coil is roughly 3.5" from the mounting frame. Like I mentioned you're talking about an adjustment of inches when the shortest wavelengths it's playing are already several feet long. Its the same effect as moving the whole subwoofer tower 3.5" forwards or backwards.

120 Hz is ~9 feet long
60 Hz is ~18 feet
30 Hz is ~36 feet
15Hz is ~72 feet

Aligning the voice coil only make sense when you're trying to align a horn tweeter and a woofer, especially since the wavelengths are much, much shorter so that way you don't need to use the crossover to adjust the phase. for a subwoofer there's nothing to be gained. especially on the servo subs, which already offer full phase adjustment from 0-180.