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Lilmike's Cinema F-20 - Page 49

post #1441 of 1695
Thread Starter 
Quote:
Originally Posted by vitaminbass View Post

lilmike, have you ever played with making one or more sections of the horn moveable/tunable? I'm sure you can do this "virtually" with software but still it would be fun to do it the old-school way. Like with the F20 you could move #8 in and out of the box. Could make that section wider than 18.5" and miter it in, them just need some type of oring or seal on the end of the cabinet.

Nope. I build it like I folded it. Seems to work well enough. I ran a LOT of simulations before I ever folded this one, and continually adjusted the sims as I completed the fold.

Once the loading is "right" there is little need for additional tuning, and it seems to me that a moving panel like that would preclude a good seal and proper bracing.
post #1442 of 1695
Please let me take a moment and defend lilmike's original design. After extensive playing around (and with all "professional" EQ yet to be done, I cannot imagine further tweaking would ever be needed. This thing pounds hard and low (especially if you have four of them haha).
post #1443 of 1695
Quote:
Originally Posted by lilmike View Post

Nope. I build it like I folded it. Seems to work well enough. I ran a LOT of simulations before I ever folded this one, and continually adjusted the sims as I completed the fold.
Once the loading is "right" there is little need for additional tuning, and it seems to me that a moving panel like that would preclude a good seal and proper bracing.

You are probably right, as it wouldn't take much slop in the assembly to make one heck of a vibration/rattle. I've always enjoyed experimenting and adjusting things and being able to do this with a computer model is new to me (but I respect the capability).

I notice at least one guy added a section of bracing all the way accross (for new build only) on the end opposite the driver, in addition to the three standard areas. Is this worthwhile or not a good tradeoff for the loss of volume? I would use 3/4" x 2" plywood scraps or a 2x4 ripped in half.

A couple years ago I did some reading on subwoofer design and one guy was talking about the bracing rasing the resonant frequency, or ringing frequency of the panels so that it was well out of the operational range of the subwoofer. Any comments on that as it applies to this unit?


Mike, thanks so much for contributing this design to the community!
post #1444 of 1695
@vitamin....I didn't brace the first one, but did the second one with 1x2 oak planks. They both sounded the same, so I didn't bother with bracing #2 & #3. In the end, it might not help, but certainly won't hurt IMHO. Just my $0.02.
post #1445 of 1695
I think the bracing you are referring to was in my build. Mike has stated that complete bracing is not optional in a sub horn.

Whether I did it right.. or wrong.. I don't know. I simply tried to create a continuous brace from one side of the sub to another so I wasn't replying on the stiffness of the plywood folds as bracing.

I did round over the edges of the bracing.. fwiw.

Tim
post #1446 of 1695
Mike's right. Hornresp assumes you're using panels of rigid unobtanium to build your horn. Any amount your baffles flex, you're going to be losing something in the way of performance vs. the Hornresp simulation. That's aside from a risk of actually having joints pull apart due to the forces involved.

That's not to say it's going to happen - my SDX horn hasn't done that after 18 months of me beating on it completely unbraced - but that issue aside if you want to get the most out of any horn design you do need to take steps to make those panels vibrate as little as possible.

At full gallop, you can actually reach into my SDX horn and feel the driver baffle vibrating just a bit. That's using 11 ply 3/4" wood on only a 300mm wide span, too... just gives you some idea about how much force is being exerted by the two SDX10 drivers against the cabinet.
post #1447 of 1695
here's the bracing i did. i also put large rubber feet on the bottom. between those modifications and the weight of the MDF, the sub is completely inert at loud volumes. i haven't performed any measurements, but I don't imagine the bracing has caused any detriment to performance.

post #1448 of 1695
thanks guys. the last photo is what I had in mind. Will likely use 3" or so sections of the leftover plywood. I have already done the "official" other three braces. The only tricky part is, getting the braces to stay in there while the glue dries, might have to think on that. For all the others I was able to use an air/brad nailer to easilly hold them in place while the PL dried. Might still be able to do that but it would be at an angle/toenailed in.

For closing up the top, I saw where one guy just put a bead of PL on the top of every section than just lowered down the lid and clamped it down. I'm not sure if I"m that brave, it would bug me not knowing if everything sealed up right and any gaps between boards (even if sealed with PL) are technically not as strong, I think anyway.

