Noise and Vibration considerations in eVTOL aircraft
eVTOL 飞机的噪声和振动注意事项

Advanced Air Mobility (AAM) comprising unmanned aerial vehicles (UAVs) and Urban Air Mobility (UAM) with less than 100 nautical miles (nm) flying range is an emerging quick mode of transport in large cities with heavy traffic jams where the possibility of using helicopters is ruled out due to dense population and annoying noise levels of these rotorcraft. With significant advances in battery technology and electric propulsion, electric vertical takeoff and landing (eVTOL) aircraft form a critical role in UAM to offer sustainable transport of passengers and cargo loads in densely populated large cities around the world.
先进空中机动 (AAM) 包括无人机 (UAV) 和飞行范围小于 100 海里 (nm) 的城市空中机动 (UAM)，是交通拥堵严重的大城市中一种新兴的快速交通方式，可以使用由于人口密集且这些旋翼机的噪音水平令人讨厌，因此直升机被排除在外。随着电池技术和电力推进的显着进步，电动垂直起降 (eVTOL)飞机在 UAM 中发挥着关键作用，为世界各地人口稠密的大城市提供可持续的乘客和货物运输。

An increase in the production and operation of eVTOL aircraft and UAVs within populated civilian areas at relatively low altitudes creates noise annoyance issues that impact human health and well-being. Additionally, as eVTOL aircraft designs evolve, researchers are still assessing the types of noise and vibration sources and the means to control them for the comfort of the pilot and passengers on board. The Federal Aviation Administration (FAA) in the USA and European Union Aviation Safety Agency (EASA) in the EU have formed working groups on Noise Vibration Harshness (NVH) to formulate strict noise regulations on UAM aircraft and ensure compliance by different eVTOL and UAM manufacturers on acceptable community noise levels (exterior noise) and pilot and passenger comfort levels during air travel (interior noise). Different research facilities under the aegis of the National Aeronautics Space Administration (NASA) at Langley Research Centre in Hampton Virginia and Glenn Research Centre in Cleveland Ohio, have been assisting FAA in developing eVTOL aircraft concept designs, developing generic codes for predicting the performance and noise signatures of these aircraft, and analyzing and characterizing the noise generated by these vehicles. Owing to multiple sources of noise in an eVTOL aircraft generating tonal, narrowband, and broadband noises under different flight modes of operation, efforts are being made to optimize the design for noise reduction at source, improve the lightweight fuselage design for better aerodynamics, range, and NVH, and explore Active Noise and Vibration Control Technologies for improved interior noise and pilot and passenger comfort.
在海拔相对较低的人口密集的民用地区，电动垂直起降飞机和无人机的生产和运营不断增加，产生了影响人类健康和福祉的噪音问题。此外，随着电动垂直起降飞机设计的发展，研究人员仍在评估噪声和振动源的类型以及控制它们的方法，以保证机上飞行员和乘客的舒适度。美国联邦航空管理局 (FAA) 和欧盟航空安全局 (EASA) 成立了噪声振动声振粗糙度 (NVH) 工作组，制定针对 UAM 飞机的严格噪声法规，并确保不同 eVTOL 和 UAM 制造商的合规性可接受的社区噪音水平（外部噪音）以及航空旅行期间飞行员和乘客的舒适度（内部噪音）。美国国家航空航天局 (NASA) 旗下的弗吉尼亚州汉普顿兰利研究中心和俄亥俄州克利夫兰格伦研究中心的不同研究机构一直在协助 FAA 开发 eVTOL 飞机概念设计、开发用于预测性能和噪声的通用代码这些飞机的特征，并分析和表征这些车辆产生的噪音。由于电动垂直起降飞机的噪声源较多，在不同的飞行模式下会产生音调、窄带和宽带噪声，因此正在努力从源头优化降噪设计，改进机身轻量化设计，以获得更好的气动性能、航程、和 NVH，并探索主动噪声和振动控制技术，以改善内部噪声以及飞行员和乘客的舒适度。

A few concepts of urban air mobility (eVTOL aircraft) designs have been proposed by NASA (Ref [1], Ref [2]), some of which are shown in Figure 1 (Ref [1]) that comprise a quadrotor aircraft, with turboshaft and electric propulsion; side-by-side aircraft, with turboshaft and electric propulsion; lift + cruise aircraft with electric and turbo-electric propulsion; a quiet single-main rotor helicopter with turboshaft and electric propulsion; and a tilt-wing aircraft with turbo-electric propulsion.
NASA 提出了一些城市空中交通（eVTOL 飞机）设计概念（参考文献 [1]、参考文献 [2]），其中一些概念如图 1（参考文献 [1]）所示，由四旋翼飞机组成，涡轮轴发动机和电力推进；并排飞机，采用涡轮轴发动机和电力推进；采用电力和涡轮电力推进的升力+巡航飞机；带有涡轮轴和电力推进的安静单主旋翼直升机；以及带有涡轮电力推进的倾斜翼飞机。

The markets and vehicle attributes identified by the NASA Emerging Aviation Markets Tiger Team include (Ref [3])
NASA 新兴航空市场老虎团队确定的市场和车辆属性包括（参考文献 [3]）

a.) Small, unmanned aircraft systems (UAS) with characteristics of i) no people onboard, ii) low-mid altitude, iii) 0-100 kts speed, iv) 10-1000 lbs payload, v) 10-1000 nm range, vi) eVTOL/hybrid propulsion.
a.) 小型无人机系统 (UAS)，具有以下特征：i) 机上无人，ii) 中低空，iii) 0-100 节速度，iv) 10-1000 磅有效载荷，v) 10-1000 海里航程, vi) eVTOL/混合动力推进。

b.) urban air mobility (UAM) with characteristics of i) max 6 pax, ii) piloted or remotely operated, iii) up to 3000 ft altitude, iv) 0-200 kts speed, v) 800-8000 lbs payload, vi) 100 nm range, vii) eVTOL/hybrid propulsion.
b.) 城市空中机动 (UAM)，其特征为 i) 最多 6 人，ii) 驾驶或远程操作，iii) 最高 3000 英尺高度，iv) 0-200 节速度，v) 800-8000 磅有效载荷，vi ) 100 nm 范围，vii) eVTOL/混合推进。

c.) thin/short-haul aircraft with characteristics of i) 9-30 pax on board, ii) up to 2 pilots, iii) up to 12.5 kft altitude, iv) 180-300 kts speed, v) 6k-30k lbs payload, vi) 200-1k nm range, vii) eCTOL
c.) 薄型/短途飞机，具有以下特征：i) 机上载客 9-30 人，ii) 最多 2 名飞行员，iii) 高度最高 12.5 节，iv) 速度 180-300 节，v) 6k-30k 磅有效载荷，vi) 200-1k nm 范围，vii) eCTOL

d.) large, unmanned aircraft systems (UAS) and High-Altitude Long Endurance (HALE) aircraft with characteristics of i) no people onboard, ii) mid-high altitude, iii) 0-250 kts speed, iv) 100-6000 lbs payload, v) more than 3000 nm range, vi) long endurance.
d.) 大型无人机系统 (UAS) 和高空长航时 (HALE) 飞机，具有以下特点：i) 机上无人，ii) 中高空，iii) 0-250 节速度，iv) 100-6000磅有效载荷，v) 超过 3000 nm 范围，vi) 长续航时间。

