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This International Standard provides descriptive statistics of the hearing threshold for populations of various ages. It specifies the following, for the range of audiometric frequencies from 125 Hz to 8000 Hz and for populations of otologically normal persons of a given age within the age limits of 18 years to 70 years inclusive:

• the expected median value of hearing thresholds given relative to the median hearing threshold at the age of 18 years;
• the expected statistical distribution above and below the median value.

The data are applicable for estimating the amount of hearing loss caused by a specific agent in a population. Such a comparison is valid if the population under study consists of persons who are otologically normal except for the effect of the specific agent. Noise exposure is an example of a specific agent and for this application selected data from this International Standard are referred to as “Data Base A” in ISO 1999.

The data may also be used to assess an individual's hearing in relation to the distribution of hearing thresholds which is normal for the person's age group. However, it is not possible to determine for an individual precisely which part of an observed hearing loss is attributable to an accumulation of detrimental effects on the hearing which increase with age, and which part has been caused by other factors such as noise.

The hearing threshold deviation as defined herein and the hearing threshold level as defined in other standards (ISO 389-1, ISO 8253-1, ISO 8253-2, IEC 60645-1) express the hearing threshold of an individual or an individual ear relative to, respectively,

• the median hearing threshold of a population of 18-year-old persons, or
• a reference zero level specified in various parts of ISO 389.

To the extent that the reference zero level represents the median of the 18-year-old population, the values of the two terms will be the same.