Up to 20dB (Silent): Almost complete silence. The sound is comparable to the rustling of leaves or a whisper in a quiet room. Ideal for office computers, home theater (HTPC), or if you want absolute silence when working at night.
20-30dB (Quiet): "golden rule". It is the typical sound of a quiet home or a library. It sounds like a soft fan in the background, which is usually completely covered if you are wearing headphones or if music is playing. The best choice for everyday use and gaming.
30dB and Above (Audible): Usually refers to coolers that operate at full power (overclocking) or small, high-speed fans. Recommended if you care only about maximum cooling performance and don’t mind the noise.