Kyoji Fujiwara

Noise barriers with reactive surfaces

Abstract The boundary element numerical modelling method is used to study the insertion loss of rectangular, T-shaped and cylindrical edged noise barriers with rigid, absorbing and soft surfaces. The most efficient design is a T-shape with a soft upper surface. It is found that a uniform series of wells in the upper surface of a T-shaped barrier produce insertion loss values equal to those of a soft surface over a significant range of frequencies. At low frequencies the insertion loss is less than that expected for a rigid upper surface. The use of wells tuned to two different frequencies widens the spectral range of the soft surface effect. It is found that the performance of the wells can be described accurately in the numerical model by a simple analytical expression for the specific impedance of a plane surface. Experimental measurements on scale models are compared with the results of numerical modelling.

Absorption characteristics of a practically constructed Shroeder diffuser of quadratic-residue type

Abstract Schroeder diffusers of the quadratic-residue type in two dimensions were constructed. The size of a unit is 45 cm × 45 cm ¢ 60 cm and the prime number is 7. The wells were constructed as follows: 3 mm thick plywood for well walls, 6 mm thick plywood for the case of the unit and 30 mm thick chipboard for the well bottoms. The reverberant sound absorption coefficient was measured in a reverberation chamber using 48 (6 × 8) units. Their absorption coefficient was very large in the frequency range below the lower limit of the design frequency. Although the reason for this high absorption efficiency has not yet become clear, the data obtained experimentally are presented in this paper.

EFFICIENCY OF A NOISE BARRIER WITH AN ACOUSTICALLY SOFT CYLINDRICAL EDGE FOR PRACTICAL USE

This paper examines the sound shielding efficiency of a noise barrier having an acoustically soft cylindrical edge, as it compares to that provided by the well-known absorptive cylindrical edge. It has been reported that the waterwheel-shaped cylinder (acoustic tubes in a radial arrangement) approximates a soft surface cylinder, and that the sound shielding efficiency of a noise barrier is improved by placing the cylinder on its edge. The efficiency of the waterwheel-shaped edge barrier is strongly frequency dependent, thus the improvement in overall sound pressure level is smaller than expected when the source is broadband noise. The present study investigates the use of varied tube depths to improve the efficiency of the waterwheel-shaped edge. It is shown that adding tubes of different depths can flatten the frequency dependence, and that such tubes are only needed in the upper half of the cylinder. These findings led to the design of a new edge device for controlling road traffic noise, whose numerical simulations suggest that it is twice as effective in overall sound pressure level as the original waterwheel with uniform-depth channels.

Traffic noise reduction at balconies on a high-rise building façade

The performance of balconies with ceiling-mounted reflectors on a high-rise building façade is examined using numerical analyses and scale-model experiments. The reflectors are designed to reflect direct and diffracted waves incident on the ceiling outside the balcony. The sound pressure reduction, provided by the reflectors, on a window surface adjacent to the balcony is evaluated at intermediate floors levels. In terms of A-weighted sound pressure levels, a balcony equipped with reflectors reduces road traffic noise by 7-10 dB(A), compared to an ordinary balcony, at incident angles of noise close to the angle for which the reflectors are designed. The efficiency is roughly the same as, or greater than, that of a balcony with an absorbent ceiling. However, it is also shown that when the vertical incident angle of the noise is smaller than the design angle of the reflectors, or the horizontal incident angle is large, efficiency is reduced.

Noise control by barriers—Part 2: Noise reduction by an absorptive barrier☆

This paper presents a method of estimating the excess attenuation of a noise by an absorptive barrier covered with sound-absorbing materials. The approximate theory of diffraction by the absorptive barrier is derived from rigorous theory for a hard barrier. A single chart, which may be very convenient for the rapid estimation of the effect of absorption (the increase of the excess attenuation caused by the absorbing treatment of the barrier) in the practice of noise control, is presented. The validity of the method developed in this paper is confirmed by comparing estimated with measured values.

Noise control by barriers—Part 1: Noise reduction by a thick barrier

Abstract This paper presents a method of estimating the excess attenuation of noise by a thick barrier. In this method, the excess attenuation of noise by a thick barrier is assumed to be composed of two parts, one being the effect of a virtual thin barrier with the same height and the other the effect of thickness. A single chart for estimating this thickness effect is offered under conditions which only permit an error of a few decibels. The validity of the method presented here is verified by comparing the estimated with the measured values. Consequently, this method may be useful for the purpose of estimating the excess attenuation of a band of noise by the barrier whose thickness is larger than half a wavelength.

Visualization of the sound field around a Schroeder diffuser

Abstract This paper presents an attempt to visualize the sound field near the surface and inside the wells of a Schroeder diffuser. The distribution of particle velocity in the sound field is analyzed numerically and compared with an experimentally measured distribution. To obtain the experimental distribution, Kundts method is employed using a two-dimensional acoustic duct and fine cork powder. The predicted and measured distributions are qualitatively in good agreement. As an application of the results, a new sound absorbing structure is developed for the low frequency range. The absorption coefficient at normal incidence is 0.5 at 63 Hz and higher than 0.9 above 80 Hz.

Analysis of sound absorption by periodic structures using a hybrid-boundary element/mode expansion method

Abstract An improved hybrid method is proposed for analyzing sound scattering by a periodic structure. Part of the scattered field formulated with the mode expansion method is combined with other components of the field formulated with the boundary integration method in one period of the structure. Structures treated by this method can have arbitrary periodic forms made of locally reactive boundaries and porous materials. The oblique incident absorption coefficient of the structure is obtained simply from the reflection factor calculated for each elemental wave of the scattered field. The accuracy of the method is demonstrated by the agreement between calculated and measured values of the normal absorption coefficient of some test structures.

Analysis of the structure-borne sound in an existing building by the SEA method

Abstract By using the SEA method, the characteristics of structure-borne sound propagation in an existing building are analyzed. The calculated results of the sound-pressure level in the room are compared with measured values. An analytical model that simplifies the building is a group of rectangular rooms. For a certain type of building, the structure-borne sound propagation may be obtained by the use of a reduced two-dimensional model. It is found that the sound-pressure level in each room shows a linear attenuation with a room-location factor that is defined by the number of unit rectangular rooms and the regression coefficient relating to room dimensions. In this calculation, effects of the main structural wall and the blocking mass at each junction are taken into consideration.

Full-scale tests of reflective noise-reducing devices for balconies on high-rise buildings

In the present report, the noise-shielding efficiency of balconies with ceiling-mounted reflectors on the façades of high-rise buildings is examined through full-scale field measurements. The reflectors are designed to reflect direct and diffracted waves incident on the balcony ceiling outside the balcony. Field measurements are conducted to investigate the performance of the reflectors on intermediate floors of a high-rise dwelling adjacent to a railway. The reflectors reduce railway noise by approximately 5 dB, in A-weighted sound pressure level, compared to an ordinary balcony. The noise-shielding efficiency of a balcony equipped with reflectors is greater than that of an absorbing balcony.