Assume there is 100 percent energy intensity on a screen at 10 feet (3.05 m) from the light source. At what distance would you have 25 percent intensity?

Step one:

Ask yourself, "What do I think is going to happen?" If there is a loss of energy, then the screen must be farther away from the light source.

Step two:

Now ask yourself, "How far away would the screen need to be?" From the lesson, you know that when distance doubles, the energy is one-fourth of its original intensity. Multiply the first distance by two.10 * 2 = 20 feet (6.1 m)

The answer is 20 feet (6.1 m).

Assume that a loudspeaker is generating 75 dB_SPL at a distance of 13 feet (4 m) away from the source outdoors. What would the level be at 52 feet (16 m) away?

Step one:

Notice that 52 feet (16 m) is exactly four times as far from the point source of energy as 13 feet (4 m). This means you can use the Inverse Square Law to quickly calculate the dB_SPL at the 52 feet (16 m) position.

Step two:

Ask yourself, "How many times has the distance doubled?"

13 * 2 = 26 feet (8 m)

26 * 2 = 52 feet (16 m)

The distance has doubled two times.

Step three:

Because we are calculating a loss, multiply -6 dB by the number of times the distance has doubled.

 -6 * 2 = -12 dB

Step four:

Now that you have calculated the amount of loss, the impact of the change needs to be determined. Add the results from step three to the original dB_SPL measurement.

75 + (-12) = 63 dB_SPL

75 - 12 = 63 dB_SPL

The sound pressure level at 52 feet (16 m) from the point source is 63 dB_SPL.