A quiet library isn't an absence of sound engineering; it's the product of a lot of it. Carpeted floors, felt-lined book returns, high ceilings that disperse rather than reflect sound, deliberately placed soft furnishings that absorb rather than bounce — every material choice in a well-designed reading room is doing acoustic work most visitors never consciously notice, precisely because it's working.
Silence as an engineered default
Left alone, a room full of hard surfaces and people amplifies itself: one raised voice makes the next person raise theirs to be heard over it, and the room ratchets louder by feedback rather than by anyone actually intending noise. Acoustic design breaks that feedback loop before it starts, absorbing the first raised voice before it becomes the reason for a second one.
Notification design has the identical feedback problem
An interface with no restraint on notifications runs the same runaway loop: one app's badge earns attention, so the next app adds a badge to compete for the same attention, and the "quiet" baseline ratchets upward until every surface is shouting for the same reason the library would be if nobody had installed carpet. Good notification design does exactly what acoustic engineering does — not eliminating signal, but absorbing the unnecessary sources before they force everything else to get louder to compete.
Silence was never the absence of design. In a library or an interface, it's usually the most expensive thing in the room.
Anyone auditing where an interface got loud might start by asking the same question a library architect asks of a noisy room: what's amplifying, and what should have been absorbed a long time ago.