Plenum Window Research Articles

Generalization of noise insulation afforded by common window designs with active noise control exposed to moving noise source

A window device that can provide noise insulation while maintaining natural ventilation is highly desirable and is especially sought after in heavily noise-polluted and hot-humid climates cities, such as Southeast Asian cities. However, a solution to provide such a window is non-trivial because the design principles guiding acoustics and ventilation are diametrically opposed. While louvre windows have traditionally been used in communal, healthcare, and some educational setting to provide a balance between acoustics and ventilation benefits, it is still sorely lacking to provide sufficient insertion loss without sacrificing ventilation entirely. A new plenum window design has recently garnered research and market interest to tackle the dichotomous problem. Additionally, the application of active noise control (ANC) in windows is gaining popularity and offers potential solutions to improve noise control without compromising ventilation. However, it remains unclear how different traditional and modern window designs compare in terms of their acoustic performance, especially when exposed to non-stationary noise sources such as urban traffic, a major noise pollutant. Thus, the present study seeks to provide a generalization of insertion loss provided by different window designs exposed to moving sources including a tangent study on ANC implementation via a time-domain approach.

A feasibility study on active sound reduction across an acoustic plenum window by cancelling source clusters on internal periphery of the window cavity.

The possibility of applying active control to reduce sound transmission across a practical plenum window is examined experimentally in the present study using measured transfer functions of all related sound transmission paths. As a result of the limited space within the window, the error microphones are located at the indoor window opening while the secondary cancelling sources are mounted along the periphery of the window void. Results show that the cancelling sources near the outdoor window opening corners and within the overlapping region of the window play more useful roles in the control. Also, the highest sound reduction is around 6 dB with six error microphones positioned either at the central region or along the periphery of the indoor window opening. However, the results with the central error microphones suggest the possibility of adopting a dual control system to enhance the low frequency performance. Control systems with fewer error microphones result in lower sound reduction. Besides, it is found that four cancelling sources, located around the outdoor opening of the window, will be enough to achieve meaningful active sound transmission reduction between 100 and 1000 Hz. Involving more cancelling sources does not result in better performance despite the added complexity.

A time-domain study on the effects of noise source incidence on plenum window with active noise control

Active noise control (ANC) applied to the built environment on windows presents itself as a potential solution to reduce noise exposure for residents living near major noise sources since windows are typically the primary path for noise intrusion on a building's facade. The plenum window design seeks to address the dual dichotomy of ventilation and acoustic dilemma that plagues hot-humid and highly noise-polluted cities. In the present study, a time-domain approach is developed to investigate the effect of varying impinging angles of noise sources on the acoustic performance of a plenum window which can represent different storeys of the receiving end of noise exposure. Furthermore, the ANC performance of a control system affected by the varying noise sources is explored in this study. While most ANC studies have limited their scope of study to frequency domain investigations, the present study takes a time-domain approach to account for the causality of the ANC system concerning the primary noise source and secondary control source, which is thus more accurate to the behaviour of a real-time ANC controller. A total of 121 simulation studies were performed, and the findings show that the plenum window's PNC (passive noise control) is affected by the incident angle of a noise source based on its tonal frequency, which is pronounced near the aperture's cut-off frequency, faperture. However, the ANC performance due to varying incident angles of primary noise remains unaffected even at high frequencies with a standard deviation, σANC < 1 dB except for the 630 Hz tonal frequency, which coincides with faperture.

