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48 TRG-TRC007-EN
notes
period four
Equipment Sound Rating
Fields of Measurement
To measure sound pressure correctly, it is important to understand the behavior
of sound in various environments, or fields.
In theory, a free field is a homogeneous, isotropic medium that is free from
boundaries. In practice, an example of a free field over a reflecting plane would
be a large open area void of obstructions, like a parking lot or meadow.
An ideal sound source, that is, one that radiates sound equally in all directions,
placed in a free field generates sound-pressure waves in a spherical pattern.
At equal distances from the source, the sound pressure is same in all directions.
As the sound waves travel farther away from the source, the area of the sphere
increases. Doubling of the distance from the source spreads the sound over
four times as much surface area.
Free Field
source
source
Figure 53