L_<eq>, L_αと大きさの関係について

抄録

Most of the noises generated in daily life are fluctuating in both level and frequency. It is important for a measure of such fluctuating noises to reflect subjective judgment, to be applied to any kind of noises, and to be easily calculated. L_<eq> and L_α are often used as the measures of noises, but, in actual noises, both are apt to take similar values, and it has not yet been decided which is more suitable. There, in the present study, two experiments were conducted in order to make clear the propriety of L_<eq> and L_α as the measure of traffic noise. Pink noise was used as stimuli, the level of which was changed simulating road traffic noise so that the values L_<eq> and L_α might be different (Fig. 1 ・Table 1). The duration was about 30 sec. These stimuli were generated with a noise generator, and recorded on magnetic tapes, after the level and time were controlled by Programmable Sound Control System. In the experiment, they were reproduced, and presented to subjects through an amplifier and a loudspeaker in the sound proof room. In Exp. I, the method of magnitude estimation was used. Subjects judged the loudness of each stimulus assigning freely a number which seemed to reflect the loudness. They were able to use any number as they liked except negative. In Exp. II, the method of subject adjustment was adopted. Steady-state pink noise with same duration was used as the comparison stimulus. Subjects controlled the level of the comparison stimulus by themselves, hearing and comparing two noises a number of time, and carried out the matching of the loudness. Both noises were repeatedly presented with 30 sec silent intervals until the subjects finished the matching. Correlation was examined with coefficient of correlation and standard error between subjective judgment and physical values for estimation-L_<eq>, L_α(L_5, L_<10>, L_<20>, L_<50>) and the loudness level based on Stevens' Mark VI(LL_E, LL_P, LL_5, LL_<10>, LL_<20>, LL_<50>). LL_E is the loudness level calculated from the energy mean of each octave band, LL_P is the energy mean of the loudness level calculated every 100msec, and LL_α is the loudness level calculated from L_α of each octave band. As a result, L_<eq>, LL_E and LL_P, which are based on the energy mean, showed higher correlation with subjective judgment and smaller standard error than L_α and LL_α in both experiments (Table 2). Consequently, the energy mean seems to be more suitable than the statistical values, such as L_α, for dealing with level fluctuating. As for the frequency weighting, both dB(A) and Mark VI treatments showed good correspondence with subject judgment, but taking the trouble for calculating LL into account, it may be concluded that L_<eq> is more suitable for the assessment of traffic noise.