![]() Free field refers to an idealized environment where sound travels freely in all directions, without interacting with anything- imagine the entire universe entirely filled with air at uniform temperature (and nothing else). For instance, the output from an audio speaker 1” across is similar to a point source if you are 5” (or more) away from it. What qualifies for “far enough away” depends on the size of the source- a simple rule of thumb is multiply the length of the widest part of the source by five. Real world sources actually do behave like point sources, provided you are far enough away. Point source refers to an ideal sound source that sends sound out equally in all directions. The math above applies to a point source in free field conditions. Dividing the distance by a factor of twenty adds 26 dB to the sound level. That means that the distance is “two times ten times away.” Doubling distance subtracts 6 dB from the level multiplying distance by ten subtracts 20 dB. For instance, the distance ratio is 20:1.
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