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Quantifying modulation in the acoustic field of a small-scale rotor using bispectral analysis
Abstract
This paper describes a methodology to quantify inter-frequency modulation in the acoustic field of a small-scale rotor. How the blade passing frequency modulates the intensity of the higher-frequency (broadband) noise content is of specific interest, as this modulation is a major factor in the human perception of rotor noise from advanced air mobility vehicles and drones. A proposed modulation-parameter is based on post-processing steps that are applicable to a single acoustic time series. First, an auto-bispectral analysis assesses the dominant nonlinear, quadratic inter-frequency coupling between the blade passing frequency and the higher-frequency noise content. Secondly, the degree of modulation is determined using a robust parameter: a correlation parameter between the (low-frequency) modulating BPF signal and an envelope of the (higher-frequency) carrier signal. Provided that a single parameter is obtained for a given acoustic time series, the directivity pattern of the modulation strength can be inferred from data available from standard acoustic measurement campaigns. For illustration, an 11 inch diameter single-rotor in hover is considered, with acoustic data taken at 420 microphone positions within a plane perpendicular to the rotor disk. It is revealed that modulation is confined to a sector , where is the rotor plane and negative angles are in the direction of the rotor-induced flow. The strongest modulation appears around . This work aids in quantifying the phenomenological description of modulation, namely that it results from the periodic advance and retreat of certain rotor blade's noise sources, relative to a stationary observer.
... In this study, the time-and/or ensemble-averaged characterization of rotor noise is augmented by utilizing metrics that preserve the temporal variation in the intensity of high-frequency noise [38]. This intensity variation is a nonlinear frequency interaction and refers, in the context of our current work, to the phase consistencies between the low-frequency BPF and higher-frequency noise. ...
... Baars et al. [38] described methods for quantifying BPFM. The various scalar metrics involved in this process are briefly summarized below, and it must be emphasized that these are computed from a single acoustic pressure time series. ...
... Finally, two modulation metrics are defined: the correlation strength, ρ a max ρτ c , and the phase ϕ a τ cm f b 2π, where τ cm is the temporal shift for which the maximum correlation value occurs. Since the correlation metric involves the BPF-harmonic, correlation maxima occur periodically (see, for instance, Fig. 11 in [38]). This implies that the temporal shift cannot be determined uniquely, but up to an integer multiple of the bladepassage period, so ϕ a ∕2π 1. ...
Acoustic spectra of rotor noise yield frequency distributions of energy within pressure time series. However, they are unable to reveal phase relations between different frequency components while these play a role in the fundamental understanding of low-frequency intensity modulation of higher-frequency rotor noise. A methodology to quantify interfrequency modulation is applied to a comprehensive acoustic dataset of a fixed-pitch benchmark rotor, operating at a low Reynolds number and at advance ratios ranging from [Formula: see text] to 0.61. Our findings strengthen earlier observations in case of a hovering rotor, in which the modulation of the high-frequency noise is strongest around an elevation angle of [Formula: see text] (below the rotor plane). For the nonzero advance ratios, modulation becomes dominant in the sector [Formula: see text] and is most pronounced at the highest advance ratio tested ([Formula: see text]). Intensity modulation of high-frequency noise is primarily the consequence of a far-field observer experiencing a cyclic sweep through the noise directivity pattern of relatively directive trailing-edge noise. This noise component becomes more intense with increasing [Formula: see text] and is associated with broadband features of the partially separated flow over the rotor blades.
... In a recent paper by Baars et al. [15], a methodology was presented for quantifying the degree of BPF modulation in the acoustic field generated by a small-scale rotor. Specifically, the post-processing technique utilized the quadratic inter-frequency coupling (phase consistency between frequencycomponents) and also a correlation parameter between the low-frequency BPF and an envelope of a threshold-based higher-frequency (carrier) signal. ...
... Specifically, the post-processing technique utilized the quadratic inter-frequency coupling (phase consistency between frequencycomponents) and also a correlation parameter between the low-frequency BPF and an envelope of a threshold-based higher-frequency (carrier) signal. The main finding of [15] was that for a rotor in hover, the BPF modulation is strongest at angles of θ ≈ −20 • (below the rotor plane). The focus of our current work is to explain the modulations effects in a dataset of a laboratory-scale rotor at advance ratios ranging from J = 0 to 0.6. ...
... kHz for a duration of T = 40 seconds (10 480 blade passages). All further details of the sensor-acquisition and spectral analysis are similar to the ones described by Baars et al. [15]. A single rotor noise spectrum is illustrated in Figure 1a and corresponds to microphone position B (and J = 0). ...
Rotor noise comprises harmonic features, related to the blade passing frequency, as well as broadband noise. Even though an acoustic spectrum yields a frequency-distribution of energy within an acoustic pressure signal, it does not reveal phase-relations between different frequency components. The latter are of critical importance for the development of accurate prediction- and auralization-algorithms, because they may result in low-frequency modulation: temporal variations in the intensity of higher-frequency rotor noise. Baars et al. recently outlined a methodology to quantify inter-frequency modulation. The current work applies this methodology to a new and comprehensive dataset of a laboratory-scale rotor at advance ratios ranging from J = 0 to 0.6. PIV measurements of the blade-induced flow disturbances complement the acoustic data to elucidate how the vortical flow structures of one blade impact the inflow of the consecutive blade. Findings of the directivity patterns of modulation are described and interpreted using existing knowledge of the specific noise sources of the low Reynolds number (small-scale) rotor.
... al [108] found broadband noise modulations of processed wind tunnel experimental data to be perceptually-important to the noise character of a small quadcopter UAS, especially for higher human-audible frequencies. Baars et al. measured the time-varying noise of a small UAS propeller to develop a noise metric that quantifies broadband noise modulation with the blade passage frequency [109], as recommended by Rizzi et al. [36]. However, Baars et al. noted that further work is required to relate their developed modulation parameter to human perception [109]. ...
... Baars et al. measured the time-varying noise of a small UAS propeller to develop a noise metric that quantifies broadband noise modulation with the blade passage frequency [109], as recommended by Rizzi et al. [36]. However, Baars et al. noted that further work is required to relate their developed modulation parameter to human perception [109]. ...
