# Prediction and Control of Broadband Noise Associated with Advanced Air Mobility--A Review
## 4 Passive Reduction of BBN
### Other Ideals  
_Trip:_ Broadband noise (BBN) is influenced by the trailing edge boundary layer thickness, which is often caused by a laminar separation bubble on the suction side of the propeller. [PERSON] et al. [81] conducted a study to investigate the primary sources of BBN for small-scale propellers operating at low Reynolds numbers. They identified that the trailing edge boundary layer thickness, due to a laminar separation bubble, is a significant contributor to BBN. To address this, they experimented with leading-edge serrated and straight trips (as shown in Figure 27a,b) positioned before the laminar separation to reduce trailing edge thickness. This modification led to a reduction in turbulent boundary-layer trailing-edge noise, with an overall broadband noise reduction of up to 4 dB observed during static tests at a rotational rate of 5000 rpm. As depicted in Figure 27c, at 3500 rpm, the serrated trip achieved greater broadband noise reduction at higher frequencies by approximately 2 dB. This suggests that the trailing edge boundary layer interaction noise may be more critical than laminar separation bubble interaction noise in this context. However, it is important to note that the effectiveness of this noise reduction method is limited to conditions where a laminar separation bubble is present, which typically occurs during cruise conditions.  
Notched Leading Edge: [PERSON] and [PERSON] [82] introduced an innovative approach by modifying a 9.4-inch DJI Phantom 2-bladed propeller to explore noise reduction techniques. They investigated the impact of adding a leading-edge notch by cutting a groove into the stock blade (as shown in Figure 28). Their findings indicate that placing the notch at an r/R of 0.90 resulted in the most significant reduction in broadband noise (BBN), though it came at the cost of a 9% increase in power required to maintain similar thrust conditions. As shown in Figure 28b, the sound pressure level (SPL) spectrum revealed a notable reduction in broadband noise (BBN) above 100 Hz compared to the stock blade, although there was a slight increase around 80 Hz. This demonstrates that the notched blade has a strong capability for BBN reduction. Their smoke flow visualization explained that the notch created a barrier between the high-pressure and low-pressure surfaces, causing the flow on the high-pressure surface to move in the opposite direction, thereby disrupting the tip vortices. While this method shows promise for noise control, it also introduces  
Figure 26: (**a**) ORG is initial propeller and (**b**) NM-AM is optimized propeller [80].  
Although this design is hard to manufacture, it still gives a good idea to replace traditional propellers in the future.  
Another straightforward application idea is an applied wing tip design to rotor tip design. [PERSON] and [PERSON] [84] investigated the noise of nine different tip shape rotors by numerical method based on a Bell UH-1H helicopter rotor and found that not all tip geometries achieved significant noise reduction, although many demonstrated good aerodynamic performance. The Eagle tip design showed the best noise reduction, with a decrease of approximately 4.5%, and also improved the figure of merit ratio by up to 17%. It is understood that tip design can influence tip vortex formation noise and potentially affect BWI and other noise sources. However, since the study did not focus on broadband noise (BBN) and lacked a detailed SPL spectrum analysis, the extent of BBN reduction and the underlying mechanisms remain unclear, indicating a need for further investigation.