When a librarian hears someone talking they tell them to be quiet. What is happening is they are hearing sound waves. Sound is a mechanical wave observed by detecting vibrations in the inner ear. Sound can travel across the library because sound can travel through air, water, wood and even steel. The waves that are produced when you talk are longitudinal waves. The speed of sound in air at STP is 331 m/s. So when you talk, it gets to the librarians very quickly. The sounds you make all have different frequencies and amplitudes. Librarians prefer sounds with low frequencies and low amplitudes. The longitudinal waves that you produce when you talk have compressions and rarefactions. If the librarian is on her way over to yell at you for talking, since she is approaching you, according to the Doppler Effect she will hear your voice at a higher frequency than your friends at your table. While in the library, create sound waves with very small amplitudes and you should be fine.