The reference intensity level of 10–12 W/m2 is a typical value for the threshold of human hearing. A sound of intensity I0 has a sound level of 0 dB, while a sound at the upper range of human hearing, called the threshold of pain, has an intensity of 1 W/m2 and a sound level of 120 dB. Thus the human ear is capable of responding to intensities ranging over 12 orders of magnitude on a logarithmic scale of intensity called sound level, SL.
Example 1.5 The intensity of waves at a distance of 20 m from their source is 25 W/m2. What is the intensity at a distance of 50 m and how do the amplitudes of the waves compare at the two places?
The amplitudes are in the inverse ratio of the distances from the sources.