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Old 21st August 2010, 01:39 PM   #91
jbell is offline jbell  United States
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Quote:
Originally Posted by CuttaBoi View Post
30-35hz would be good!
build the big box then... 2 sheets 3/4" ply -- easily hits mid 30's with authority.
jbell's set of four tapped horns Or even build labsubs....

The goal here was specifically the best output in a single sheet, that I could easily build for the church's/schools that I do installs for. The entire reason for a single sheet, was primarily weight driven. If you limit yourself to a single 42lb auroco sheet of plywood, you are pretty much guaranteed a 50lb cabinet with a 3015lf installed. Finding a pack friendly and attractive size was a side bonus.

The real benefit of this last design, is I finally have a sound similar to the fury box, in a quick/simple box build. I'm beginning to see folding of TH's like hitting keys on a piano. The more dimensions that line up, the more it's like hitting octaves of the same note... and you have a note that sticks out, and other notes that are muffled... The more you can vary the dimensions, the more it's like hitting a dense chord... all the notes speak. My cube is a good example of that. I sacrificed 80hz to get 40hz... 80hz sounds a little stuffy, 40hz comes through clear -- that's primarily how I got more 40hz out of the cube than hornresp says I should get. This last design doesn't have any 2 horn paths the same length, and because of the 'not quite' 90 angles, I accomplished something similar to scott's 12degree angle reflectors. 29 tall, 21 wide, 24 front to back, and the 27degree baffle angle making all the short paths of different lengths... it all makes this box sound good. I'm happy with it.
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Old 21st August 2010, 02:28 PM   #92
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Originally Posted by jbell View Post
martinsson:
What I assumed was cabinet flex on my initial 28v readings, was soggy ground and rain robbing 2db.
I must have access to the worst ply in the world here - I was getting enough flex with 17mm ply horn designed around a 12" driver to significantly distort the impedance plot above 80 Hz (and by extension the response above that area). I had to resort to bracing and doubling the thickness of the panel closest to the driver to address that, and I can still detect quite a bit of vibration on one of the box's external panels. And your design uses 1/2" ply for a larger box holding a 15" driver and you're not running into panel flex issues? I want some of that ply
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Old 21st August 2010, 02:40 PM   #93
AndrewT is offline AndrewT  Scotland
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Hi,
plywood has a very different stiffness in one direction compared to the other direction. I seem to recall that the grain of the two outer layers runs parallel to the long dimension.

If a panel is flexing, could it be that the less stiff direction is being used, whereas cutting it from a ply in the other direction could much reduce the Xmax of the wood panel?

Would bonding a layer of carbon fibre reinforced plastic (CFRP) to both faces improve the panel's stiffness to such an extent that a thinner plywood (and a lighter/cheaper) speaker, results in good performance?
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Old 21st August 2010, 02:49 PM   #94
jbell is offline jbell  United States
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Originally Posted by Brian Steele View Post
I must have access to the worst ply in the world here - I was getting enough flex with 17mm ply horn designed around a 12" driver to significantly distort the impedance plot above 80 Hz (and by extension the response above that area). I had to resort to bracing and doubling the thickness of the panel closest to the driver to address that, and I can still detect quite a bit of vibration on one of the box's external panels. And your design uses 1/2" ply for a larger box holding a 15" driver and you're not running into panel flex issues? I want some of that ply
While auroco is not baltic birch -- it's 5 even thickness plys, and the best thing available to me short of baltic.

My rule of thumb is any panel over 12" needs to be braced, and in this design it is. A steel grill will brace the mouth, so all is covered.

Running my hand over the exterior panels while running sine waves at 28v is very encouraging. Yes there are small vibrations, but they are small and nothing I've not seen in any other pa cabinet. (I really don't care if my impedance peaks don't quite line up with hornresp, and that I'm losing a little efficiency due to some flex -- this is pa, not studio.) In short, if I had additional ply avail to me for this design -- would I spend it on braces? There are a couple internal panels I think I'd feel better with a brace or two, but that's it.
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Old 21st August 2010, 03:16 PM   #95
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Quote:
Originally Posted by jbell View Post
my hand over the exterior panels while running sine waves at 28v is very encouraging. Yes there are small vibrations, but they are small and nothing I've not seen in any other pa cabinet. (I really don't care if my impedance peaks don't quite line up with hornresp, and that I'm losing a little efficiency due to some flex -- this is pa, not studio.) In short, if I had additional ply avail to me for this design -- would I spend it on braces? There are a couple internal panels I think I'd feel better with a brace or two, but that's it.
Actually the loss of efficiency is more important to PA than studio use .

Seriously, a distorted impedance plot suggests that there could also be higher distortion around the affected frequencies (a vibrating box panel does not make a good linear transducer).

The plywood generally available locally is called "viola ply". It isn't void-free, and it's not very sturdy. There's another type called "marine ply" available at significantly higher cost - I might try that next. I also recently got access to some MDF - but I'm saving that for another project .

I've gotten the best results using 1/2" fiberglass bonded to 3/4" ply, but the resulting box ended up being very heavy for its size, and likely not very suitable for PA use...
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Old 21st August 2010, 03:46 PM   #96
soho54 is offline soho54  United States
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Quote:
Originally Posted by Brian Steele View Post
There's another type called "marine ply" available at significantly higher cost - I might try that next.
That would be a better option, but they make you pay for the "marine" part. It will be void free though.
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Old 21st August 2010, 10:30 PM   #97
PASC is offline PASC  Brazil
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Hi!

Well done jbell!!

What helps me to identify where the panel flexes is put some thicker construction dry sand over the inspected panel;
Now a sweep test tone...
Where the problem (flexing) is the sand will jump, dance...
Save the same sand, turn the box, put it over the next panel...

Regards,
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Old 21st August 2010, 10:40 PM   #98
freddi is offline freddi  United States
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is jbell's TH drawing at it's official level? - if so would like to send it to a friend - what woofers does it like best?

Last edited by freddi; 21st August 2010 at 10:45 PM.
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Old 21st August 2010, 10:49 PM   #99
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Not sure about the drawing being offical yet. But the driver is the 3015lf.
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Old 21st August 2010, 10:54 PM   #100
freddi is offline freddi  United States
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thanks so loaded weight could be as low as 60lb -?
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