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Author Topic: Push pull double 18  (Read 9785 times)

Ivan Beaver

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Re: Push pull double 18
« Reply #20 on: March 20, 2018, 05:39:24 pm »


My take away was I couldn't detect the difference with the LAB 12's until after they exceeded Xmax, and then the PP sounded a little worse. If you tend to push the drivers past Xmax, having them sound worse when they do help you "put on the brakes" before they break ;^).
But that is ONLY if you are paying attention to the sound.

Many people don't pay any attention, until it stops.  And then it is to late. :(
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David Morison

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Re: Push pull double 18
« Reply #21 on: March 21, 2018, 04:07:17 am »

Here's a distortion sweep at 120dB made the same day, that gets above the noise floor a little. 
Look at the port shelf talking  ;D   It's why i stopped...

Hi Mark,
Could you help interpret that a little please?

The curves seem to show that 2nd harmonic has the highest distortion, followed by 3rd etc, pretty much as expected.
However the trace IDs below that have an oddly high percentage figure for the 4th harmonic even though that trace appears lower than 2nd & 3rd - is there an explanation for that please?

Thanks,
David.
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Mark Wilkinson

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Re: Push pull double 18
« Reply #22 on: March 21, 2018, 08:51:09 am »

Hi Mark,
Could you help interpret that a little please?

The curves seem to show that 2nd harmonic has the highest distortion, followed by 3rd etc, pretty much as expected.
However the trace IDs below that have an oddly high percentage figure for the 4th harmonic even though that trace appears lower than 2nd & 3rd - is there an explanation for that please?

Thanks,
David.

Hi David, I don't really have a good interpretation, other than the observation the box was audibly buzzing/rattling at the bottom of REW's sine sweep. 
The shelf port was just screwed in, albeit fairly tightly, for tuning verification (30Hz target). 120dB isn't that loud, but it was obvious it was loud enough to rattle the port without bracing or glue. The buzz rattle sounded like low 100Hz ish....maybe 4th order??   
I only took the two sweeps posted, one at 110dB, then at 120. If 120 had stayed clean, I would have tried for 130 just being anxious to get a preliminary look.. 
It was kinda silly for me to post any distortion plots before bottling her up, but all the 2nd order reduction talk lured me in  :)

I swear, I think I spend as much time and work doing bracing and cursed grills, as box design/construction.  What a comparative pain.
Hope to finish up port bracing today.

But hey, the second nicest thing I like about this about this sub, is the driver grill is only 12 1/2" x 20" !!!
Maybe I'll finally get one not to rattle first try ...

Do you have any particular techniques you like for discerning resonances?  Slow manual sine sweeps is about the best technique I've found so far.
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Uwe Riemer2

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Re: Push pull double 18
« Reply #23 on: March 21, 2018, 10:33:28 am »

Mark,
you are putting a lot of effort into this design, but did you consider the effect of gravity on the drivers in the long run?

I would, because I have considerable experience in repair of a triple chamber bandpass design with the same driver orientation. The loss of center position of otherwise fine drivers hurts every time I see it.

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Mark Wilkinson

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Re: Push pull double 18
« Reply #24 on: March 21, 2018, 01:38:38 pm »

Mark,
you are putting a lot of effort into this design, but did you consider the effect of gravity on the drivers in the long run?

I would, because I have considerable experience in repair of a triple chamber bandpass design with the same driver orientation. The loss of center position of otherwise fine drivers hurts every time I see it.

Hi Uwe, thanks for the heads up. I've wondered if cone sag might be an eventual problem.

How do you tell when there is a loss in center position?

Do you think loss of center occurs when in storage, when playing, or both?
It's made me wonder how manufactures store their drivers in the wharehouse.....

If this is an issue, and anybody with experience here please do chime in...

I've had the idea that I could build the next ones with slot ports on top and bottom, so the subs could stand on either end, and rotate the hang direction. 
Or I could run the slot vertically, but I'm hoping to minimize width, and going vertical would make the box substantially wider.
But I like what I have unless.......

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Art Welter

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Re: Push pull double 18
« Reply #25 on: March 21, 2018, 02:19:48 pm »

I've wondered if cone sag might be an eventual problem.

1)How do you tell when there is a loss in center position?
2)Do you think loss of center occurs when in storage, when playing, or both?
3)It's made me wonder how manufactures store their drivers in the wharehouse.....
4)If this is an issue, and anybody with experience here please do chime in...
Mark,

I had already mentioned cone sag to you prior to your construction on 3/3/18, was a bit surprised when you oriented the cones horizontally in your PPSL. "Done is done", but  since Uwe brought it up again..
1) You will see the sag in the surround after time stored horizontally.
Distortion will increase as the cone is harder to push up than down. After sag, the driver will require more power to achieve Xmax in the up stroke than the down stroke. Eventually, the driver may hit Xlim (flap, clack, or break) on the down stroke with far less power than in it's original state.
2) Gravity (and rust) never sleeps.
3) Warehouse storage is usually for as short a term as possible, but often the wrong way (horizontally). There are examples of "new old stock" with 1/2" sag posted on line.
4) The softer  and heavier the suspension, the more the problem, and the sooner it becomes problematic.

