Novel plenum window with sonic crystals for indoor noise control

Abstract

A novel plenum window with incorporation of sonic crystals (SCs) was investigated using numerical and experimental methods in the present studies. Additional insertion loss (IL) that could be obtained by the 3 SCs was computed through a commercial software and was validated through experimental results from reverberation rooms. Experimental data showed that the SCs could attenuate more traffic and construction noises compared to white noise. When the noise source directly faced the opening of the plenum window, the performances of the SCs in reduced traffic and construction noises were improved significantly especially at low and middle frequency ranges (from 250 Hz to 1250 Hz and from 800 Hz to 2500 Hz for traffic and construction noise, respectively), which covered the designed center and resonant frequencies of the SCs (1000 Hz and 1164 Hz, respectively). For such a case, the SCs were able to attenuate additional 4.2 dBA and 2.1 dBA of traffic and construction noises, respectively, at 1000 Hz. Peak IL was found to be at 630 Hz which was about 5.4 dBA and 5.5 dBA for traffic and construction noise, respectively.

Introduction

Environmental noises such as traffic and construction noises are often a major problem in a small and densely populated country like Singapore. The severely of this issue increases dramatically in recent years due to increased population and human activities. Consequently, many residential units are built along the expressway and many construction works are conducted nearby these areas. Some efforts were made by the government to mitigate the noise level at these residential or commercial areas. For example, barriers and enclosures were built to partially shield the construction sites or expressways. However, these efforts may not always be successful in implementation due to various issues such as visual intrusion, safety and space limitation. Therefore, an innovative plenum window with sonic crystals (SCs) was designed, fabricated and tested in the present studies for mitigating the traffic and construction noises exposure in the residential areas.

The idea of SC appeared in the 90s and it was defined as periodic distribution of sound scatterers in fluid to attenuate sound wave by their acoustic band gaps [1], [2]. The phenomenon of band gaps was explained by the destructive Bragg interference and the center frequency ($f_c$) of the band gap is determined by:

$$f_c=\frac{c}{2d_s\sin \theta },$$

where c is the speed of sound in air, $d_s$ is the distance between adjacent sound scatterers and $\theta$ is the incidence angle relative to the front plane of the array of sound scatterers. SCs have numerous applications and it have been widely studied by many researchers in recent years. Martinez-Sala et al. [3] demonstrated that it was possible to improve the sound attenuation obtained from a mass of trees by arranging them in a periodic lattice like SCs. They believed that the position of the attenuation peaks obtained along a certain range of frequencies would depend on the type of lattice used. Lagarrigue et al. [4] studied the acoustic transmission coefficient of a resonant SC made of hollow bamboo rods through experimental and theoretical methods. Their results showed that a SC made from a natural material with some irregularities could exhibit clear transmission bandgap. Koussa et al. [5] studied the acoustic performance of a SC assisted noise barrier using two-dimensional (2D) Boundary Element Method. They concluded that SC assisted barriers presented additional acoustic properties of absorption and resonance and hence, achieved 6 dBA of sound attenuation in comparison to the traditional barrier. Aa and Forssen [6] designed a complex graded index SC structure which was formed by varying the lattice constant and the cylinder radius. The SC structure was examined numerically in a 2D domain with traffic noise scenario consisting of 95% light and 5% heavy duty vehicles driving at 70 km/h. They obtained noise reduction of 4.2–.3 dBA with such a configuration.

Plenum window was defined as partially opened double glazing window where its inner and outer window openings are staggered so that sound cannot directly propagate across it [7]. Outdoor air is allowed to ventilate the indoor space because the gap between the two glass panes together with the opening form an air passage. Tang et al. [8] found that the ventilation window was able to offer an additional acoustical protection of 12 dBA to 13 dBA compared to the conventional openable window. In addition, they also concluded that the introduction of sound absorption materials on the top internal surface of the window cavity could gain an extra 2 dBA of insertion loss (IL). Sondergaard and Olesen [9] studied the sound insulation of a supply air window with open vents in the laboratory for different combinations of opening area of the vents and noise absorbing materials. Their results showed that it is possible to improve sound insulation by 8–16 dB by using a supply air window with an opening area of 0.35 ${\text{m}}^2$. Tong and Tang [10] investigated the acoustical IL of a plenum window in a semi-anechoic chamber. They found maximum a IL of 14 dB for the case where the orientation of the building facade relative to the line source was fixed. Yeung et al. [11] achieved a noise reduction of 8 dBA with the use of noise absorptive material on a plenum type window at a housing area. Wang et al. [12] designed a new ventilated window which combined the new wind wall designs and multiple quarter-wave resonators to make a balance between ventilation and acoustical performances. They claimed that this new ventilated window was able to achieve transmission loss of 10–22 dB in the frequency range of 500 Hz–4 KHz. Tang [13] examined how a small inclination of the indoor side window pane could affect the sound insulation of the plenum window. He concluded that there was a 2 dB sound insulation increment when the source was close to the window and the window panes were non-parallel. Tang et al. [14] conducted a full scale field measurement of the acoustical IL of the plenum windows. They found that plenum windows were able to achieve 7.1 dBA–9.5 dBA of IL compared to side hung casement windows. Tang et al. [15] conducted a series of experiments to understand how the implementation of active noise control would affect the sound transmission across a plenum window. They found that the active control system with two loudspeakers located symmetrically about the plenum cavity horizontal centerline facing directly the incoming noise had the best performance.

A simulation model was developed by Yu et al. [16] in order to predict the sound insulation performance of ventilation windows in some buildings. They claimed that in the low-to-mid frequency range, the cavity resonance effect could control the sound reduction index of the double glazing. Tang [17] investigated the potential improvement of plenum window noise reduction by installing rigid circular cylinder arrays into the window cavity through finite-element method. His results showed that the cylinders improved the broadband noise reduction across a plenum window regardless of the direction of sound incidence. The influences of the panel thickness, the air-gap spacing between two glass panes and the opening size on the sound transmission loss of a plenum window were investigated by Du et al. [18]. They claimed that the transmission loss varied little with changes in air-gap spacing. However, it could be improved by the increment of the glass thickness and by decrement of the opening sizes if the half wavelength of impinged sound was smaller than the air-gap spacing. The noise reduction performances of a louver window and a plenum window with similar ventilation performance were compared by Ji et al. [19]. The results showed that plenum window was more effective than louver window in reducing outdoor noise. In order to develop a simple empirical prediction model for traffic noise transmission loss across a plenum window, Li et al. [20] conducted a parametric study inside the building acoustics testing chambers of the Hong Kong Polytechnic University. They found that a model which assumed percentage reverberant field attenuation and frequency-independent diffraction directivity could give best prediction for traffic noise transmission loss.

It is obvious that plenum window can overcome the major shortcomings of conventional noise barrier or enclosure where it still allows air to pass through the window while it is fully staggered. It can be directly installed on the window frame of a building and it is transparent. However, traditional plenum windows are not able to attenuate noise based on selected frequency range. Therefore, the main objective of the present study is to design a plenum window with SCs which specially targeted in reducing traffic and construction noises. Numerical method was used first to estimate the acoustical performance of the novel plenum window. Thereafter, the novel plenum window was fabricated and was tested in a reverberation room in order to validate the performance of the window in laboratory.