Experimental study of active noise control for a full-scale plenum window in a domestic apartment

Plenum windows, also known as ventilation windows, allow for natural ventilation in living spaces while providing noise reduction to improve the health and living comfort of occupants, especially in noise-polluted and tropical/sub-tropical cities. However, while plenum windows can provide acceptable levels of natural ventilation, the passive noise performance of plenum windows when fully opened is still lacking as compared to the conventional closed windows- especially in the low frequencies experimentally. Active noise control (ANC) has shown good performance in several applications for low-frequency control. Thus, the application of ANC using a single-channel and decentralized dual-channel ANC implementation, informed by prior simulation studies is developed and field tested for a plenum window in a domestic setting built to reproduce the environment of a high-rise apartment. The experiments show that the average global attenuation benefit in the apartment room due to a single-channel ANC is 2.9 dB in the 100 Hz – 500 Hz range while the decentralized dual-channel ANC provides up to an average of 4.8 dB reduction in the 100 Hz – 500 Hz range or an average reduction of 3.8 dB in the 100 Hz – 800 Hz range. The decentralized dual-channel ANC is also tested under different noise types and is effective in low-frequency control. Furthermore, the dual-channel ANC remains robust even with an additional mechanical ventilation system operating inside the plenum window, improving the natural ventilation.

Optimization of single-channel active noise control performance in a plenum window using the surface impedance approach.

The use of active noise control (ANC) implementation in plenum window design is investigated in this study. Various simulated configuration of a single-channel ANC is performed using the surface impedance approach (SIA) in order to optimize ANC performance. Based on a systematic search procedure, the optimal control source placement is found for a control source localized at the central bottom and central depth of the plenum window, near the window's inlet from which primary noise is impinging. The optimized ANC configuration provides an average attenuation benefit of 9.2 dB between 200 and 630 Hz. Error sensor location in the plenum window cavity is not crucial for the ANC system and does not need to be rigid. A dual-channel ANC system with control sources at both sides of the plenum window can extend the frequency of control to ∼800 Hz with an average attenuation of 7.6 dB. Additionally, an experimental case study using a real-time ANC system is conducted with a built-to-scale plenum window in an apartment informed by findings from the SIA simulation, demonstrating the usefulness of the SIA in ANC optimization process.

The sound insulation across the plenum window with staggered sound scatter arrays

In this study, experiments and simulations were carried out by a scaled-down model in a semi-anechoic chamber to investigate the acoustic effect of the staggered sound scatter array on the sound insulation of a plenum window. The 1/4 scaled-down plenum window has a gap of 98 mm and the diameter of the sound scatter cylinder is 19 mm. Firstly, the reverberation times of the verified case and reference case were presented to explore the possible effect of the installment of the sound scatter array on the reverberation of the room model. Next, the effect of four sound scatter array arrangements on the sound insulation of the plenum window was analyzed experimentally and numerically. Results show that the installment of the staggered sound scatter array can enhance the sound insulation of the plenum window. Below 600 Hz, enhancement of the staggered sound scatter array on the sound insulation of the plenum window is lower than 2.2 dB. Beyond 600 Hz, the extra transmission loss increases with the frequency and is up to around 6.0 dB. In addition, the trapped mode was observed inside the window cavity and it excited a second higher gap mode which helps improve the sound insulation of the plenum window.

A pilot study on the sound insulation performance of plenum doors

Demands for natural ventilation is inaugurated recently, particularly after the COVID-19 outbreak. Various ventilating windows have been proposed, one of which a plenum window has been studied for these years. Since the COVID outbreak shops and restaurants often leave doors opened for ventilation reasons during their business hours. However, by this, the indoor sound environment is suffered by incoming noises from streets. Therefore, it is desirable that a sound insulation structure enabling natural ventilation is developed. In this study, we propose a door with plenum structure which is an application of the same principle of plenum windows to a door. Doors and windows are of different sizes, and under different conditions. Therefore, in this paper, as a pilot study, we employed finite element method to analyse the sound insulation characteristics of a plenum door. Results show that a plenum door, although its sound insulation is not very high, shows sound insulation performance to a certain extent. Also, a parametric study is given to show the effect of parameters including that of sound absorption linings.