I plan on transcribing the exact location of the sections to the lid with a framing square, drilling screw holes every 6" from the underneath side on the lid, confirming I"m hitting a board with a dry run, then applying the PL and closing it up. Flush trim bit, 1/4" roundover and It will be nearly finished. I plan on needing EQ so at least to start out I will use the BFD. Long term maybe a Berry DSP amp. Need to research how many channels they have in the para EQ section.

I think some of the pro build companies that have a CNC router cut all the parts to an exact fit then use a brad nailer to tack together while the glue dries, and don't use screws. I'm still stuck on using screws for most of the build but then I don't have a table saw and there are minor gaps here and there. I do use a router with guide board, and hand planer to cheat/tweak things in when required. The degree of flatness between sections on top when you have all but the lid on illustrates how things came out for you.
post #1449 of 1695
After reading thaddeussmith's build thread I went out and bought a pocket screw kit and use it in my build just like they did. You can see the screws in the braces in his pic above. I build one side of mine with no screws showing that faces into the room. The last side I put on I had marked and screwed from the outside onto a nice bead of pl and it worked great. The pocket screw jig works great. I use it all the time now.
post #1450 of 1695
Quote:
Originally Posted by vitaminbass View Post

The only tricky part is, getting the braces to stay in there while the glue dries, might have to think on that. For all the others I was able to use an air/brad nailer to easilly hold them in place while the PL dried. Might still be able to do that but it would be at an angle/toenailed in.

if you'll notice, i used pocket hole screws for everything, including the braces. problem solved. as for the final side, tons of PL with pocket screws in the horn opening and driver compartment, and then lots and lots of clamps/weight for 48 hours. plenty of PL squish-out (technical term) meant that a solid seal was accomplished.

post #1451 of 1695
Quote:
Originally Posted by ovrrdrive View Post

After reading thaddeussmith's build thread I went out and bought a pocket screw kit and use it in my build just like they did. You can see the screws in the braces in his pic above. I build one side of mine with no screws showing that faces into the room. The last side I put on I had marked and screwed from the outside onto a nice bead of pl and it worked great. The pocket screw jig works great. I use it all the time now.

haha, beat me to it.
post #1452 of 1695
After you outlined the footprint of the interior panels on panel 1, you should have clamped the side panels to each other and predrilled the screw holes in a mirror image. Once you are ready to install your final panel "lid", first lean it up against a wall and put your sheetrock screws in about 3/4 the way, then lay that panel on your cabinet as a dry run. You will be able to see if your screws will hit their intended target when you go to screw them in for real.
post #1453 of 1695
Quote:
Originally Posted by thaddeussmith View Post

haha, beat me to it.

Hey at least I gave you credit! lol
post #1454 of 1695
Quote:
Originally Posted by dutchswan0311 View Post

After you outlined the footprint of the interior panels on panel 1, you should have clamped the side panels to each other and predrilled the screw holes in a mirror image. Once you are ready to install your final panel "lid", first lean it up against a wall and put your sheetrock screws in about 3/4 the way, then lay that panel on your cabinet as a dry run. You will be able to see if your screws will hit their intended target when you go to screw them in for real.

I saw that technique and for me, had "missing the target" written all over it. I'm not using a drill press so not all screw holes are straight. And yes I use a square to try to keep the sections straight as I build but still...you haven't seem me work with wood. I'm a metal guy. Here's an example of my world of woodworking: started translating points to the back side of the lid, got a ways into it and realized I needed to be doing a mirror image (measuring from the other corner) since I'm marking on the back side of the lid. DUH! Those pencil marks will come right off with the belt sander.

Pocket screws...have the tool, and it's sitting right where I put it when I got home from the store last year. Well, I tried a practice run just holding a couple 2 x 4s togeither and created noting but splits and a mess. I'm sure it's a great tool in the right hands. MIne is the $20 or $30 version with a single hole you have to clamp to the board.

I'm done with the additional braces. Took about an hour, roundover on the edges, used the air nailer and a screw or two at an angle to hold them in place while the PL dries.

I've been putting healthy beads of PL on all joints as in, a fillet all around after the part is in place. Does that add significantly to the strength or is the real work done by the thin layer between the two components? Instructions said one tube of PL could do the whole thing and I"m a ways into the 2nd one so I suspect I'm going heavy with it.

Maybe one more night and I'll be done. Well, need to make the cover for the access panel yet.

