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Discussion Starter · #1 ·
I've torn apart my classic truck to find 1) an enormous hollow cavity in the rear quarter panel 2) an empty vent out from the bottom and 3) some conveniently cut openings. The vent is quite large. Both driver (shown) and passenger side are identical. Don't worry about the black vertical bar w the hose coming out of it. I'd relocate that. Would this space work for an infinite baffle setup?

I have been tearing my hair out trying to find a place for the subs that would not take up too much of the cargo space. Is this the solution? Presumably I would need some rings to push the sub out a bit, deaden the heck out of the quarter panel and maybe even spray inside. It's pretty echoey steel. What do you think?


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See if you can get an estimate on the surface area of the "vent" as that will predicate whether this would work as an IB space. Try to get measurements at the "tightest" or most restrictive area of the vent space.
 

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Interior volume doesn't matter.

The opening will only matter for figuring out what size driver you can use without enlarging the 9" dimension by cutting metal.

With IB cone area is king. Most use 18s or bigger but it can be done with 12s - however - even a 12 needs an 11"+ cutout. So start deciding if you want to cut the hole bigger or simply use that for the vent.

Critically though; What you need to figure out is what size the "vent" is to the atmosphere and if it's only in the rear quarter panel or are there any other venues for air to get back into the cabin. The dimensions where it opens out in the rear fender well or anywhere between the opening on the interior and the outside where there's a constriction less than the 5x19 opening.

Infinite baffle means that the front of the cone is totally isolated from the back. In order for a confined space to be considered IB there has to be a vent to atmosphere with at least the same area as the Sd of driver.

Most 15s are ~130 in²
Most 12s are ~80 in²
5 x 19 = 95 in²
So with a 12 you'd be good but a 15 would be considered restricted.

If you have at least that much vent all the way to the outside then you look for a driver with a Qts of 0.7 but if the area is less than that then it becomes restricted and you need a lower Qts driver.

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I agree with above

However a long throw sub and power can make up for some lack of cone area. But it's certainly easier with more cone area.

Mounting depth may be an issue or solution.
To get the depth you will likely need you'll have to build it out a bit however that makes it easier to use bigger subs. You'll want at least a double baffle anyway.
 

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And brace those quarter panels. You don't want any movement on that subs mounting surface. The rest doesn't really matter if it doesn't vibrate. Your not building a pressurized box.
 

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Discussion Starter · #8 ·
Interior volume doesn't matter.
I am a neophyte here so maybe you can address this: if interior volume doesn't matter, why do we see so many infinite baffle setups where the speakers are between the cab and the trunk? Doesn't the volume of the trunk matter? Or do we assume that all of these trunks are actually quite vented?
 

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Discussion Starter · #9 ·
And brace those quarter panels. You don't want any movement on that subs mounting surface. The rest doesn't really matter if it doesn't vibrate. Your not building a pressurized box.
Do you mean that the vent size is less relevant as the mounting surface becomes more rigid?
 

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No. The vent size is relative to the amount of air the subwoofer is moving. In the simplest terms that's the cone size. The bracing will stiffen the mounting surface so the sub doesn't waste energy fighting against a moving panel it's mounted to.

They are in-between for simplicity. All you need to do(which isn't easy) is stop the sound waves from the front of the cone from interacting with the back. Air volume doesn't matter (as long as it's venting one side to the atmosphere) because the point of ib is using only the waves the speaker makes with no reinforcement from box pressure.
 

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Discussion Starter · #11 ·
if interior volume doesn't matter, why do we see so many infinite baffle setups where the speakers are between the cab and the trunk? Doesn't the volume of the trunk matter? Or do we assume that all of these trunks are actually quite vented?
But then we're back to this question. Are thousands of car audio people with this setup just using the term "IB" incorrectly when they should be saying "really big sealed enclosure"? @Theslaking
 

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But then we're back to this question. Are thousands of car audio people with this setup just using the term "IB" incorrectly when they should be saying "really big sealed enclosure"? @Theslaking
Trunks are vented, and very large. As the enclosure gets bigger and bigger it begins to behave as IB where the enclosure has little impact on the frequency response. If the trunk is big enough it might not even need to be vented in order to behave close enough to IB for the frequency range the sub is playing. A vented trunk isn’t technically IB, but it behaves close enough unless there are several really big subs.
 

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Discussion Starter · #16 ·
As the enclosure gets bigger and bigger it begins to behave as IB where the enclosure has little impact on the frequency response.
Interesting! So then the volume of the enclosure is relevant? I estimate the internal volume of the quarter panel is about 7.5 ft^3. That's 1/2 of an accord trunk. Shouldn't that be about similar performance with a single 12" driver given the size of the vent? Or are trunks even more vented, relatively?
 

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The volume of the quarter panel doesn't matter if the atmosphere vent allows the same amount of air to pass as the cone is moving each stroke.

7.5 cu is small enough to make it not IB if the vent is not up to the task.
 

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Discussion Starter · #18 ·
The volume of the quarter panel doesn't matter if the atmosphere vent allows the same amount of air to pass as the cone is moving each stroke.
Understood. That makes sense. But let's take your build as an example:

7.5 cu is small enough to make it not IB if the vent is not up to the task.
My quarter panel volume is about half the volume of your trunk. You've definitely got more than 2x the amount of air pushed by your cones than I would have. Are the vents in your trunk enormous? or are you relying on the size of your trunk somewhat? If the latter, then I guess it's just down to a close volume calculation.

If I decide to move forward with the IB build, I think the largest cone I could do is 12". What specs should I be looking for?
 

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You can sort of model it in winisd. Make the box roughly the size of the quarter panel enclosure and decrease Ql to like 1 (default 10) to simulate leakiness. Ql not terribly sensitive once it gets below 5 or so.

trunk baffle can be modeled the same way but with a higher Ql and bigger enclosure.
 

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My trunk is probably 16 cu + 4x8" vents. So I use the leaky box theory. I was willing to cut my vents bigger though.

I tested it (that pic is current state but not the first version) by simply popping my trunk open. No difference in response in the cabin.

I also filled every cavity and blocked every air passage.
 
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