Precision Measurements of Equal-loudness Level Contours

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The purpose of this research is to get Japanese fundamental hearing data for renewal work of international standard ISO 226 (Acoustics - Normal equal-loudness level contours). Two kinds of laboratory work were carried out throughout these two years.(1) Effect of the sound field difference between free field and diffuse fieldThe frequency characteristics of 40 phon equal-loudness level, threshold of hearing and head transfer function were measured both in free field (in anechoic room) and in diffuse field (in reverberation room). The frequency range was 250 Hz to 2 kHz.Comparing the level differences of these characteristics obtained in both fields, following results were obtained;(a) the shape of level difference of 40 phon equal-loudness level and threshold of hearing are similar to that of head transfer function,(b) the values of the level difference of threshold of hearing are 3 to 5 dB lower than that of head transfer function.(c) frequency dip in 800 Hz is found newly in these three characteristics, while there was no dip in ISO 226.(2) Reexamination the error of constant methodThe points of subjective equality (PSE) using constant method are affect-ed by the experimental condition. Following knowledge are obtained;(a) the dispersion of PSE from 5 trials in same conditions is about 3 dB, this value is almost same as the level step of test stimuli,(b) PSE are influenced by the center level of test stimuli, and the responses of subjects are deviated to center level.