The noise sources in a UAM (eVTOL) aircraft differ significantly from that of existing rotorcraft, with aerodynamically generated noise of the rotating blades being the dominant noise source compared to the motor-generated noise, which are significantly quieter than their combustion engine counterparts.
UAM (eVTOL) 飞机的噪声源与现有旋翼飞机的噪声源显着不同，与电机产生的噪声相比，旋转叶片的空气动力学产生的噪声是主要噪声源，而电机产生的噪声比内燃机同类产品安静得多。

In an eVTOL aircraft, the distributed electric propulsion (DEP), as the name indicates, comprises multiple rotors or propellers integrated across the aircraft and is enabled by the relatively low weight and high torque of the electric motors and power delivery systems. In certain eVTOL aircraft designs, the designers take advantage of the reduced weight and complexity of DEP and tile the rotors during the transition from vertical to forward flight to realize a simple mechanism for tilting the rotor nacelles or wings (Ref [3]). The eVTOL designers have been relying more on large number of smaller rotors operating at higher shaft speeds and lower torque than over fewer larger rotors operating at slower speeds because a) more torque is required to produce the same thrust at lower tip speeds, b) electric motors are perfectly capable of producing torque over a wider range of shaft speeds at high efficiencies, and c) lower blade tip speeds result in much lower aerodynamic noise – essentially taking advantage of DEP, improved motor torque-speed curve at high efficiency, and less NVH issues due to lower blade tip speeds. Few other eVTOL manufacturers have evolved a different design for DEP using high specific torque motors (Nm/kg) and larger rotors to reduce the disk loading and hence lower power requirement during hover and vertical flight take-off/landing.
在电动垂直起降飞机中，分布式电力推进 (DEP) 顾名思义，包括集成在飞机上的多个旋翼或螺旋桨，并通过相对较轻的重量和较高扭矩的电动机和动力传输系统实现。在某些电动垂直起降飞机设计中，设计人员利用 DEP 减轻的重量和复杂性，并在从垂直飞行到向前飞行的过渡期间平铺旋翼，以实现用于倾斜旋翼机舱或机翼的简单机构（参考文献 [3]）。 eVTOL 设计者更多地依赖于以较高轴速度和较低扭矩运行的大量较小转子，而不是在较低速度下运行的少量较大转子，因为 a) 在较低叶尖速度下需要更多扭矩才能产生相同的推力，b) 电动电机完全能够在更宽的轴速范围内以高效率产生扭矩，并且 c) 较低的叶尖速度可显着降低空气动力噪声 – 本质上是利用 DEP，以高效率改进电机扭矩-速度曲线，并且更少由于叶片尖端速度较低而导致 NVH 问题。很少有其他 eVTOL 制造商开发出不同的 DEP 设计，使用高比扭矩电机 (Nm/kg) 和更大的转子来减少磁盘负载，从而降低悬停和垂直飞行起飞/着陆期间的功率需求。

For vertical lift, it is anticipated that a larger number of rotors will be used to lift UAM vehicles rather than the one or two rotors used on conventional helicopters and tiltrotors. The rotors on a typical eVTOL aircraft may be operated with variable rotation speed, have lower tip Mach number to optimize performance and noise, and may use different onboard propulsion mechanisms for different flight operations, thus resulting in disparate rotor noise comprising markedly different frequency content and temporal character compared to a conventional rotorcraft.
对于垂直升力，预计将使用更多数量的旋翼来提升 UAM 飞行器，而不是传统直升机和倾转旋翼机上使用的一两个旋翼。典型的电动垂直起降飞机上的旋翼可以以可变转速运行，具有较低的叶尖马赫数以优化性能和噪声，并且可以针对不同的飞行操作使用不同的机载推进机构，从而导致不同的旋翼噪声，包括明显不同的频率内容和与传统旋翼机相比的时间特征。

Fluid-structure interaction coupling between the rotors and airframe components in an eVTOL aircraft results in multiple noise sources and signatures. As shown in Figure 2 (Ref [3]), the important noise sources include Blade-Vortex Interaction (BVI) noise, Blade-Wake Interaction (BWI) noise, Fuselage-Wake Interaction (FWI) noise, Blade-Airframe Interaction (BAI) noise, turbulence ingestion noise, and thickness, steady loading, and broadband noise. In the case of co-axial rotors spinning in opposite directions, we also have rotor-rotor interaction noise. For rotors embedded in ducts, a design could be achieved for lower NVH with additional treatment of acoustic materials on the inside walls of the stators.
eVTOL 飞机中转子和机身部件之间的流固耦合会导致多种噪声源和特征。如图2（参考文献[3]）所示，重要的噪声源包括桨叶-涡流相互作用（BVI）噪声、桨叶-尾流相互作用（BWI）噪声、机身-尾流相互作用（FWI）噪声、桨叶-机身相互作用（BAI）噪声。 ) 噪声、湍流吸入噪声以及厚度、稳定载荷和宽带噪声。在同轴转子以相反方向旋转的情况下，我们还会产生转子-转子相互作用噪声。对于嵌入管道中的转子，可以通过对定子内壁上的声学材料进行额外处理来实现更低的 NVH 设计。

The rotating blades typically used in eVTOL lift and propulsion result in three distinct noise sources with different magnitudes and frequencies depending on the flight mode of operation —thickness, loading, and high-speed-impulsive (quadrupole) noise. The speeds of rotating blades in an eVTOL aircraft are controlled to achieve a lower tip Mach number to optimize performance and lower the high-speed impulsive noise. Consequently, noise due to unsteady loading arising from the closely coupled rotors and airframe components appears to be the dominant NVH issue, as thickness noise is unlikely to be as high as loading noise sources. This is also confirmed by the classification of noise sources in a rotorcraft, as shown in Figure 3 (Ref [4]), wherein the dominant loading noise sources are marked in blue for differentiation. As eVTOL aircraft flight operations include the transition between vertical and horizontal models of flight with complex interactions between the rotors and airframe components during a wide range of operating conditions, aerodynamic loading becomes the dominant noise source (unsteady, narrowband and broadband), as shown in Figure 3 (Ref [4]).
通常用于电动垂直起降升力和推进的旋转叶片会产生三种不同的噪声源，这些噪声源具有不同的幅度和频率，具体取决于飞行操作模式：厚度、负载和高速脉冲（四极）噪声。 eVTOL 飞机中旋转叶片的速度受到控制，以实现较低的叶尖马赫数，从而优化性能并降低高速脉冲噪声。因此，紧密耦合的旋翼和机身部件产生的不稳定负载引起的噪声似乎是主要的 NVH 问题，因为厚度噪声不太可能与负载噪声源一样高。旋翼机噪声源的分类也证实了这一点，如图 3（参考文献 [4]）所示，其中主要的负载噪声源被标记为蓝色以进行区分。由于 eVTOL 飞机飞行操作包括垂直和水平飞行模型之间的转换，以及在各种操作条件下旋翼和机身部件之间复杂的相互作用，空气动力载荷成为主要噪声源（不稳定、窄带和宽带），如图所示图 3（参考文献 [4]）。

