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Droogne

Skramlike sub: some design questions.

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I understand wanting to go smaller on the box design. But after the last 3 weekends using the SKRAM with a B&C 21SW152 4ohm and a QSC PL380, I'm convinced that lugging around the box is well worth the output the original SKRAM design puts out. It is heavy yes... it take 2 people to load it onto a snowmobile deck on my truck, yes.... it takes up alot of space in my garage for storage, yes.... but when you crank it up with some Meyer Sound UPJ's, its worth the tried and tested design that Ricci put a ton of time into designing. 

 

If you have a alot of free time on a CNC router and free time in your shop building this and want to experiment then this is why I love this conversation. Can't wait to see your results. My experience with the original design is pretty ear blowing for bass music as well as for all around boring club music.. which I will never let happen again on my system after the shit that was played on it last weekend out of my control....

Ricci, the forum is still limiting me to 0.41 megs of image upload. would love to share photos, but the forum won't let me

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LW1400 has a cone that is rather light and flexible for a 21". I'd be careful putting it into a cabinet which increases the pressures on the cone. 

S1 has a marginal impact in this type of design. Whether 300cm or 30cm. I wouldn't worry about it much. Especially at the cost of making the cab heavier or more complex. 

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9 minutes ago, Ricci said:

LW1400 has a cone that is rather light and flexible for a 21". I'd be careful putting it into a cabinet which increases the pressures on the cone. 

S1 has a marginal impact in this type of design. Whether 300cm or 30cm. I wouldn't worry about it much. Especially at the cost of making the cab heavier or more complex. 

Ok, so a simple vented should be more suitable for the LW1400? 

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On 3/9/2020 at 11:56 AM, peniku8 said:

Do you really think it's an issue with the modern pro drivers? Most pro style cabs are simple vented direct radiators, which are stored on the wheels, so the driver ends up being in a horizontal position most of the time. I've never seen any manufacturer talk about that. If it's really an issue, I think we'd see the advice of storing them differently more often.

It's probably not a huge issue but I have seen a number of drivers with large shifts from coil center. Some were current drivers. I left a JBL 2269H sitting on it's face with 700g+ of magnets on the cone for a year to get it back to center rest position. It was probably 5 or 6mm inwardly set when I got it. It can and does happen. Back in the day it was probably a bigger issue with much shorter xmax and looser suspensions. On a modern woofer with 15mm xmax and a really tight suspension an offset of  1 or 2 mm isn't a big deal. Be more careful on drivers with a softer suspension, shorter xmax and heavier MMS. If the woofer takes a big offset it will still play but the sound may be degraded somewhat and the functional xmax shortened. If you can't hear it does it matter though...?

I guess my point was ideally you'd keep the driver vertical, but sometimes the tradeoffs for that outweigh a horizontal driver position. I wouldn't worry about it too much. It's way down the list of priorities IMO. 

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13 minutes ago, Droogne said:

Ok, so a simple vented should be more suitable for the LW1400? 

Yes. 

It's not that the LW1400 would not work, but it would not be my first choice and I would want to be careful with the overall volume until figuring out what it could take. 

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4 hours ago, Ricci said:

Yes. 

It's not that the LW1400 would not work, but it would not be my first choice and I would want to be careful with the overall volume until figuring out what it could take. 

Well, then I'll build the x2 subs for the NLW9600 and test the LW1400 in it too. Otherwise I could just sell them, or build some sort of cab (with the LW1400) to support the NLW9600. 

 

18 hours ago, Tahoejmfc said:

I understand wanting to go smaller on the box design. But after the last 3 weekends using the SKRAM with a B&C 21SW152 4ohm and a QSC PL380, I'm convinced that lugging around the box is well worth the output the original SKRAM design puts out. It is heavy yes... it take 2 people to load it onto a snowmobile deck on my truck, yes.... it takes up alot of space in my garage for storage, yes.... but when you crank it up with some Meyer Sound UPJ's, its worth the tried and tested design that Ricci put a ton of time into designing. 