... However, the precise modulation range was found to be sensitive to the chosen directivity function model. Baars et al. also found that the greatest modulation occurred near the rotor plane for a small UAS propeller [109]. ...
https://etda.libraries.psu.edu/catalog/19898zug117
This thesis makes contributions to two important aspects of computing the time-varying rotor noise of electrical vertical take-off and landing (eVTOL) aircraft for urban air mobility (UAM) applications: the time variation of rotor broadband noise, and noise due continuously-varying rotor angular speed. The noise prediction software PSU-WOPWOP was updated to compute the broadband noise spectrum as a function of observer time within a rotor period. Time-varying broadband noise spectra and directivity computations for helicopter main rotors in forward flight show significant modulation with the blade passage frequency, due to convective amplification and retarded-time effects. Qualitative agreement between noise predictions and flight test data was obtained. The noise prediction software PSU-WOPWOP was also updated to compute the discrete frequency noise of rotors with continuously-varying rotational speed, which may require accepting input kinematics and aerodynamic loading data at unevenly-spaced source time intervals. Order of magnitude analysis and numerical simulations demonstrated that thickness noise modulations caused by variable rotor speed are primarily due to the time variation of instantaneous rotor speed, rather than angular acceleration, for typical rotor speed variations expected for manned eVTOL aircraft.
... These unsteady blade motions with variations in their angular velocity result in tonal noise with a higher blade passing frequency (BPF) harmonics and differences in the directivity pattern compared to the tonal noise of steady rotations [8]. In RPM-controlled rotors, the broadband noise is modulated by the interaction of the acoustic and the BPF harmonics [9]. Typically, DEP systems-based eVTOL exhibits a lower rotor tip speed than conventional helicopters, indicating the importance of broadband noise [5]. ...
... An APC 11 × 4.7 SF rotor was used to validate the ability of the model to predict noise directivity. The noise directivity patterns of the broadband noise (1)(2)(3)(4)(5)(6)(7)(8)(9)(10)(11)(12)(13)(14)(15)(16) in hovering flight at 2000, 4000, and 6000 RPM were obtained [32]. Microphone arrays were placed at a distance of 1.62 m from the rotor center every 10 • from -60 • to 60 • (0 • is rotor disk plane). ...
This study performed a real-time prediction of RPM-controlled multirotor noise using a comprehensive multirotor noise assessment (CONA) framework. The key objectives of this study include the synthesis of frequency-modulated multirotor noise and assessment of flyover and takeoff noise under wind conditions. The CONA framework predicted the rotor tonal and broadband noise for a planned mission profile. Gusty wind conditions were efficiently simulated using a Dryden wind turbulence model. When tracking the mission profile under wind conditions, the multirotor noise was characterized by a non-stationary signal with frequency and amplitude modulations. These characteristics induced variations in the noise impact, such as the psychoacoustic metrics. To clarify the effects of wind conditions, prediction-based psychoacoustic analysis were performed for cross- and plus-type quadrotor configurations. The results revealed that the RPM-controlled mechanisms of each configuration were different, indicating that gusty wind conditions exhibited complicated effects on noise impact owing to its mean flow velocity and directions. Moreover, the high-resolution time-frequency analysis illustrated the modulation characteristics of the non-stationary noise signal. The prediction-based psychoacoustic analysis of multirotor noise revealed that the CONA framework facilitated the perception-based evaluation of multirotor configurations in aerial vehicle designs and operations.
... As such the waveform is separated into its low and high frequency harmonics using an arbitrarily chosen cutoff frequency of f c 10ω 5 . This adhoc separation follows the same two-source approach introduced by Baars et al. [34,35] and Baars and Ragni [36] which separates the rotor noise signal (Figs. 5b and 5c top) into one comprising a modulating component (middle) and a carrier component (bottom). ...
Several higher-order statistical metrics are used to evaluate the significance of waveform nonlinearities in the sound field of a laboratory-scale coaxial, corotating rotor in hover. These comprise the magnitude-squared coherence, skewness, and kurtosis of the pressure waveform and its time derivative, number of zero crossings per rotation, a wave steepening factor, and the quadrature spectral density and its integral. A unique feature of this rotor setup is the constructive and destructive interference of sound waves produced by neighboring blades, which are incubators for signal distortion effects. Waveform distortions are evaluated for changes to rotor index angle, the separation distance between the upper and lower rotors, as well as changes to rotor speed for different observer positions. Significant sensitivities in the kurtosis of the pressure waveform and its time derivative, the number of zero crossings, and the integral of the quadrature spectral density are shown for changes in rotor index angle, observer position, and rotor speed; stacking distance appears less important at affecting changes to these metrics. The trade space between these metrics and rotor figure of merit demonstrates how changes to the rotor index angle can invoke relatively small changes in rotor performance while generating large changes in acoustic waveform nonlinearities.
... A similar approach has previously been applied with promising results for sUAS sound synthesis during hovering operation in an anechoic environment [4]. Amplitude modulation of the synthesised broadband components was included to reproduce the phase-coupling effect between the BPF tones and the higher frequency broadband component ( > 1000Hz) [13]. ...
... 8 Furthermore, inter-frequency modulation at BPF harmonics can cause modulation in the broadband noise of a rotor. 9 Due to the complicated wake interactions and changes in tonal noise characteristics induced by multiple smaller rotors, the contribution of broadband noise to total noise footprint can be relatively significant. 6 The variations in rotational speed of each rotor, influenced by flight conditions and atmospheric environments, can accentuate frequency modulation (FM) and amplitude modulation (AM) in the noise generated by RPM-controlled multirotor configurations. ...
This study presents a comprehensive numerical framework for auralizing multirotor noise during complex maneuvers, incorporating rotor tonal and broadband noise prediction, synthesis, and psychoacoustic analysis. Validation studies for various flight scenarios, including single rotors and quadrotors, confirm the capability of this framework in the prediction-based psychoacoustic analysis of multirotor configurations. The primary focus of this research is to assess the flyover and takeoff noise of multirotors under diverse operating conditions. When tracking the mission profile under gusty wind conditions, the multirotor noise is characterized by frequency and amplitude modulations induced by rotational speed control for each rotor. A high-resolution time-frequency analysis is conducted for the tonal noise to highlight these modulation characteristics. Additionally, the prediction-based psycho-acoustic analysis is performed for cross-and plus-type quadrotor configurations to clarify the effects of gusty wind profiles and flight control. The results demonstrate that variations in the rotational speed of each rotor, influenced by mission profiles and gusty wind conditions, contribute to the distinct acoustic characteristics of multirotor configurations in both physical and psychological aspects. The synthesized noise time signal and modulation characteristics provide valuable insights into the impact of operational environments on psychoacoustic metrics and annoyance.
... For instance, the sUAS Yn has significantly higher values of Fluctuation Strength at microphones M3 and M7. These correspond to angles ±30 • , and are consistent with the angles of maximum emission of amplitude modulated sound in small-scale rotor noise found by Baars et al. [83]. The sUAS M3 presents the lowest values of Fluctuation Strength. ...