The dominant loading noise in an eVTOL aircraft can be broadly classified into deterministic and nondeterministic sources, as seen in Figure 3 (Ref [4]). The deterministic loading is further subdivided into steady and unsteady, while the nondeterministic loading sources can be divided into narrowband and broadband noise sources. The steady, deterministic loading is due to lower harmonic loading and is referred to as “loading noise”. Thickness noise and steady loading noise together are also known as “rotational noise” (as marked in Figure 3). The unsteady deterministic loading noise arises from a variety of unsteady loading sources, including nonuniform inflow, blade-vortex interaction, rotor-rotor or rotor-airframe interactions, and unsteady response to control inputs. Nondeterministic narrowband loading noise (or narrowband random noise) is caused by the turbulence ingestion of atmospheric or turbulent wake into the rotor or propeller. The rotation of a rotor blade “chops” several times through a turbulent eddy resulting in a narrowband random noise source consisting of broad spectral peaks around the blade passage frequency (BPF) and its harmonics. The nondeterministic broadband noise is caused by unsteady turbulent loading due to blade-wake interaction (BWI), turbulent boundary layer (TBL) causing blade-self noise, and turbulence ingestion with atmosphere and wakes.
eVTOL 飞机中的主要负载噪声可大致分为确定性来源和非确定性来源，如图 3 所示（参考文献 [4]）。确定性负载又可分为稳态和非稳态，非确定性负载源又可分为窄带噪声源和宽带噪声源。稳定的、确定性的负载是由于较低的谐波负载而产生的，被称为“负载噪声”。厚度噪声和稳定负载噪声一起也称为“旋转噪声”（如图 3 所示）。非稳态确定性负载噪声源自各种非稳态负载源，包括不均匀流入、叶片-涡流相互作用、旋翼-转子或旋翼-机身相互作用以及对控制输入的不稳定响应。不确定性窄带负载噪声（或窄带随机噪声）是由大气或湍流尾流进入转子或螺旋桨的湍流引起的。转子叶片的旋转多次“切碎”湍流涡流，从而产生窄带随机噪声源，该噪声源由围绕叶片通过频率 (BPF) 及其谐波的宽谱峰组成。不确定性宽带噪声是由叶片-尾流相互作用 (BWI) 引起的不稳定湍流载荷、引起叶片自身噪声的湍流边界层 (TBL) 以及大气和尾流的湍流吸收引起的。

In Figure 4, the overall sound pressure level vs blade tip Mach number is presented (Ref [3]) with the assumption that the rotor has a diameter of 3 m with a solidity of 0.2 and the noise levels predicted for a “hovering” operation condition with a fixed thrust of 2500 N. The observer is assumed to be at a distance equivalent to 10 rotor radii from the rotor hub (i.e., 15 m) in the plane of the rotor. As the tip Mach number is varied from the lower value of 0.3 (near stall) to a higher value of 0.8, the variation of the overall sound pressure levels (OASPL) and the contributions of thickness, steady loading, and broadband noise could be studied in Figure 4. The thickness noise (blue curve) rises almost linearly and is the dominant noise for tip Mach numbers > 0.6. The steady loading noise (green curve) rises moderately for tip Mach numbers > 0.5 and has a high contribution for tip Mach numbers > 0.4. The broadband noise (red curve) raises moderately for tip Mach numbers > 0.5. The trends in thickness, loading, and broadband noise indicate that the radiated broadband noise is caused primarily due to “unsteady” surface pressure fluctuations while the thickness and loading noises are the result of “steady” convective amplification. Also, the trend in broadband noise (red curve) indicates higher values for tip Mach number < 0.5 due to an increase in turbulent boundary layer (TBL) noise as the angle of attack of the blade sections increases, eventually leading to flow separation. The essential takeaway from Figure 4 is that there is an “optimal” tip speed for a given rotor geometry, below which additional tip speed reductions will be ineffective in reducing noise or will even increase it. At this “optimal” tip speed, both deterministic and nondeterministic unsteady loading noise sources will dominate. Many eVTOL rotor designs have evolved to operate near the “optimal” point, at tip Mach numbers at or below 0.5, to achieve lower noise, which is found suitable in lightly loaded condition such as the cruise mode of operation. Hence, the critical aspect of achieving low NVH during eVTOL operations is to mitigate the unsteady loading noise sources (showcased in blue in Figure 3) operating at optimal tip Mach number.
图 4 给出了总声压级与叶尖马赫数的关系（参考文献 [3]），假设转子直径为 3 m，实度为 0.2，并且预测“悬停”操作的噪声级固定推力为 2500 N 的条件。假设观察者位于转子平面内距转子轮毂 10 个转子半径的距离（即 15 m）。当尖端马赫数从较低的值 0.3（近失速）到较高的值 0.8 变化时，可以研究总体声压级 (OASPL) 的变化以及厚度、稳定载荷和宽带噪声的贡献如图 4 所示。厚度噪声（蓝色曲线）几乎呈线性上升，是叶尖马赫数 > 0.6 的主要噪声。对于叶尖马赫数 > 0.5，稳定加载噪声（绿色曲线）适度上升，并且对于叶尖马赫数 > 0.4 有较高贡献。对于尖端马赫数 > 0.5，宽带噪声（红色曲线）适度升高。厚度、负载和宽带噪声的趋势表明，辐射宽带噪声主要是由于“不稳定”表面压力波动引起的，而厚度和负载噪声是“稳定”对流放大的结果。此外，宽带噪声的趋势（红色曲线）表明尖端马赫数 < 0.5 的值较高，这是由于随着叶片部分攻角的增加，湍流边界层 (TBL) 噪声增加，最终导致流动分离。图 4 的主要结论是，对于给定的转子几何形状，存在一个“最佳”叶尖速度，低于该叶尖速度的额外降低将无法有效降低噪声，甚至会增加噪声。 在这种“最佳”叶尖速度下，确定性和非确定性不稳定负载噪声源都将占主导地位。许多 eVTOL 旋翼设计已发展到在“最佳”点附近运行，叶尖马赫数等于或低于 0.5，以实现更低的噪音，这适合轻负载条件，例如巡航运行模式。因此，在 eVTOL 操作期间实现低 NVH 的关键是减轻在最佳尖端马赫数下操作的不稳定负载噪声源（图 3 中的蓝色部分）。