 

If you have a alot of free time on a CNC router and free time in your shop building this and want to experiment then this is why I love this conversation. Can't wait to see your results. My experience with the original design is pretty ear blowing for bass music as well as for all around boring club music.. which I will never let happen again on my system after the shit that was played on it last weekend out of my control....

Ricci, the forum is still limiting me to 0.41 megs of image upload. would love to share photos, but the forum won't let me

Thanks for reminding me to look back at the Skram. I have been modelling so much I lost track where it all/I began. I particulary noted the larger volume (which means reduced porth length) which helped boost the low end. Secondly, it made me realise I could still achieve my idea of a modular sub existing of 2 parts (1 part driver+fronthorn+ports and 1 part pure volume). The connecting opening in the 2 boxes would also be where the driver will be inserted from). I will post a pic of this design tomorrow. 

Another 'advantage' will be an extra way to modify the setup. 

1: tunable slot by splitting it up in 4. 

2: modifiable volume.

This is of particular interest to me, as I can now do this:

FULL SETUP. Tuned to 31hz. 130db +-0dB down to 32hz. Horn mouth velocities around 8m/s at RMS.

image.png.d321e291f27c8c6e2c2ff2942c18ae1e.png

ONLY BOX 1 (no additional volume, so ~140L smaller). Tune also 31hz. 130 +-0dB down to 36hz. Increased port velocities (below 36hz so compression less crucial, XO can help to reduce noise if present)

image.png.ace608248d498480abd604d912aaadc1.png

This smaller version is clearly inferior, and I would never build the sub like this. However, I have multiple situations where we would quickly want to bring a single sub, without too much hassle). Its output is still overkill for a lot of situations like this (I'm thinking homewarmings/parties etc).

 

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image.png.4f2bcf8fef91dff61998f684589694dc.png

Final design @Ricci @peniku8. EDIT: now with accurate drawing 

MAIN BOX

So a 65 (depth) x 60 (width) x 85cm (height) main box, which will accomodate all the vital parts (driver, front horn and port). There is around +10cm of clearance behind the driver. Port in yellow, horn moth opening in green. 

 

SECONDARY BOX

A second box (in yellow) measuring 65 x 60 x35cm to add volume. They are in connection via the vertical red line. This opening will be just as big as it needs to be to insert the driver into the main box. The 2 boxes will be joined together by 8 clamps and a double of layer of sealing material between (to make them airtight, and to make sure they dont rattle).

image.png.8bfd967003a40a8cf416cbf66eaa4120.png

31hz tune. 350cm² added to S2.

image.png.fce6af9e40fb5a4eb6ef961376aaf66d.png

image.png.53f689f7c04303428543f482844652e0.png

Horn mouth velocity at Xmax limited output at RMS (reduction of the output between 50 and 100hz a reduces it down well below 6m/s). 

 

 

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Designing this thing on paper is pretty hard, I'm sure. Whenever I have a cab idea, I'm putting the whole thing together in Inventor and grab the measurements off the actual model, which I then input into HR. It's a lot of work for a concept which might be really bad, but it leaves very little room for error. This way you can create a very accurate feedback loop to get to the desired result.

What's the actual frequency response of the cab? Did you find semi inductance parameters somewhere?

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34 minutes ago, peniku8 said:

Designing this thing on paper is pretty hard, I'm sure. Whenever I have a cab idea, I'm putting the whole thing together in Inventor and grab the measurements off the actual model, which I then input into HR. It's a lot of work for a concept which might be really bad, but it leaves very little room for error. This way you can create a very accurate feedback loop to get to the desired result.

I have to admit I have no clue about how to use it. Drawing is pretty easy, and quick. As long as I have a design that doesnt change in the depth, I can do it 2D. 

