Sound transmission across plenum windows with non-parallel glass panes

Transcription

1 Sound transmission across plenum windows with non-parallel glass panes S.K. TANG 1 1 The Hong Kong Polytechnic University, Hong Kong, China ABSTRACT Plenum windows, which take the form of a partially opened double glazing window, have attracted the attention of both government officials and researchers in the past several years because of their high sound insulation loss and their allowance for reasonable level of natural ventilation. However, it is very difficult to improve their sound insulation capacity as the gaps between the window panes are always restricted because of the building facade thickness. Application of sound absorption is basically not practical unless the material is transparent and thin. In this study, an attempt is made to examine how a small inclination of the indoor side window pane can affect the overall sound insulation of the plenum window. It is found that there is a 2dB sound insulation increase when the window panes are non-parallel and the source is relatively close to the window. The improvement becomes less obvious as the source distance increases, but improvement can still be observed by an appropriate revision of the window pane inclination Keywords: Sound insulation, Transmission I-INCE Classification of Subjects Number(s): INTRODUCTION Noise from ground traffic has long been a major source of pollution in dense built environment. Within the already very congested urban space, conventional noise mitigations, such as noise barrier and enclosure, are not effective. Setback is basically not possible as it will mean scarifying the very precious urban land. Closing windows or using double glazing windows (1) are not solutions as they do not allow for natural ventilation and are not recommended by the local authority. Noise mitigation devices which can be attached to a building façade are therefore attractive alternatives. While the balcony is not very effective unless sound absorption materials are installed within its cavity (2,3), the plenum windows are shown to be able to offer good sound insulation and at the same time allow for a reasonable level of natural ventilation (4). This plenum window design (See Figure 1) has attracted attention in the past few years. (a) (b) Figure 1 Plenum window design. (a) Vertical plenum window; (b) PolyU hostel Red rectangles : window openings; arrow : air movement path 1 shiu-keung.tang@polyu.edu.hk 1168

2 A plenum window is a partially opened double glazing window (5). The outer and the inner window openings are staggered so that sound cannot propagate directly across it. The gap between the two glass panes together with the openings forms an air passage, allowing outdoor air to ventilate the indoor space. Figure 1 shows the schematics of this window design and a practical application adopted by The Hong Kong Polytechnic University (PolyU) Homantin Student Hostel. Tong et al. (6) reported some findings from a field mockup tests using a busy trunk road as the noise source. So far, the additional acoustical benefit of replacing a conventional casement window (which meets the minimum ventilation requirement of local authority) by a plenum window is between 8 to 9 dba for traffic noise control application. There have been efforts on enhancing the acoustical protection of the plenum window. Kang and Brocklesby (7) studied the effect of micro-perforated absorbers (MPA) and Tang (8) investigated how sound absorption can improve the performance of the PolyU plenum window. However, as a window, daylighting penetration and aesthetics are the most important issues. The conventional opaque sound absorption is not applicable. The small gap size also limits the application of the MPA. The conclusion of Tang (8) is that the MPA has to be used together with conventional sound absorber for maximal performance of the plenum window. Since the space within the plenum window void is limited and high transparency for view and daylight penetration is essential, there is not much room for the improvement of plenum window acoustical performance. Since internal reflection may help reducing sound transmission across a façade device (9), the effects of inclination of the inner glass pane on the sound transmission loss across the plenum window are examined by using a scale model inside the anechoic chamber of PolyU in the present study. The effects of the source distance d on the sound transmission loss are also studied. 2. EXPERIMENTAL SETUP All the experiments in this study were carried out insider the PolyU anechoic chamber. A scale model of scale down ratio 1: 4 was adopted. Figure 2 shows the setup. The receiver chamber was a 1:4 scaled down model of the PolyU reverberation chamber as in Tong and Tang (10). The model was made of 18mm varnished plywood panels. The source was an array consisted of ten 6-inch aperture loudspeakers. The loudspeakers were so tilted such that their normal axes pointed directly to the height level of the outer window pane edge. Scale model Plenum Window h d Loudspeaker array Figure 2 The scale model and the sound source 1169

3 The plenum window used in this study was of the vertical type (7,8) and its size was 500mm (Height, h) by 300mm (Width) by 150mm (Depth). The window panes were made of 3mm thick acrylic sheet and its frame 18mm varnished plywood panels. The outer window pane was kept vertical, while the inner window pane could be tilted at three different inclination angles (θ) with the vertical plane : -5º, 0º and +5º as shown in Figure 3. Since the gap size, g, was limited at 100mm, the inclination angle θ of the inner window pane was also limited to within ±5º. The inner window opening was fixed at 200mm. The window sill was at a height of 200mm above the chamber floor. Figure 3 Inclination angle definition The acoustical performance of the plenum window was described in term of insertion loss, IL. In this study, IL was defined as the reduction in noise level inside the model receiver room compared to the reference case where the window was fully opened (that is, with the window panes removed) after correction for the reverberation effect : RC i, ref IL = + i NLi, ref NLi, plenum 10 log10, (1) RCi, plenum where the suffices ref and plenum denotes quantities related to the reference case and the test plenum window case respectively, NL the noise levels inside the model, RC the room constant and i the ith 1/3 octave band. NLs were measured using 13 microphones uniformly spanned over the entire receiver room volume. The data acquisition system was the Brüel & Kjær 3560D PULSE system with a sampling rate of samples per second per channel. The corresponding A-weighted traffic noise insertion losses, IL A, were obtained by applying the EN normalized traffic noise spectrum (11) to IL i : ( R i ILi ) 0.1Ri IL A = 10log , (2) i= 1 i= 1 where Rs are the normalized traffic noise spectral weightings. The loudspeaker outputs were kept constant throughout the present study. In the foregoing discussions, the sound frequencies are scaled back to the full scale model. 3. RESULTS AND DISCUSSIONS The reverberation inside a receiver room can be increased by replacing a casement window with a 1170

