# Prediction and Control of Broadband Noise Associated with Advanced Air Mobility--A Review
## 4 Passive Reduction of BBN  
### Leading- and Trailing-Edge Serrations  
[PERSON] et al. [75] presented a comparative numerical and experimental study of LE serrations (see Figure 21) with different morphologies based on a P17 \(\times\) 5.8 carbon fiber propeller, which has a diameter of 431.2 mm. Figure 22 shows the spectrum of different propellers at 2400 rpm and 3200 rpm, respectively. An obvious BBN reduction in the sawtooth propeller was achieved between 2 kHz to 12 kHz. To investigate the reduction mechanism, they conducted numerical simulations, which revealed that the LE serrations generated vortices that influenced the boundary layer turbulence. Additionally, the LE serrations were shown to reduce velocity fluctuations, contributing to the reduction of broadband noise (BBN). Finally, they performed a multi-rotor hovering test using three LE serration propellers, with better performance than the single propeller test. The results indicate that the sawtooth propeller outperformed the others, not only in noise reduction but also in thrust generation. Similar research was conducted by [PERSON] and [PERSON] [76], who examined the effects of leading-edge serrations on various propellers. Their findings indicate that propellers with LE serrations could achieve significant noise reduction across variable-pitch propellers and under different flight conditions.  
Figure 21: Blade designs with different LE serrations [75].  
Figure 20: Artificial neural network (ANN) model configuration [72].  
[PERSON] et al. [77] investigated the trailing-edge serration of a propeller by experimenting at four different RPMs. They designed three types of serrations: half flat tip, quarter flat tip, and rectangular for a 2-blade propeller with a 0.225 m radius, as shown in Figure 23. Figure 24 illustrates the SPL at 2000 RPM, where all the serrated propellers achieved a noticeable reduction in broadband noise (BBN). However, at 3000 RPM, only the half flat tip propeller maintained effective noise reduction, while the other two designs actually increased BBN at low-mid frequencies. Additionally, the serrations led to a decrease in thrust, with the half flat tip propeller experiencing the smallest loss, approximately 16.8%. Consequently, they concluded that the half flat tip design was the better choice. [PERSON] et al. [78] conducted similar experimental work, focusing on the impact of sawtooth serrations of a 2-blade propeller with an 8-inch diameter. Their findings indicate that the depth of the serration plays a significant role in BBN reduction. Specifically, larger serration depths resulted in a reduction in BBN by 3 dB at 3 kHz, with the reduction increasing to 5 dB at 10 kHz. However, they also reported that serration wavelength or size may lead to tonal noise increase, and noise reduction benefits are most pronounced at lower tip speeds.  
Figure 23: Geometry of (**a**) baseline, (**b**) half flat tip, (**c**) quarter flat tip, and (**d**) rectangular serrations [77].  
Figure 22: SPL results of different LE serrated propellers at (**a**) 2400 RPM and (**b**) 3200 RPM [75].