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Old 31st December 2011, 10:59 PM   #1
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Question 2x12" box design

Hi there, This is my first time go easy :P

I need some help designing a sub box. I am new to this so Im totally stumped on designing boxes. Ive got a couple of 12" Fusion cs-sw120s, 250rms. I have no idea of what i want, but i know i want it to be big and bassy. People have been telling me that the lower tuned the box is the deeper sound you will get? is this true lol

Anyways here are the details for my 2x12" design! this is for 1 of the speakers

Peak: 1000
RMS: 250
Frequency Response 27Hz-1kHz
Impedance: 4 Ohms
Sensitivity:88dB

Ported
Volume [litres] 50
Volume [cu.ft.] 1.77
Port Diameter 76.2 [3"]
Port Length 250mm [9-27/32"]
Optimum Tuned Frequency [Hz] 33.6
Mounting depth 136mm [5-11/32"]
Mounting Diameter 277mm [10-15/16"]

Just remember thats for one sub and the car I am putting it into has 33-34inches of boot space [wide].
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Old 1st January 2012, 01:45 AM   #2
mdocod is offline mdocod  United States
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The T/S parameters found here: 12? Subwoofer (CS-SW120) | FUSION Electronics

...are not quite complete enough to do ideal simulations.

I was able to work backwards, and came up with an estimated Fs of 29hz (143g moving mass) to match the claimed 88dB sensitivity, however, I don't know if that claim is based on 2.83V or 2.00V so this could be off a bit. Alternatively, if they are talking about a 88dB USPL rating (@2.83 into 4Ohm) then the driver likely has a moving mass of more like 210g and a Fs of 24 hz. I'm inclined to believe that either is possible. Based on the claimed range of 30-1000hz reproduction, I am inclined to believe the 29hz Fs is more likely to be closer to the "real" specification but either way it doesn't matter.

I will take the 250W RMS claim with a grain of salt but use it for the sim anyways, and also, since no Xmax value is listed, I will estimate it to be 5mm.

---------

The charts are based on 2 drivers total, 3.54ft^3 (total), and a pair of 3" diameter ports tuned to ~33hz (as per "suggested")

The first chart shows a theoretical maximum SPL in open space at 1 meter based on an Xmax limited Pe of ~200W [100W per driver] (requires a ~30hz high pass to protect driver from over-excursion).

The second chart shows the Pe as a function of Xmax and thermal, so you understand where the 100W per driver limit is derived from.

The 3rd chart reveals the flaw in the box design. From ~30-50hz, at 100W per driver, the port noise is going to be very audible. Anything below ~40ft/s is considered acceptable in most builds. In a trunk, a little higher is probably alright, but your deeper bass notes are probably going to make whooshing noise. Below 30hz is is only applicable if you leave it without a high-pass, though even without a high pass, there is generally very little musical content down there anyways. I think at minimum I would move up to ports of roughly 4" x 15" instead of the suggested ~3" x 9".

There are lots of directions you can take this. It depends on your listening goals, how much amplification power you have available (or want to make available) and how much trunk space you have available (or are willing to give up). For many practical reasons, you will see that most sub boxes in cars are smaller sealed boxes. Smaller boxes will generally translate to higher amplification needs to reach the same listening levels, however, amplification these days is relatively inexpensive, and large complex ported boxes are heavy and awkward.

Eric
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Old 1st January 2012, 11:48 PM   #3
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Thanks Eric, you really went to a decent length to make your point, I appreciate that. So for best resaults im going to want a box with 3.54ft of total air volume [would that best work as 1.77 in two seperate sections?], and tuned to 33Hz.

Needs: mainly techno crap. but from time to time, old school metallica and some of the newer metal/rock

The car Im using is a station wagon. boot space:
max height: 17" [to top of back seat]
max width: 34" [side to side of boot]
length faily unlimited: 41"

The max i would build to would probably be:
height: 17"
width: 34"
length: 18"/19"

I would like to have it LOUD! but i still want some boot space. i wouldnt mind if the specs where something like this:

I had a go, but its probably rubbish

2x12" sub box by Computerdude - Google 3D Warehouse
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Old 1st January 2012, 11:49 PM   #4
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Thanks Eric, you really went to a decent length to make your point, I appreciate that. So for best resaults im going to want a box with 3.54ft of total air volume [would that best work as 1.77 in two seperate sections?], and tuned to 33Hz.

Needs: mainly techno crap. but from time to time, old school metallica and some of the newer metal/rock

The car Im using is a station wagon. boot space:
max height: 17" [to top of back seat]
max width: 34" [side to side of boot]
length faily unlimited: 41"

The max i would build to would probably be:
height: 17"
width: 34"
length: 18"/19"

I would like to have it LOUD! but i still want some boot space. i wouldnt mind if the specs where something like this:

I had a go, but its probably rubbish

2x12" sub box by Computerdude - Google 3D Warehouse
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Old 4th January 2012, 09:59 AM   #5
mdocod is offline mdocod  United States
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Model looks fine to me. Though I don't know the effective port cross section and length, as long as it is tuned right with enough port cross section it will work fine. If you're willing to give up that much space that would be a nice naturally rigid design with plenty of vent space to keep chuffing to a minimum.

You just have to weigh your options. The maximum box size you are talking about there will encompass over 5ft^3 of space when finished. You could just shove the pair in a total of 1.5ft^3 sealed and it would be fine IMO.

Eric
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Old 5th January 2012, 04:16 AM   #6
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Default Help me please

Could you design a box for me on google sketch, or give me dimensions? vented 30-33Hz @ 3.5Cu.ft I got plenty of boot space i really don't know what im doing and i dont wanna waste heaps of cash on a box i came up with in 10 minutes lol that might or might not work. thank you. i hope iam not asking too much. I just need some help, i dont ask for help much but getting these speakers to work optimized is important. Thanks Eric for all your help so far.
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Old 5th January 2012, 12:36 PM   #7
mdocod is offline mdocod  United States
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I haven't used sketchup in so long it would take me way to long to re-learn.

In order to save on some wood, the only change to the box design you came up with that I would suggest is to remove the baffle that splits the 2 ports as they are lead to the front of the box and just adjust the position of things such that where the 2 ports come together to form a single larger port, the cross section there should equal the combined cross section of 2 ports. Your approach is fine.

Assuming an overall box size of 17x34x19, build the port with the same approach as you had in mind there with the googe sketchup, just eliminate the center baffle that separates the 2 ports, and eliminate the switch-backs in the port design. The port should be 15.5" high (front baffle less 1.5" for wood thickness on either side), 4" wide, split into two 2" wide sections at the rear that form the top of a "T" shape for the port looking at it from the side. By my math, if you run the top legs of the T over to each "end" of the box leaving a 2" gap before the side-wall, the tuning should line up just fine and the vent air velocity will actually be lower than the goal. I estimate the resulting tuning frequency to be in the neighborhood of 35hz.

If you want to get the tuning frequency down to 30hz, do the exact same layout but reduce the port dimensions to 15.5" x 3" (1.5" where it is split at the rear), and run the length of the top of the "T" out to within 1.5" of the side wall. That will increase the box space and lower the tuning frequency of the port, which will help bottom end, however, port air velocity will potentially be higher (still below goal IMO so not a big deal).

Anything in the 3-4" port width will work fine. (divide by 2 for where it splits at the top). Other slight changes in box dimensions are not likely to make a meaningful change so don't worry about it too much.

Eric

Last edited by mdocod; 5th January 2012 at 12:44 PM.
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