... It is important to note that the motor-only noise (dashed green line) depicted a significant energy content at around 10.3 kHz, with a magnitude larger than 50 dB/Hz. This high-frequency noise component appeared in the vicinity of approximately f = 18 × Ω, which may have been related to the 18 electromagnetic poles of the motor used in the experiment, similarly reported by Baars et al. [32]. This additional energy content from the motor may generate an additional noise source of a similar frequency range for laminar and grid turbulent cases, as seen in the spectrum. ...
This paper presents an experimental investigation into the effects of turbulence ingestion on the aerodynamic noise characteristics of rotor blades in edgewise flight. A small-scaled, two-bladed rotor was used in the study. The test utilised two turbulence-generating grids, to generate turbulence inflows with different characteristics, and to compare them to the baseline configuration of the laminar inflow. The experiments were set at forwarding edgewise flight configuration, with freestream inflow velocity ranging from 10 m/s to 22 m/s. Simultaneous measurements of far-field acoustic pressure and load were conducted, along with a separate flow measurement using particle image velocimetry. The acoustic spectra demonstrated a larger contribution to the tonal noise radiation at blade passing frequency, and to the broadband noise radiation at the mid-frequency domain, due to turbulence ingestion. However, the broadband responses in the high-frequency domain were comparable between the tested laminar and turbulence inflow cases, with similar broadband humps featuring in the acoustic spectra. The directivity patterns of the overall sound pressure level showed that the noise radiation was lowest near the plane of rotation, and highest downstream. Turbulence ingestion effects could also be seen in the elevated noise levels throughout the observation positions for the grid inflow cases, particularly at larger advance ratios.
... The psychoacoustic metrics, such as loudness, sharpness, and tonality (ECMA-418-2, 2020;Zwicker and Fastl, 2013), aim at quantifying annoyance relative to the listener's position. Psychoacoustic metrics have been used to evaluate annoyance of UAS (Christian and Cabell, 2017;White et al., 2017), to optimize blade spacing design (Torija et al., 2021;Torija et al., 2022), and to quantify the BPF modulation (Baars et al., 2021). ...
This paper proposes an experimental setup for measuring the sound radiation of a quadrotor drone using a hemispherical microphone array. The measured sound field is decomposed into spherical harmonics, which enables the evaluation of the radiation pattern to non-probed positions. Additionally, the measurement setup allows the assessment of noise emission and psychoacoustic metrics at a wide range of angles. The obtained directivity patterns using a third-order spherical harmonic decomposition (SHD) are shown to exhibit low distortion with respect to the original measurements, therefore, validating the SHD as an adequate representation strategy. Furthermore, the noise emissions are evaluated, and the highest noise emission is observed in the [Formula: see text] azimuth direction. The exterior spherical acoustic holography description is employed to evaluate psychoacoustic metrics at arbitrary far-field positions and validated on a reference microphone. The estimated psychoacoustic metrics are closely related to the target metrics, which allows for sound quality analysis at any point external to the drone.
... Frontiers in Aerospace Engineering frontiersin.org 07 noise depending on the noise measurement location (Baars et al., 2021). In addition, flow recirculation can affect the noise measurements in indoor experiments (Whelchel et al., 2020). ...
Drones, which have become increasingly popular in recent years, produce a lot of noise due to the movement of their propellers. When flying near humans, especially as in urban situations, noise suppression is critical. It has been demonstrated that noise can be minimized by increasing propeller lift per unit rotation speed and decreasing propeller rotation speed by expanding propeller area or designing the airfoil shape. This study developed a new structure, serrated Gurney flap, by merging the Gurney flap, which is the trailing-edge structure of an airfoil, and the serration, which is the low-noise structure found in an owl feather, and studied its performance through experiments and numerical simulations. The results indicated that the structure can boost the propeller’s lift coefficient while reducing the vortex separation induced by the Gurney flap and suppress propeller noise by slowing the propeller. Further modification of its structure may result in improved efficiency as well as decreased noise level.
... 197 To date, most rotor broadband noise analyses-whether predicted or measured-have focused on time-averaged broadband noise spectra, meaning that the broadband noise sound pressure level (SPL) spectrum is integrated over a time segment on the order of a rotor period. However, in the past 3 years, the time variation of rotor broadband noise has recently increased attention [202][203][204][205][206][207] due to its apparent importance to human perception of helicopter main rotor noise, as identified by Christian, Caston, and Greenwood. 106 As reviewed by Gan (Ref. ...
A new class of electric aircraft is being developed to transport people and goods as a part of the urban and regional transportation infrastructure. To gain public acceptance of these operations, these aircraft need to be much quieter than conventional airplanes and helicopters. This article seeks to review and summarize the aeroacoustic research relevant to this new category of aircraft. First, a brief review of the history of electric aircraft is provided, with an emphasis on how these aircraft differ from conventional aircraft. Next, the physics of rotor noise generation are reviewed, and the noise sources most likely to be of concern for electric aircraft are highlighted. These are divided into deterministic and nondeterministic sources of noise. Deterministic noise is expected to be dominated by the unsteady loading noise caused by the aerodynamic interactions between components. Nondeterministic noise will be generated by the interaction of the rotor or propeller blades with turbulence from ingested wakes, the atmosphere, and self-generated in the boundary layer. The literature for these noise sources is reviewed with a focus on applicability to electric aircraft. Challenges faced by the aeroacoustician in understanding the noise generation of electric aircraft are then identified, as well as the new opportunities for the prediction and reduction of electric aircraft noise that may be enabled by advances in computational aeroacoustics, flight simulation, and autonomy.
... 13,14 The tonal noise transforms to frequency-modulated noise due to RPM variations, and complex frequency modulation (FM) and amplitude modulation (AM) characteristics are significant factors in the DEP system. Baars et al. 15 quantitatively examined the acoustic field modulation of a single rotor by performing a bispectral analysis. Jeong et al. 16 observed the FM characteristics and phase effects by the RPM fluctuation during multirotor hovering, which is a dominant factor in noise directivity pattern. ...
