I've added a big 12" sub to the trunk which has enormous bass when standing in front of the open trunk, but only mild bass inside the cabin.

Clearly, the sound deadening material in the trunk on the fuel tank works very well. How can I port in just the bass from the trunk without opening up the cabin to road noise or fuel pump noise?

 

Simply put, you can't. The road and fuel pump noise, are present in the trunk. As soon as you create an opening into the trunk, you introduce the noise. The only way possible to achieve that would be to literally port the sub box directly into the cabin, which would require awful creative engineering. If you only ported the box into the cabin, you're looking at a specific frequency response that you'll hear most, but would at least be better than what you have.

The only way to open the trunk at all and seal the road noise out, would be to seal off the box and opening together, and isolate that all from the fuel pumps and rests of the trunk space. And that's not going to happen unless you completely re-engineer the placement of the fuel tank to give yourself a way to do it.

ALLLLL that aside. What you could do, is open the rear shelf, remove the factory sub, or however you want to create that space. Then, dynomat the entire trunk to try to minimize road noise (at a cost of some weight). Finally, you need to come up with some sort of... for lack of a better word, blanket.. to enclose the fuel pump assemblies on the top of the gas tank. Basically, sound insulation. it's the only way to minimize the noises from them, and the outside, but still give a path for the sound to travel.

 

Excellent information, thank you - that's way more effort than I'm up for, lol

I guess that I just read too many posts about people putting a sub in the trunk (some clearly smaller than my 12") and saying that it was clearly audible in the cabin, only to be disappointed to realize the volume difference is easily 3-4x between inside the cabin and in front of the open trunk.

 

There is actually another option. I run a sealed box, and it is mounted very solidly to the car. There is very little bass when standing at the trunk, but plenty in the cabin, as the bass is basically transferred through the chassis of the car, rather than into the trunk airspace.

 

I'd be pretty impressed if a majority of bass is being transferred through the box through chassis to cabin. That being said, it wouldn't be unusual to have little bass when standing behind the trunk but lots outside just due to the way waves bounce around. When my eclipse had a sealed sub setup in it, the bass was phenomenal inside the car, but simply standing outside of it with the trunk open was somewhat disappointing.

On the other side, if you do want to transfer vibrations to your panels or seat or whatever, there are "tactile transducers" which basically replace the cone of a speaker with a weight, and vibrate whatever they're attached to. I have one I haven't used in a while, they're pretty neat. Just make sure to have a low pass set, otherwise your music starts talking to you through your hips, a bit unnerving..

AuraSound AST-2B-4 Pro Bass Shaker Tactile Transducer

 

I run two Memphis Audio 12's in a ported box, facing outwards(towards bumper) & I have ample quality/clean bass in the cabin.

 

Mine is pointed in the reverse, straight into the sound deadening material over the fuel tank. This makes the installation much less vulnerable to objects moving around in the trunk and I figured the direction of the cabin would be good (since the sound needs to pass through the insulation regardless.)

If I were building an enclosure fit the x308 I'd engineer a bandpass design with port at the very top going straight over the top of the fuel tank.

 

Mine were installed to breathe better & utilize the trunk's acoustics, I rarely have anything in my trunk, so no space issues there. I ran the same sub set-up in my departed XJ8L, yet the bass sounds better in the base lol.

I tried front-firing the box in my base 308 but I noticed it was being reflected/bounced back towards the rear/bumper. Rear firing, bounces it back to the cabin much better I've found.

 

It's called 'cabin gain', and it's basically what I was mentioning above. It's caused by resonances, and the bass being transferred through the chassis.
I couldn't say whether the majority of the bass you get in the cabin is purely acoustic, or whether it's caused by resonances, or bass being transmitted through the chassis, but the end result is the same, plenty of noise inside the car, and very little outside.
Which is perfect, because that's exactly where you want it

 

I removed my factory...ahem...bass and put a piece of black foam (not the really dense part) from one of my pelican cases in its place under the speaker grill to allow the bass to come through. I have absolutely no fuel pump or road noise, but the foam allows the base to push through.