# Signal Enhancement of Helicopter Rotor Aerodynamic Noise Based on Cyclic Wiener Filtering
## 1 Introduction  
The noise problem has become an important factor affecting the lives of residents. [PERSON] examined the effects of background noise on infants and children [1]. [PERSON] investigated the impact of background noise on school students aged between 11 and 18 years, and their level of annoyance was often related to background noise [2]. [PERSON] analyzed the effect of low-frequency noise on cognitive performance of young people aged 19 to 29 years, based on changes in their heart rates [3]. [PERSON] studied the effects of exposure to road, railway, airport, and recreational noise on blood pressure and hypertension, especially for people aged 37 to 72 years [4]. Furthermore, literature [5] investigated seafarers' cognition and attitude towards ship noise emission and raised seafarers' perceptions of the impact of noise on their health. In addition, if the background noise is too strong, the target signal will be overwhelmed, affecting our ability to obtain information. When the background noise is too strong, on the one hand, it affects the health of the listener, on the other hand, in order to obtain the target signal more effectively, some methods need to be used to filter the background noise. During the flight of the helicopter, there is a strong background noise, which will limit the detection distance of the helicopter. Therefore, in order to achieve helicopter detection at a longer distance, it is necessary to filter the background noise. Helicopters, as an air transport platform, usually make full use of terrain and other conditions to fly at low or ultra-low altitudes when performing rescue missions. They have the advantage of being difficult to be detected by radar, infrared, and other detection methods. The helicopter adopts the rotor to provide lift, propulsion, and control force, with a unique vertical take-off and landing, as well as hovering, high maneuverability, and low-altitude and low-speed flight performance [6]. However, the rotor generates strongmedium- and low-frequency noise, the attenuation in the air is slow, and the propagation distance is long [7; 8; 9]. The rotor aerodynamic noise presents time-varying, non-stationary, and multi-harmonic characteristics under the influence of various conditions such as the atmosphere, mountainous environments, fuselage scattering, and flight state when the helicopter is flying in complex environments, such as mountainous areas [10]. Therefore, the investigations of helicopter rotor aerodynamic noise have become a crucial topic of research nowadays. Helicopter flight noise is mainly derived from the main rotor, tail rotor, and engine. Among them, the former two belong to rotor noise, and the latter is non-rotor noise. The engine noise is related to the high-frequency part and attenuates rapidly in the air. Meanwhile, the noise reduction of the engine can be achieved through the engine design. The aerodynamic noise of the main and tail rotor is mainly in low frequency and slowly attenuates in the air; it is the focus of helicopter noise research [11]. The SNR (signal-to-noise ratio) is extremely low when the helicopter noise signal is received over long distances due to attenuation, since it propagates through the atmosphere. However, the frequency component of rotor aerodynamic noise is low and propagates far in the air during the long-distance detection of the helicopter. Hence, it is the key to helicopter detection. According to the different generation mechanisms and propagation directions of various noises, helicopter rotor aerodynamic noise can be divided into thickness noise, loading noise, blade vortex interaction (BVI) noise, high-speed impulse (HSI) noise, and broadband noise [8; 9]. Thickness noise and loading noise are also commonly referred to as rotation noise [7]. The rotation process of the blade aerodynamic force and the rise of the air volume pulsation constitute the low frequency part of the rotor noise.