struggling with enclosure dimensions

Hi All,
I have a JBL GT4-10 Sub, i'm looking in to making my own box for it, to go in the boot of the car.

i've been round and got various programs that work out box dimensions. but i've not a clue...the dimensions it gives me for the spec of the sub are strange, ie. the front face of the box, where the sub would be mounted, i have less than 20mm either side...

my maths skills are practically non-existant so i can't work it out my self, well won't because i'll mess it up =\

so if anybody knows what they're doing when working out dimensions....i'd be very grateful :)

Specifications

RMS Power Handling :250W
Peak Power :1000W
Frequency Response :28Hz - 500Hz
Sensitivity :90dB
Impedance :4 Ohms
Voice-Coil Diameter :50mm
Cut-Out Diameter :232mm
Mounting Depth :150mm
Overall Diameter :258mm
Vas :36.10L
Qts :0.42
Fs :29.6Hz
Qes :0.45
Qms :6.45
Sealed Enclosure (vol) :21L
Sealed Enclosure (vol) :50L
^^ eh? sealed enclosure two different numbers for the same thing =\

any help on this would be fantastic...
:)
 
Greets!

Well, not having a clue what car you have, or even if I did I still wouldn't know its transfer function or what the max size cab its boot will accept, etc. for that matter, any recommendations I make are purely speculative.

Assuming a typical midsize American/J.A. Pan sedan works well with a ~40Hz acoustic F3, so based on this I'd use ~1.46ft^3. This equates to ~13.63"/side if a cube and ~22.00" x 13.63" x 8.43" if a golden ratio rectangle (all dims inside). Since it's sealed and going into a confined space, it doesn't matter what side you put the driver on.

If you want a slower roll off (more LF), then stuff it with up to a couple of lbs of polyfil, or if more midbass 'slam' is desired, then reduce the cab's Vb by adding sealed bags of kitty litter or similar to suit.

GM
 
GM said:
so based on this I'd use ~1.46ft^3. This equates to ~13.63"/side if a cube and ~22.00" x 13.63" x 8.43" if a golden ratio rectangle (all dims inside). Since it's sealed and going into a confined space, it doesn't matter what side you put the driver on.

GM [/B]


hey,

that is fantastic, thanks for your help!
I will definatly let you know how i get on with this, I will try the cube with 13.63" per side.
 

micb

Member
2004-08-03 1:44 am
UK
The JBL GT 4 is a great budget car audio driver.

Have listened to a few of them in cars, a good size to use is the JBL recommended 0.8 gross cubic foot, once you take into consideration the driver displacement you get bang on the Qtc 0.707 alignment.

You can either buy an off the shelf prefab box or build you own.

I can give you the dimentions for a box no problem if you are working to space contraints.

IMPORTANT.

When doing area calcuations rectagles or squares or other shapes you must not forget there is a difference between "outside" an "inside" space dimentions.

The thickness of the material is a factor it reduces airspace, this can be as much as a cubic foot, bigger boxes can be even more.

e.g.

Internal measurments 11.5x11.5x10.5 = 1388.6 cubic inches.

1728 inches in a cubic foot so:

1388.6/1728 = 0.803 cubic foot.

Our wood (18mm - 3/4 inch MDF) takes up space so add 1.5 inches (2x 3/4 of a inch) on each dimention so for example.

Externaly that works out as 13x13x12 inches.

If you want the invidual wood cuts goto this page and scroll down to the "Enclosure Design Helper".

http://www.bcae1.com/spboxnew2.htm

Any questions just feel free to ask.
 
The last statement is very true concerning inside dimensions is crucial, there is usually a 10% area where the sub will respond very similar however once you leave the 10% and you'll have a sub that doesn't sound to good. For instance I have a truck I was going to build a .75" MDF box for and discovered that the MDF would take up almost .8 ft^3. There wouldn't have been enough room for the sub even. Good luck.