LilMike question: Is it ok to make the access panel flush on the inside, having the flange on the outside? This will make the sealed chamber slightly larger, I suspect this isn't significant but not sure.
post #1455 of 1695
Thread Starter 
Quote:
Originally Posted by vitaminbass View Post

I saw that technique and for me, had "missing the target" written all over it. I'm not using a drill press so not all screw holes are straight. And yes I use a square to try to keep the sections straight as I build but still...you haven't seem me work with wood. I'm a metal guy. Here's an example of my world of woodworking: started translating points to the back side of the lid, got a ways into it and realized I needed to be doing a mirror image (measuring from the other corner) since I'm marking on the back side of the lid. DUH! Those pencil marks will come right off with the belt sander.
Pocket screws...have the tool, and it's sitting right where I put it when I got home from the store last year. Well, I tried a practice run just holding a couple 2 x 4s togeither and created noting but splits and a mess. I'm sure it's a great tool in the right hands. MIne is the $20 or $30 version with a single hole you have to clamp to the board.
I'm done with the additional braces. Took about an hour, roundover on the edges, used the air nailer and a screw or two at an angle to hold them in place while the PL dries.
I've been putting healthy beads of PL on all joints as in, a fillet all around after the part is in place. Does that add significantly to the strength or is the real work done by the thin layer between the two components? Instructions said one tube of PL could do the whole thing and I"m a ways into the 2nd one so I suspect I'm going heavy with it.
Maybe one more night and I'll be done. Well, need to make the cover for the access panel yet.
LilMike question: Is it ok to make the access panel flush on the inside, having the flange on the outside? This will make the sealed chamber slightly larger, I suspect this isn't significant but not sure.

Beads of PL around the joint can't hurt, but likely won't help too much. It is a pretty tough glue - I've not seen glue break, it is the wood every time I have tested.

Access panel flange is not an issue at all. Do what works for you. Make sure it is sealed - the change in volume is insignificant.

BTW - I'm with you - I'd rather work with metal than wood.... Those projects will come....
post #1456 of 1695
well it's together and glue drying. I used 2 full tubes of PL so I must be putting it on heavy. Once I had the holes in the lid marked and drilled, I put a few screws in the corners to suspend the lid up off the rest of it...then with the lights off ran a flashlight along through all the holes...makes it easy to see where the screws are going to hit. They all lined up great but I did have one that hit a void and wouldn't hold, but it will be ok. I put the screws 7-8 inches apart and that seemed to give me a good even clamp down.

It would be fun to build two of these...one with no bracing at all and then a 2nd one heavilly braced...and the run full analysis on them.

I seem to recall somewhere in the 49 pages, putting in dampening material was briefly discussed, polyfill etc. and the word was that it would not be a good idea. I assume this was in the throat, but what about putting some in the sealed chamber? If I understand the design correctly, the sealed side's job is just to dis-allow the back wave from coming around an cancelling out the front wave. In a perfect world there would be no vibration or resonance of the sealed chamber...like using concrete for a sub cabinet. Also, in a conventional sealed sub (not a horn) , sound (some of it higher frequency such as harmonics) reflects off the inside of the sealed chamber and penetrates the driver's cone. But then in the F20, that sound has to go through a bunch of reflections to make it out. Hmmmmm...
post #1457 of 1695
Thread Starter 
As far as the damping? Don't worry about it. I have never bothered. Maybe someday I will do it and measure the effects. Until then - I don't see the need.

As far as in the throat? Bad idea. Just costs SPL. Use active EQ instead, make the sub and room work together.
post #1458 of 1695
Can't remember if it was mentioned in this thread...if I am running sine wave tests (not too long at one time) and monitor RMS voltage at the sub, what would be a safe voltage to stay under? I calculated 32V makes 300W using 3.5 ohms DCR but then isn't impedance what really matters? And that varies with freq. I'm just looking for some estimate on a "safe" voltage. Running it with an EP2500 for now, with the sub sitting up on sawhorses in the middle of the room I'm surprised what it puts out with just 2 volts.
Edited by vitaminbass - 12/16/12 at 3:28pm
post #1459 of 1695
Thread Starter 
If I had the recommended 20 Hz highpass in place, I really wouldn't worry much under the thermal limit of the driver.

For the MFW - this is about 32 volts.
For the RSS390 HF or the DVC, this is about 45 volts.

However - I do not recommend high-power sine wave testing.