On the Hybrid Modelling of Rigid Sonic Crystal Embedded in Plenum Chamber at Low Frequencies Using the Effective Medium Approach and Finite Element Method

This study explores the application of homogenization schemes in modelling acoustic wave propagation in a finite array of circular scatterers embedded into a 2D cavity. The cavity is integrated into the 2D waveguide attached to the cavity inlet and outlet. Square-lattice sonic crystal (SC), composed of rigid and radially symmetric circular cylinders, is used to represent the finite array in this study. The model is aimed at informing the novel designs of plenum windows with superior soundproofing performance. The sound transmission performance is analysed with hybrid numerical simulation using the Finite Element Method (FEM) and homogenization scheme. A theoretical homogenization method based on the Effective Medium Approach (EMA) is reviewed and implemented on the modelling of the SC array inside the cavity at low frequencies and then implemented in FEM. Comparison of the transmission loss (TL) reveals a high degree of matching between settings with and without homogenization at low-frequency range. It is observed that performance of the hybrid model maintains a high degree of matching provided the cavity modes are dominated by a single characteristic scale in the low frequency. This includes the case of a narrow plenum chamber with only a single cylinder placed inside the cavity with a width of one lattice constant. This homogenization method is proven to be more computationally efficient in FEM compared to the modelling of exact geometry of the problem.

Incorporation of Resonators Into Plenum Window

A plenum window with incorporation of Helmholtz resonators in between two glass panes was tested in a reverberation room. The effects of jagged flap on reducing strength of diffracted sound was also investigated in the present studies where white, traffic and construction noises were examined during each set of experiment. When the noise source was located at the central line of the plenum window, the plenum window with Helmholtz resonators was able to mitigate 8.5 dBA, 8.9 dBA and 8.2 dBA of white, traffic and construction noises, respectively, compared with the case of without window. These amounts of noises that attenuated by the plenum window were slightly higher than the case where noise source was diverged 30X away from the plenum window. The effects of jagged flaps on the acoustical performance of the plenum window were negligible. The Helmholtz resonators had the best performance in the frequency region between 900 Hz to 1300 Hz where in this frequency range, the plenum window with Helmholtz resonators was able to attenuate additional 1.7 dBA, 1.9 dBA and 1.6 dBA of white, traffic and construction noises, respectively, compared with the case of without resonators.

Application of active noise control for plenum window in a domestic setting

Plenum windows, also known as ventilation windows allow for natural ventilation in living spaces while providing noise reduction to improve the health and living comfort of occupants, especially in noise polluted and tropical/sub-tropical cities. However, while plenum windows can provide acceptable levels of natural ventilation, the passive noise performance of plenum windows when fully opened is still lacking compared to the conventional closed windows at low frequencies. Active noise control (ANC) has been shown to provide good performance in several applications in low-frequencycontrol. Thus, the application of active noise control through a single-channel and dual-channel ANC implementation is to be developed and demonstrated in a field test for a plenum window set in a domestic home built to reproduce the environment of a high-rise apartment. The experimental application will be based on the optimal configurations for single and dual-channel ANC informed by prior simulation studies.

Experimental investigation on the enhancement of plenum window noise reduction using solid scatterers.

The sound transmission across plenum windows installed with rigid non-resonant cylindrical scatterer arrays was investigated in detail using scale-down model measurements carried out inside a fully anechoic chamber. The arrays have manifested to some extent the acoustical behaviors of virtual sonic crystals. The maximum cross section blockage ratio was 0.6. The effects of plenum window gap, array configuration, and scatterer diameter on the sound transmission characteristics were also examined. Results indicate that the window cavity longitudinal modes and the gap modes control the sound transmission characteristics at low frequencies. The upper bound of this frequency range increases with decreasing gap width. Within this frequency range, the scatterers have negligible effect on the sound transmission. At higher frequencies, the array configurations with scatterer(s) attached to the window walls result in stronger sound reduction. There are relatively higher sound transmission loss improvements around the frequencies where a full bandgap is observed. There are wide bandgaps in various lattice directions, and the present results suggest that they play a role in the broadband improvement of sound reduction.

