Since atmospheric absorption coefficients depend strongly on environmental temperature and humidity, they vary in response to changing meteorological conditions during a year. It is necessary to examine the fluctuation of the atmospheric absorption coefficients in order to predict outdoor sound propagation more accurately. Attenuation coefficients for atmospheric absorption were calculated from hourly meteorological data observed at Nagoya Observatory during a year. The calculated results show that the attenuation coefficients at low and high frequencies are higher in winter than those in summer, although the attenuation coefficients at middle frequencies become high in summer, and that the attenuation coefficients at all frequencies in daytime tends to be higher than those at night. Moreover it has been clarified quantitatively that the attenuation coefficients in a sunny day are higher than those in a rainy day. As a result of this study, it has been found that the attenuation coefficients for atmospheric absorption vary strongly due to the changes of real atmospheric conditions during a year in Japan.
Outdoor sound propagation is remarkably influenced by many physical factors. Source characteristics such as the intensity and spectrum affect the propagation distance of the sounds, and relate deeply to evaluation methods of the propagated sound to receivers as well. Meteorology and atmospheric condition are also important parameters for sound propagation outdoors. Due to the complexities, further investigations are necessary, though these parameters can be introduced and reflected to several prediction methods nowadays. In this report, excess attenuation of some different sounds after outdoor propagation ranging from hundreds to hundreds of thousands of meters are presented, and the effects of the meteorology and atmospheric conditions on the outdoor sound propagation are discussed. Especially, the measurements on asphalt are compared with the predictions by the parabolic equation (PE) method using vertical sound-speed profiles estimated from the meteorological data such as atmospheric temperature, wind speed and direction.
Measurements of sound propagation from a point source over a playing field were made in August, September and December in order to know the property of outdoor sound propagation close to the ground. In the measurements, two loud speakers were set at both ends of a base line over a distance of 200m. The four receiving points were set at the distance of 40m to 160m from each loud speaker. The profiles of wind and temperature were observed at various heights up to 2m above the ground. The measurements were made during daytime and after dark in the seven days and a total of 262 tests were carried out. The results of the measurements indicated that sound levels change strongly not only due to wind and temperature gradients in the atmosphere, but also due to the acoustical property of the ground surface over a distance of about 100m from a source. Moreover the measured values were compared with those calculated by a theoretical model proposed by K. B. Rasmussen. It has been found that the measured values are in good agreement with the calculated ones and the theoretical model is useful as one of the simulation methods of outdoor sound propagation.
In the neighborhood of airports, especially on the side of runway, the sound of aircraft take-offs and landings can be heard sometimes as loud or other times as weak. No doubt, it depends on meteorological conditions such as wind and temperature, but there are few investigations that statistically analyzed the relationship between levels of aircraft noise and meteorological conditions. Here, we report a result of analysis on long-term observations of meteorological conditions and aircraft noise during take-off roll on the runway. The data was obtained on the side of a runway using an unattended noise monitoring system installed at Narita Airport. The result shows that sound level becomes lower on downwind condition, while it becomes higher on upwind condition. As for temperature condition, the sound level is high when the temperature gradient is inversion, being compared with that on lapse condition. Moreover, it seems to be appropriate to use vector wind calm and temperature gradient neutral as representative for long-term average noise level.
Noise attenuation performance of embankment was investigated by numerical analysis using the finite difference time domain (FDTD) method. The visualization of transient sound propagation over embankments has revealed that the sound wave travels along their shoulder slopes. From the quantitative examination on the insertion loss, it has been found that the noise attenuation effects in low frequencies become lower as the shoulder slope of the embankment becomes gentler. Even if the embankment has finite acoustic impedance (in many cases, embankments are covered with lawn or grass), it is considered that the deterioration of noise attenuation effect in low frequencies remains because the acoustic impedance is generally large. When assessing their noise reduction effects, such deterioration of the performance must be carefully considered.
In order to clarify a mechanism that a low frequency sound occurs from road bridges, we have studied measurement results obtained from tests where test cars passed on bridges in the preservice road sections. As a result, it is ascertained that the low frequency sound occurring from road bridges emerges with bridge vibration, which is a dipole sound source, and that it depends on vehicle velocity and weight. Furthermore, in accordance to the measurement results obtained from the tests where vehicles passed on a bridge joint step, it has been confirmed that the low frequency sound less than 8-10Hz is affected by bridge span vibration and that the low frequency sound more than 8-10Hz is affected by bridge joint noise.