Sound intensity spans a trillion-fold range, forcing a logarithmic scale

Evidence

The sound intensity represents the flow of energy through a unit area. In other words, it represents the watts per unit area from a sound source, and this means that it can be related to the sound power level by dividing it by the radiating area of the sound source. As discussed earlier, sound intensity has a direction that is perpendicular to the area that the energy is flowing through; see Figure 1.10. The sound intensity of real sound sources can vary over a range, which is greater than one million-million (1012) to one. Because of this, and because of the way we perceive the loudness of a sound, the sound intensity level is usually expressed on a logarithmic scale. This scale is based on the ratio of the actual power density to a reference intensity of 1 picowatt per square meter (10−12 W m−2). — David M. Howard, Jamie Angus, Acoustics and Psychoacoustics, 177

Categorization

• DEFINE: Defines sound intensity as watts per unit area and grounds why the SIL scale exists.
• XREF: Connects to the earlier decibel/bel explanatory structure in the chapter and to the soundscape notions of signal vs. noise levels.