# Prediction and Control of Broadband Noise Associated with Advanced Air Mobility--A Review
## 3 Methods for Prediction of BBN  
### Experimental Observations  
Inflow Turbulence Noise: Several experimental investigations of propeller noise under different non-uniform flows have been conducted. [PERSON] et al. [11] researched the noise of the third-party 1206 APC propeller (12-inch diameter) under the cylinder wake (see Figure 6a). They found the cylinder wake increased the BBN level at all ranges, especially at the mid-range of the frequency, as shown in Figure 6b. Similar research was conducted by [PERSON] et al. [12], whose results demonstrated that both tonal and broadband noise levels increased under the influence of the cylinder wake. Specifically, the broadband noise increased by 12 dB, while the tonal noise experienced a 1 dB increase for a 2-blade propeller. These findings corroborate the hypothesis that inflow turbulence significantly contributes to BBN, highlighting the impact of non-uniform flow conditions on propeller noise characteristics. In Figure 7b, we present the results by [PERSON] et al. [13], who studied the effect of distributed five-bladed propellers' noise under the influence of three different grid turbulence inges (Figure 7a) at varying advance ratios. All the grid turbulence scenarios caused significant variations in the broadband noise (BBN), particularly within the mid-to-high frequency range. Notably, grid 3, which generated the highest turbulence intensity (twice as strong as grid 1), resulted in the greatest increase in BBN. They also observed the haystacking phenomena at the blade passing frequency at \(\theta=60^{\circ}\) with the increased turbulent length scale. [PERSON] et al. [14; 15] conducted grid turbulence ingestion for a single two-bladed propeller at forward inflow and edgewise directions, respectively. Both resulted in a larger contribution of turbulence ingestion to the broadband noise radiation at the low-to-mid-frequency. However, the inflow condition still has a higher variation of BBN above 1 kHz, whereas the edgewise condition did not show a similar trend. Another aspect of turbulence ingestion, specifically, boundary layer turbulence, was investigated by [PERSON] et al. [16]. They examined the effects of varying the turbulence ingestion by adjusting the tip gap between the plate and a 2-bladed propeller with a 12-inch diameter to study the haystacking phenomenon, as illustrated in Figure 8a. Their findings indicate that increasing thrust led to more pronounced haystacking, which was more evident at perpendicular angle observer points. Additionally, they discovered that a lower tip gap, which results in greater ingestion of boundary layer turbulence, caused a significant increase in broadband noise (BBN) and the prominence of haystacking peaks. However, once the tip gap was reduced to 10 mm, further decreases did not result in a substantial increase in BBN, as depicted in Figure 8b. Figure 6: (**a**) Schematic diagram of experiment with the cylinder wake and (**b**) PSDs measurements by Mic 40 at \(U_{\infty}=20\) m/s and 9000 RPM with and without cylinder wake [11]. BWI noise: BWI dominates the spectra in the mid-frequencies for flight conditions. [PERSON] et al. [17] conducted a single 2-bladed propeller noise test at an edgewise inflow condition, that is, forward flight. As seen in Figure 9a-d, a strong wake structure behind the propeller was generated by the flow, which resulted in localized acceleration zones, as indicated by the increased velocity. Moreover, with the increasing tilting angle, the flow velocity increased in the wake of the blade. Meanwhile, as shown in Figure 9e, the mid-frequency noise remained comparable, which suggested BWI noise. The phenomenon was also observed in multi-rotor forward flight condition noise tests conducted by [PERSON] et al. [18], which further evidenced that noise behavior was strongly governed by rotor-wake interactions. Additionally, propellers operating in ground effect exhibit tip vortex interactions that contributed to BWI noise. This was also observed by [PERSON] et al. [19] and [PERSON] et al.