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Old 16th August 2011, 05:45 AM   #1
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Default Ultralight PA Sub with 268G

Hmm. New to blogs. But I have to say I read a bunch as I designed my latest project. The latest build was a PA style sub with very specific objectives; A PA sub that I could carry with one hand.

The best commercial options I could find all seemed to me to have flaws. The primary one being that they were not lightweight... At least by my definition.

I built a 6+ cubic foot enclosure out of common foam board insulation with beveled latex glue joints and many reinforceing cross braces. I covered that with thin aluminum commonly used for roof valleys/gutters ( did I mention that controlling cost was also an objective).

1/2 inch by 1/16 aluminum angle to protect the corners, and a leather dogbone handle and the cabinet is complete. Oh yeah... painted it with truck bed liner from the hardware store.. Carefull, nothing will remove that without also removing the skin from your fingers...

JBL 268G (at an amazing 6 1/2 lbs) and an inexpensive bash 300 watt amp brings the whole thing to finish.

I am sure that it is not the most amazing sub ever... Win-Isobeta specs it as -14 at 20 hz. But I bet it is close to the best per pound at 28.5 lbs for an 18" self powered PA sub...

Thoughts? Comments? It's OK I can take it.

Steve
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Old 16th August 2011, 06:38 AM   #2
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Originally Posted by stevemcpreston View Post
Thoughts? Comments? It's OK I can take it.

Steve
TTIUWP
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Old 16th August 2011, 09:14 AM   #3
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Pictures? You may have to use rope or other material to bind the sub and not allow it to walk away! What about panel flex at PA levels? It would result in serious loss of SPL.
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Old 16th August 2011, 01:19 PM   #4
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On the origional test - signal sweeep 10 to 200 hz (when it was bare aluminum with no feet) it did vibrate across the floor at random - mostly around 40 hz. With 4 heavy duty rubber feet on it, it stays in place well.
As for panel flex, yes it's a problem. Let's just say you don't want to set your beer on it while it's playing. Only way I can think to get around that is to totally rebuild it with honeycomb aluminum. Don't have the spare cash for that right now.
Still it sounds pretty good for a total price under $500.
Steve<a href="http://www.freeimagehosting.net/92067"><img src="http://www.freeimagehosting.net/t/92067.jpg"></a>
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Old 16th August 2011, 01:26 PM   #5
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Sorry about the links above. Try this one.
Steve
http://www.freeimagehosting.net/92067
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Old 16th August 2011, 10:29 PM   #6
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That may qualify for the prestigious 'Highest Loss Enclosure ever built' award.
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Old 17th August 2011, 04:42 AM   #7
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Enlighten me. I understand that with midbass, midrange and treble flexing of the cabinet reduces spl by allowing the sound to travel in full space or at least half space. The cabinet direct the sound functionally making it 1/8 space increasing spl in front of the speakers.
Having spent long hours sitting behind various DJ setups I can only say that if you think your wood cabinets direct the low bass forward you are dreaming (at least with any cabinet that is 'portable')....

A ridigid cabinet in a sub controlls the sub cone/coil preventing it from overexcursion... However, JBL differential drives have a breaking coil in the center and I powered it below the max power because I knew the box would flex...

Ok. So my true dream enclosure would be steel lined reinforced concrete, which could come close to making subs perform like 1/8 space spl... but the point is I don't want to pack that around.

This is an experiment in trade offs. If I have to pack 2 of these to match 1 concrete bunker style so be it.

Steve
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Old 17th August 2011, 01:25 PM   #8
jbell is offline jbell  United States
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There are some sub designs that are NOT omnidirectional... the attached measurements of my ss15 show that. (It's a 50lb sub... so a bit heavier than yours, but has REALLY good output.)

However in the panel flex department, additional bracing internal with adhesive/foam board, etc should help alot.

My appreciation for something 'out of the ordinary' that seems to fit your needs. Just a thought -- For future builds you can approximate what you get with 'honeycomb' board by adhering 1/8" ply to inside/outside of the foam board.


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Old 17th August 2011, 10:25 PM   #9
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Quote:
Originally Posted by stevemcpreston View Post
Enlighten me. I understand that with midbass, midrange and treble flexing of the cabinet reduces spl by allowing the sound to travel in full space or at least half space. The cabinet direct the sound functionally making it 1/8 space increasing spl in front of the speakers.
Having spent long hours sitting behind various DJ setups I can only say that if you think your wood cabinets direct the low bass forward you are dreaming (at least with any cabinet that is 'portable')....

A ridigid cabinet in a sub controlls the sub cone/coil preventing it from overexcursion... However, JBL differential drives have a breaking coil in the center and I powered it below the max power because I knew the box would flex... Steve
sorry, bad quip, the back-wave energy is contained within and damped by panel treatments so that the box does not turn into a distortion generator. You can conceivably make a very light but highly damped box out of carbon fiber with fractal lattice dampers.
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Last edited by revboden; 17th August 2011 at 10:31 PM.
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Old 17th August 2011, 11:07 PM   #10
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I've spent quite some time thinking about this since i saw this thread. Now I want to make just such a box using a lattice made of Tinker-toys or something similar and the thinest material possible. I spent about three hours yesterday trying to figure out how to make a carbon fiber sphere... I figured it out but would need 3500$ in carbon fiber...
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