Trunks can be very noisy places. Get somebody to take you for a short ride with you in the trunk. If your rear seat backs fold down, open the pass through and take a drive. Mostly tire and exhaust noise. In many things the only thing between you and the rear tires and exhaust is some sheet metal and your rear seat backs.
It's worth having a plan. There are two basic ways to deal with a trunk. The best noise reduction solution is to combine the two. We don't often have a choice.
Isolate the Trunk From the Passenger Compartment
If we can keep the trunk noise inside the trunk, we won’t really care that it is there. The technique suffers a little since you want the barrier to be as close to the noise source as possible, but can work well. There may be no choice if you’re working on a car that will have an exposed painted trunk.
A big advantage is that you will use a lot less material and have an easier project. It would be a good idea to read the Car Audio Considerations section below.
Don’t forget the rear deck in this. You’ll want to cover the partition side to side and from the back seat platform, up and over the rear deck.
Barrier Layers on Trunk Floor and Quarter Panels
>Here you are combining the passenger compartment air space and isolating both from the outside world. If you won’t be doing both treatments, this is probably the one to use for the best performance. If you have a sub-woofer in a box in the trunk, you must use this approach.
Car Audio Considerations
People put sub-woofers in boxes in trunks. I don’t know why, but they do. To me, this is like putting your home theater’s sub in your neighbor’s house. The trunk is by far the noisiest area in any car. If you hear the sub, you’re going to hear the noise, unless you turn it up really loud. Now you have a new problems.
>Luxury car trunks are referred to as “bass traps” by car audio enthusiasts. That’s because the passenger compartment is so well isolated from the trunk acoustically. It’s pointless to put a sub in a box in a trunk like that.
>IB sub(s) in a car with a trunk makes more sense to me.