Broken box(es) - Home Theater Forum and Systems -

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post #1 of 10 Old 12-08-09, 09:41 PM Thread Starter
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Broken box(es)

I built a box (read: 2) for my buddy's car. It's running a JL Audio 12w7 with a 1000w amp.

I built the first one with no bracing and it blew the sides out and the slotted port extension in the first two days he was using it. I've been learning from this experience haha. Built a box for my 15" Titanic MKIII and its holding up great.

I build another one the other weekend and i doubled up the sides and put triangle bracing in all the corners with some bracing around the ports. I also used construction adhesive and predrilled all the holes.

He says it's still rattling at high db's and he thinks it's coming from the port. It is also sliding around in the back of his Denali.

I wish i had pictures of the inside to show you guys but I'm tired of redoing it when I know the quality of work I'm capable of isn't that (or maybe).

How do you guys make these boxes so strong?

Oh and if someone knows of a good article on bass horns. HOLLER

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post #2 of 10 Old 12-09-09, 03:55 AM
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Re: Broken box(es)

Its worth noting that simply making the box out of thicker material (but use at least 1/2" material, preferable 3/2's) isnt as effective as adding good bracing. To make a box strong, get some good adhesive (which also serves to seal the enclosure) and glue all the panels together and use screws as well, pre drilling them as you have done previously. Adding bracing is good, but the best sort is windowed bracing rather than just blocks in the corners.

Port noise could be due to the fact the port is too small and the air speed inside it is too great. It may also be cabinet resonance filtering down the port, or possibly mechanical driver noise from the voice coil assembly being heard via the port. You need to ensure the port is of correct size, your not over driving the the driver (too many watts) and you do your best to prevent cab resonances. You can line your box with foam, or use acoustic wadding to help with cabinet resonances, also building the box slightly out of square (trapezoidal) will further help acoustics of the box.

Hope that gives you a couple of things to think about.
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post #3 of 10 Old 12-09-09, 12:29 PM
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Re: Broken box(es)

More info and photos on the box.

What are the dimensions of the box?

What is the diameter and length of the port?

did you put any stuffing in the box?

Any leaks?

Did the driver maybe get damaged after the explosion?
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post #4 of 10 Old 12-09-09, 12:36 PM
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Re: Broken box(es)

How do you guys make these boxes so strong?
With proper bracing, lots of wood glue and either clamps or screws.
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post #5 of 10 Old 12-09-09, 12:39 PM Thread Starter
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Re: Broken box(es)

I built it straight from the JL Audio site. Althought it suggests 1000w to be in the danger zone, would the two combined be destroying the box/subwoofer?

I was using 3/4" MDF and doubled up both of the sides. I haven't tried looking for 1" MDF yet. Any big name suppliers carry this?

There is no stuffing inside, and I haven't heard it in person when it "rattles". He conveniently goes to school 3 hours away.

When I tested it, no leaks on the first, broken box or the second box.

Would modeling the box in WinISD perform better than manufacturers specs. I have seen some cases where the manufacturer suggests smaller enclosures than ones to maximize potential.

Thanks guys
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post #6 of 10 Old 12-09-09, 12:45 PM
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Re: Broken box(es)

With a box that size, you shouldn't *need* bracing, but bracing is always good. No offence, but I'm thinking your build is a bit suspect.

Do a google image search for Subwoofer Bracing to get some ideas.

Looking at the link you've provided, and looking at the Top Down view of the ported box, I think you should build a skeleton type brace that wraps around the driver. I grabbed this in a quick google search. Notice how the bracing behind the driver has a big half moon cutout, but still touches all the panels around it. I imagine for your friends box, a skeleton bracing like that, horizontal but as far away from the opening of the port as possible.

Also, did you round over the entry and exit of the port?
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post #7 of 10 Old 12-09-09, 01:18 PM Thread Starter
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Re: Broken box(es)

Nope...router broke a month ago and is getting replaced.

Alright, no offense taken. It's one of those things. I thought I had learned from the first mistake and I hate building this box for him and it breaking again.

I'd rather see the error of my ways then spend days trying to argue against it.

I thought about building some bracing that would cradle the driver but without the router and a warm garage it wasn't feasible.

