# Prediction and Control of Broadband Noise Associated with Advanced Air Mobility--A Review
## 5 Active Noise Control Technologies of BBN  
Reducing noise radiation through passive control can negatively impact aerodynamic performance. Therefore, an alternative approach is to integrate passive control into the propeller system design, optimizing it for regular flight operations. ANC should be engaged only in specific situations when noise levels temporarily exceed acceptable limits, such as near a vertiport or when the aircraft encounters turbulence. As mentioned earlier, the dominant source of broadband noise (BBN) is boundary layer turbulence. Active noise control (ANC) techniques aim to manage the boundary layer, including methods such as suction and blowing control. Several studies have investigated these techniques for airfoil noise reduction. [PERSON] et al. [85] conducted experiments using suction at the upper front area of an airfoil to reduce boundary layer thickness. Their findings demonstrate a reduction in trailing-edge noise by 3.5 dB. [PERSON] et al. [86] achieved even greater noise reduction by incorporating trailing-edge blowing into the airfoil section. Their numerical simulations indicated a maximum noise reduction of 20 dB. Ingested turbulence impinging the leading edge generates leading-edge noise [22]. A more effective approach is to control the flow at the leading edge. [PERSON] et al. [87] investigated the leading-edge blowing effect of rod wake ingestion noise of an airfoil. They designed single and double-orifices airfoils mounted behind the rod (see Figure 30) and evaluated them in a wind tunnel for 15 and 25 m/s inflow conditions with 0 and 10 angles of attack. They found the blowing could slightly reduce the low frequency BBN, while increasing mid-to-high-frequency BBN at 25 m/s inflow condition, as shown in Figure 31. The increase in self-noise was attributed to leading-edge (LE) blowing, suggesting that LE blowing might not be an optimal solution due to the additional high-frequency broadband noise (BBN) it introduces. However, it is not definitive that LE blowing is inherently problematic, as the effectiveness is significantly influenced by the orifice design. Therefore, further investigation is needed to better understand its impact. Few studies have been conducted for turbine rotors. A trailing edge rotor blowing blade is shown in Figure 32, which was evaluated by [PERSON] [88] in the Aeroacoustics  
Figure 29: Toroidal propeller [83]. Figure 31: SPL spectrum of blowing with **(a)** single-row orifices at AOA = 0\({}^{\circ}\), **(b)** single orifices at AOA = 10\({}^{\circ}\), and **(c)** double-row orifices at AOA = 10\({}^{\circ}\)[87]. Figure 32: TERB hollow fan blade with pressure side [88]. Dielectric barrier discharge (DBD) plasma actuators could produce electric wind by ionization of the air particles and provide another ideal to control the boundary layer flow. [PERSON] et al. [90] conducted an experimental study by installing two copper electrodes along the leading edges of helicopter model blades with a 0.73 m diameter, as shown in Figure 34a. The blades were set at a constant pitch angle of 12\({}^{\circ}\), which might induce some surface separation. The experiment aimed to suppress vortex shedding from the leading edge using downward electric winds generated by plasma, potentially altering the separation and reducing pressure perturbations on the blade. Figure 34b illustrates a significant decrease in broadband noise at low-to-mid frequencies, attributed to the actuation-induced separation control. The study also found that this actuation not only reduced the overall sound OASPL of BBN by 8 dB but also improved the figure of merit by 25%. Tonal noise was reduced by only 3 dB, further supporting the conclusion that dielectric barrier discharge (DBD) actuation effectively reduces broadband noise (BBN), particularly when separation is present, and BBN is the dominant noise source.