# Active Control of Low-Frequency Noise through a Single Top-Hung Window in a Full-Sized Room
## 1 Introduction  
The World Health Organization (WHO) has recently put forth strong recommendations to reduce urban transportation noise levels. These recommendations are underpinned by a rapidly growing body of evidence associating a myriad of health risks with environmental noise exposure [1]. In high-rise urban landscapes, traditional noise mitigation approaches, such as noise barriers, have limited efficacy in the low-frequency range and are effective only for the lower floors of a high-rise building that are in its shadow zone [2]. Although noise mitigation at the receivers' end, such as on building facades, is inefficient, it is usually the only form noise control available for land-scarce, high-density urban cities. The sustainability push for naturally ventilated buildings also increases noise control difficulty, as noise propagates easily through any opening in the facade.  
Noise control for naturally ventilated buildings has been traditionally passive, wherein physical elements have been employed to obstruct or dampen the acoustic waves as they enter the facade [3, 4].  
The main limitation of effective passive interventions, such as louvres and plenum windows, appears to be the restriction of airflow. Moreover, passive elements are designed with the noise control perspective of reducing the total acoustic pressure in the building interior by treating all sounds that enter the facade opening as waste to be reduced. Although the indoor acoustic environment for naturally ventilated buildings is dependent on context, there has been some evidence showing a preference of some external sound sources (e.g., natural sounds, human voices) over others (e.g., heavy traffic) while no indoor sound sources were present [5]. With the introduction of international standards on soundscapes [6; 7; 8], it is imperative that indoor acoustic comfort is approached with a perceptual perspective [9]. One overlooked factor in an indoor environment is the concept of control, which is challenging for naturally ventilated buildings. For instance, opening windows to lower the temperature increases the influx of exterior noise. The selective control of external noise sources [10; 11; 12] without obstructing airflow appears to be achievable with an active noise control (ANC) system designed for facade openings [13].  
By projecting an opposing sound wave with the same amplitude at the precise time and space of the disturbance to be controlled, ANC has been successfully integrated into automobile cabins [14; 15], aircraft cabins [16], and headsets [17] to reduce low-frequency noise. The ANC principle has been recently adapted for facade openings to provide significant noise control without obstruction to natural ventilation [18; 19; 20; 13]. Even though the proposed method where noise sources are distributed across the entire aperture allows for optimal control [21], the limited size of the loudspeakers in such a layout inherently limits noise control performance in the low frequency range.  
To improve low-frequency active control performance, larger loudspeakers were affixed to a full-sized, but smaller, top-hung ventilation window. Active control of recorded urban noise was evaluated in a full-scale room of a mockup residential apartment. For comparison, the active control performance was benchmarked to the passive insulation provided by fully closing the window.