Using High Impedance Headphones With Audio Interfaces
Here's what you need to know about headphone specifications - CNET
High Impedance Headphones for Crystal and Regen/shortwave Radios - Etsy UK
Archimago's Musings: MEASUREMENTS / MUSINGS: Headphone impedance, sensitivity, efficiency, and amplifier output impedance. (And a quick thought on Darko's "All You Need Is...")
Magnetic HEADPHONE IDEAL for Crystal or Tube Radio High Impedance 3200 Ohm | eBay
Headphone Impedance And Sensitivity Explained | The Master Switch
David Clark H10-66 High, Low Dual Impedance Headset