The International Standard Reference Zero for Pure-Tone Audiometers And Its Relation to the Evaluation of Impairment of Hearing

Abstract

International reference zero levels for pure-tone audiometers have been agreed upon and recommended by the International Organization for Standardization (ISO) and are under consideration for inclusion in a new American Standard for Audiometers. The new ISO levels are desirable because (1) the shape of the threshold contour is more accurate, (2) the threshold levels are reproducible from laboratory to laboratory and from country to country, and (3) they represent the only apparent escape from permanent ambiguity and confusion in international exchange of audiometric information. The new ISO levels differ significantly from the American Standard values of 1951, in terms of which our present rules and laws for calculating percentage impairment of hearing for compensation purposes are written. The apparent difficulties arising from the numerical differences can be easily overcome, however, because it is possible to go from the 1964 ISO scale to the 1951 ASA scale by a simple subtraction before beginning such calculations. For the average of the three frequencies 500, 1 000 and 2 000 c/s the difference to be subtracted is 11 dB.