Inverse-square law

The law of distance or distance law describes the amount of decrease of a physical quantity as a function of the distance to the source or the transmitter. Prerequisite is a free field without reflecting boundary.

For energy sizes - the 1 / r ² law

Among these variables include not only radiation intensities ( energy quantity ) eg X-rays, radioactivity, solar radiation ( visible light radiation ) or other in all directions propagating electromagnetic waves and the sound intensity.

The energy E, which assumes a three-dimensional space in a uniformly radiating source, is spread over a spherical surface, which is proportionally larger with the square of the distance r from the source. The radiation intensity I, that is, the " power per unit area " (P / A), therefore, increases with 1/r2 from:

The intensity thus falls in the distance is doubled to a quarter of the initial value. This corresponds to a decrease in level by 6 dB.

In general, the level can decrease calculated as follows:

Or

Here lg is the logarithm to base 10

For effective values ​​of linear field sizes - the 1/r-Gesetz

These variables include, for example, the acoustic field quantities such as sound pressure, sound velocity and Schallauslenkung as sound field sizes. The effective values ​​of these quantities take inversely proportional with the distance from the sound source, that is, with 1 / R:

When the distance is doubled, the values ​​thus fall to half of its initial value. This corresponds - as with the square sizes - a decrease in level by 6 dB. Thus, here goes for the level change in dB:

Or

Level changes can be given about ie without knowledge of whether it is the measured variable to a quadratic or linear size. For a determination of the physical units such knowledge must be available.