Standardization of equal -loudness level characteristics

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Hearing experiments to derive equal-loudness level contours were carried out in a large anechoic room. In the experiments, the equal-loudness level contours of pure tones at 20 phon, 40 phon and 70 phon as well as the threshold of hearing of pure tones in a free field were measured. We get the following results with the experiments.(1) The equal-loudness level contours at all the levels examined show discrepancy from the present international standard, ISO 226, by more than 10 dB in lower frequency range than 1 kHz. This means that the -present standard should be totally revised.(2) All the data of the threshold of hearing obtained were presented to TC43 of TSO (International Standardization Organization). It is decided that a new international standard ISO 226 Part 1 is to be published with other data and other data from Germany and Denmark. We contribute this standardization remarkably because only our group could present such data that covered all over the audible frequency range.(3) We found that a small peak of around 2 dB exists between 1 kHz and 2 kHz for both the equal-loudness contours and the threshold of hearing. Comparing this peak with individual head transfer functions (transfer function of a acoustic path from a sound source to the listener's external ear), we found that the peak can be explained by a corresponding dip in the head transfer functions. This shows that the head transfer functions is an important factor to determine individual equal-loudness contours and threshold of hearing.