If a sound source is operating in an enclosed space, the SPL will vary with the distance that the measuring microphone is from the source. At certain close ranges, the levels will obey the inverse square law and at these distances, there will exist approximately a free-field. At greater distances, the reduction in measured level with increased distance will be less than predicted by the inverse square, and finally a region will be reached where the level is almost constant regardless of the distance, and this is called the reverberant field. The area between the free-field and the reverberant field is called the far-field. Its extent is a characteristic of the directionality of the sound source as well as of the acoustics of the room.

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