TESTING WITH HIGH POWER SINE WAVES WILL DO THIS TO A COIL FASTER THAN YOU CAN TURN IT DOWN!
post #1460 of 1695
When did is happen? Not when I was there, right?

JSS
post #1461 of 1695
Thanks.

usually if at high power and I'm doing tones and not sweeps, I give it a few seconds, enough to take readings, and then rest. From what I understand, high frequencies can be worse than low (within reason) because less air is being pumped through to cool the coil. I've got an old Sansui 80W I could hook up and that should be pretty safe as well (instead of the EP2500). Amazing how an 80Wx2 amp used to weigh 40 lbs and now a 3000W amp weighs 7 lbs (such as an Inuke etc).
post #1462 of 1695
Thread Starter 
Quote:
Originally Posted by maxmercy View Post

When did is happen? Not when I was there, right?
JSS

No - that was a long time ago....testing one of my first horn builds.

About 2X RMS at ~50 Hz let all the magic smoke out.... Fortunately I had a few replacement drivers.
post #1463 of 1695
The crown xls-1500 that i power mine with will shut down before doing that to a coil. Impressive though!
post #1464 of 1695
Thread Starter 
Coil still had continuity....
post #1465 of 1695
Thread Starter 
Quote:
Originally Posted by vitaminbass View Post

Thanks.
usually if at high power and I'm doing tones and not sweeps, I give it a few seconds, enough to take readings, and then rest. From what I understand, high frequencies can be worse than low (within reason) because less air is being pumped through to cool the coil. I've got an old Sansui 80W I could hook up and that should be pretty safe as well (instead of the EP2500). Amazing how an 80Wx2 amp used to weigh 40 lbs and now a 3000W amp weighs 7 lbs (such as an Inuke etc).

I really don't see a reason to do tones at high level any more. I learned that lesson already.

I do pulses or sweeps now. I'll push a driver past limits when it is mine and I know I can get a replacement.
post #1466 of 1695
I almost fried two channels of my amp doing high testing sine waves myself. It started smoking but I was lucky enough to not damage anything known. It still sounds great at reference. If anyone wants to know the F-20 with a MFW-15 is about half the DTS-10. So duals would equal the DTS-10 at 20hz and above. The DTS-10 wins below 20hz but also costs more than duals, even 3. Great design and great bang for the buck.
post #1467 of 1695
huh...on the amp I would think you would get a thermal trip by the time there is smoke?


I don't have enough preamp input level to do much damage with my current rig. MP3 player to BFD to EP2500, the mp3 player only puts out .18mv at max so the EP is just loafing along.

I used to always bring the sub to the room where I have my good computer (with REW, soundcard etc.) but this sucker is big...might have to bring a computer out to the workshop. first thing the wife said when she saw it: "that thing isn't going into the living room, right?"
post #1468 of 1695
Nope, the amp just kept pushing wattage out. It is a 1998 amp and probably does not have all the protections new ones have. It still works though like nothing happened. This was playing a 15khz sine wave for a long time at high levels. Like I said before, I could hear that tone in another room while both doors were shut! It was loud
post #1469 of 1695
Hey guys is this a sound that the woofer would make because of an air leak? I built 2 F-20 using MFW-15 drivers and one of them does this when its getting pushed a little hard. I checked and fixed a leak in the access panel already. I have the woofer mounted on weatherstripping. I checked that and added corded weather seal (the kind that looks like clay) where the gaps for screws are. I also tightened the screws down again on the woofer. Some started to loosen. When I was checking the screws one of the washers actually was loose enough to move. After doing all of that it still makes this noise, The woofers were used in sealed cabs before the F-20 and I never heard the noise before. I will swap out the woofer tomorrow and try again.

Click on the link smile.gif http://s596.beta.photobucket.com/user/louisdamani/media/20121217_2245391_zpsf9183d8f.mp4.html
post #1470 of 1695
did the original seal on the woofer go bad? I just went with that, and it seemed to seal up great. For the access panel I routered out a shallow, 3/8" wide track...then inserted some 1/2" round foam...the kind you use to fill gaps before caulking. As much as that foam compresses, I probably could have just layed it under the cover and let it squish. You can use a ligher of a small pc. of paper to check for leaks on the cover. You are sure it's the sub vibrating, and not just something nearby?

At 20 hz and modest input, my CD player(which is 16 ft away) starts skipping. It is sitting on a sturdy workbench that's tied into the wall.
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