An international collaborative research group, comprising Prof. Yôiti Suzuki of Research Institute of Electrical Communications, Tohoku University, a coordinator; Dr. Kaoru Ashihara, a senior researcher of Technology Information Department (previously, senior researcher in the Institute for Human Science and Biomedical Engineering), National Institute of Advanced Industrial Science and Technology (AIST); Dr. Hisashi Takeshima, Associate Professor of Electronics Department, Sendai National College of Technology; Dr. Kenji Ozawa, Associate Professor, Faculty of Engineering, University of Yamanashi; and five foreign researchers from Germany, Denmark and the United States, has been engaged in research works on the precision determination of loudness level contours for sounds, for three years from fiscal 2000, supported by a grant-in-aid from the New Energy and Industrial Technology Development Organization (NEDO).

A draft for international standards based on the results of this work was approved by the International Organization for Standardization (ISO), and formally enacted as an full revision of ISO 226:1987, on August 15, 2003.

An equal-loudness-level contour represents a frequency characteristic of the sensitivity of human auditory system, to be drawn by connecting sound pressure points sounding identically loud for different frequencies, representing an equal sensation contour in the sound- pressure-level and frequency plane. Thjs respresents one of the fundamental characteristics of auditory sense, which was internationally standardized as ISO 226 since 1961. As it was found later, however, that the old ISO 226 involved substantial errors, a continuing work to revise has been carried out since 1985, without attaining to a finish, despite strong demands from various fields.

The new equal-loudness-level contours based on the results of works by the international collaborative research group have been adopted as the International Standard, ISO 226:2003.

The new standard is characterized as following:

• While the previous standard depended on the British data in 1950s, the new one is based on international collaboration efforts involving Germany, Denmark, Japan, UK and USA. In particular, Japan has made the greatest scientific contribution, providing about 40 % of the total data.
• Between the new and the previous standards, very large differences are recognized up to about 15 dB (decibels) for a wide area of frequency region lower than 1 kHz. A difference of 10 dB means 10-fold difference in sound energy, and that of 15 dB corresponds to 30-fold difference. (Fig.1)

Fig. 1. Comparison between the new and the previous characteristics of equal-loudness-level contours. Remarkable differences are observed in the low frequency range.

The new standard is expected to play a crucially important role as the basic data for supporting the development of technologies for high precision audio signal reproduction suited for the DIGITAL era, such as techniques for efficiently compressing digital signals of music, determination of optimum frequency characteristics of high-definition audito reproduction systems, and so on.

The equal-loudness-level contours are closely related to the measurement and evaluation of noise. Currently, the noise level is measured through the "A" frequency weighting characteristics, which would reflect the frequency characteristics of human auditory sense. As a result of the present work, it was found that the "A" weighting well conformed with the equal-loudness-level contours. This provides valid grounds for the current noise evaluation method, and will serve as an important foundation for developing means for more accurate methods to evaluate and rate various noises.