# Review of Control Technologies for Quiet Operations of Advanced Air-Mobility
## 2 Passive Control Technologies
### Tonal Noise Reduction  
Historically, a great number of works have been undertaken to passively mitigate the acoustic signature of rotors and small-scale propellers of Unmanned Aerial Vehicles (UAV) and other AAM:  
_Ducting:_ [PERSON], et al. [7] researched the use of ducts with acoustic absorption materials to reduce the noise. They tested two ducts - one with perforated internal walls and one without perforations. Both ducts showed deleterious effects in thrust, with a 10-20% reduction in thrust, and overall noise level increase of ~5-10 dB. _Stacking:_ Two propellers on top of each other was found to increase the overall propulsive efficiency [8; 9]. Loss of thrust and propulsive efficiency was observed in the front (or top) propeller [9], while an increase in thrust and propulsive efficiency was observed for the bottom propeller. [PERSON] et al. [10] investigated the effects of ducts on coaxial rotors in hover using high-fidelity CFD [10]. They performed some qualitative acoustic analysis on the various configurations evaluated and saw that placing the duct around the lower propeller of the coaxial system resulted in reduced acoustic signature (see Figure 1). Clearly, here we see that there are some acoustic benefits to ducting and may need critical placement for peak noise cancellation. _Blade Count and Spacing:_ For the same thrust, increasing the number of blades reduces the tonal noise. Increasing the number of blades from two blades to four blades resulted in 10 dB reduction, but also increases the frequency of the noise [11]. This may increase the broadband noise floor, and hence increase the perceived noise level. Unequal blade spacings were found to diminish tones at the BPF harmonics [11]. Reducing the diameter of the propeller reduces the tip Mach number for a given rotational rate and the noise as well. Figure 1: Pressure fluctuations for different configurations (counter-rotating). Left: open rotors, Center: duct around lower rotor, and Right: duct around upper rotor [10]. A tip Mach number reduction from \(\text{M}_{\text{T}}=0.6\) to \(\text{M}_{\text{T}}=0.53\) was seen to have an acoustic energy difference of 6 dB [11]. _Phase Control:_ [PERSON] et al. [12] explored the potential of using Phase Control for noise reduction experimentally by utilizing a dual-rotor test stand. The two-bladed CF125 rotors were separated by a hub separation distance of 0.4 meters. Using a timing belt and mechanical adjustments, phasing and rotational rates are set for each of the blades. Noise measurements were performed using microphones installed in the Structural Acoustics Loads and Transmission (SALT) facility [13]. Using the experiments as validation, they developed a numerical model for further phasing studies of more configurations, varying the propeller count from 2 to 8 rotors. They showed that the overall directivity of the blade passage frequency noise can be modified, with a reduction as high as 28.6 dB recorded [12]. They also conclude that the deviation from the rotational rate between the two systems should not exceed 0.5% for a 6 dB blade passage frequency noise reduction. [PERSON] et al. [14] also found similar results numerically by varying the phase angles between the rotors and found quiet zones in the in-plane inter-boom bisections up to 20 dB. Similar work was done by [PERSON] et al. [15] experimentally and numerically but focused more on stacked rotor performance. They utilized two sets of 3-bladed rotors, with at tip radius of 158.75 mm, and tested various axial and azimuthal offsets between the two rotors. They found that the configuration with the ratio of stack clearance distance to rotor radius (\(\Delta\text{Z}/\text{R}\)) = 0.15 to be the most efficient due to rotor power loading with a power savings of 6.9% relative to the coplanar case. The stacked rotor configurations showed increases in low frequency tonal noise with increasing axial separation and decreasing azimuthal offset. All the stacked rotor configurations showed broadband levels lower than that of the baseline rotor.