CCS - Compact Cheater Sub

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This project started after reading about and finally building Art Welter’s Keystone sub (thank you Art and thank you all for a great read!). The simulations and measurements suggested one Keystone sub would be about as loud as 2 BFM T48’s in half the pack space. Having built 2 at 21” wide and going by ear (outside SPL measurements are difficult for me, but I hope to be able to do some measurements this summer) I would say that is about right.

So now I am tempted to scrap/sell the T48’s and reuse the 3015LF’s (old version) in a more compact design. I have read the SS15 thread and as much info I could find on Martinson’s THAM15, but my urge for DIY keeps nagging me so I started thinking about a new design.

As the title says, I am trying to come up with a (relatively?) compact design, compact in packing space that is. So I drew up this:

[image]Image:Compact Cheater Sub FLH B 03|none[/image]

Using a triangular form it’s possible to reduce packing space by using the room inside the horn. The speaker can be tucked away inside the horn of the 2nd speaker for transport. This means this design needs to be built in pairs, but that already was my intention. With two cabinets you can easily “V-plate”, like the t48 more or less demands, and “cheat” your way into a bigger horn without needing the complete woodworks and space. So that’s “cheating” part of this design. See the configuration on the right in the image above.

I know my SketchUp skills are pretty much non existing, but I hope I got the basic idea across. Now for the fun part: Hornresp. I did some sims and thought this would be quite acceptable (if feasible):

[image]Compact Cheater Sub FLH B 02a|none[/image]

[image]Compact Cheater Sub FLH B 01a|none[/image]

I know, this is the sim for a FLH and the SketchUp image shows a TH, but it's just to show my idea...
Genuine 40Hz reproduction at 135dB with 450 Watts for each cab (so 900W into 4 Ohms total). The difficulty for me is translating the Hornresp data into a physical design. I more or less guesstimated the dimensions and I hope I’m not too far of. It looks like an easy build as well, with only one fold. But to be honest I haven’t calculated the areas yet. These are just the first thoughts I had...
 
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just a guy

Member
2006-05-12 6:59 pm
This is actually a pretty clever idea, it takes Bill's V plate to a whole other level by making the V plate responsible for MOST of the cab volume and cutting your pack space into a small fraction of what it would be.

But I have some practical criticism of Bill's V plate concept.

First, placing a board over the horns isn't going to do much. It's not going to seal airtight and it's going to vibrate right off and out of place. It's also not going to prevent the two separate cabs from "walking" out of place in reference to each other. Even though the drivers are opposed they will NOT cancel each other's forces unless they are held rigidly in place by the enclosure. If the enclosure is not a single rigid piece there is no force cancellation from the dual opposed layout. And you would need a plate on the bottom as well or you most likely won't have an air tight seal on the bottom of the subwoofer.

So you would have to use weather stripping or similar to seal it air tight and bolt everything together with a plate on the top AND bottom. That's going to dramatically increase your set up time.

Even with everything bolted together it's still going to be a remarkably weak enclosure. There's no bracing and EVERYTHING is going to vibrate, the V plate itself AND the two separate cabs have no structural integrity as a whole.

Vibrations mean lost energy and lost energy means lost spl.

Unless everything is air tight, braced and structurally strong it's simply not going to perform as the sims indicate it should.

Think about it in simpler terms. Make a sealed or ported box. Cut it in half. Place the two halves side by side and run the sub. There's going to be air leaks, the internal pressures will push the two halves apart subtly (microscopically) and it won't take long until the half with the driver attached simply "walks" away from the other side due to the rocking motion of the heavy cone moving back and forth. There's no way it can work unless you use weather stripping and bolt it together to make it a single air tight unit.

So while this may seem like a good idea at first glance, unless you put plates on the top and bottom attached with bolts and use weather stripping to seal up everything air tight and use a significant amount of bracing to attach all the parts together in a rigid and structurally significant way, it's just not going to perform as a sim would predict. The sim assumes everything is air tight and infinitely rigid.
 
