When measuring sound energy levels in the AV industry, the decibel formula is used with two measurements. From the lesson on decibels, you know that decibels indicate a relationship, or ratio, between two values of the same unit. If you performed the 20 Log calculation on the distance values from the table Inverse Square Law: Area of a Projected Image, the results would be identical to those shown in Inverse Square Law: Sound Pressure Loss Over Distance table.

Notice that the table Inverse Square Law: Sound Pressure Loss Over Distance below appears very similar to the table Inverse Square Law: Area of a Projected Image. The distances and energy intensity change are the same. However, now we are measuring in decibels instead of area. When decibels are used to measure sound pressure level, remember: as distance doubles from a point source of sound, the result is a 6 dB loss. So, with a point source, the change in the level of sound energy is 6 dB per doubling (farther away) or halving (closer towards the source) of distance.

The Inverse Square Law: Sound Pressure Loss Over Distance and Inverse Square Law: Area of a projected Image tables demonstrate that no matter what the environment (e.g. a rectangular image or sound pressure), as distance increases, a fixed amount of energy is more thinly distributed.

If you would like to see mathematical demonstration of this principle, refer to the second example in Sample 10 and 20 Log Calculations. In this example, distance quadruples, resulting in a 12 dB loss (-6 dB for each doubling of the distance.) The animation also demonstrates this.