A comprehensive multirotor noise assessment framework is developed to predict the noise of rotational-speed-controlled rotor configurations in real-time. The key objectives are to synthesize the frequency-modulated multirotor noise and analyze the frequency modulation (FM) characteristics. The framework includes modules associated with the flight control, aerodynamics, time reconstruction, noise prediction, and time-frequency analysis (TFA). In addition to the hybrid blade element momentum model, the aerodynamics module contains a linear inflow model, a Beddoes wake model, and an unsteady aerodynamic correction model. The convective form and source-time dominant algorithms are used in the acoustic analogy for tonal noise prediction. The FM characteristics are identified using the synchrosqueezing-based high-resolution TFA for strongly non-stationary signals. The framework is verified through validation and verification studies for diverse rotor configurations and flight conditions. During the cruise flight of the multirotor, the tonal noise exhibits simultaneous frequency and amplitude modulations. In wind gust conditions, these modulations result from rotational speed variations, acoustic wave interference, and Doppler shifting. By clarifying the non-stationary noise signal in diverse flight environments, the proposed framework can facilitate noise assessment in the perception-influenced design stage of multirotor configurations.
... The relationship between SPL and distance, the altitude, and the orientation of the UA are studied with respect to the receptor [42]. Correlation-based techniques have also been applied to quantify modulation in the acoustic field of a small-scale rotor [43]. Bi-spectral analysis has been used to reveal the correlation between the modulation strength parameters and the degree of time variation of the amplitude, with the Blade Passing Frequency (BPF) being the modulating signal and the higher-frequency noise being the carrier signal. ...
This paper addresses one of the recognized barriers to the unrestricted adoption of Unmanned Aircraft (UA) in mainstream urban use—noise—and reviews existing approaches for estimating and mitigating this problem. The aircraft noise problem is discussed upfront in general terms by introducing the sound emission, propagation, and psychoacoustic effects. The propagation of sound in the atmosphere, which is the focus of this paper, is then analysed in detail to isolate the environmental and operational factors that predominantly influence the perceived noise on the ground, especially looking at large-scale low-altitude UA operations, such as in the envisioned Urban Air Mobility (UAM) concepts. The physics of sound propagation are presented, considering all attenuation effects and the anomalies due to Doppler and atmospheric effects, such as wind, thermal inversion, and turbulence. The analysis allows to highlight the limitations of current mainstream aircraft noise modelling and certification approaches and, in particular, their inadequacy in addressing the noise of UA and, more generally, UAM vehicles. This finding is important considering that, although reducing noise at the source has remained a priority for manufacturers to enable the scaling up of UAM and drone delivery operations in the near future, the impact of poorly considered propagation and psychoacoustic effects on the actual perceived noise on the ground is equally important for the same objective. For instance, optimizing the flight paths as a function of local weather conditions can significantly contribute to minimizing the impact of noise on communities, thus paving the way for the introduction of full-scale UAM operations. A more reliable and accurate modelling of noise ground signatures for both manned and unmanned low-flying aircraft will aid in identifying the real-time data stream requirements from distributed sensors on the ground. New developments in surrogate sound propagation models, more pervasive real-time sensor data, and suitable computing resources are expected to both yield more reliable and effective estimates of noise reaching the ground listeners and support a dynamic planning of flight paths.
In the last decade, the drone market has grown rapidly for both civil and military purposes. Due to their versatility, the demand for drones is constantly increasing, with several industrial players joining the venture to transfer urban mobility to the air. This has exacerbated the problem of noise pollution, mainly due to the relatively lower altitude of these vehicles and the proximity of their routes to extremely densely populated areas. In particular, both the aerodynamic and aeroacoustic optimization of the propulsive system and of its interaction with the airframe are key aspects of unmanned aerial vehicle design that can signify the success or the failure of their mission. The industrial challenge involves finding the best performance in terms of loading, efficiency and weight, and, at the same time, the most silent configuration. For these reasons, research has focused on an initial localization of the noise sources and, on further analysis, of the noise generation mechanism, focusing particularly on directivity and scattering. The aim of the present study is to review the noise source mechanisms and the state-of-the-art control strategies, available in the literature, for its suppression, focusing especially on the fluid-dynamic aspects of low Reynolds numbers of the propulsive system and on the interaction of the propulsive system flow with the airframe.
In 2018, NASA formed an Urban Air Mobility Noise Working Group (UNWG) to assemble noise experts from industry, universities and government agencies to identify, discuss, and address urban air mobility (UAM) noise issues. This paper presents a set of high-level goals intended to address barriers associated with UAM noise that may hamper their entry into service. It summarizes the current practice, identifies gaps in the current practice, and makes recommendations to address the gaps to achieve the high-level goals in four areas of interest: Tools and Technologies, Ground and Flight Testing, Human Response and Metrics, and Regulation and Policy.
The present work deals with the modeling of the aerodynamic sound generated by the propellers of small-size drones, taking into account the effects of horizontal forward flight with negative pitch and of installation on supporting struts. Analytical aeroacoustic formulations are used, dedicated to the loading noise. The fluctuating lift forces on the blades are expanded as circular distributions of acoustic dipoles, the radiated field of which is calculated by using the free-space Green's function. This provides descriptions of the sound field, valid in the entire space. The stationary mean-flow distortions responsible for the lift fluctuations and at the origin of the sound are estimated from existing numerical flow simulations and from ad hoc models. Installation and forward-flight effects are found to generate much more sound than the steady loading on the blades associated with thrust. Therefore, the models are believed reliable fast-running tools that could be used for preliminary low-noise design through repeated parametric calculations, or for noise-impact estimates corresponding to prescribed urban traffic.
The forward flight stage of multicopters accounts for a relatively large amount of flight time in comparison with hovering, ascending, and descending conditions; therefore, understanding its aerodynamic and aeroacoustic performance is significant for preliminary design, aerodynamic optimization, and control methodologies. By setting different rotation speeds, shaft angles, and freestream velocities according to realistic operation conditions, an investigation of a full-scale isolated rotor in a low-speed anechoic wind tunnel is conducted. When the freestream velocity increases and the shaft angle decreases, the thrust, power, moments, and sound pressure level (SPL) of the rotor increase by maintaining the same rotation speed. Through appropriate normalization methods, the loading coefficients and SPL collapse to one surface with respect to the advance ratio and shaft angle, and third-order polynomials are applied to fit nondimensional data. Using the fitted polynomials, power consumptions and noise emissions for generating the required lift during the level flight are calculated from different parameter combinations, and optimal solutions are suggested.
