I see nothing wrong with bracing as that is what car manufactures use for structural integrity. Think about the under side of a hood or trunk lid. See the web of bracing?

Yeah, but those things you suggested as an example are flat areas that were welded before they were painted or put in the car.

Bracing is going to take a lot of time, not to mention space which I would like to retain.

Expanding foam on the other hand sounds like a good idea. I don't mind the mess or the clean up. It will be just like bracing but cheaper and faster.

I have four holes in total that I don't really need. One around each wheel well in the back and one where there would have been a second door on a four door car, behind the B-pilar.

 

The hole is like a small cave, going up and down from what you can see, a few inches in both directions.

I wonder if it will be a waste, if I add CLD tiles and then foam, or just one of the two.

Edited February 9, 201411 yr by The enD

A brace could be as simple as a piece of 10mm round/square stock contoured and glued to the inside of the panel, effectively cutting the span in half. The same panel would then require twice as much inertia to resonate. I am aware of the pockets you are referring to. I did the expanding foam in the very front of the side pockets as that area is relatively in line with the front of my baffle. I foamed it enough that it was filling the void above the strut/tire and stopped there. To be honest with you Teo I built the sealed wall that separated my trunk from the pass compartment first and worried about the noise behind the enclosure afterwards. Mine required no further sound deadening in the trunk as the pass compartment has a barrier in between stopping the sound waves. The barrier is key as Sean pointed as low frequency sound waves are longer than the material you are using.