Ported Enclosure Advice

[IonicOwl]

Hey everyone, newb on the forum and recently started taking an interest in car audio.

I tend to put most of my money and effort into the mechanical aspect of my car, so my audio setup is on the budget side.  A friend of mine gave me a JBL GTO 1264, 12" 300W (RMS) 4-Ohm single voice coil subwoofer.  Here's my first attempt at a 1 cuft sealed enclosure, as recommended by JBL on their data sheet, I made a few years ago.  It's been carpeted since:

Not a woodworking masterpiece, but it sort of worked.  I've been using it for a few years and I've always suspected that something's not right about it.  I put the lack of low end down to it being a sealed box, but it's not even that punchy, so I'm thinking that the recommended volume from JBL might be too small.  I've therefore decided to rebuild the box, but this time go for a ported setup.

JBL recommend a 2 cuft ported enclosure, but I'm not falling for that one again, so I've used WinISD to calculate the box volume I need.  I've ran into a few question marks though, which is why I've started this thread, so I hope someone can point me in the right direction.

The main issue I've faced is getting the TS parameters from the data sheet into WinISD in the first place.  My driver isn't on the built-in database, so I've tried my best to add it in myself.  Problem is that either some parameters in the sheet use different notation than WinISD expects, or the data sheet just doesn't include them at all.  That means I've had to make an educated guess using information from various forum posts, or leave it blank and hope for the best.

These are the TS parameters from the data sheet:

(I'd paste the text in, but the PDF won't copy all characters for some reason, the full PDF is on my Google Drive https://drive.google.com/file/d/0B4d_sTx7kSuZVDhkOFpqd29EdWtwTEpfY2tXT3J2MkxqNnJZ/view?usp=sharing)

...And this is what I got from WinISD:

The yellow line is my existing sealed enclosure (looks weird), and the green line is the proposed ported enclosure.  Before I go to the effort of designing the enclosure and gathering materials, can anyone with better experience than me see anything wrong with this?  I really don't want to mess this up a second time.  Any help will be massively appreciated.

Driver: JBL GTO1264 (300W RMS - 4-Ohm SVC)

Amp: Bass Face DB1.1 monoblock

[Joe X]

Took the parameters to my software, in-car performance for sealed 1 cf and ported 2 cf @ 34 Hz (green plot) is as follows:

 

 

I actually think their recommendation is fine is just a matter of properly calculating internal net volume, not sure if your sealed box is 1 cf net internal volume or not.

 

 

[IonicOwl]

Thanks for the reply Joe.  Sorry it took so long to reply, I thought I'd turned on email notifications for replies.

The net volume point has been a problem for me too.  On their data sheet, JBL suggest a 1.75 cuft enclosure  (not 2 cuft like I mistakenly put in my OP), but they say "includes driver/port displacements".

 Is that to say that I have to add the driver/port displacement to the recommended 1.75 cuft volume, or have they done the calculation for me and expect me to make a 1.75 cuft box, then add the port/driver, leaving an actual internal volume of less than 1.75 cuft?  If it's the former then why have they not included the driver displacement in the datasheet?

Hope the above makes sense.

[Joe X]

For those low power 12s 0.1 cubic feet of driver displacement will be about right and they mean net volume with 1.75 so you need to add those displacements to calculate the gross volume which is the internal volume of the box you need to build.

Here's a sample spec for a box made with 0.75" wood:

Enclosure Specifications:

Fb = 34 Hz
Vb = 1.75 ft^3

External Height = 14 in
External Width = 23.25 in
External Depth = 14 in

Port Quantity and Dimensions:

Number of Ports = 1
Pipe Port Diameter = 4 in
Pipe Port Length = 13.8 in

If your calculations arrive to the same net volume and tuning you will know you have understood  how to do the job right.

 

[IonicOwl]

Ah that's a little confusing.  When I think of gross vs net, I think of it in financial terms, where gross pay is money before deductions, and net pay is what you're left with.  It sounds like it's the other way round, where gross, not net volume, is the final volume after port and driver displacement is taken into account.

So with a target Vb of 1.75 cuft, should I be making a box with something closer to 1.9 cuft, which ends up back at 1.75 after I've dropped the driver and port in?

[Joe X]

In the case I left you  0.114 +0.1 = 0.214 so 0.214+1.75 =1.964 cubic feet gross, that will be the internal volume of the box you build. And yes you work your way inside out with sub boxes.  

[IonicOwl]

22 hours ago, Joe X said:

Enclosure Specifications:

Fb = 34 Hz
Vb = 1.75 ft^3

External Height = 14 in
External Width = 23.25 in
External Depth = 14 in

Port Quantity and Dimensions:

Number of Ports = 1
Pipe Port Diameter = 4 in
Pipe Port Length = 13.8 in

If your calculations arrive to the same net volume and tuning you will know you have understood how to do the job right.

That's reassuring, I've just fired up WinISD again and compared numbers with the 1.75 cuft box you came up with, and it's very close:

Vb = 1.74cuft

Fb = 34Hz

Port Dia = 4"

Port Len = 13.48"

 

I'm going to have a play in Sketchup and get an actual box design set up.  I'll post it up in here once I have something.

Thank you so much for the help on this Joe, you've gone above and beyond