The demand in micro-air vehicles is increasing as well as their potential missions. Either for discretion in military operations or noise pollution in civilian use, noise reduction of micro-air vehicles is a goal to achieve. Aeroacoustic research has long been focusing on full scale rotor-crafts. At micro-air vehicle scales however, the hierarchization of the numerous sources of noise is not straightforward, as a consequence of the relatively low Reynolds number that ranges typically from 5000 to 100,000 and low Mach number of approximately 0.1. This knowledge, however, is crucial for aeroacoustic optimization and blade noise reduction in drones. This contribution briefly describes a low-cost, numerical methodology to achieve noise reduction by optimization of micro-air vehicle rotor blade geometry. Acoustic power measurements show a reduction of 8 dB(A). The innovative rotor blade geometry allowing this noise reduction is then analysed in detail, both experimentally and numerically with large eddy simulation using lattice Boltzmann method. Turbulence interaction noise is shown to be a major source of noise in this configuration of low Reynolds number rotor in hover, as a result of small scale turbulence and high frequency unsteady aeroadynamics impinging the blades at the leading edge.
Unmanned aerial systems (UASs) are an increasingly popular technology with a rapidly expanding range of uses. Contra-rotating propellers are used as propulsors on some multi-rotor unmanned aerial vehicles (UAV). Contra-rotating propellers have the benefit of reducing the UAV's plan size and adding redundancy in case of component failure. However, contra-rotating propellers are notoriously noisy in other applications, but the noise from contra-rotating multi-rotor UAVs has not been studied. This paper presents results from an experimental investigation of a single static contra-rotating propeller mounted in an anechoic chamber. The effect of propeller diameter, propeller spacing, and blade number were investigated. The rotational speed of the upstream and downstream propellers was varied over a large range to generate a data set containing noise and efficiency measurements at many different operating points.
This paper that attempts to characterize the high-frequency components of in-flight, near-field noise measurements of a Bell 206B helicopter. This noise is shown to be largely broadband, and displays a pattern of modulation that appears to be an audible and salient feature of the noise. The development of a frequency domain technique is given that uses very short (in time) discrete Fourier transforms with windowing and overlap in order to produce a representation of how the 1/3 rd octave band spectrum of the noise varies over time scales shorter than a blade passage. The output of this method can be used to resynthesize the source noise. This resynthesis evidences natural variations that are perceptually similar to those seen in the original recording, and which would not be predicted by contemporary rotorcraft noise analysis techniques. These concepts are then applied in an attempt to synthesize helicopter source noise from a physics-based prediction that includes such modulating high frequency components. The elevated significance of broadband noise on future prediction efforts is discussed.
The current paper describes a computational workflow for the prediction of the acoustic impact of vertical takeoff and landing vehicles hovering over an crowded, urban area. The aeroacoustic sources are predicted by using the high-fidelity CFD solver SIMULIA PowerFLOW 6-2019 based on a Lattice-Boltzmann/Very-Large-Eddy-Simulation method. The acoustic propagation in quiescent air is extracted directly from the CFD simulation performed in a domain encompassing both the vehicle and the nearby urban city block. The effects of sound scattering due to the presence of several buildings is analyzed by comparing the noise levels on the surface of the buildings with levels extracted on the same surface from another solution performed without buildings. The potentiality of this technology in the optimization of a flight corridor is highlighted.
https://ntrs.nasa.gov/citations/20200002436
As interest grows in rotor- and propeller-driven electric vertical takeoff and landing (eVTOL) aircraft for the Urban Air Mobility market, there is a potential for previously studied concepts to reemerge due to the opportunities afforded by novel technologies and operating modes. One such concept is the “stacked” rotor, which consists of multiple co-rotating rotors positioned co-axially with a small axial offset. The goal of the work presented in this paper is to determine whether stacked rotors offer a compelling advantage for eVTOL aircraft in terms of both performance and acoustic characteristics. Results are presented for new experimental tests and computational modeling of multiple stacked rotor configurations, and comparisons are made with conventional rotor configurations. Testing of thirteen separate configurations— each using the same blade shape—revealed a configuration that resulted in an increase in the rotor power loading efficiency by more than 7% and reduced noise by more than 3 dBA when compared with a conventional rotor with all blades located in the same rotational plane.
This paper describes computational workflows for the evaluation of the acoustic environmental impact of vertical takeoff and landing vehicles. An operational scenario of a vehicle undertaking an approach maneuver over a flat terrain is considered. The aeroacoustic sources are predicted by using the high-fidelity CFD solver SIMULIA PowerFLOW based on the Lattice-Boltzmann/very-large-eddy-simulation method. An integral approach based on the Ffowcs-Williams and Hawkings analogy is used to compute the noise on a hemisphere centered around the vehicle. Finally, the on-ground noise footprint is computed by using the hemisphere noise extrapolation approach. In view of future trajectory/maneuver optimization studies, different trimmed trajectories are considered in order to show the effect of both the aero-mechanical setting of the vehicle and its instantaneous position on the perceived noise levels along a flyover event. Trajectories are described by a time sequence of waypoints, vehicle pitch angle, tilt angles of the propulsion units and rotational speed of the rotors. A high number of transient flow solutions covering a broad flight envelope is used to compute the noise hemisphere database. Interpolation of the instantaneous noise hemisphere along an arbitrary trimmed trajectory is carried out prior the noise extrapolation from the hemisphere to the ground. The influence of the trajectory on noise is finally discussed in connection with the flow physics of some specific conditions occurring during the maneuvers, revealing the importance of multiple vortex-body interactions in a complex multi-rotor system.
The spatial evolution of acoustic waveforms produced by a laboratory-scale Mach 3 jet are investigated using both 1∕4 and 1∕8 in. pressure field microphones located along rays emanating from the postpotential core where the peak sound emission is found to occur. The measurements are acquired in a fully anechoic chamber, where ground or other large surface reflections are minimal. Various statistical metrics are examined along the peak emission path, where they are shown to undergo rapid changes within 2 m from the source region. An experimentally validated wave-packet model is then used to confirm the location where the pressure amplitude along the peak emission path transitions from cylindrical to spherical decay. Various source amplitudes, provided by the wave-packet model, are then used to estimate shock formation distance and Gol'dberg numbers for diverging waves. The findings suggest that cumulative nonlinear distortion is likely to occur at laboratory scale near the jet flow, where the waveform amplitude decays cylindrically, but less likely to occur farther from the jet flow, where the waveform amplitude decays spherically. Direct inspection of the raw time series reveals how steepened waveforms are generated by roguelike waves that form from the constructive interference of waves from neighboring sources as opposed to classical cumulative nonlinear distortion.
