# Review of Control Technologies for Quiet Operations of Advanced Air-Mobility
## 2 Passive Control Technologies
### Broadband Noise Reduction  
_Serration and Trip Effects:_ [PERSON], et al. [16] investigated the main sources of broadband noise for small scale propellers at low Reynolds numbers and found it to be mainly a result of the trailing edge boundary layer thickness caused by a laminar separation bubble on the suction side of the propeller. They used a leading-edge trip and serrations (Figure 2a,c) placed prior to the laminar separation to reduce trailing edge thickness which resulted in a decrease in turbulent boundary-layer trailing edge noise. They reported an overall broadband reduction of up to 4 dB in static tests at a rotational rate of 5000 rpm (see Figure 2b,d). Figure 2d shows that at the same operational conditions, the serrations had more broadband noise reduction at higher frequencies by about 2 dB. This may be a result of the effective magnitudes of the trailing edge noise sources, i.e., the trailing edge boundary layer interaction noise may be slightly more critical than the laminar separation bubble interaction noise. An important takeaway mentioned is that this method would only be effective for cases where there was a presence of a laminar separation bubble, which was mentioned to be the case only for cruise conditions, and thus would perform poorly for take-off conditions where blade loading is highest. [PERSON] et al. [17] investigated the effects of trailing edge serrations on broadband noise reductions UAVs. They perform these tests on an experimental test rig with an 8-inch diameter 2-bladed propeller. They noted that an increase in pitch angles did increase the overall sound pressure levels both in the tonal and broadband noise levels of UAV propellers. Reduction in broadband noise levels were achieved using trailing edge serrations. The results suggested that the noise reduction is due to the interaction between the serrations and the trailing edge boundary layer, which is a dominant source of high-frequency broadband noise. The depth of the serration also has an effect in the amount of reduction in SPL that can be achieved with the larger depths resulting in higher BBN reduction of 3 dB at 3 kHz and increases to a 5 dB reduction at 10 kHz (see Figure 3d, where Case 3 is highest depth/amplitude of serration). Additionally, the effectiveness of this technique is reported to diminish with increasing rotational rates, and thus is only recommended at lower tip speed rotors/propellers. It is important here to note that although the targeted high-frequency broadband noise was reduced, there may be an increase in the tonal noise levels as seen in Figure 3d. As such, care must be taken in selecting the right serration wavelength/size. Similar work was done by [PERSON] et al. [18], where they focused on the effect of the saw-toothed serration on the strength of the trailing wake, and subsequently the overall noise signature of the propeller. They reported an approximately linear reduction in SPL measurements for an increase in serration depth, with a 46.67% chord depth serration cut resulting in approximately 28% decrease in noise. Another novel idea was proposed by [PERSON] [19] and implemented by [PERSON] and [PERSON] [20]. They utilized a DJI Phantom 2-bladed propeller with a diameter of 9.4 inches ad a pitch of 5 inches. The noise measurements are performed below the rotor and are traversed from hub to tip. They examined the effects of adding a leading-edge notch to the stock blade by cutting a grove in the blade (see Figure 4). They reported that placing the notch at r/R of 0.90 resulted in 25% (\(\sim\)30 dB) reduction in peak SPL at a detriment of 9% increase in power required for similar thrust conditions. The main idea behind the notch approach was to disrupt the tip vortices rolling off the blade tips which is one of the major sources of propeller aerodynamic noise. This approach has a potential for noise control but again, has a penalty of thrust and power increase. Additionally, this may lead to some structural limitations for scaling to larger rotors.