Splitting sub enclosure between trunk and cabin?

[Laserface 10+ year member]

I want to place an 8" sub between the rear seats in my 1991 BMW 850i. I've attached 2 images, one showing the rear seats, and another showing the opening in the trunk/cabin wall.

Having the sub between the seats on the cabin side like I want wouldn't allow for a properly sized enclosure.

If I were to mount the sub cabin side, sealing it to the pictured opening, with a properly sized enclosure sealed to the trunk side of the opening, will it function properly?

Click to read more...

 

[Popwarhomie 5,000+ posts]

4th order bandpass. Port through ski hole.

 

[Laserface 10+ year member]

The issue I'm having is that a box on the cabin side can only be 11"x10"x5.5". I can only get around 0.15 cubic feet in the cabin. For aesthetics, I'd prefer to have the sub as close to the bulkhead as possible with a shallow mount sub. This led to my idea of having the rear bulkhead be part of the enclosure.

The idea would be just enough of a box on the inside to hold a 8" shallow mount sub sealed to the opening. On the trunk side, would be a box of appropriate volume, also sealed to the opening. Together, they'd make a sealed sub enclosure. I'm just not sure if this is viable. The opening in bulkhead will act as a baffle, but I'm not sure how that would alter function.

Unless you're proposing the 4th order bandpass box in the trunk, with the port into the cabin?

 

[Laserface 10+ year member]

Here is a quick mock up I did in SketchUp of the design I'm proposing. I've read that shape isn't as important as volume in box design, but I can't find any examples of someone designing a box like this. Both sides will be sealed to the trunk wall, and form two halves of a sealed enclosure. Will the opening in the trunk wall disturb the function of the box?

 

[Popwarhomie 5,000+ posts]

Unless you're proposing the 4th order bandpass box in the trunk, with the port into the cabin?

Yes. That the best option for vehicles designed like that.

 

[Laserface 10+ year member]

Yes. That the best option for vehicles designed like that.

In this situation, what should be on the cabin side of the port? Currently there is a hard plastic box that formerly contained a first aid kit. Mine is missing. Is it best to have some sort of vent like a speaker grille on the cabin side?

EDIT: Did a quick SketchUp of a box that should:
-Fit in the trunk with minimal space loss
-Satisfy the manufacturer's recommended sealed box volume
-Have a 1 to 1 ported and sealed volume

 

[shredder2]

Op sounds like a 4th skipassed is alien to him.

Idk... to me it looks like a fail Unless done right or something noobs tell themselves is cool. Placing the driver towards the center of the cabin never ends well unless walled usually unless the trunk is sealed off.
You gonna lose space in the trunk to do it right, no way around that fact I think. Placing part of the enclosure in the backseat/trunk is... well... idk.
Doing a 4th is proven time and time again in BMW with a trunk/blowthru.. why get off on a tangent?
A single 8' sub even in a 1:1 4th will not be TOO huge imo... Unless you're one of those guys Op who is used to a sealed footprint for the enclosure.
Bass takes SOME space... but backseat infringement in my book is awfully bad.
Jmo

 

[Laserface 10+ year member]

I appreciate the input.

Op sounds like a 4th skipassed is alien to him.

It IS alien to me. I'd never heard of a "4th" until Popwarhomie mentioned it.

Idk... to me it looks like a fail Unless done right or something noobs tell themselves is cool. Placing the driver towards the center of the cabin never ends well unless walled usually unless the trunk is sealed off.
Doing a 4th is proven time and time again in BMW with a trunk/blowthru.. why get off on a tangent?
A single 8' sub even in a 1:1 4th will not be TOO huge imo... Unless you're one of those guys Op who is used to a sealed footprint for the enclosure.

I'm not looking for anything huge. I'm redoing the 30 year old factory system, and I see a chance to make an upgrade. I just want to add a little bass. Trunk space is more important than bass. That's why I was proposing a shallow mount 8 with a small, sealed box. I'm just not sure if my half and half box would function properly. I guess it seems like I'm trying to reinvent the wheel here.

Bass takes SOME space... but backseat infringement in my book is awfully bad.

