鹿児島空港周辺の航空機騒音

抄録

Noises of 4 types of aircrafts on domestic routes were measured at 14 points shown in Fig. 1 in Kagoshima-city (a medium-sized city situated in the southernmost prefecture in Japan) for six days from 18 to 23 Oct. 1965, during which the weather was fine, with a wind velocity of less than 5 m/sec, temperature randing from 17 to 23℃, humidity randing from 34 to 48%. The measuring groups were as follows : 8 groups moving from point to point with sound level meters, 1 group with a magnetic recorder, 2 groups at B and C points, with a simple altimeter. The method of a sound level measurment is shown in Fig. 2. Observers start observations at the moment when they catch sight of aircraft. Tab. 2 shows the average sound level of each type of aircraft at 14 points in dB(A) at its taking-off and landing. From this table the mean differences of sound level between types of aircraft and between take-off and landing may be estimated. Fig. 3 shows the average octave band levels at point A, and by applying the A weighting charactor of sound level meter to these spectra, we obtained the full line spectra shown in Fig. 5. The spectra in dotted line shown in Fig. 5 were obtained by correcting absorption by air at a temparature of 20℃ and a humidity of 42%. The above atmospheric conditions correspond to what is shown in Tab. 5. The correction data were taken from ISO-Draft (Draft secretariat proposal for a procedure for describing aircraft noises around an airport). In Fig. 5 relative sound levels are shown in dB(A), based on the sound level at point A (the distance r from the path of aircraft to point A is nearly 200m). It seems that no correction is required for Heron (H). Tab. 4 shows PN-dB estimated from sound levels of Fig. 3 and the difference between PN-dB and sound level dB(A). The value of the above difference is about 13 as already known. Now we tried to work out the contours of equal sound level dB(A) for Heron. Five planes of this type shown in Tab. 5 were selected because the courses of these planes were reported to be very similar. They flew along the courses shown by the curved Gothic line in Fig. 4 and climbed at an inclination of about tan α=0. 11. Now we are able to calculate the distance r from the path to each measuring point. By substituting r and sound levels in dB(A) at points E and F into Formula (I), we can determine the power level PWL(A) of this type of aircraft. Formula (I) corresponds to nondirectional sound propagation in a free field. The mean value of two PWL(A) thus obtained was 142. 5 dB. By Using the formula (I) again we were able to calculate the contours of equal sound level. Fig. 4 shows this result which was drawn along a straight course on the assumption that the planes flew straight on and we referred to Fig. 4 for working out the acutal contours shown in Fig. 6. The contours in Fig. 6 was used to estimate the sound level at each measuring point. They are shown by line (a) in Tab. 6. We compared these estimated levels which actually measured values shown by line (b) (from Tab. 5) and line (d) (from Tab. 2). The defference between estimated and observed values is shown by lines (c) and (e). Errors, of course, arise from many factors : (I) permitted errors of sound level meters, (2) personal errors of observers, (3) change of places where observations are made (most of measurements were conducted on flat housetop of two- or three-storied buildings (4) the fluctuations of atomospheric conditions, (5) change of the pathes on which aircraft flew. The last would be the largest cause of errors. Fig. 7 shows the contours for Convair-240 (CV), worked out by the same procedure. The examination of Tab. 6 and Fig. 7 indicates that errors are within the range of ±4dB (except one point). The values of NNI (proposed in the ISO draft) were calculated by rough method. In Formula (2) dB(A)+13 was used to represent L instead of PN-dB and the total number of flights in one day (from morning till evening) to represent N.