34 minutes ago, peniku8 said:

What's the actual frequency response of the cab? Did you find semi inductance parameters somewhere?

I also have to admit I cant find them.. I know it is not ideal, but no way around it. Everything I read is that the NLW9600 is a very good driver. 

Normal FR

image.png.49a785babdadd6885c79c6277ddda573.png

 

LaVoce SAN 214.50 with Le activated for reference:

image.png.7ab841d109e4b46b77ab5111d7de9762.png

image.png.e017249116df12a471d157c77af387d4.png

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(For the LaVoce/DS115 (for example in the future) I would use a somewhat bigger front horn to boost that dip between 50 and 80hz. Would result in around +-1dB extra output in general.)

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A couple of things that I see to double check.

Not sure the horn section has 133cm of length? It's a little hard to judge from the drawing scale. Could be close.  

Also seems like the L12 length of 37cm is possibly a little too long. SKRAM is 28cm. Is there an extra 9cm length there? Again hard to tell for sure from the drawing. 

I think you may need 2 or 3cm more on the overall height to get the overall volumes in the sim. Depends on bracing, etc... though. 

Vent doesn't look long enough to effectively be 96cm to me? Based on your dims and port area provided the panel that connects at 90deg to the main vent panel would need to be about 26cm long to make a 96cm effective vent length. 

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I'm unsure if the drawing is accurately to scale, but 90cm of port length seems close enough if you add half of the port height.

This is a very large port. I like to have at least 100cm² per litre of displacement. For the 21DS115 that is 5l and I ended up using ~550cm² in a design that prioritized space efficiency. Low port compression is nice and everything, but I'd try to see if more back chamber volume isn't more useful in this case. I also think a meter of port length is quite a lot. A port this long on a 6th order bandpass would have me keep a close eye on group delay for sure.

The SKHorn essentially uses less than half as much port area with 50% more capable drivers and the compression results are very tolerable until output exceeds 130db from 30Hz and up.

Cover the top part of the cab in Warnex inside out so you can use it as transport box for something else 😊

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1 hour ago, Ricci said:

A couple of things that I see to double check.

Not sure the horn section has 133cm of length? It's a little hard to judge from the drawing scale. Could be close.  

I'll double check. In any case, the length didn't change a lot to the FR/velocities. 

Quote

Also seems like the L12 length of 37cm is possibly a little too long. SKRAM is 28cm.

 

Quote

Is there an extra 9cm length there? Again hard to tell for sure from the drawing. 

S1 does not start at the edge of the woofer, for construction/design reasons. 

Quote

I think you may need 2 or 3cm more on the overall height to get the overall volumes in the sim. Depends on bracing, etc... though. 

Tomorrow I'll do an exact calculation of the volumes taken in by the bracing. I might add some height. Easy because height isn't so much an issue anymore. Because I split up the sub, I can transport it easily, few cm won't change that. 

Quote

Vent doesn't look long enough to effectively be 96cm to me? Based on your dims and port area provided the panel that connects at 90deg to the main vent panel would need to be about 26cm long to make a 96cm effective vent length. 

 

As peniku noted, I added 50% of the height, so 13cm/2. 

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1 hour ago, peniku8 said:

I'm unsure if the drawing is accurately to scale, but 90cm of port length seems close enough if you add half of the port height.

Correct, is exactly what I did. 

Quote

This is a very large port. I like to have at least 100cm² per litre of displacement. For the 21DS115 that is 5l and I ended up using ~550cm² in a design that prioritized space efficiency. Low port compression is nice and everything, but I'd try to see if more back chamber volume isn't more useful in this case.

I'll double check. However, I still think this is not too much or anything. Especially when you close of a part of the port for differential tuning. Also, when taking the port bracing its more like 850 than 920.

Also, my restrictions were not so much space, rather footprint and carryability. Splitting it in 2 and keeping the footprint between 60x65 does that. 

