Sensitivity determines the loudness of your earphone at any level of volume. It is the effectiveness with which the electrical signal is converted to a sound signal. It is measured in decibels and shall be denoted as ‘DB’. More the sensitivity, more will be the loudness. Further, it also denotes the sound signal generated by 1MW power. This ensures the effective power output in terms of sound. That is why some earphones shall not be loud even if you tune it to the maximum volume and a few others shall gain momentum in volume within a slight change of volume.