# Active Control of Low-Frequency Noise through a Single Top-Hung Window in a Full-Sized Room
## 4 Discussion and Conclusions  
An active control method for controlling low-frequency urban noise while maintaining natural ventilation in a building was devised. Four loudspeakers were affixed to a frame surrounding a top-hung ventilation window in a full-sized mock-up apartment room. The active control system with a fully opened window achieved similar noise reduction to a fully closed window for aircraft noise, whereas active attenuation for traffic and motorbike noise was not too far off from passive insulation. In an extreme example, active attenuation of low-frequency compressor noise significantly outperformed that of a fully closed window. This highlights a potential scenario for low-frequency noise control through apertures beyond the domestic setting. However, it is important to note that the upper frequency limit of the active control is dependent on the distance between the control loudspeakers [21; 24] and appears to be related to the smallest dimension of the opening, as reported in [25]. In the top-hung window shown, the theoretical limit would be based on one wavelength of the shortest opening (\(\sim\)0.4 m), which is about 850 Hz. Besides the frequency limit, the directivity of the control loudspeaker becomes critical as the frequency increases, which would have implications on the placement of the loudspeakers and be worth further investigation. Moreover, only a single top-hung window was affixed with the active control system, potentially allowing low-frequency noise to leak into the interior through other, albeit closed, window panels. Expansion of the system to all available top-hung windows in the window system could be investigated next. Despite the limitations, the attenuation results obtained from a single top-hung window are promising, especially for noise types with dominant low-frequency content, such as jet aircrafts and compressor noise. Although placement of control sources at the edge of the window frame was previously found to not be optimal [21], a tradeoff to obtain improved low-frequency performance and reduced visual obstruction could be worthwhile. This ANC window system can also be a solution to the urban planner's dilemma of balancing the proximity and orientation of residential buildings to urban transport infrastructure. Optimizing the orientation of naturally ventilated buildings for wind flow, solar heat gain, and air temperature [26] would not be bogged down by the consideration of noise propagation. Moreover, the ANC window also provides an avenue for the indoor soundscaping of naturally ventilated buildings, whereby unwanted noise can be removed using selective algorithms [11; 12] and wanted sounds can be introduced or amplified via the same loudspeakers that deliver the anti-noise. This element of controllability with an ANC system aligns well with the concept of adaptive acoustic comfort, suggested by [PERSON] et al. [9], which could be worth further investigation. Conceptualization, B.L. and W.-S.G.; methodology, B.L. ; software, D.S. ; investigation, B.L. and D.S. ; validation, B.L., V.B., S.W. and D.S. ; writing--original draft preparation, B.L. ; writing--review and editing, D.S., V.B., S.W. and W.-S.G.; supervision, W.-S.G.; funding acquisition, W.-S.G.; project administration K.L. and I.L. All authors have read and agreed to the published version of the manuscript. Figure 9: Attenuation in dB as a function of frequency during ANC ON (window opened) and ANC OFF (window closed) for (**a**) aircraft, (**b**) motorbike, (**c**) traffic, and (**d**) compressor noise types. **Funding:** This research is supported by the Singapore Ministry of National Development and the National Research Foundation, Prime Minister's Office under the Cities of Tomorrow (CoT) Research Programme (CoT Award No. COT-V4-2019-1). Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not reflect the views of the Singapore Ministry of National Development and National Research Foundation, Prime Minister's Office, Singapore.