I’m autistic with relatively high sound sensitivity. I’m wondering if another autistic person in this community used any soundproofing in an apartment that actually reduced noise coming from a busy street. There are a lot of different products out there and I don’t want to buy something that doesn’t work.
You forgot impedance layers and intentionally making them mismatch, which causes considerable energy loss between them without necessarily having to rely on a lot of mass.
Wood > low density foam > wood causes a ton of reflection.
Wood > pliable/stretchy rubber > wood causes a lot of sound energy to turn into deformation forces on the rubber, converting it to heat.
I didn’t forget, I just lumped those under “mass” for simplicity because that’s what they are, masses. Sure they work more effectively through the mix of materials but like I said, they can engineer a soundproof room in a room if they want but that’s beyond the scope of a simple comment. Two layers of wood like you have in both examples is a not inconsequential amount of mass.
Also need to be careful with any design because the triple leaf effect is real and weird and doing layers like that inside a room might not have the desired effect, especially if they overlook coupling to the floor/ceiling and just created a drum instead of a dampener.