Thanks again!
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post #8 of 10 Old 12-09-09, 02:22 PM
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Re: Broken box(es)

the 'port rattling' is likely something called 'chuffing', when the air velocity approaches 25 ft/sec you will probably hear chuffing, it is similar to blowing over the mouth of a beer bottle, but likely with a lower frequency (since it is a subwoofer port...) Anyways, you need to make the port a larger diameter which will also require a longer length to maintain the same tuning frequency. Consequently you will need to make the box a bit bigger unless you have an externally mounted port (to account for the internal volume taken up by the larger port size). WinISD Pro would be a great help to you since there is a page that shows the predicted port velocity... Be sure to change the signal level (watts) to whatever you want to run...

BTW, you mentioned the "danger" zone JL advertises, that refers to the voice coil's limits as far as the amount of heat it can dissipate; so running the sub using more than 1000 watts will be more likely to 1) push it between its mechanical limits of excursion and 2) potentially overheat the sub if it is continually driven at over 1000 watts (for a long period of time). I have a 13w7 that I have hooked to 2 Crown K2 amplifiers (each voice coil on a channel, one channel of each amp running one voice coil... better heat dissipation, the other channel runs a 'mid' as the car people would call it), given the impedance it can see up to 2750 watts total and my sub works just fine ... you just have to be aware of how much power your sending it over a long period of time, transients (short bursts) should be fine over 1000 watts...

They mainly say the "danger thing" because they wont warranty the sub if you are running over 1000 watts (I think it actually says that on the site too) to cover their asses

Forgot: the wood glue is what gives a box its strength, if you can cut your box edges at 45 degree angles (to maximize the surface area for gluing) it will help too... cover the entire surface of each joint with wood glue (don't just make a line of glue, spread it evenly) after pre-drilling some holes and then screw it together, the screws are more to hold things in place while the glue dries... you can also then (on the inside of the box) run a bead of glue in the corners to give a bit more strength; my buddies dad is a master carpenter and he recommended the original "TiteBond" wood glue to me, idk what glue you were using... According to the bottle of TiteBond, it dries stronger than the actual wood, so if you do the 45 degree edges, and get an even coating of glue, in theory the edges will have more strength than a cross section of the board... or just start using steel water pressure tanks like I have done takes a lot of dampening and stuff tho; a future project is to do a sealed TC LMS-Ultra 18" in a water heater tank...

Last edited by Chester; 12-09-09 at 02:32 PM.
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post #9 of 10 Old 12-09-09, 02:46 PM
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Re: Broken box(es)

It sounds to me like you aren't giving the 12w7 sub a proper box.

Just to get your head in the right place. You need to realize the w7 series of jl audio subs are 2nd only to the lms series of tc sounds in measured sound quality at their particular sizes. They are also capable of incredible SPL. The 8w7 is better than even a typical 12" sub. The 10" moves up to being equal to the 15" subs out there. the 12w7 is like having an 18" sub. This is due to the incredible capabilities of the driver which is a marvel of subwoofer engineering.

IOTW- You got an amazing sub.

Now you need to match box to meet the subs potential. Since this sub is actually used in commercial home theater subs sealed. I'd suggest you build a sealed design for your car. You don't need extension for music and you will get a much easier build. A good sealed box is 2.2 cuft.

For this box use Birch Ply. and glue it with titebond original(strongest wood glue according to 3rd party tests). Next use Dense Oak bracing on every axis at 6" to 8" intervals. When done your sub's interior should look like a matrix. Be sure to leave a gap for the driver's long voice coil.

next you will need to line every wall with cloth wrapped 4" thick rockwool.

Once you do this. Use gasket tape to line the front baffle driver cut out and mount your sub.

This is how I make an insanely good sealed box sub. Don't worry about it being sealed. This thing can fill a house in a sealed box. Your car will be easy. If you want to add strength to the joints you can try biscuits or dowels.
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post #10 of 10 Old 12-16-09, 09:09 PM Thread Starter
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Re: Broken box(es)

Thanks...I don't have much experience on the subwoofer side of things. More so on the woodworking.

Sounds like I should be modeling the subwoofer in WinISD or equivalent program instead of building to manufacturer specs.

Building a box with birch is my next project (as soon as the weather takes a turn for the better).

I was using Titebond and it has done well. The matrix idea is rough on my brain but is getting better. I tend to attempt everything in my head and am very about measurements; but I rush building boxes overall. I built the last one in 2 days since I'm only home in my garage from school every few weekends.

Thanks for the advice. I'm just glad I was brought up being comfortable with constructive criticism.
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