Quote

I also think a meter of port length is quite a lot. A port this long on a 6th order bandpass would have me keep a close eye on group delay for sure.

The SKHorn essentially uses less than half as much port area with 50% more capable drivers and the compression results are very tolerable until output exceeds 130db from 30Hz and up.

True. I modelled based on those results. However, I'll see if I can't make it shorter. Because I need the subs to be 120cm long (as I realised they are perfect speakers stands for my mains) I can't make the subs much smaller. Maybe a better volume vs port size ratio might be good. But the 60x65x120 size is fixed. 

Quote

Cover the top part of the cab in Warnex inside out so you can use it as transport box for something else 😊

Maybe 😛 but if I want to brace it, that will be somewhat harder. 

Maybe I'll open up those second boxes on 2 sides, so I can add more than one box. Interesting. 

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If you reverse the butterfly layout on both subs, you could join the two volume boxes to a simple case. Brace it a little and add some PE foam and you have a mixer case or case for mic stands w/e you're carrying around with you

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20 hours ago, Droogne said:

Correct, is exactly what I did. 

I'll double check. However, I still think this is not too much or anything. Especially when you close of a part of the port for differential tuning. Also, when taking the port bracing its more like 850 than 920. 

I already factored in the extra proximity effect when I made my comments. That panel will need to be about 26cm to get the effective vent length in the sim. Looks like it is about 18 -20cm at most. 

Port bracing should always be accounted for. It reduces your effective vent area and multiple vents also tune a little higher than one big vent. 

At the end of the day we are probably talking a difference of a few Hz difference in the tuning, but even that can make a difference. 

S1 starts at the wall where the panels pinch together. Do you have 8 or 9 cm there before getting to the driver? 

Parallel wiring with speakons depends...You have a few options. You could make a splitter Y cable and run a speakon to each cab direct from the amp. You can also put 2 jacks on a cab and wire the second jack in parallel and use it to daisy chain to a second cab. This is much more common.

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MAIN BOX: 284.6L internal

SECOND BOX: 111.24L internal

TOTAL: 395.8L

 

INTERNAL PANNELS

horn, port: 19.4L

bracing (calculated 10.7L) 15L to give some reserve

DRIVER: 15L

 

395.8 - 49.2 = 346.64L

HR schematic: 359.5L

So yes, I miss around 13L (with a 880cm x 92.6cm port, which was the adjusted value from te original 922.4 to allow the voume taken in by the port bracing.)

 

As I want to keep the size the same, and I think I can manage with a smaller port I adjusted to: 

793cm² x 85cm which brings the HR calculated volume to 345L. 

 

 

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19 hours ago, Ricci said:

I already factored in the extra proximity effect when I made my comments. That panel will need to be about 26cm to get the effective vent length in the sim. Looks like it is about 18 -20cm at most. 

image.png.f2d8d326b18ac6ffb1f89bce0431f61c.png

Ok using the port size listed in the post above (793cm²):

- first part of the port (black line) is 65 (the depth) - 1.8 (backpanel) - 15.50 (height of the port) = 47.7cm

- second part (blue line)/the corner = 15.5cm (port height)

- third part (red line) then should be 85cm - 47.7 - 15.5 - 7.75 (50% of port height) = 14.05cm

Quote

Port bracing should always be accounted for. It reduces your effective vent area and multiple vents also tune a little higher than one big vent. 

How do I take the multiple vents in account? I know I have, but couldnt find out how.

Quote

At the end of the day we are probably talking a difference of a few Hz difference in the tuning, but even that can make a difference. 

I try to get as close as I can. 

Quote

S1 starts at the wall where the panels pinch together. Do you have 8 or 9 cm there before getting to the driver? 

Parallel wiring with speakons depends...You have a few options. You could make a splitter Y cable and run a speakon to each cab direct from the amp. You can also put 2 jacks on a cab and wire the second jack in parallel and use it to daisy chain to a second cab. This is much more common.

