That's a pretty substantial claim that spending, potentially, thousands of dollars is the, potentially, only way to create a cost-effective solution. And that one must employ an acoustic consultant to solve a problem with noise.
I'll start small: If i build a fort with a few couch cushions around me, i will hear a lot less white noise. white noise that is already in the room, being reduced by the foam/foam-like stuffing of the cushions. But it is being reduced to my ears so long as they are on the inside. If i remove one of the cushions so there's say a door, a window, or a sky light the overall noise will increase, but not substantially. so removing a piece will only partially reduce the integrity of the sound reduction. obviously i'd have to model an exact situation to find the exact noise reduction loss and nobody's here for that, but i'd put all of the money that my kids will ever earn on the fact that in 100% of cases having some of the cushions in place would still lead to noise reduction.
Moving a little bigger: if we were to have a round shower curtain rod, suspended in the middle of the room, unattached to any wall, and try hanging a thin plastic shower curtain, a thick quilt/foam pad, heavy drapes, or nothing... obviously hanging the quilt/foam/drapes is going to reduce the noise, that is already in the room, more-so than the thin plastic (or a window film) which would reduce the noise slightly more than nothing at all.
Moving to the room size: at some point the shower curtain rod thing would grow until it is the size of the room. it will still reduce any noise that comes from outside of it, and that comes in from the outside of the room. it'll probably be less effective than when it was smaller, because now the inside of it is a hollow chamber reflecting the "noise" back in on itself. but it will still remove and reduce some to a lot of noise depending on as you said the method the noise gained access to a room (if it is a sound wave travelling through a solid medium, down the railway supports, through the ground, into the foundation of your building, up the steel/wood beams supporting the building, thus vibrating the drywall of the walls as if it were the membrane of a speaker/headphones) and also depending on the frequency of the sounds wave. but the point is that no matter what the circumstance it will still make a difference, and in most cases a noticeable/substantial difference. if nothing else it acts like a baffle, muting the extremes in volume of the sound.