Decibels
It turns out that human ears can tell if one sound is louder than another only if the intensity differs by about 1 dB. This does indeed turn out to be a useful intensity measure.
Another example: suppose one sound is 1 x 10-6 W/m2, and another sound is twice as intense at 2 x 10-6 W/m2. What is the difference in decibels?