In Figures 5 through 7, the noise signatures and representative pressure wave propagations are captured for a piston propeller aircraft, a helicopter, and an eVTOL aircraft, respectively (Ref [5]). In Figure 5 for piston propeller aircraft, the rotors have small diameters and fast tip speeds, leading to intense high-frequency pressure waves. The helicopter noise signature in Figure 6 is characterized by large diameter, medium tip speed, impulsive very low-frequency pressure waves, and rotor-wake interactions. The eVTOL aircraft noise signature in Figure 7 is a result of large blade diameter, slow tip speed, and low-intensity low frequency pressure waves. It is an order of magnitude lower than the noise signatures in Figures 5 and 6. The pressure waves in Figure 6 clearly indicate that Blade-Vortex Interaction (BVI) is the most significant source of aerodynamic interaction noise in a helicopter and is generated by the interaction between the rotor and its own wake. The aerodynamic noise in an eVTOL aircraft, as depicted in Figure 7, becomes more pronounced during descending and manoeuvring flight when a rapid fluctuation of aerodynamic loads is experienced from the rotor blades passing near the tip vortices formed by preceding blades, resulting in the radiation of highly impulsive noise.
在图 5 至图 7 中，分别捕获了活塞螺旋桨飞机、直升机和 eVTOL 飞机的噪声特征和代表性压力波传播（参考文献 [5]）。在图 5 中，活塞螺旋桨飞机的转子直径较小，叶尖速度较快，从而产生强烈的高频压力波。图 6 中的直升机噪声特征具有大直径、中等叶尖速度、脉冲极低频压力波以及旋翼尾流相互作用等特征。图 7 中的 eVTOL 飞机噪声特征是由大叶片直径、慢叶尖速度和低强度低频压力波造成的。它比图 5 和 6 中的噪声特征低一个数量级。图 6 中的压力波清楚地表明桨叶涡相互作用 (BVI) 是直升机中空气动力相互作用噪声的最重要来源，并且由转子与其自身尾流之间的相互作用。如图 7 所示，电动垂直起降飞机中的气动噪声在下降和机动飞行期间变得更加明显，此时转子叶片经过前叶片形成的叶尖涡流附近时会经历气动载荷的快速波动，从而导致辐射高度脉冲噪声。

With increased urbanization, residents living in large cities and towns continuously experience vehicular pollution from traffic snarls and annoying noise from vehicle driving and honking. Upon UAM becoming a reality in the near future with a large number of eVTOL aircraft plying in dense neighbourhoods, residents may be subject to additional annoyance from UAM noise pollution, necessitating a hybrid noise standard addressing both ground and upper space noises. Numerous studies carried out over the past few decades clearly confirmed that considerable noise damage is caused by traditional aircraft, supplemented by the fear and fright of residents living near airports. Regulatory authorities ought to formulate stricter noise standards for the successful deployment of UAM in urban spaces. As shown in Figure 8 (Ref [6]), UAM vehicles fly over the upper spaces of cities in the range of 300 to 600 m that can be blocked by buildings, the tallest of which is Burj Khalifa at 828 m. Hence, it is inappropriate to introduce and use aircraft noise standards tailored to open airports for UAM noise standards.
随着城市化进程的加快，居住在大城市和城镇的居民不断遭受交通拥堵以及车辆驾驶和鸣喇叭带来的恼人噪音带来的车辆污染。随着大量电动垂直起降飞机在密集社区中飞行，UAM 在不久的将来成为现实，居民可能会受到 UAM 噪声污染的额外困扰，因此需要制定混合噪声标准来解决地面和高空噪声问题。过去几十年进行的大量研究明确证实，传统飞机造成了相当大的噪音损害，再加上居住在机场附近的居民的恐惧和恐惧。监管机构应制定更严格的噪声标准，以确保城市空中交通在城市空间的成功部署。如图8（参考文献[6]）所示，UAM车辆飞越城市高层空间，范围为300至600 m，这些空间可能被建筑物阻挡，其中最高的是哈利法塔，高达828 m。因此，城市空中交通噪声标准引入和使用针对开放机场量身定制的飞机噪声标准是不合适的。

The noise level of commercial aircraft complies with the effective perceived noise in decibels (EPNdB)- 20 (Stage 2) standard established in 1960, with the aim of reducing it to the EPNdB-30 (Stage 5) level by 2030. Commercial aircraft manufacturers and airports are continuously developing noise-reduction technologies to ensure compliance with the EPNdB-30 standard. In the case of UAM, as the noise standards are still evolving due to a lack of flight data within the city, the U.S. Federal Aviation Administration (FAA) announced guidelines in 2022 on how to build a UAM vertiport, the basic infrastructure of UAM, and formed UAM Noise Working Groups to develop the noise standards. In a joint effort, FAA and NASA have been working with the eVTOL aircraft manufacturers to build concept prototypes and encourage participation in UAM to establish the necessary certification support for UAM commercialization and operation. In the EU, EASA is taking a similar approach by a) working on regulations on UAM vertiport operations and pilot licenses, b) proposing VTOL and airframe standards and preparing regulations related to pilot certification, and c) evolving plans to approve the use of UAM for small cargo transportation, such as courier services paving the way for commercializing UAM passenger transportation.
商用飞机的噪声水平符合 1960 年制定的有效感知噪声分贝 (EPNdB)-20（第 2 阶段）标准，目标是到 2030 年将其降低至 EPNdB-30（第 5 阶段）水平。 商用飞机制造商机场正在不断开发降噪技术，以确保符合 EPNdB-30 标准。就UAM而言，由于缺乏城市内的飞行数据，噪音标准仍在不断发展，美国联邦航空管理局（FAA）于2022年宣布了关于如何建设UAM垂直起落场的指南，这是UAM的基础设施，并成立了 UAM 噪声工作组来制定噪声标准。 FAA 和 NASA 一直在与 eVTOL 飞机制造商合作，构建概念原型并鼓励参与 UAM，为 UAM 商业化和运营提供必要的认证支持。在欧盟，欧洲航空安全局正在采取类似的方法，a) 制定 UAM 垂直起落机场运营和飞行员执照的法规，b) 提出 VTOL 和机身标准，并准备与飞行员认证相关的法规，以及 c) 制定批准使用 UAM 的计划适用于小型货物运输，例如快递服务，为城市空中交通客运商业化铺平道路。

UAM noise standards account for and regulate only the loudness and pitch of a sound, comprising three elements: loudness, pitch, and timbre. The current proposal on noise standards for UAM recommends the usage of the EPNdB index, which is a combination of loudness (dB) and pitch (Hz). As of date, there are no regulatory standards for sensory properties related to timbre. It is highly recommended that the noise standards for UAM be expanded to include timbre also, as shown in Figure 9 (Ref [6]). Accordingly, the requirements to expand noise standards for each UAM flight stage considering timbre are captured in Figure 10 (Ref [6]).
UAM 噪声标准仅考虑和规范声音的响度和音调，包括三个要素：响度、音调和音色。目前关于 UAM 噪声标准的提案建议使用 EPNdB 指数，该指数是响度 (dB) 和音调 (Hz) 的组合。迄今为止，还没有与音色相关的感官特性的监管标准。强烈建议将 UAM 的噪声标准扩展为包括音色，如图 9 所示（参考文献 [6]）。因此，图 10（参考文献 [6]）列出了考虑音质的每个 UAM 飞行阶段扩展噪声标准的要求。

For better NVH in UAM aircraft, multiple approaches could be taken, including a) Source noise reduction technologies, b) aircraft design, c) low-noise aircraft operations, and d) Active Control
为了提高 UAM 飞机的 NVH 效果，可以采取多种方法，包括 a) 源噪声降低技术，b) 飞机设计，c) 低噪声飞机运行，以及 d) 主动控制

