“Reducing noise in the academic library: The effectiveness of installing noise meters” ($) in the paywalled Library Hi Tech. The paper…

“explore[s] the effect of an electronic [light-up] noise-monitoring device (NoiseSign) at reducing noise levels in quiet study areas in an academic library.”

Perhaps their NoiseSign could also trigger a powerful signal-blocker that would block all phone connections within 50 feet for thirty minutes. The resulting peer-pressure on talkative teens could be enormous.