The Concept of Intensity – A short tutorial

Summary: I made a video describing the concept of intensity and how it relates to sound and area. The difficulty people have is what area ? What formula do I use? It is not specified in the book. Most people do not understand the range and how the volume of the sphere is related to intensity or decibels in general. There are also two short answer questions which are good to see if one understands the concept of intensity and its relation to sound and area.