The design configuration of an eVTOL aircraft decides the noise source mitigation strategies for that design. Typically, blade geometry, blade tip Mach number, number of blades per rotor, and rotor spacing define the source noise signature. While the increase of blade thickness may facilitate better propulsion, thickness noise also increases accordingly, with a 6 dB increase for doubling of thickness. The tonal and broadband components of loading noise are impacted by surface pressure (loading), the rate of change of surface pressure, and the speed (Mach number) with higher noise levels for higher mean surface pressure and a higher rate of change of surface pressure (loading).
eVTOL 飞机的设计配置决定了该设计的噪声源缓解策略。通常，叶片几何形状、叶片尖端马赫数、每个转子的叶片数量以及转子间距定义了源噪声特征。虽然叶片厚度的增加可能有利于更好的推进，但厚度噪声也相应增加，厚度增加一倍，噪声增加 6 dB。负载噪声的音调和宽带分量受到表面压力（负载）、表面压力变化率和速度（马赫数）的影响，噪声水平越高，平均表面压力和表面压力变化率就越高（加载中）。

Source noise reduction strategies for UAM include (Ref [3])
UAM 的源噪声降低策略包括（参考文献 [3]）

a.) For isolated rotors and propellers
a.) 对于隔离转子和螺旋桨

– Increasing the number of blades
– 增加叶片数量

– Optimizing the blade aerofoil shape
– 优化叶片翼型形状

– Optimizing the blade platform shape
– 优化叶片平台形状

– Reducing the speed for lower tip Mach number
– 降低速度以降低尖端马赫数

– Smoother design of leading and trailing edges to prevent wakes and turbulence
– 前缘和后缘设计更平滑，可防止尾流和湍流

b.) For rotor-airframe interactional noise effects,
b.) 对于旋翼-机身相互作用噪声效应，

– Increasing the rotor/airframe separation distances
– 增加旋翼/机身间隔距离

– Preferably placing the rotors above the airframe supports
–最好将旋翼放置在机身支架上方

– Avoiding close proximity of pusher propeller configurations to a fuselage, rotor wake, or wing wake
– 避免推进式螺旋桨配置靠近机身、旋翼尾流或机翼尾流

c.) For rotor-rotor interactional noise effects,
c.) 对于转子-转子相互作用的噪声效应，

– Adjusting the rotor blade rotational speed or phase relative to other rotors
– 调整转子叶片相对于其他转子的转速或相位

– Adjusting the relative rotation direction between rotors
– 调整转子之间的相对旋转方向

– Maintaining appropriate inter-rotor distance
– 保持适当的转子间距离

d.) Sound barrier and sound absorptive treatments with refraction, reflection, and absorption of acoustic waves
d.) 通过声波的折射、反射和吸收进行声屏障和吸声处理

IV B) Aircraft Design IV B) 飞机设计

For aircraft design that produces optimal NVH, it is important to integrate the multidisciplinary design, analysis, and optimization of aircraft with the system noise prediction tools so that correct trends due to configuration changes could be established early in design with faster turnaround time. Additionally, the design trends due to configuration changes and the influence of specific noise sources on overall NVH help understand the “sensitivity analysis” and build a decision matrix that considers different trade-offs. Design methods such as gradient-based optimization and finite difference methods help determine the sensitivities of overall noise level to different design variables. Alternatively, surrogate models that may help set the design direction based on sensitivity analysis could be used for “quick” design direction, though these surrogate models still need test data or other computational models along with the challenge of accurately modeling aeroacoustics using these surrogate models.
对于产生最佳 NVH 的飞机设计来说，将飞机的多学科设计、分析和优化与系统噪声预测工具相结合非常重要，以便可以在设计早期建立由于配置变化而产生的正确趋势，并缩短周转时间。此外，由于配置变化以及特定噪声源对整体 NVH 的影响而产生的设计趋势有助于理解“灵敏度分析”并构建考虑不同权衡的决策矩阵。基于梯度的优化和有限差分方法等设计方法有助于确定总体噪声水平对不同设计变量的敏感性。或者，可以将有助于基于灵敏度分析设定设计方向的替代模型用于“快速”设计方向，尽管这些替代模型仍然需要测试数据或其他计算模型，以及使用这些替代模型精确建模气动声学的挑战。

The low-noise aircraft operations include operational planning and trajectory optimization. For rotorcraft, it is suggested during take-off to depart at a high rate of climb and maintain a high altitude during the cruise to maximize propagation distances. During approach and landing, more than 6 dB of noise reductions are found to be achieved by suitably tailoring deceleration and flight path angles.
低噪声飞机运行包括运行规划和航迹优化。对于旋翼机，建议起飞时以高爬升率起飞，并在巡航期间保持高高度，以最大限度地延长传播距离。在进近和着陆过程中，通过适当调整减速和飞行路径角度，可以实现超过 6 分贝的噪音降低。

eVTOL aircraft noise levels have benefited from continuous enhancements in technologies enabling noise reduction at source (motors, engines, propulsors, etc.). Owing to stringent weight targets and from the perspective of safety (high crashworthiness standards) and aesthetics (stylish with excellent fit and finish), aircraft cabins and associated components use strong, lightweight, and flame-retardant composite fiber materials, resulting in unsteady rotor-airframe interaction noise (Figure 3, Ref [4]) becoming a major noise source and posing a challenge in addressing cabin interior noise levels and sound quality. A schematic of typical noise and vibration sources and their propagation paths into an aircraft cabin is shown in Figure 11 (Ref [7]). The noise sources include turbulent boundary layer (TBL) noise and propulsion-system-induced engine/motor noise on the outer surface of the cabin. The vibration transmission is through the mounting points while noise transmission paths include window transmission, structure-borne transmission, and airborne transmission. Akin to automotive interior noise control, traditional “passive” noise control measures such as the use of structural damping, insulating blankets, acoustic absorption, rubber isolation of vibrating parts, etc. would still be applicable for eVTOL interior noise control but with strict design constraints on weight and payload capacity.
eVTOL 飞机的噪声水平受益于源头（电机、发动机、推进器等）降噪技术的不断增强。由于严格的重量目标，并从安全（高耐撞标准）和美观（时尚、出色的贴合和表面处理）角度考虑，飞机机舱和相关部件采用坚固、轻质、阻燃的复合纤维材料，导致旋翼不稳定。机身相互作用噪声（图 3，参考文献 [4]）成为主要噪声源，并对解决机舱内部噪声水平和声音质量提出了挑战。图 11 显示了典型噪声和振动源及其进入飞机机舱的传播路径的示意图（参考文献 [7]）。噪声源包括客舱外表面的湍流边界层 (TBL) 噪声和推进系统引起的发动机/电机噪声。振动通过安装点传播，噪声传播路径包括窗户传播、结构传播和空气传播。与汽车内部噪声控制类似，传统的“被动”噪声控制措施，如使用结构阻尼、隔热毯、吸声、振动部件橡胶隔离等，仍然适用于电动垂直起降飞机内部噪声控制，但有严格的设计限制关于重量和有效负载能力。

Active control such as Active Noise Control (ANC) or Active Structural Acoustic Control (ASAC) will be more useful in eVTOL aircraft as the traditional passive control approaches using sound insulators and absorbers may not be effective against low-frequency noise and also add significant weight to the aircraft. 
主动噪声控制 (ANC) 或主动结构声学控制 (ASAC) 等主动控制将在电动垂直起降飞机中更加有用，因为使用隔音器和吸音器的传统被动控制方法可能无法有效对抗低频噪声，并且还会显着增加重量到飞机上。

