"Relative energy" is an important concept, as we shall see shortly. Relative energy is calculated by raising ten to the power of the decibel level divided by ten. This process is used in adding and subtracting sound levels. In addition, the relative energy is directly proportional to the numbers of equal noise sources under consideration in an assessment. For example, if one noise source emits a certain amount of noise energy at a given distance, seventeen equal sources would produce seventeen times that noise energy at that distance. It is very important to remember that the relative energy is actually an energy density, or energy per unit area, and is not only dependent on the source or number of sources, but also on the distance from the source.