I have two 12" JBL GT4 subs, and i am going to start my boot build soon....

I am going to box them seperatly, one in each corner of the boot...anyway my question is?

How close does the volume of the box have to be to what JBL say it should be?

i can calculate the volume of the box simply, if i keep the box a traditional shape, e.g. cube or prism, but i want to fibre glass one side to the shape of the boot :cheese: (if that make sense) and this would make it hard for me to calculate the volume :cry:

Thanks in advance for your Help

Paul (beginner) :confused:

Paul, Theoretically your box(es) should be exactly 1.00 cubic feet each (internally).
However, in practice a tolerance of say +/- 5% will make no noticable difference.

If your box is (say) 12"x12"x11.5" internally then no problem.
If it's more like 12"x10"x6" then it will sound pants !! :chunks:

If you're really worried, people have measured their completed boxes by removing them from the car and filling them with a measured amount of water. Yours should hold 27 to 29 litres (each).
Note: Make sure you remove the subs first !!! :lol:
Oh, and dry them well before use.

If it works out a little small, just give 'em more power to play with....lurvely !

Hope that helps !

Tall Paul :)

Instead of water, you can use those little foam balls they fill bean-bags with. :)

Thanks alot Tall Paul & bibby...so +-5% of icubic foot should be ok! how much difference does it make to the sound out of curiousity? if i go bigger/smaller

i prefer the bean bag idea, sounds less messy, and once i get the fibreglass out it will get messy!!

one more quetion whats the best stuff out there to bond fibre glass to mdf?

Thanks again

Paul (beginner)

one more quetion whats the best stuff out there to bond fibre glass to mdf?


Paul (beginner)

more fiberglass

Very good nik_coupe , ""more fiberglass""

i know the fibre glass stuff is strong but i didn't know whether i there was anything out there to bond the two together better than just fibre glass!!

Just asking

Paul (beginner)

One more quetion?

1 cubic foot, is that just for the box, what i mean is, does that it include to speaker itself or do i need to calculate that aswell?

Thanks again

Paul R

1 cubic foot, is that just for the box, what i mean is, does that it include to speaker itself or do i need to calculate that aswell?

On a lot of box reccomendations, they will specify whether or not driver displacement has been accounted for... If it does not, I usually assume that it does not take displacement into account, and to oversize the box to take this into account

For the record however, for the most part, manufacturers box specs are pretty meaningless, simply an approximate guide to get the speaker working... It takes into account nothing of the environment it's working in, power available, or other components in the install. Happering the T/S perameters into BBP6, and looking at the response, group delay etc. will help you tailor the performance to what you need, even in a sealed box

Happering the T/S perameters into BBP6, and looking at the response, group delay etc. will help you tailor the performance to what you need, even in a sealed box

only one thing i can do in response to this
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the name is Paul (beginner), that all went over my head! sorry, you can nottake anything for granted with my intelligence!

But thanks for the help anyway.

Paul (beginner)

Ah sorry chap...BBP6 is a box design program, it allows you to put in the details of your sub (the T/S perameters), and it'll show you a graphical representation, of how the sub will respond, and perform in the box you have chosen... you can alter the box, and see how it will affect performance, until you are happy with how things look :)

BBP6...this would be one of those programs i dont have :(

Never mind, i will just have to play with the cardboard mock ups until i get it right :(

but it will all be worth it in the end, to finally have two 12" subs, which i have wanted for so long!!! :shiny:

Thanks alot

Paul (beginner)

Paul, this already includes the displacement of your sub.
It assumes you mount it on the outside of the box with the magnet sitting inside the box. i.e. build a 1ft net (internal volume) box then drop the sub into it and away you go. Add 200 to 300w rms and you'll be grinning for hours !

Go for it.

Tal Paul :)

Thanks for clearing that up, so it is 1 cubic foot +-5% and that includes the sub being mounted normally ( speaker mouted outside the box with the magnet inside the box!!Great :shiny:

I will hopefully be running the two subs from a rocky 400a4 amp! which i have been told will run 2x250rms! can not wait!!

I will post the results of the box as soon as it is done!

I promise this is the last question....
Does it matter how thick the mdf is that i use?

Thanks again

Paul (Beginner)

Just use a thickness that will maintain rigidity of the box... with a small sealed design like that, you should be able to get away with 12mm MDF if needs be, but 18mm would be a much better bet

18mm it is then, i aint going to compimise on sound for the sake of 6mm!

Thanks alot

Paul R