The back seat in this car is beyond useless. When comfortable in the driver's seat, there is 1" of rear leg room. One inch. It's very unlikely there will ever be a passenger back there. I believe "backseat infringement" is a little harsh for what I have planned. The car is undergoing a bit of a restoration, and will be in the upholstery shop anyway. It would be easy for me to build something similar to what's in the attached photo before it goes to upholstery shop. It would have the same footprint as the current (empty) first aid container, level with the bolsters on the rear seats.

Given context, do my box designs make more sense? I'm more interested in making the best of the minimal space I'm willing to lose, than I am with maxing out the bass.

 

[Jeffdachef 5,000+ posts]

If you have roadside assistance you might want to look at a spare wheel enclosure. It'll be a lot better than your current ideas. The only way you'll get any result louder than your door speakers with an 8 is with the 4th order bandpass and that box will be very big.

Why are your current ideas bad? Because of phase cancellation. Too far from a proper loading wall aka the trunk lid results in Incident waves colliding with the reflected waves pretty much negating all bass so you literally feel like you wont even have a sub. We've fixed too many setups like this through the years. People blame the gear but its literally the positioning, its very important acoustics wise to have the sub in the right orientation and setup.

 

[Laserface 10+ year member]

If you have roadside assistance you might want to look at a spare wheel enclosure. It'll be a lot better than your current ideas. The only way you'll get any result louder than your door speakers with an 8 is with the 4th order bandpass and that box will be very big.

Why are your current ideas bad? Because of phase cancellation. Too far from a proper loading wall aka the trunk lid results in Incident waves colliding with the reflected waves pretty much negating all bass so you literally feel like you wont even have a sub. We've fixed too many setups like this through the years. People blame the gear but its literally the positioning, its very important acoustics wise to have the sub in the right orientation and setup.

Thank you. I'm not really looking for design suggestions, but rather a critique of my design. If it's bad, it's bad. My designs were 100% made to fit within space constraints with little regard for function. With the space I've given myself to place a sub/enclosure, it seems I'll be putting in a lot of effort for minimal or no gain. I'm not willing to lose my spare tire. I think the best option for me is simply to rebuild the factory system with the best components I can source and be done with it.

 

[Jeffdachef 5,000+ posts]

Thank you. I'm not really looking for design suggestions, but rather a critique of my design. If it's bad, it's bad. My designs were 100% made to fit within space constraints with little regard for function. With the space I've given myself to place a sub/enclosure, it seems I'll be putting in a lot of effort for minimal or no gain. I'm not willing to lose my spare tire. I think the best option for me is simply to rebuild the factory system with the best components I can source and be done with it.

lets say this, you would be no better off with that sub idea than if you never installed it at all. Thats how bad that alignment is.

 

[Laserface 10+ year member]

lets say this, you would be no better off with that sub idea than if you never installed it at all. Thats how bad that alignment is.

Which questionable design are you talking about? With my 2 halves of a whole design, the only thing that can be adjusted in that situation is the volume of the trunk half. It is what it is. With that design though, would it actually function as a sealed box? Or is the big metal baffle in the middle of the "box" going to destroy waves, or frequencies or whatever? Asking, you know, for science.

By alignment, do you mean the sub being directly behind the port? In my "4th order" design, the sub can moved anywhere inside the box, and not just placed directly behind the port. The port is stuck in the middle, though.The attached picture shows this, along with the max dimensions I could fit within my constraints. 34"Lx14"H with a width between 6-9".

Both of my designs, even if they were viable, are compromised from the beginning because I'm limiting myself to shallow mount subs.

 

[shredder2]

A 4th order bandpass for a single 8" sub need not be too large, granted the one pictured below is a lower power one but having a 4th with the port positioned in the Skipass with the remainder of the opening sealed off is usually how its done for most Bmw in my exp... and it works well.

That ^^^^ ain't too large imo, and should reinforce the lower frequencies pretty good with a lower powered setup.

 

[SlugButter]

Working on a 4th order for an 8 now with the port on the outside of the box. A 4 inch circle into the cab is all you would need.