4 plenum window provided that the window opening size w h/2. The maximum w was h/2, above-which some sound might be able to pass directly across the plenum window. In the present study, the reverberation times (RTs) were estimated by averaging the results obtained from the 13 microphones inside the receiver room. Figure 4 illustrates the frequency variations of RTs. As a preliminary study, w was fixed at h/2 throughout the experiment. This opening window size is comparable to that of the PolyU hostel plenum window. 1.0 Reverberation Time (sec) θ = 0º θ = +5º θ = -5º Open window One-third Octave Band Centre Frequency (Hz) Figure 4 Reverberation times of the receiver room One can observe that the small variation of inclination angle, no matter it is positive or negative, tends to reduce the RTs at frequencies below the 1000Hz one-third octave band. However, the presence of the plenum window did make the receiver room more reverberant in general, and thus Eq. (1) is essential for reflecting the actual acoustical protection of the plenum window as the reverberation tends to increase the noise level inside the receiver room θ = 0º θ = +5º θ = -5º IL A (dba) Source Distance (m) Figure 5 Variation of plenum window insertion loss with source distance Figure 5 shows the combined effects of w and d on the A-weighted traffic noise insertion losses IL A. The source distances have been scaled back to the full scale condition. It can be observed that tilting the inner window pane can improve the acoustical performance of the plenum window in 1171

5 general. When the source is close to the window, the increase in IL A is ~2dBA and for 6m d 8m, the improvement can be up to 3dBA. In general, a positive inclination angle tends to produce better insertion loss for d > 4m in general. The direction of the inclination does not affect the IL A at small source distances. The positively inclined inner window pane tends to reflect more direct and diffracted sound back toward the edge region of the outer window pane and thus the better IL A in general. However, the effect is not so significant compared to its negative counterpart. There are two source distances d = 4m and 10m where the inclination does not improve the IL A. The exact reason for this is not clear and it is left to further investigation. 4. CONCLUSIONS A series of scale model experiments was derived in this study to investigate how the inclination of the inner window pane of a plenum window can help improve the acoustical performance of the window. The scale down ratio adopted was 1:4. Owing to the small gap size of a plenum window, the inclination angle was limited to within ±5º. The installation of the plenum window increased the reverberation inside the receiver room compared to the case of open window. The inclination of the inner window pane reduced the reverberation strength at lower frequencies. The A-weighted traffic noise insertion loss of the plenum window in general decreased with increasing source distances. The inclination also helps increasing the insertion loss in general. The improvement was around 2dBA for short source distance and can be as high as 3dBA as the latter increased. However, there were source distances at which the improvement was not significant and it is left to further investigation. ACKNOWLEDGEMENTS This work is supported financially by a grant from the Research Grant Council, the Hong Kong SAR Government under Project no. PolyU152164/15E. REFERENCES 1. Tadeu AJB, Mateus DMR. Sound transmission through single, double and triple glazing. Experimental evaluation. Appl Acoust. 2001;62(3): Tang SK. Noise screening effects of balconies on a building façade. J Acoust Soc Am. 2005;118(1): Lee PJ, Kim YH, Jeon JY, Song KD. Effects of apartment building façade and balcony design on the reduction of exterior noise. Bldg Environ. 2007;42(10): Kang J. An acoustic window system with optimum ventilation and daylighting performance. Noise Vib. Worldwide 2006;37(1), Ford RD, Kerry G. The sound insulation of partially open double glazing. Appl Acoust. 1973;6(1): Tong YG, Tang SK, Kang J, Fung A, Yeung MKL. Full scale field study of sound transmission across plenum windows. Appl Acoust. 2015;89: Kang J, Brocklesby BM. Feasibility of applying micro-perforated absorbers in acoustic window systems. Appl Acoust. 2005;66(6): Tang SK. Acoustical protection of a plenum window installed with sound absorptions. Proc INTER-NOISE 2015; 9-12 August 2015; San Francisco, USA Paper no Ishizuka T, Fujiwara K. Traffic noise reduction at balconies on a high-rise building façade. J Acoust Soc Am. 2012;131(3): Tong YG, Tang SK. Plenum window insertion loss in the presence of a line source - a scale model study. J Acoust Soc Am. 2013;133(3): BS EN , Road traffic noise reducing devices Test methods for determining the acoustic performance Part 3. Normalized traffic noise spectrum. BSI: London;