Takafumi Shimizu

Effects of an air-layer-subdivision technique on the sound transmission through a single plate

Many studies on the sound transmission through a single plate have been carried out theoretically and experimentally. The transmission-loss characteristics, in general, follow mass law. Therefore, increasing mass of a plate is a fundamental measure to improve the insulation performance. This method, however, has limitations and might not be a reasonable alternative in current standards. Furthermore, the transmission loss at the critical frequency of coincidence is deteriorated significantly even if the mass is rather large. In this paper, the effect of the air-layer-subdivision technique is studied in detail from the viewpoint of the sound transmission problem of a single plate. An analytical model of an infinite single plate with a subdivided layer is considered and the improvement of the transmission loss is estimated. The limitations of the technique are clarified with some parametric studies. In order to validate the predictions, an experiment was carried out. The transmission loss of a glass board with the air layer subdivided by acryl partitions was measured in the experiment. They were in good agreement with the theoretical ones near and above the coincidence.

Suppression of diffracted sound by green walls

Green walls covered with vegetation are used in urban areas to purify the air, improve the appearance of the landscape and reduce heat-island effects. They also improve the noise environment. It has been reported in previous studies that the surfaces of green walls have good sound-absorbing performance. However, the contribution of the greenery to the sound-insulation performance of green walls, which are also used as noise barriers, has not been verified quantitatively. In this study, the enhancement of the sound-absorption performance of a wall due to greening was verified via measurements using actual green walls and the results were compared with those of a steel noise barrier, which has limited sound-absorption performance. Numerical analyses using the boundary element method (BEM) were also conducted. As a result, it was confirmed that the sound-absorbing effect generated by wall greening reduces the diffracted noise, thus contributing to sound-insulation performance enhancement in areas above the noise barrier.

Optimization of acoustic absorption by green walls made of foliage and substrate

Green walls may absorb sound and contribute to noise reduction in urban areas. Recent experiments demonstrate that a foliage layer placed above a substrate layer may lead to a significant increase of acoustic absorption coefficient in a broad frequency range. However, the physical origin of this improvement remains unclear. In this work, measurements are carried out in an impedance tube on foliage, substrate, and foliage/substrate samples using the three-microphone two-load method. Acoustic absorption coefficient and surface specific impedance are measured in rigid backing condition between 100 and 1000 Hz. Effective speed of sound and characteristic impedance are also experimentally determined for foliage and substrate. For foliage/substrate samples, a good agreement is obtained between measured acoustic absorption coefficients and calculated ones using the effective properties of foliage and substrate layer and matrix manipulations. Analysis of results reveals that absorption coefficient spectrum is mai...