Revision of ISO 226 “Normal Equal-Loudness-Level Contours” from 2003 to 2023 edition: the background and results

As significant errors were reported in 1985 for the international standard related to equal-loudness-level contours (ELLCs) for pure tones, the earlier international standard was fully revised in 2003 as ISO 226:2003, after 18 years of revision work. Twenty years later, the standard has been revised again as ISO 226:2023. One motivation for the revision was to reflect the lowering of the threshold of hearing at 20 Hz by 0.4 dB in ISO 389-7:2019. In addition, the following two points of substance were revised: (1) implementation of the power exponent relating loudness perception to physical intensity formulated in an academic paper published in 2004, which describes the derivation of ELLCs relating to ISO 226:2003, and (2) adoption of mathematical expressions that preserve the appropriate number of significant digits. In this review, the process of the revision and the technical details of the changes are described. The differences from the 2003 edition are only 0.6 dB at most, and the 2023 standard can be regarded as the same as the 2003 edition in terms of practical use.