Parametric study of acoustic windows using a Green’s Function transfer matrix method

The Green’s function dependent transfer matrix method was applied to model sound transmission loss of a plenum window with a given configuration. The analytically calculated transmission loss reasonably followed that of the experimental data obtained from a previous study. The troughs in the calculated transmission loss spectra were found aligning well with the natural frequencies of the acoustic modes inside the plenum chamber. Vectorization of the calculations shows a 10-fold acceleration in computation, enabling desktop evaluation of transmission losses of plenum window systems with various configurations. Utilizing the analytical model, a parametric study of varying the three crucial design parameters, including the gap width, the overlapping length and the area ratio, between the outlet and inlet openings of the plenum window was conducted. The results demonstrated that increases in the latter two design parameters would improve the transmission loss performance of the plenum window at low frequencies, while increasing the gap width would compromise its transmission loss performance. Also, an increase in the overlapping length would widen the frequency range coverage with elevated transmission loss performance.

Different alternative retrofit to improving the sustainability of building in tropical climate: multi-criteria decision-making.

With the growth of the number of old buildings in urban cities, there is an imperative demand for retrofitting those buildings to minimize their energy consumption and maximize their sustainability. This article seeks to provide a multi-criteria assessment of different retrofitting scenarios in the Malaysian context, focusing replacement of windows. Four different criteria assessed operation energy usage, global warming potential (GWP) emission, embodied energy, and the cost of each alternative. Life cycle analysis is used for each scenario using the Energy Plus software program to estimate the energy demand. The preliminary result showed that a louvered window is unsuitable for operational energy usage compared to other options. In embodied energy and GWP, double-glazing shows an optimal choice by 532 MJ kg/m2 and 101 kg/M2 CO2 between the other two alternatives for retrofitting. However, in the operational energy category, triple glazing has the best performance by 1.06 kW/a day. Finally, comparing the cost of each other options, plenum windows have the lowest rate by 825 kg/M2 MYR. Thus, multi-criteria decision-making (MCDM) is used to select the most sustainable window for buildings. The result shows that the best option is a double-glazing window, followed by a plenum window. This study revealed the requirement for utilization of MCDM handles to guarantee the correct choice of design strategies for the best decision.

On the acoustical performance of plenum window installed with rigid circular cylinders at low frequency

The low-frequency acoustical performance of plenum window with an array of rigid circular cylinders installed is analysed with the Finite Element Method (FEM) in the current study. This study includes the effect of vibrational glass boundaries, cavity resonance, the opening width of the window, and the arrangement of cylinders, on the sound transmission loss (TL) of the plenum window. The relationship between the above factors with TL is also investigated statistically and simple optimization of the geometry has been done based on the results. Results have shown that including the mechanical vibration of glass does not result in a significant difference with previous results, but an average decrease of 1 dB in TL is observed near the eigenfrequencies of the glass boundary. The effect of opening width on TL is not trivial but frequency-dependent due to the cavity resonance formed between the overlapping regions. Lastly, the introduction of rigid cylinders causes a shift of significant TL to lower frequency regardless of the opening width. The installation of them also seems to have destroyed the formation of cavity resonance, which results in much smaller TL.

Noise reduction of plenum window with add-in dual staggered sound scatterer arrays

In present study, a 1:4 scaled down model was used to explore the noise reduction across the plenum window with add-in dual staggered scatterer arrays (sonic-crystal). Reverberation time inside the model space was measured firstly to eliminate the effect of the possible reverberation variation on the sound transmission loss of the plenum window. Two sonic-crystal arrays, the two-by-two and two-by-three scatterer arrangements, were adopted for measurement. A total of four arrays was thus tested after the staggering. Computational simulation was conducted for the sound field inside the plenum chamber to study the noise reduction mechanism of the present window system. Results show that the noise reduction of the plenum window was improved by varying degrees due to the placement of the dual staggered sonic-crystal. The Installation of the dual staggered sonic-crystal increased the sound energy reflections out of the plenum window inlet and decreased the sound energy that passed through the plenum window cavity. At the same time, the resonances inside the window cavity also contributed to the sound transmission loss of the plenum window. The noise reduction across the plenum window was enhanced. The improvement was between ~2 to ~2.7 dBA.

