In a crowded restaurant, diners usually talk with their companions at their own table. But the sound of their conversations bounces off walls and reflects to other patrons, creating background noise. Each individual speaker wants to be heard over that noise, so they end up talking a little bit louder, which again increases the overall din. Eventually—barring an interruption—the system gets loud enough to reach the limit of the human voice.
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Source: Phys.org