TL;DR "The sensitivity of headphones is usually between about 80 and 125 dB/mW and usually measured at 1 kHz." and note that since dB is logarithmic, every 6dB is 2x the energy (human-perceived "doubling of volume" is typically ~10dB; see also: https://en.wikipedia.org/wiki/Fletcher%E2%80%93Munson_curves )
TL;DR "The sensitivity of headphones is usually between about 80 and 125 dB/mW and usually measured at 1 kHz." and note that since dB is logarithmic, every 6dB is 2x the energy (human-perceived "doubling of volume" is typically ~10dB; see also: https://en.wikipedia.org/wiki/Fletcher%E2%80%93Munson_curves )