Noise reduction of enhanced acoustic balconies on a high-rise building block

A new device called 'enhanced acoustic balcony' is installed in a new housing estate in Hong Kong. It is intended to help reduce the impact of traffic noise on the residents. This balcony is basically an enlarged form of a plenum window and with three openings. Apart from the outdoor air inlet, there is the balcony door and a side-hung window on the interior balcony wall for natural ventilation of the indoor space. Sound absorption of NRC 0.7 is installed on the balcony ceiling and its sidewall facing the incoming traffic noise and an inclined panel is installed outside the balcony to provide noise screening. A site measurement of its noise reduction is carried out in the present study in a newly completed housing block. A 28 m long loudspeaker array is used as the sound source. The indoor noise levels are measured according to ISO standard. The results show that the difference between indoor and outdoor noise levels in the presence of this balcony form varies over a relatively narrow range between 10 to 13 dBA for an elevation angle from 25 to 60 deg. There is a weak increase of the noise level difference with elevation angle.

Prediction of sound reduction index of plenum window by the surface impedance approach

The paper predicted the sound reduction indices (SRIs) of plenum windows with various configurations by the surface impedance approach (SIA). Assuming that the glass panes of plenum windows are rigid, two spaces connected through a plenum window can be taken as a three-coupled space. In terms of a multiply connected space, the SIA enables to characterize the acoustical properties of each sub-space by its own descriptors – blocked pressure and surface impedance across the coupling interface surface between any two sub-spaces. With the descriptors identified, the sound radiation in the entire space can be predicted accordingly. The sound radiation due to a point source in an acoustical space consisting of a rectangular space, a plenum window and a semi-free space was successfully predicted by the approach. Providing the diffuse sound field is generated in the rectangular space, the SRIs of plenum windows could be determined with the predicted averaged sound pressure level in the diffuse source space and the predicted sound intensity level across a prescribed measuring surface in the receiver space, i.e., the semi-free space according to the standard ISO 15186–1. The SRIs of five plenum windows were studied by the SIA. The predicted results are in good agreement with the measured ones obtained in the laboratory environment, which indicates the feasibility of the SRI prediction by the SIA.

Noise reduction of plenum windows on the façade of a high-rise residential building next to heavy road traffic

Extensive traffic noise transmission loss measurements were carried out inside selected residential units of a standalone 32-storey housing block located next to a very busy and noisy main trunk road in the present study. A total of 35 units, which were all equipped with plenum windows, was surveyed. These plenum windows are intended to help reduce noise exposure of the residents and at the same time allow for a reasonable level of natural ventilation. The results show that the traffic noise transmission losses of the unit façades installed with the plenum windows adopted in this housing block vary between 10.6 and 13.0 dBA and are only weakly dependent on elevation from the trunk road. The results also validate in-situ the prediction model established previously by the authors using laboratory and site mockup data. Generalized models for both empirical and experimental estimation of the traffic noise transmission loss across a residential flat unit façade installed with multiple plenum windows are developed. The differences between their estimations are well within engineering tolerance.

Solving Noise Pollution Issue Using Plenum Window with Perforated Thin Box

In the present study, a conventional plenum window was incorporated with perforated thin box in order to enhance its performance at frequency range which centralized at 1000 Hz as most of the common noise sources at city nowadays are centralizing around this frequency. The entire studies were conducted in a reverberation room. The effectiveness of jagged flap on mitigating diffracted sound was also studied. Three types of noises were examined in the current study—white noise, traffic noise and construction noises. The experimental results showed that the plenum window with perforated thin box could reduce 8.4 dBA, 8.7 dBA and 6.9 dBA of white, traffic and construction noises, respectively. The jagged flaps did not have significant effect on the plenum window’s noise mitigation performance. When frequencies were ranging from 800 Hz to 1250 Hz, when compared with the case of without perforated thin box, it was found that the perforated thin box had good acoustic performance where it was able to reduce additional 1.6 dBA, 1.6 dBA and 1.2 dBA of white, construction and traffic noises, respectively.

Novel plenum window with sonic crystals for indoor noise control

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.