Yeah, shelves are like a mini stage for sound waves. If you pile thick, heavy books, you’re basically throwing a wall of material that absorbs mid‑range frequencies – great for killing booms, but it can also dampen the room’s natural warmth. Thin, light stacks let higher frequencies bounce more, so you end up with a livelier ambience but more echo. The trick is to mix the two: place a dense, heavy stack on one side to anchor low end, and a lighter, more open arrangement on the opposite side to keep the high end bright. Also, vary the spacing between shelves – tighter gaps cause more diffraction, which can smooth out harshness, while wider gaps let sound pass through more cleanly. Think of the books as a tunable filter: adjust the stack size, material, and gaps until the room’s response hits the sweet spot. And trust me, I’ve spent a night humming a 432‑Hz hum to test just how much a single book can change the vibe.