The schematic of a potential setup to achieve ANC in an aircraft is shown in Figure 12 (Ref [8]) wherein each of the secondary loudspeakers generates anti-noise signals to create a “zone of quiet” with more than 10 dB of noise reduction in each ear with the size of the zone being about 1/10^th of the wavelength () of the sound in a diffuse sound field. The required error signal is determined from a Laser Doppler Vibrometer (LDV) measurement of the vibration of a small membrane pick-up located close to the ear canal. The user movement is tracked by a camera that actively controls the galvanometer-driven mirrors to steer the laser beam and maintain its position on the membrane.
图 12（参考文献 [8]）显示了在飞机上实现 ANC 的潜在设置示意图，其中每个辅助扬声器都会生成抗噪声信号，以创建一个“安静区域” ，降噪效果超过 10 dB在每只耳朵中，该区域的大小约为扩散声场中声音波长 () 的^1/10 。所需的误差信号是通过激光多普勒振动计 (LDV) 对靠近耳道的小膜拾音器的振动进行测量来确定的。用户的运动由摄像头跟踪，摄像头主动控制检流计驱动的镜子来控制激光束并保持其在薄膜上的位置。

An experimental setup of ANC in an aircraft using feedforward control based on a filtered-x least mean square (LMS) algorithm is shown in Figure 13 (Ref [8]). Numerous studies are carried out in Ref [8] with different positions of the head, changes in LDV signal, different locations of the secondary speakers, etc. to ensure the robustness of the ANC performance. A representative sound pressure level (dB) difference with ANC off and on with the reflective membrane placed near the ear canal is shown in Figure 13 with significant noise reduction across the frequency band.
图 13（参考文献 [8]）显示了飞机中使用基于滤波 x 最小均方 (LMS) 算法的前馈控制的 ANC 实验装置。参考文献[8]针对头部的不同位置、LDV信号的变化、副扬声器的不同位置等进行了大量研究，以确保ANC性能的鲁棒性。图 13 显示了在耳道附近放置反射膜时 ANC 关闭和打开时的代表性声压级 (dB) 差异，整个频段的噪音显着降低。

An application of ASAC using piezoelectric actuators and sensors is shown in Figure 14 (Ref[9,10]) wherein a total of 199 piezoelectric elements (1” by 1” by ¼”) were bonded onto the fuselage frame caps with a distance of 1/8” between neighboring piezoelectric elements formed into three large groups of actuators. The disturbance signal was generated using speaker-ring systems shown in Figure 14 a) to simulate the propeller noise operating at 910 rpm generating a Blade Passage Frequency (BPF) of 61 Hz. A feedforward control using a filtered-x LMS algorithm was applied using accelerometers as error sensors as shown in Figure 14 b).
使用压电执行器和传感器的 ASAC 应用如图 14（参考文献[9,10]）所示，其中总共 199 个压电元件（1” x 1” x 1/4”）粘合到机身框架盖上，距离为相邻压电元件之间的1/8”形成三大组致动器。使用图 14a) 中所示的扬声器环系统生成扰动信号，以模拟以 910 rpm 运行的螺旋桨噪声，生成 61 Hz 的叶片通过频率 (BPF)。使用 Filtered-x LMS 算法进行前馈控制，并使用加速度计作为误差传感器，如图 14 b) 所示。

The acceleration spectra measured at accelerometer 3 and noise spectra measured at row 1 aisle seat are shown in Figure 15 a) and 15 b), respectively with > 20 dB reduction achieved in both cases for 900 rpm, 61 HZ BPF, showing the effectiveness of ASAC for interior noise control.
在加速度计 3 处测量的加速度谱和在第 1 排过道座位处测量的噪声谱分别如图 15a) 和 15b) 所示，在 900 rpm、61 HZ BPF 的情况下，两种情况下均实现了 > 20 dB 降低，显示了有效性ASAC 用于室内噪声控制。

Another application of ASAC involves using smart foam actuators for aircraft interior noise control that offers the benefits of both passive and active (aka hybrid) control (Ref [11]). Smart foam actuators comprising a sound-absorbing foam with an embedded distributed piezoelectric polyvinylidene difluoride (PVDF) layer designed to operate over a broad range of frequencies are affixed to the surface of the aircraft.  While the acoustic foam acts as a passive absorber and targets high-frequency noise sources, the PVDF serves as the active component and is activated to provide secondary noise signals at low frequencies to cancel the primary noise sources. An active noise control (ANC) methodology based on adaptive feedforward filtered-x Least-Mean-Squared (LMS) control algorithm is used to drive the smart foam actuators affixed at different locations on the skin of the aircraft to reduce the sound pressure levels at an array of microphones using accelerometers on the panel as the reference signal for the feedforward algorithm. The smart foam details and mounting on a structure are shown in Figure 16 while a schematic of smart foam treatment in the aircraft and evaluation in a test facility is shown in Figure 17 (Ref [11]).
ASAC 的另一个应用涉及使用智能泡沫执行器进行飞机内部噪声控制，它具有被动和主动（又称混合）控制的优点（参考文献 [11]）。智能泡沫执行器由吸音泡沫和嵌入的分布式压电聚偏二氟乙烯 (PVDF) 层组成，设计用于在广泛的频率范围内工作，固定在飞机表面。 声学泡沫充当被动吸收器并针对高频噪声源，而 PVDF 充当主动组件并被激活以提供低频二次噪声信号以消除主要噪声源。基于自适应前馈滤波 x 最小均方 (LMS) 控制算法的主动噪声控制 (ANC) 方法用于驱动固定在飞机蒙皮不同位置的智能泡沫执行器，以降低飞机蒙皮处的声压级。使用面板上的加速度计作为前馈算法的参考信号的麦克风阵列。智能泡沫细节和在结构上的安装如图 16 所示，而飞机中智能泡沫处理和测试设施评估的示意图如图 17 所示（参考文献 [11]）。

As shown in Figure 16, the acoustic foam is shaped to hold the PVDF in the form of a half-cylinder and each side of the PVDF is affixed to the foam using a light application of spray glue. To improve the radiation efficiency of the smart foam element and thus increased volume velocity and radiated sound pressure level, each end of the PVDF film is glued to a balsa anchor that ran the full width of the smart foam element thus creating fixed boundary conditions (Figure 16, Ref [11]).
如图 16 所示，声学泡沫的形状可将 PVDF 固定为半圆柱体，并且使用少量喷胶将 PVDF 的每一侧固定到泡沫上。为了提高智能泡沫元件的辐射效率，从而提高体积速度和辐射声压水平，PVDF 薄膜的每一端都粘在轻木锚上，该锚遍布智能泡沫元件的整个宽度，从而创建固定的边界条件（图16，参考文献 [11]）。

