Future air traffic at low altitude will be based on novel air vehicles, such as drones and air-taxis. While drones, e.g., for delivery of goods may be processed decentralized, passenger air-taxis will be operated at vertiports close to city centers, e.g., connected to big rail hubs, close to civil airports, but also in smaller communities in the vicinity of larger cities. In any case it can be expected, this novel air transportation industry will create aircraft noise especially in those areas, where currently no significant noise from airports or en-route airliners is given, its characteristics will also be quite different from the well-known air traffic noise. The large variety of aircrafts’ technical layouts, designs and operational modes, which in some cases differ massively from current commercial aviation, require a very detailed knowledge of the noise sources and their changes of their characteristics while operating the vehicles at low altitudes. Besides still not officially applicable requirements for noise certification, the discussion about using noise metrics, which are the best to describe IAM/UAM/AAM noise, is still ongoing. But not only classical noise levels, commonly used in aviation industry, which in general can be directly derived from measurements or simulations, are being discussed more-and-more in the context of low-altitude vehicles, especially when the question of public acceptance comes into the picture. This requires the inclusion of non-acoustic parameters that make noise impact assessment an important part of future infrastructure and land-use planning together with an appropriate traffic management.

The presentation is going to address the abovementioned topics and related challenges about noise, trying as much as possible to provide a relevant summary of current questions which are in the focus of the aircraft industry, aircraft operators, and communities.

Rotor noise is a primary noise source for aircraft such as helicopters and eVTOLs, and wind tunnel testing serves as a crucial means for validating rotor noise reduction designs. Research activities on whole-path wind tunnel testing technologies for rotor aerodynamic noise based on an 8-meter-scale large aeroacoustics wind was given in this paper. From the perspective of noise source identification, a 4-meter aperture, 130-channel microphone phased array was developed, along with a rotating sound source identification algorithm based on signal reconstruction. Regarding noise propagation, a low-noise in-flow microphone support structure was designed and a 22-channel scanning U-shaped in-flow microphone array was developed. For near-field noise reception, surface-mounted microphones with embedded installation methods were adopted to achieve acoustic load measurement on aircraft surfaces. In terms of far-field noise reception, a hemispherical far-field noise characterization array was designed to enable high-resolution measurement of noise directivity. A unified data acquisition and processing platform was employed to collect, process, and correct various noise signals. The BO-105 model was utilized to complete whole-path wind tunnel testing verification covering noise source identification, in-flow acoustic propagation, and near/farfield acoustic reception of rotor noise, achieving a seven-time repeatability accuracy of 0.59dB for far-field directivity measurements.

A number of standard methods for measuring noise from aircraft recommend using ground-board mounted microphones. These comprise a microphone mounted on or in a rigid ground-board. The purpose of the ground-board is to produce a ground reflection effect which minimises the difference between acoustic measurements made over different ground surfaces. Nevertheless, ground-board mounted microphone measurements are still somewhat affected by the characteristics of the surrounding ground surface. This talk will describe recent work to investigate the physics of this effect, numerical and experimental methods for deriving/measuring corrections for adjusting measurements to a free-field equivalent level, and ground-board mounted microphone designs which minimise the dependence of measurements on the local ground surface.

In the context of the rapid expansion of Urban Air Mobility aircraft projects, a need for tools 
development has been identified, notably to control the noise generation. Effectively, this aspect
is known to be one key parameter toward public acceptance. Moreover, thanks to the use of 
individual electric motors, UAM aircraft are characterized by the possibility to consider a large 
number of configurations, making the situations even more complex.
In order to support government agencies and industries tackling this challenge, ONERA is
currently working on the following aspects of UAM noise, which will be introduced in this 
presentation.
Firstly, fast prediction tools are required to investigate numerous configurations. At ONERA, 
they are based on the lifting-line approach (coupled to either prescribed or free-wake method 
or even vortex particle method) chained to a Ffowcs-Williams and Hawkings integral.
For the validation of these fast prediction tools, and for addressing more complex cases, high-
fidelity numerical simulations are needed. They can be based on either the Navier-Stokes 
equations or the lattice Boltzmann method. During this talk, comparisons of applications of 
those methods will be illustrated.
Another way to characterize noise emission is to rely on measurements of propeller noise, for 
example in an anechoic room. Some activities related to this subject will then be presented.
Finally, the last point discussed during this talk concerns the psycho-acoustic aspect of the 
problem, especially for UAM operated in an urban environment. In this field, ONERA is not 
only working on the simulation of noise propagation in complex environments via ray tracing 
methods but also on studies/enquiries on noise annoyance and nuisance based on human tests 
in a listening room.