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Okay, so how about this...

Using a top plate and bottom plate (as noted above), have a piece of 3"x2" (or whatever) that attaches to the bottom plate (we'll assume it attaches by magic for now), but stands a several mm taller than the cabinets. You have threaded inserts/t-nuts/whatever, and screw to the 3"x2", and also to each cabinet.
The offset heights would put the top plate under some tension, which will help with energy loss, by pushing any resonances right out of the pass band.
Some good weather stripping would be needed, but when everything's bolted down, I think it'll be pretty good. The mouth of the horn is a fairly low pressure environment compared to what's going on inside, so I don't think perfect air-tightness is necessary - see Art's horn extenders for some inspiration.

Sure, it adds a bit of setup time, but so do any horn extenders. This one should really be modelled as a TH, though ;)

Chris
 
It's not going to seal airtight and it's going to vibrate right off and out of place. It's also not going to prevent the two separate cabs from "walking" out of place in reference to each other.
I know from experience the V-plate needs to be bolted down firmly to both cab. Otherwise it'll rattle like hell! Nothing a few T-nuts and butterfly bolts can't handle :). On top of that it needs wheather striping, preferably rubber that doesn't compress completely (also to prevent rattling). So that part will be fairly airtight.
That's going to dramatically increase your set up time.
That's true. But it still beats having to drive back for cabs because it didn't all fit in the car :D.

Even with everything bolted together it's still going to be a remarkably weak enclosure. There's no bracing and EVERYTHING is going to vibrate, the V plate itself AND the two separate cabs have no structural integrity as a whole.

Vibrations mean lost energy and lost energy means lost spl.

Unless everything is air tight, braced and structurally strong it's simply not going to perform as the sims indicate it should.
I totally agree. The V-plate needs to be braced properly. It could do double duty as a small podium for the lead singer ;)
Maybe a bottom brace would be needed, but I doubt a full bottom V-plate would make enough difference to validate the additional setup difficulties. I realise this construction will not be the best possible one, it is a compromise. And the SPL will suffer. Question is by how much?
 
This one should really be modelled as a TH, though ;)
Yeah, I know. For the sim above to be consistent with my 3D sketch I should have put a back chamber around the speaker, but I really struggle with 3D software. Never got the hang of it, I'm afraid :xeye:.

I already did some sims with a TH. In fact, I did those before I did the FLH. But I realised that a TH wouldn't benefit from a lower fc when used in multiples, like this FLH (which comes only in pairs).

But here you go. This is my second TH simulation. It's doesn't go as low as the FLH. It's quite a difference actually. But it's more compact.

[image]CSS TH B-01|none[/image]

[image]CCS TH B 02|none[/image]
 
Basically everything in that link is WILDLY optimistic. He states 3 db gain from the V plate, 6 db gain from wall (1 pi) loading and 12 db gain from corner (0.5 pi) loading. And the graph at the top shows the 65 hz low knee of a single cab dropping to 45 hz when stacked. Good luck with all that.

He is a marketer, a showman that's for sure. This is why he likes to funnel conversation from open forums into his own forum that he can heavily moderate. Stuff like facts can be a very inconvenient truth that he likes to shield his potential customers from.
 
Basically everything in that link is WILDLY optimistic. He states 3 db gain from the V plate, 6 db gain from wall (1 pi) loading and 12 db gain from corner (0.5 pi) loading. And the graph at the top shows the 65 hz low knee of a single cab dropping to 45 hz when stacked. Good luck with all that.

It would be nice to see some measurements to back those points up.

Adding a piece of wood like that to complete the 'V" is likely not going to get close to the gain he's suggesting, unless it's properly damped and braced. If I was going to do it, I'd include a vertical center brace, then sandbag it :).

If used as I suggested, the "V" should extend the path length a bit, which will produce a drop in the resonance frequency.
 
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