Your drivers have a fairly light, stiff suspension, but if you have a choice, follow my prior advice re-posted below.

Otherwise, rotate the drivers when you notice a few mm of sag.

Cone sag can also occur vertically, the diagonal sag eventually causing voice coil rubbing problems.
Oh, well...

Cheers,
Art





« Last Edit: March 21, 2018, 02:41:44 pm by Art Welter »
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Mark Wilkinson

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Re: Push pull double 18
« Reply #26 on: March 21, 2018, 04:19:49 pm »

Mark,

I had already mentioned cone sag to you prior to your construction on 3/3/18, was a bit surprised when you oriented the cones horizontally in your PPSL. "Done is done", but  since Uwe brought it up again..
1) You will see the sag in the surround after time stored horizontally.
Distortion will increase as the cone is harder to push up than down. After sag, the driver will require more power to achieve Xmax in the up stroke than the down stroke. Eventually, the driver may hit Xlim (flap, clack, or break) on the down stroke with far less power than in it's original state.
2) Gravity (and rust) never sleeps.
3) Warehouse storage is usually for as short a term as possible, but often the wrong way (horizontally). There are examples of "new old stock" with 1/2" sag posted on line.
4) The softer  and heavier the suspension, the more the problem, and the sooner it becomes problematic.

Your drivers have a fairly light, stiff suspension, but if you have a choice, follow my prior advice re-posted below.

Otherwise, rotate the drivers when you notice a few mm of sag.

Cone sag can also occur vertically, the diagonal sag eventually causing voice coil rubbing problems.
Oh, well...

Cheers,
Art

Hi Art,  the reason I said I've been wondering about cone sag is because of your prior post, I remember it well... :)

This box isn't a done-is-done situation.  It's a prototype for an eventual 4 boxes, to let me see how well the PPSL idea works.
Letting the plenum bolt up directly to the cabinet walls made for really easy construction.  Heck, sawdust is barely over two weeks old. 
Whether I have to build 3 more of the same, or 4 from an altered design, doesn't really matter much at the margin. 

I do like the width and the way the box handles though.
But enough warning about horizontal orientation, and I'll certainly go vertical....

or split the slot port in two, half on top, and half on bottom, where I just rotate the sub every so often...
What do think about that idea?   Thx

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Uwe Riemer2

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Re: Push pull double 18
« Reply #27 on: March 21, 2018, 04:48:47 pm »


How do you tell when there is a loss in center position?

Do you think loss of center occurs when in storage, when playing, or both?
It's made me wonder how manufactures store their drivers in the wharehouse.....

I've had the idea that I could build the next ones with slot ports on top and bottom, so the subs could stand on either end, and rotate the hang direction. 
Or I could run the slot vertically, but I'm hoping to minimize width, and going vertical would make the box substantially wider.
But I like what I have unless.......

Some baskets allow a parallel look along the spider, this is the easiest way to spot it.

Loss of center position occurs in storage and playing, I have no data, what is more severe.
But the benefit of push/pull might be outweighted by the asymmetrical load of gravity.

Build one box with the slot vertically and then compare the two designs,
or use the existing design on the side.

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Art Welter

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Re: Push pull double 18
« Reply #28 on: March 21, 2018, 05:17:27 pm »

I do like the width and the way the box handles though.
..or split the slot port in two, half on top, and half on bottom, where I just rotate the sub every so often...
What do think about that idea?   Thx
If you like the box ergonomics the way it is, build more of the same, and flip 'em every once in a while when stored when you notice sag.

Splitting the bass reflex port would also work OK, and stiffen up the cabinet a bit, but then you have two dirty bottoms instead of one, and upside down plugs half the time...




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Ivan Beaver

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Re: Push pull double 18
« Reply #29 on: March 21, 2018, 06:36:56 pm »

If you like the box ergonomics the way it is, build more of the same, and flip 'em every once in a while when stored when you notice sag.


The old Maryland Sound "clams" (Karlson couplers) would exhibit bad cases of cone sag.

On one side they would be hanging out of the basket, and on the other side they would be depressed into the basket-because of the mounting angles.  Especially in clubs where the were installed (like the old Hammerjacks in Baltimore for example.)
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A complex question is easily answered by a simple-easy to understand WRONG answer!

Ivan Beaver
Danley Sound Labs

PHYSICS- NOT FADS!
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