Focusing on helicopter aeroacoustics, this report introduces the new progress of relevant research from the aspects of aerodynamic noise prediction methods, sound generation mechanisms, propagation characteristics, and suppression technologies. It mainly includes: prediction methods for helicopter aerodynamics and noise, sound generation mechanisms and propagation characteristics of helicopter aerodynamic noise, helicopter aerodynamic noise suppression technologies, as well as the development trends and existing issues of helicopter aeroacoustics

The low-altitude economy and urban aircraft industry are growing rapidly; however, the deployment of low-altitude aircraft has led to increasing noise pollution concerns. In this presentation, low-altitude aircraft flyover noise impacts on urban areas with complex wind conditions are investigated through a comprehensive simulation framework. Effects of the atmospheric boundary layer on sound transmissions are first examined by employing an improved Gaussian beam tracing method for sound propagations in inhomogeneous flows. The results reveal that even in a steady stratified wind, the airflow refraction and ground reflection will lead to complex sound propagation features, altering the spatial distributions of sound intensity and frequency spectrum. Further, noise transmissions in a typical urban canyon scenario with complex airflows around buildings are simulated. It is observed that the aircraft noise intensity undulates violently at the upwind side of the buildings due to strong wind refractions, and there are downwind noise intensities ripples caused by airflow wakes behind buildings. These studies highlight the substantial impacts of urban airflows on noise propagations, emphasizing the necessity of integrating urban wind patterns into air vehicle noise assessment models and noise-aware flight planning strategies.

In this talk, we will showcase the aeroacoustic research achievements from our team at Peking University over the past five years. First, we will introduce our Wiener-Hopf modeling results for wave scattering, by incorporating machine learning techniques, and for sound radiation from an elliptic-shaped duct. This research lays the foundation for subsequent mode detections in measurement and health monitoring applications. Following that, we will discuss our near-field aeroacoustic source analysis methodology based on the acoustic analogy. We are also making progress in advancing fiber-optic acoustic imaging technology for drone noise sources visualization. Most recently, we have embarked on an investigation into the psychoacoustics of low-altitude aircraft, performing perceptual assessments of propeller noise under various operating conditions and developing active noise control strategies to minimize annoyance levels.

With the increasing maturity of unmanned aircraft systems (UAS) and their indispensable role in the low-altitude economy, concerns over noise emissions have become crucial among the public. This study aims to develop an efficient noise prediction model for multi-rotor UAS considering the geometrical installation effect and complex flight motions. The hybrid model combines multiple aerodynamic principles to simulate rotor tip vortex evolution and blade aerodynamic loadings. The stationary airframe is modeled using the surface singularity method to capture its influence on rotor wake development. These interactions induce unsteady flows on the rotor blades, which act as the primary loading noise sources in the acoustic analogy and are evaluated using unsteady airfoil theories. Based on this approach, the noise emissions at a specified observer location can be efficiently computed. We will first focus on the noise prediction for simplified rotor systems, including isolated and coaxial rotors in flight conditions, single rotor interacting with supporting strut, etc., and the results are compared against high-fidelity numerical simulations for validation. Then, to demonstrate the capability of the proposed framework, a full multirotor UAS will be studied. This research provides a practical and efficient tool for preliminary UAS noise assessment, with potential applications in urban air mobility and low-noise UAS design.