International standard reference zero for audiometers.

Abstract

This is a detailed report on the technical activities of ISO's Technical Committee on Acoustics No. 43, Working Group on Threshold of Hearing, which led to the ISO Recommendation R389, “Standard Reference Zero for the Calibration of Pure-Tone Audiometers,” November 1964. The activities described are the determinations of the transfer factors from loudness balancing experiments between the five earphone-coupler combinations R389, the incorporation of the transfer data into the computation of the reference equivalent threshold sound-pressure levels (RETSPL) in R389, and the details of the weighting of the original threshold determinations. A statistical analysis estimates the average standard deviation of the RETSPL in R389 to be 2 dB due in large part to the variance in the transfer factors. The present USASI standards are definitely outside the ISO uncertainty limits at all frequencies. The standard deviation of the differences between adjacent columns in R389 (or the equivalence of the RETSPL for the different earphone-coupler combinations) was estimated to be 2.5 dB. To improve accuracy in audiometry, it is suggested that the variance due to the transfer factors be eliminated by agreement on one standard earphone type.