Tonal noise from fans mainly comes from the periodic unsteady blade forces and/or vane forces due to the interaction between the rotor and its environment. To distinguish the different interactions leading to tonal noise in the acoustic spectrum, this paper presents an original method consisting of rotating a quasistatic part, e.g. a stator or an obstruction in the environment of the rotor. An analytical model has been developed to take into account relative rotation motions between a number of rotors. This model shows that the acoustic radiation due to the interaction between a rotating stator and the rotor shifts in the spectrum at frequencies ω = mB (k) Ω (k) −nΩ (l) , where m is the harmonic order, B (k) is the number of rotor blades, Ω (k) is the angular velocity of the rotor, Ω (l) is the angular velocity of the moving quasistatic part and n is the order of the unsteady forces decomposed in circumferential Fourier series. Using this modulation effect, every circumferential mode n radiates at different frequency in the acoustic spectrum. The radiation due to the interaction between the rotor and all the other static components remains at frequencies mB (k) Ω (k) .
A model is proposed for predicting the presence of cumulative nonlinear distortions in the acoustic waveforms produced by high-speed jet flows. The model relies on the conventional definition of the acoustic shock formation distance and employs an effective Gol'dberg number A for diverging acoustic waves. The latter properly accounts for spherical spreading, whereas the classical Gol'dberg number F is restricted to plane wave applications. Scaling laws are then derived to account for the effects imposed by jet exit conditions of practical interest and includes Mach number, temperature ratio, Strouhal number and an absolute observer distance relative to a broadband Gaussian source. Surveys of the acoustic pressure produced by a laboratory-scale, shock-free and unheated Mach 3 jet are used to support findings of the model. Acoustic waveforms are acquired on a two-dimensional grid extending out to 145 nozzle diameters from the jet exit plane. Various statistical metrics are employed to examine the degree of local and cumulative nonlinearity in the measured waveforms and their temporal derivatives. This includes a wave steepening factor (WSF), skewness, kurtosis and the normalized quadrature spectral density. The analysed data are shown to collapse reasonably well along rays emanating from the post-potential-core region of the jet. An application of the generalized Burgers equation is used to demonstrate the effect of cumulative nonlinear distortion on an arbitrary acoustic waveform produced by a high-convective-Mach-number supersonic jet. It is advocated that cumulative nonlinear distortion effects during far-field sound propagation are too subtle in this range-restricted environment and over the region covered, which may be true for other laboratory-scale jet noise facilities.
A unique routine, capable of identifying both linear and higher-order coherence in multiple-input/output systems, is presented. The technique combines two well-established methods: Proper Orthogonal Decomposition (POD) and Higher-Order Spectra Analysis. The latter of these is based on known methods for characterizing nonlinear systems by way of Volterra series. In that, both linear and higher-order kernels are formed to quantify the spectral (nonlinear) transfer of energy between the system's input and output. This reduces essentially to spectral Linear Stochastic Estimation when only first-order terms are considered, and is therefore presented in the context of stochastic estimation as spectral Higher-Order Stochastic Estimation (HOSE). The trade-off to seeking higher-order transfer kernels is that the increased complexity restricts the analysis to single-input/output systems. Low-dimensional (POD-based) analysis techniques are inserted to alleviate this void as POD coefficients represent the dynamics of the spatial structures (modes) of a multi-degree-of-freedom system. The mathematical framework behind this POD-based HOSE method is first described. The method is then tested in the context of jet aeroacoustics by modeling acoustically efficient large-scale instabilities as combinations of wave packets. The growth, saturation, and decay of these spatially convecting wave packets are shown to couple both linearly and nonlinearly in the near-field to produce waveforms that propagate acoustically to the far-field for different frequency combinations.
The growing use of small UAVs and MAVs has increased the interest in determining the performance of small-scale propellers that power them. These small-scale propellers operate at much lower Reynolds numbers than propellers for larger aircraft; typical Reynolds numbers for small-scale propellers are less than 100,000 based on the velocity and chord at the 75% span station. The low Reynolds numbers of the propellers make predicting the performance of these propellers difficult as the performance of the propellers generally change with Reynolds numbers. In order to understand the Reynolds number effects on small-scale propellers, the performance data of twenty-seven off-the-shelf propellers and four 3 D-printed propellers were measured in both static and advancing-flow conditions. The diameters of these propellers ranged from 2.25 in to 9 in. From these tests, the performance of the propellers was shown to improve as the Reynolds number increased. This improvement was seen as either an increase in the thrust coefficient, a decrease in the power coefficient, or both. For propellers in an advancing flow, performance improvement was also seen as an increase in the propeller efficiency. Results from some of the propellers tested showed an efficiency improvement of around 10%. Nevertheless, the maximum efficiency achieved by the small-scale propellers is still low when compared to typical values of larger propellers.
While much research has been carried out on propellers for full-scale aircraft, not much data exists on propellers applicable to the ever growing number of UAVs. Many of these UAVs use propellers that must operate in the low Reynolds number range of 50,000 to 100,000 based on the propeller chord at the 75% propeller-blade station. Tests were performed at the University of Illinois at Urbana-Champaign (UIUC) to quantify the propeller efficiency at these conditions. In total, 79 pro-pellers were tested and the majority fit in the 9-to 11-in diameter range. During the tests, the propeller speed (RPM) was fixed while changing the wind-tunnel speed to sweep over a range of advance ratios until reaching the windmill state (zero thrust). To examine Reynolds number effects, typically four RPM's were tested in the range 1,500 to 7,500 RPM depending on the propeller diameter. Propeller efficiencies varied greatly from a peak near 0.65 (for an efficient propeller) to near 0.28 (for an exceptionally poor propeller). Thus, these results indicate that proper propeller selection for UAVs can have a dramatic effect on aircraft performance.
Ecological awareness increases energy production by wind turbines, but the considerable associated annoyance risk requires carefully designed noise control strategies when they are to be installed in densely populated regions. Well-chosen operational restrictions may reduce noise annoyance while preserving cost-effectiveness. This re-search project investigates the relationship between the inhabitants' wind turbine noise annoyance, exposure indi-cators, operational characteristics and environmental variables for a specific industrial site near a residential area. In contrast to most other research on wind turbine noise annoyance, in this study a six–month field experiment is conducted including regular on-line annoyance reports, continuous 1/3-octave band noise level registrations, periodic sound recordings, electricity production per minute and meteorological observations. Logistic regression reveals that the risk of high annoyance not only depends on the rotational speed, but also on the wind turbines' nacelle position relative to the location of the dwellings, i.e. the wind direction. This directivity effect can be captured when noise parameters such as the background noise level caused by other sources and a so-called fluctuation-indicator are introduced. Background noise measurements incorporate wind induced vegetation and road traffic noise; their LA95-level is inversely related to the risk of high annoyance. The fluctuation-indicator is calculated from the 1/3–octave band spectra to capture the periodic part of wind turbine noise; here the risk of high annoyance increases with increasing fluctuation strength. These detailed findings can be used for designing operational restrictions that limit noise annoyance while keeping production as high as possible.
