Why can sound be heard from a room by an person even before he or she enters the room? 

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Sound waves are longitudinal mechanical waves that are produced by the vibrations between particles. Mechanical waves, such as sound waves, travel through a medium. A medium is the substance through which energy, such as a wave, is transmitted. Thus, the energy from the sound waves is carried by the air through which the sound travels. In this way, sound from another room can be heard by an individual even before he/she enters the room.

The speed at which sound travels through a medium depends on the phase of the medium. Sound travels the fastest through solids. Because the particles of solids are so close to one another, they can easily collide into one another to cause the vibrations that make sound waves.  Likewise, sound travels the slowest in gases. The distance between the gas particles makes it hard for the gas particles to collide with one another and vibrate to produce sound waves.

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