I take 53.5cm into account for the driver surface, so shortest distance is 53.5/2 = 26.75. However, I have placed the driver somewhat more to the left external pannel (pink). This increases S2.

The front horn length: 64.1cm = 85cm - 2* 1.8cm (external pannels) - 1* 1.8cm (pannel between port and horn) - 15.5cm

image.png.c66e49b7b5e7daed1bf10bd7ca233461.png

L12: 35.1cm = (64.1cm / 2) + 3cm

L23: 18.84cm = 64.1cm - 35.1cm (L12) - S3 height (10.15 = S3 area / depth = 573cm²/56.4cm)

L34: 

     black line: 20.3cm = 10.15*2 (S3 height *2)

     red line: 33.65cm = 64.1cm - S3 height - S5 height

L45: 20.32cm = S5 area/depth = 1146cm²/56.4cm

S2 area: 372cm² =  (L12 / (L12+L23) ) * S3 area 

image.png.f752714f04883a68d13485ac048a505f.png

Based on  HR I get a front horn that takes in 71L which seems to fit with a simple check: 64.1cm (length) * 20.3cm (height) * 56.4cm (depth) = 73L 

 

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@Ricci

Hey, as the mouths of the port and horn are both located on the ground, dont you get a kind of corner loading effect on the port and front horn? The tuning of the port especially is way lower in HR. Consider you put a few subs next to each other, dont you create a 'wall' aka 1pi, even though youre in a 2pi situation? 

 

Was hoping to rush the design and get the wood this weekend, but alas.. all shops closed in the weekend due to the crisis.

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Sort of but it's more complicated than that. HR considers each boundary an infinite wall. This isn't true even with a bunch of cabs outdoors. 

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13 hours ago, Ricci said:

Sort of but it's more complicated than that. HR considers each boundary an infinite wall. This isn't true even with a bunch of cabs outdoors. 

True.

However, doesnt the "+50% of the height" also apply to a vent when it radiates horizontally at floor level? 

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It will drop the tuning, but probably not by a noticeable amount. The cab will still be like 3cm off the ground and there are no side walls in addition to that.

Also, the air pressures outside the cab and inside the cab differ, which probably also has an affect on how the geometry behaves, which is probably also insignificantly small. 
With my cab, I got the tuning spot on when I added 75% of the vent width to the length, with a vertical port. But that‘s only due to the weird port entry geometry and the fact that the port is only about two port widths from the rear wall. And the handles...

Your port is pretty long to begin with, so the difference will be like 1Hz or so. I‘d not care much.

F4E8BBE6-F1CD-4FA1-A53E-2BF3D909BD3A.jpeg

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1 hour ago, peniku8 said:

It will drop the tuning, but probably not by a noticeable amount. The cab will still be like 3cm off the ground and there are no side walls in addition to that.

Also, the air pressures outside the cab and inside the cab differ, which probably also has an affect on how the geometry behaves, which is probably also insignificantly small. 

With my cab, I got the tuning spot on when I added 75% of the vent width to the length, with a vertical port. But that‘s only due to the weird port entry geometry and the fact that the port is only about two port widths from the rear wall. And the handles...

So not 50% of height, but rather 75% of width? Because my port is also significantly wider. Because that would mean +25cm? Or do you mean because the port is vertical, the width is the shortest dimention. But still, rather 75% than 50%?

1 hour ago, peniku8 said:

Your port is pretty long to begin with, so the difference will be like 1Hz or so. I‘d not care much.

 

Allright, the drivers arived today and I ordered the necessariy parts (connectors, handles..). How much filling do you suggest for this design? And do you add internal volume due to the filling (I've read +20% somewhere, but I never do).

 

 

 

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It was +75% of the shorter dimension in my case, since the port opens to the side of the longer dimension in my cab.

It's a guess, but the tuning of yours will probably come in between +50% and +75%.

What are you trying to achieve with the filling?

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