In this paper, the noise shielding efficiency of barriers with an acoustic device mounted on their top edge for reducing sound diffraction is described. The authors have already found that the intrinsic efficiency of the device, which is related to the noise-reducing mechanisms, is a function of the angles of the source and receiver but independent of their radii. In the present paper, a novel procedure based on the previous finding is applied to determine the acoustical efficiencies of practical edge-modifying products in the near field, and the results are utilized in calculations to predict sound diffraction behind the edge-modified barriers in the far field. It is proved that the novel method provides an accurate prediction.
The Acoustical Society of Japan’s prediction model for road traffic noise, ASJ RTN-Model 2003, which is generally used for the evaluation of the ‘Environmental Quality Standards for Noise in Japan’ (EQS), estimates the average noise level as the representative value in the evaluated area. However, because the noise level at each building changes greatly depending on the arrangement of buildings, it is necessary, for the evaluation of EQS, to consider the distribution of noise levels in an evaluated section. From that perspective, a new concept to adopt the most frequent level of insertion loss of road traffic noise as a representative value of the evaluated section is proposed. A simple method to predict the most frequent level of insertion loss is presented that is based on the authors’ simulation method of the insertion losses attributable to detached houses. Furthermore, the validity of the proposed equation for evaluating EQS is verified by comparing this method with ASJ RTN-Model 2003, which consists of identical parameters. The result confirms that the proposed prediction equation is appropriate for evaluating EQS.
As a calculation method of road traffic noise at the roadside area of a semi-underground road, the “hypothetical point source method” was proposed in the road traffic noise prediction model “ASJ RTN-Model” proposed by the Acoustical Society of Japan. In this method, noise radiation from a straight semi-underground road is simply modeled as sound propagation from a hypothetical point source that has a directivity specific to the dimensions of the structure and is assumed to be at the center of the mouth. In the ASJ RTN-Model 2003, the directivity characteristics were determined on the basis of the results of a scale model experiment, and the variation of the applicable dimensions of the road structures was limited. In this paper, the directivity characteristics of the hypothetical point source are examined by wave-based numerical analysis to extend the variation. In the numerical analysis, a Fourier-type transformation technique from a two-dimensional field to a three-dimensional field is applied to the calculation results obtained by the two-dimensional finite-difference time-domain method. In addition, the practical expression of the sound power of the hypothetical sound source is investigated.
To accurately estimate the noise at a signalized intersection, it is necessary to precisely reproduce the traffic volume, signal cycle and traffic noise for each vehicle behavior and driving state. Precise reproduction requires considerable effort, such as continuous calculations of vehicles and the setting of parameters such as engine speed, engine load and velocity. A simple method that involves using A-weighted sound power levels (LWA) under nonsteady running conditions has already been proposed for estimating noise at signalized intersections in a previous paper. In this study, the authors developed two simple methods for predicting noise in which the effects of acceleration and deceleration by signals is reflected. One method is based on a microsimulation traffic model, in which equivalent continuous A-weighted sound pressure levels (LAeq) is calculated by adding the noise of vehicles passing a green signal and the noise of vehicles decelerating and stopping at a red signal then accelerating when the signal turns green. The other method is even simpler and involves the assumption that an intersection zone is an unsteady running section and that LWA for a nonsteady running section is larger than that for a steady running section. Noise predicting by the three simple methods is compared with actual measurements at 10 sites. The two new methods had slightly improved accuracy relative to the measured results.
In urban areas, medium and high-rise buildings are usually constructed on both sides of an arterial road and a viaduct road often exists above it. There are many reflections by building facades, the pavement and the underside of the viaduct road in such places. Therefore the roadside noise levels may exceed the recommended limits in city street canyons. In this paper, a simple calculation model for estimating the mean increase in noise level owing to multiple reflections has been proposed in order to predict road traffic noise in city street canyons more accurately. This model is based on a diffusion method. We have also investigated the effects of multiple reflections on noise propagation using a 1:40 scale model of a city street canyon with a viaduct road. As a result of this study, it has been found that the mean increase in noise level owing to multiple reflections by building facades and the underside of the viaduct road in built-up urban areas is up to about 8 dB. Comparisons between the results of experiments and calculations show that the simple calculation model gives results consistent with experimental ones.
In this study, the new road traffic noise prediction method applicable to Japanese and Dutch various road surfaces was developed. Firstly, the A-weighted sound power levels of road vehicles were measured for actual roads in the Netherlands paved with various surfaces. With regard to the levels on dense asphalt concrete, the differences between Dutch and Japanese data were not significant, therefore it was found that the common sound power calculation model can be used in both Japan and the Netherlands. On the other hand, the levels for low-noise road surfaces in the Netherlands were 1 to 7 dB lower than those for dense asphalt concrete, therefore the different sound power calculation models were required to be constructed for such surfaces. Secondly, based on the above results, a sound power calculation model applicable to each road surface was developed. By integrating the model with the dynamic traffic flow calculation model, the new road traffic noise prediction method was constructed. Using the method, the road traffic noise in 32 urban areas including low-noise road surfaces in Japan and the Netherlands was calculated. As a result, the calculated levels correspond well with the measured levels (the differences between them were 1.3 dB on average).