Complex sentence: The severity of a hearing loss is ranked according to the additional intensity above a nominal threshold that a sound must be before being detected by an individual; it is measured in decibels of hearing loss, or dB HL.
Split into simpler sentences: The severity of a hearing loss is ranked according to ranges of nominal thresholds in which a sound must be so it can be detected by an individual. It is measured in decibels of hearing loss, or dB HL.