In acoustic imaging practise, where two-dimensional area measurements are performed, spatial sampling is a little addressed subject. Especially aliasing due to high spatial sampling at short sensor distance with respect to the source of interest, is poorly discussed in open literature. This paper will discuss the theory behind optimal spatial sampling for Fourier acoustics and offers insights into aliasing in acoustic images. The discussion on spatial sampling theory will include the Nyquist sampling theorem and the application of given theory for acoustic imaging. This will include a transformation from time-frequency considerations in one dimension to position- wavenumber relations in two dimensions. Also, the function of Fourier Transforms in Planar Near-field Acoustic Holography (PNAH) is discussed, and a formula for acoustic image resolution is given. Finally, two-dimensional aliasing is clearly illustrated by means of an analytical example. Using sampling theory, aliasing theory and basic knowledge of PNAH data processing, an inequality is derived that couples physical resolution limits with spatial sampling limitations.
In this study, hover performance and acoustic measurements are taken on two different isolated rotors representative of small-scale, rotary-wing unmanned aircraft systems (UAS) for a range of rotation rates. Each rotor system consists of two fixed-pitch blades powered by a brushless motor. For nearly the same thrust condition, significant differences in overall sound pressure level (OASPL), up to 8 dB, and directivity were observed between the two rotor systems. Differences are shown to be in part attributed to different rotor tip speeds, along with increased broadband and motor noise levels. In addition to acoustic measurements, aeroacoustic predictions were implemented in order to better understand the noise content of the rotor systems. Numerical aerodynamic predictions were computed using the unsteady Reynolds-averaged Navier Stokes code OVERFLOW2 on one of the isolated rotors, while analytical predictions were computed using the Propeller Analysis System of the Aircraft NOise Prediction Program (ANOPP-PAS) on the two rotor configurations. Preliminary semi-empirical frequency domain broadband noise predictions were also carried out based on airfoil self-noise theory in a rotational reference frame. The prediction techniques further supported trends identified in the experimental data analysis. The brushless motors were observed to be important noise contributors and warrant further investigation. It is believed that UAS acoustic prediction capabilities must consider both rotor and motor components as part of a combined noise-generating system.
This paper discusses an effort to determine useful scaling laws for predicting the sound pressure levels at various observer positions for small scale rotors in hover. Surveys of rotor thrust, rotor torque and near-field acoustics are acquired using a series of identically shaped two-bladed rotors ranging between 8 and 18 inches in diameter. Rotor speeds were varied between 50 and 90 revolutions per second which equate to blade tip Mach numbers as high as 0.56.Common sound metrics like overall sound pressure level, sound pressure level and blade-pass frequency harmonic noise were correlated with both dimensional variables (rotor thrust, torque, power and blade tip speed) and non dimensional variables (thrust and power coefficients, Reynolds number, blade tip Mach number and figure of merit)pertaining to rotor performance. The findings revealed a new correlation parameter for predicting rotor noise based on the product of figure of merit with blade tip Mach number. It is postulated that an increase in rotor collective translates to an increase in loading noise. So long as the flow remains attached, the increased loading noise is expressed by an increase in figure of merit. This new correlation parameter is shown to provide a reasonable collapse of all rotor noise data. In order to account for the directivity of the sound field, a second order response surface model of this new correlation parameter was developed and validated.
Small scale unmanned aerial vehicles using multirotor propulsion systems have received considerable attention for a wide range of military and commercial applications in recent years. In the multirotor configuration, the rotor interaction phenomenon occurs severely because the rotors are located in close proximity to each other. Therefore, the separation distance between the adjacent rotor tips has a strong effect on the wake structures and flow fields, which consequently play an important role in determining the aerodynamic performance and noise level of the multirotor vehicle. In the present study, numerical simulations of a quadcopter under hover flight conditions are conducted to investigate the mutual rotor-to-rotor interactional effects on the aerodynamic performance, wake structures, and sound pressure level using the nonlinear vortex lattice method with the vortex particle method and acoustic analogy based on Farassat’s formulation 1A. Calculations for the multirotor configurations with different separation distances show that the average thrust force decreases significantly and force fluctuation is found to increase dramatically as the rotor spacing gets smaller. In addition, the wake geometry and induced flow structure behind the rotor tend to be radially dragged down toward the center of the vehicle due to the existence of the adjoining rotor, which consequently results in strong wake-to-wake interaction and the formation of asymmetric wake structures although the multirotor operates under the hovering condition. It is also observed that unsteady loading introduced by rotor interaction leads to a considerable increase in the sound pressure level, particularly the normal direction of the rotor plane.
The multi-rotor type unmanned aerial vehicle (UAV) controls the movement through the rotational speed of each rotor. Since the phase of each rotor is different and minute variations of the rotational speed are always accompanied during flight, frequency modulation of the drone noise occurs. In this study, a new approach, called the rotation per minute (RPM) fluctuation correction method, was developed to consider the frequency modulation effects through correcting the signals predicted by the classical rotorcraft noise prediction methods. For verification of the suggested method, two experiments were performed for a single isolated rotor and a full vehicle. The measured spectra of the isolated rotor were used for verifying the spectrum before the correction. Also, the spectra corrected by the RPM fluctuation correction method were compared and verified with the noise of the full vehicle, which has frequency modulation effects.
With the increased prominence of multicopter micro aerial vehicles, more importance has been placed on the aerodynamic and acoustic performance of these systems, as their small-scale and lower Reynolds number regime provide results that are different from full-scale rotors. A computational methodology was employed in order to study the aerodynamic and aeroacoustic performance from different small-scale rotors used in a multicopter configuration. Three rotor design variables (twist, taper, and pitch) were investigated in order to understand their influence on aerodynamic and acoustic performance of a hovering rotor. Variables such as rotor rotation rate and rotor radius were kept constant. Common aerodynamic performance metrics such as the ratio of coefficient of thrust to coefficient of power and figure of merit were used to assess aerodynamic hover performance of the designed rotors. Acoustic performance was assessed by recording acoustic pressure in the far-field at 2 separate receivers. Acoustic results are presented in the frequency domain as one-third octave band data and as in overall sound pressure level. Flow fields and pressure contours were calculated and displayed in order to help explain aerodynamic and acoustic results. From the results, insights are provided for rotor designs that are more aerodynamically and acoustically efficient in hover. Specifically, rotors that provided lower values of disc loading and higher values of power loading were typically more acoustically efficient. Using greater rotor twist and taper increased both aerodynamic and acoustic performance.
