Hearing loss is defined by the International Organization for Standardization (ISO) as a deviation or a change for the worse of the threshold of hearing from normal, where “normal” refers to the median hearing threshold level (HTL) for an equivalent, representative population free from all signs or symptoms of ear disease, obstructing ear-canal wax and who have no history of exposure to noise, ototoxic substances or familial hearing disorders. Normative hearing thresholds are conventionally based on those of a typical 20-year-old population of males and females. Hearing thresholds can be expected to differ with respect to age and between sexes. The term sound-induced HL is sometimes used in order to be more inclusive of sounds that are not noise in the sense that noise is ...