This pair of 300 OHM earphones is a popular item and designed to be rugged for the aviator headsets.
Altitude Compensation: 50,000 feet
Frequency Response: 100 to 3000Hz
Impedance: 300 OHMs
Ground Level Sensitivity: 102dB min @ 1mW rms input at 1kHz
Stray Magnetic Field: 5 degrees @ 12 inches