A schematic of smart foam treatment in an aircraft and evaluation in a test facility is shown in Figure 17 (Ref [11]) which depicts the excitation of exterior fuselage panels by a) aerodynamic and turbulent boundary flow, b) engine/propulsion, and c) propulsor/wake interaction, resulting in noise radiation into the aircraft cabin. An array of smart foam actuators is affixed between the fuselage skin and interior trim panel of the aircraft which can be activated and controlled to act as secondary noise sources to cancel the primary noise radiation into the aircraft cabin. Similar to the schematic shown in Figure 12 (Ref [8]), the schematic in Figure 17 (Ref [11]) also uses a multi-channel filtered-x LMS feedforward control algorithm and produces a “zone of quiet” with more than 10 dB noise reduction inside the aircraft cabin. In the Mach 0.2 NASA Langley Research Center (LaRC) wind tunnel test facility shown in Figure 17, the aircraft interior is modeled using an anechoic chamber comprising a 5X3 array of microphones (with 4 error microphones) enabled by four control channels consisting of six smart foam actuators mounted to a single section of the six-bay panel and excited by the wind tunnel flow.
图 17（参考文献 [11]）显示了飞机中智能泡沫处理和测试设施评估的示意图，其中描述了 a) 空气动力学和湍流边界流、b) 发动机/推进系统和c) 推进器/尾流相互作用，导致噪声辐射到飞机机舱内。一系列智能泡沫执行器固定在飞机的机身蒙皮和内饰面板之间，可以激活和控制这些执行器作为辅助噪声源，以消除进入飞机机舱的主要噪声辐射。与图 12（参考文献 [8]）中所示的原理图类似，图 17（参考文献 [11]）中的原理图也使用多通道过滤 x LMS 前馈控制算法，并产生一个“安静区域”，其中超过机舱内噪音降低 10 分贝。在图 17 所示的 0.2 马赫 NASA 兰利研究中心 (LaRC) 风洞测试设施中，飞机内部使用消声室进行建模，该消声室包括 5X3 麦克风阵列（带有 4 个误差麦克风），并由四个控制通道（包括 6 个智能麦克风）启用。泡沫执行器安装在六隔间面板的单个部分上，并由风洞流激励。

In Figure 18 (Ref [11]), sound pressure levels are shown in the as-is situation, with passive control (effect of only foam), and with hybrid control (active + passive) using smart foam actuators. As can be seen from the figure, passive control (green curve) is effective only in the mid to high-frequency range (> 500 Hz) while the hybrid control (blue curve) is effective across the entire frequency range with significant noise reduction of peaks around 185 Hz and 640 Hz, resulting in about 6 dBA attenuation in 400-1000 Hz frequency range, as shown in the 1/3^rd octave band results.
在图 18（参考文献 [11]）中，显示了原样情况下的声压级，采用被动控制（仅泡沫的效果），以​​及使用智能泡沫执行器的混合控制（主动 + 被动）。从图中可以看出，被动控制（绿色曲线）仅在中高频范围（> 500 Hz）有效，而混合控制（蓝色曲线）在整个频率范围内有效，降噪效果显着185 Hz 和 640 Hz 附近的峰值，导致 400-1000 Hz 频率范围内约 6 dBA 衰减，如^1/3倍频程结果所示。

The noise reduction trends in Figure 18 confirm that smart foam actuators utilize passive material characteristics of the foam to realize a) noise attenuation through energy dissipation of higher frequencies in the sound absorber and b) vibration attenuation through energy dissipation at lower frequencies in the form of damping of panels on which the foam elements are affixed. This is clearly observed by comparing the blue and green curves a) at the 186 Hz peak, where energy dissipation is due to panel damping and b) at 640 Hz, where energy dissipation is due to sound absorption.
图 18 中的降噪趋势证实，智能泡沫执行器利用泡沫的被动材料特性来实现 a) 通过吸声器中较高频率的能量耗散来实现噪声衰减，b) 通过以较低频率的能量耗散来实现振动衰减固定泡沫元件的面板的阻尼。通过比较蓝色和绿色曲线，可以清楚地观察到这一点：a）在 186 Hz 峰值处，其中能量耗散是由于面板阻尼；b）在 640 Hz 处，其中能量耗散是由于声音吸收。

A few concept vehicles for UAM have been presented along with potential markets and vehicle attributes by the NASA Emerging Aviation Markets Tiger Team. Different kinds of noise sources in a typical eVTOL aircraft are presented including tonal, narrowband, and broadband. Classification of noise sources in rotorcraft is presented in terms of thickness, loading, and high-speed impulsive noise sources with unsteady loading dominating the noise of these aircraft. The variation of overall sound pressure level (OASPL) vs Tip Mach Number is presented along with the details of thickness, loading, and broadband noise variations and the need for an optimal tip Mach number to balance performance and NVH. A comparison of noise spectrums and Pressure Wave Propagation is done for a piston propeller aircraft, a helicopter, and eVTOL aircraft and shows how Blade-Vortex Interaction (BVI) is the most significant source of aerodynamic interaction noise, especially for propeller aircraft and helicopters. Noise standards for UAM are presented briefly with suggestions on the expansion of UAM noise standards considering timbre as an important element of sound along with the requirements to expand noise standards for each UAM flight stage. For improved NVH in eVTOL aircraft, design directions on noise source reduction technologies, better aircraft design, and low-noise aircraft operations are presented followed by recent developments in active control technologies of sound and vibration in terms of Active Noise Control (ANC) and Active Structural Acoustic Control (ASAC) using piezoelectric actuators and sensors and smart foam elements using accelerometers and microphones as error sensors. In both ANC and ASAC, a zone of quiet with more than 10 dB noise reduction could be achieved for improved NVH in the aircraft cabin. In the case of smart foam actuators, energy dissipation is observed through damping in the lower frequencies and through sound absorption at higher frequencies.
NASA 新兴航空市场 Tiger 团队展示了一些 UAM 概念车辆以及潜在市场和车辆属性。介绍了典型电动垂直起降飞机中的不同类型的噪声源，包括音调、窄带和宽带。旋翼飞机噪声源的分类是根据厚度、载荷和高速脉冲噪声源进行的，其中不稳定载荷在这些飞机的噪声中占主导地位。总体声压级 (OASPL) 与尖端马赫数的变化以及厚度、负载和宽带噪声变化的详细信息以及平衡性能和 NVH 所需的最佳尖端马赫数一起呈现。对活塞螺旋桨飞机、直升机和电动垂直起降飞机的噪声频谱和压力波传播进行了比较，显示叶片涡相互作用 (BVI) 如何成为空气动力学相互作用噪声的最重要来源，特别是对于螺旋桨飞机和直升机。简要介绍了 UAM 噪声标准，并考虑到音色作为声音的重要元素，提出了扩展 UAM 噪声标准的建议，以及扩展 UAM 各飞行阶段噪声标准的要求。为了改善电动垂直起降飞机的 NVH，提出了噪声源降低技术、更好的飞机设计和低噪声飞机运行的设计方向，以及主动噪声控制 (ANC) 和主动噪声控制方面的声音和振动主动控制技术的最新发展。使用压电执行器和传感器的结构声学控制 (ASAC) 以及使用加速度计和麦克风作为误差传感器的智能泡沫元件。 在 ANC 和 ASAC 中，可以实现降噪超过 10 dB 的安静区域，从而改善飞机机舱内的 NVH。对于智能泡沫执行器，通过较低频率的阻尼和较高频率的吸声来观察能量耗散。

