Why do crystal radios need high impedance headphones? (10-Analog Electronics)

Example problem: What benefit is offered when the earpiece is high impedance rather than low impedance? Modern radios use low impedance ones - let's see why they used to use high impedance ones instead. Aaron Danner is a professor in the Department of Electrical and Computer Engineering at the National University of Singapore. http://danner.group Video filmed and edited by Cheryl Lim. @randomcheryl