1) ISO : ISO Recommendation R1999, Assessment of Occupational Noise Exposure for Hearing Conservation Purpose, 1971.
2) U.S. NIOSH : Federal Register, vol. 36, p. 10466, May 29, 1971.
3) ACGIH : Threshold Limit Values of Physical Agents Adopted by ACGIH for 1976.
4) Japan Association of Industrial Health : Recommendation on acceptable conc. and others (1976), Jap. J. Ind. Health, 18 : 342-353, 1976 (in Japanese).