The medium through which sound is travelling is very important. Sound waves (vibrations) travel more quickly through dense materials like water and steel than through air (which is not very dense). The higher the density of the material, the faster the transmition of sound.

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Shea says this:

• Speed of sound dependent on the medium: faster in denser media
• Faster in water than in air
• Faster in steel than in air (that's why we listen to the steel rail to hear when the train is coming)
• What happens in a large theatre during the time it takes for sound to reach the audience?
• Most of us can easily see when film is two frames out of sync
• Pull up the sound by one frame in motion picture release prints, so the sound comes early
• In a large theatre the film will be out of sync for some, in sync for others