For UAM to be successful in densely populated urban areas, ultra-quiet aircraft operations are a must that could be achieved using a strong multidisciplinary research effort combining research in aerodynamics, acoustics, numerical methods, active controls, advanced materials, low-noise flight operations, and autonomous flying capabilities, to name a few. It is observed that unsteady loading will dominate the noise of these aircraft. Depending on the design configuration of an eVTOL aircraft (rotors, fixed vs tilt wing, etc.), strong aerodynamic interactions between the lifting and propulsion components and the airframe occur, resulting in annoying noise signatures. Only recently, aero-acousticians have started understanding the complex interactions between rotors or propellers and turbulent wakes generated by upstream aerodynamic components in eVTOL aircraft, while enough research has been carried out over many decades on blade-vortex interaction noise, blade-wake interaction noise, and rotor-airframe interaction noise for propeller aircraft and rotorcraft such as helicopters. A multidisciplinary approach is required to accurately build UAM noise source models using computational aeroacoustics by building on the prior research regarding helicopter BWI and propeller turbulence ingestion noise. A hybrid noise standard for UAM that includes the timber element of sound and that incorporates the noise standards from the automotive industry may be required to address the noises experienced by urban residents from both on-road vehicles and low-flying eVTOL aircraft. Further research needs to be carried out on Active Control technologies for cost-effective and scalable solutions to address NVH issues in eVTOL aircraft.  
城市空中交通要想在人口稠密的城市地区取得成功，飞机的超静音运行是必须的，可以通过结合空气动力学、声学、数值方法、主动控制、先进材料、低噪声飞行运行等方面的研究，通过强大的多学科研究工作来实现，以及自主飞行能力等等。据观察，不稳定载荷将主导这些飞机的噪声。根据电动垂直起降飞机的设计配置（旋翼、固定翼与倾斜翼等），起重和推进部件与机身之间会发生强烈的空气动力学相互作用，从而产生恼人的噪音特征。直到最近，航空声学学家才开始了解电动垂直起降飞机中转子或螺旋桨与上游空气动力部件产生的湍流尾流之间的复杂相互作用，而几十年来，人们对叶片涡流相互作用噪声、叶片尾流相互作用噪声进行了足够的研究，以及螺旋桨飞机和旋翼飞机（例如直升机）的旋翼-机身相互作用噪声。需要采用多学科方法，在有关直升机 BWI 和螺旋桨湍流吸入噪声的先前研究的基础上，使用计算气动声学准确构建 UAM 噪声源模型。可能需要制定城市空中交通混合噪声标准，其中包括声音的木材元素和汽车行业的噪声标准，以解决城市居民从道路车辆和低空飞行的电动垂直起降飞机所感受到的噪声。需要对主动控制技术进行进一步研究，以获得具有成本效益和可扩展的解决方案，以解决 eVTOL 飞机的 NVH 问题。

1. Johnson, W., and Silva, C. “NASA concept vehicles and the engineering of advanced air mobility aircraft,” The Aeronautical Journal (2022), 126, pp. 59–91 doi:10.1017/aer.2021.92
1. Johnson, W. 和 Silva, C.“NASA 概念飞行器和先进空中机动飞机的工程”，《航空杂志》(2022 年)，126，第 59–91 页 doi:10.1017/aer.2021.92

2. Silva, C.  et. al, “VTOL Urban Air Mobility Concept Vehicles for technology development,” AIAA Aviation Forum, 2018 Aviation Technology Integration, and Operations Conference, AIAA 2018-3847, June 25-29, 2018, Atlanta, Georgia, https://doi.org/10.2514/6.2018-3847
2.席尔瓦，C.等。 al，“用于技术开发的垂直起降城市空中交通概念车辆”，AIAA 航空论坛，2018 年航空技术集成和运营会议，AIAA 2018-3847，2018 年 6 月 25-29 日，佐治亚州亚特兰大，https://doi.org /10.2514/6.2018-3847

3. Rizzi, S.A. et. al, “Urban Air Mobility Noise: Current Practice, Gaps, and Recommendations,” October 2020, NASA/TP–2020-5007433, https://ntrs.nasa.gov/api/citations/20205007433/downloads/NASA-TP-2020-5007433.pdf
3. Rizzi，SA 等。 al，“城市空中交通噪声：当前实践、差距和建议”，2020 年 10 月，NASA/TP–2020-5007433， https: //ntrs.nasa.gov/api/itations/20205007433/downloads/NASA-TP- 2020-5007433.pdf

4. Greenwood, E. et. al, “Challenges and opportunities for low noise electric aircraft,” International Journal of Aeroacoustics 2022, Vol. 21(5-7) 315–381, https://doi:10.1177/1475472X221107377
4. Greenwood，E.等人。 al，“低噪声电动飞机的挑战和机遇”，《国际航空声学杂志》2022 年，第 1 卷。 21(5-7) 315–381， https://doi:10.1177/1475472X221107377

5. Joby Aviation, “How quiet is the Joby aircraft during flyover?” https://www.youtube.com/watch?v=itP8-3j2UZI
5. Joby Aviation，“Joby 飞机在飞越期间有多安静？” https://www.youtube.com/watch?v=itP8-3j2UZI

6. Kim, J.H. “Urban Air Mobility Noise: Further Considerations on Indoor Space,” International Journal of Environmental Research and Public Health, 2022, 19, 11298, https://doi.org/10.3390/ ijerph191811298
6. Kim, JH“城市空气流动噪声：对室内空间的进一步考虑”，国际环境研究与公共卫生杂志，2022, 19, 11298，https://doi.org/10.3390/ijerph191811298

7. Dimino, I. and Aliabadi, F., ”Active Control of Aircraft Cabin Noise,” September 2015, https://doi.org/10.1142/p996

8. Xiao, T. et. al, “Ultra-broadband local active noise control with remote acoustic sensing,” Nature Scientific Reports, (2020) 10:20784, https://doi.org/10.1038/s41598-020-77614-w

9. Zimcik, D. G. “Active control of aircraft interior noise,” RTO AVT Symposium on Habitability of Combat and Transport Vehicles: Noise, Vibration and Motion, Prague, Czech Republic, 4-7 Oct. 2004

10. Grewal, A. et. al, “Active cabin noise and vibration control for turboprop aircraft using multiple piezoelectric actuators,” Journal of Intelligent Material Systems and Structures, vol. 11, June 2000, pp. 438-447, doi:10.1177/104538900772664602

11. Griffin, J.R., “The control of interior cabin noise due to a turbulent boundary layer noise excitation using smart foam elements,” M.S. Thesis, Virginia Tech (VPI&SU), October 2006, https://vtechworks.lib.vt.edu/bitstream/handle/10919/32804/SmartFoamETD.pdf?sequence=1

Cover image source: AutoFlight

About the author

Dr Arunkumar M. Sampath works as a Principal Consultant in Tata Consultancy Services (TCS) in Chennai. His interests include Hybrid and Electric Vehicles, Connected and Autonomous Vehicles, 5G/6G, Cybersecurity, Functional Safety, Advanced Air Mobility (AAM), AI, ML, Data Analytics, and Data Monetization Strategies.

Also Read: Seamless edge computing in connected vehicles by Dr Arunkumar M Sampath