Other
Basic Car Audio Electronics: Inductor
This site contains a definition for what an inductor is, has a drawing of what a conductor looks like and of the symbol for an inductor, and distinguishes between the three types of inductor core types used for audio.
Curated OER
Air Core
This site contains a definition for what an inductor is, has a drawing of what a conductor looks like and of the symbol for an inductor, and distinguishes between the three types of inductor core types used for audio.