The findings from a test campaign aimed at understanding the thrust and acoustic performance of a small-scale, coaxial, corotating rotor in hover are discussed. Measurements of axial thrust are acquired alongside surveys of the near-field acoustics at eight observer positions above and below the tip path plane of the upper rotor. The variables of interest are the index angle and stacking distance between the upper and lower rotors, as well as rotor speed. Standard metrics like thrust coefficient, sound pressure spectrum level, and the sound pressure level of the various blade harmonics and subharmonics are studied. Human ear effects are also considered using the A-weighting standard, which is shown to have a profound influence on the perceived noise levels. The analysis demonstrates how rotor thrust and sound pressure levels are more dependent on rotor index angle than stacking distance. However, the index angle where the peak thrust coefficient resides is more sensitive to stacking distance at low rotation speeds than it is at high rotation speeds. Various rotor speed and index angle combinations of the stacked rotor are also shown to produce the same rotor thrust, all the while resulting in uniquely different sound levels of the various rotor harmonics.
In this study, acoustic measurements of a hover condition are taken on isolated rotor–airframe configurations representative of small-scale, rotary-wing unmanned aircraft systems (UAS). Each rotor–airframe configuration consists of two fixed-pitch blades powered by a brushless motor, with a simplified airframe geometry intended to represent a generic multicopter arm. In addition to acoustic measurements, computational fluid dynamics–based aeroacoustic predictions are implemented on a subset of the experimentally tested rotor–airframe configurations in an effort to better understand the noise content of the rotor–airframe systems. Favorable agreements are obtained between acoustic measurements and predictions, based on both time- and frequency-domain postprocessing techniques. Results indicate that close proximity of airframe surfaces results in the generation of considerable tonal acoustic content in the form of harmonics of the rotor blade passage frequency (BPF). Analysis of the acoustic prediction data shows that the presence of the airframe surfaces can generate noise levels either comparable to or greater than the rotor blade surfaces under certain rotor tip clearance conditions. Analysis of the on-surface Ffowcs Williams and Hawkings source terms provides insight as to the predicted physical noise-generating mechanisms on the rotor and airframe surfaces.
An experimental campaign was undertaken to identify the effects of flow recirculation on an isolated rotor's acoustic emissions in a closed anechoic chamber. It is shown that flow recirculation results in a significant increase in higher harmonic noise, with an increase of more than 15 dB in some harmonics. This increase in noise is due to the nature of testing in a closed facility and does not represent the acoustic emissions of a similar rotor in hovering flight outdoors. Future measurements of rotors and full unmanned aerial vehicles in closed anechoic chambers must acknowledge recirculation effects and attempt to mitigate the effect on reported measurements.
A first principles understanding of the sound field produced by multirotor drones in hover is presented. Propeller diameters ranging from 8 to 12 in. are examined and with configurations comprising an isolated rotor, quadcopter, and hexacopter configuration. The drone pitch, defined as the ratio of drone diameter to rotor diameter, is the same for all multirotor configurations and is valued at 2.25. A six-degree-of-freedom load cell is used to assess the aerodynamic performance of each configuration, whereas an azimuthal array of 1/2 in. microphones, placed between two and three hub-center diameters from the drone center, is used to assess the acoustic near field. The analysis is performed using standard statistical metrics such as sound pressure level and overall sound pressure level and is presented to demonstrate the relationship between the number of rotors, the drone rotor size, and its aerodynamic performance (thrust) relative to the near-field acoustics. © 2018 by the American Institute of Aeronautics and Astronautics, Inc.
Open rotor noise derives from a variety of different sources that can result in tone or broadband noise. In this chapter we discuss these different sources and present analytical methods for the prediction of some of the most important for low Mach number flows. Time and frequency domain approaches to the prediction of loading noise and thickness noise are presented. Amiet's approximation for the prediction of broadband rotor noise is introduced. The chapter concludes with a discussion of the haystacking phenomenon and the prediction of rotor noise when there is significant blade-to-blade correlation.
This is the text of an introductory lecture intended to provoke discussion of some of the problems likely to face participants in the Workshop as they develop the subject of Computational Aeroacoustics (CAA). Many of the points were already made by the author in a lecture at the 1st IMACS Symposium on Computational Acoustics (Crighton, 1988). Nonetheless, it is appropriate to repeat and amplify them in a Workshop devoted specifically to CAA and not to the wider problems of acoustics — and in particular, in a Workshop in which it is hoped that interest and expertise will have been brought together in more than the critical mass needed to really make CAA take off as a significant tool in aeroacoustics investigations.
Accurate measurements of jet noise spectra at model scale-over a wide range of frequencies critically depend on the quality of the jet rig as well as the measurement system. The many aspects of the instrumentation system that require careful consideration to ensure the acquisition of good data are discussed. The-proper choice of the microphones and their orientation, the dynamic range requirements, and the de sign of the electronics for minimizing the magnitude of the corrections that must be applied to the raw spectra are among the many issues pertaining to jet noise measurements investigated. The reasons for the upturns in the jet noise spectra at the higher frequencies in recent tests are examined to identify the contributing factors, and some recommendations are provided to avoid this problem. Spectral measurements of simple point sources, simple distributed sources, and model scale jets with normal and grazing incidence microphones on traversing arrays, as well as on fixed poles at different distances from the sources, are used to evaluate these factors. It is also established that a distance of similar to 35 nozzle diameters ensures measurement in the true far field for jet noise.
A listening test has been performed to investigate the relationship between human annoyance and the amplitude modulation of wind turbine noise. To obtain sound samples for the listening test, sound from a 1.5 MW wind turbine in Korea was recorded. The strength of the amplitude modulation of the sound samples was defined in terms of the modulation depth spectrum, which was approximated by assuming that the sound samples are sinusoidally amplitudemodulated. The stimuli for the listening tests were created by reducing the modulation depth spectrum of the sound samples.A total of 30 participants were involved in the listening tests. The results of the listening tests indicate that the equivalent sound level and the amplitude modulation of wind turbine noise both significantly contribute to noise annoyance.
A method for calculating the far-field noise produced by an airfoil in
rectilinear motion through a turbulent flow field is extended to the
case of a rotating blade, thus giving a prediction method for a
propeller or helicopter rotor with a turbulent inflow. The method
incorporates accurate airfoil response functions including the effects
of compressibility and noncompactness. A high frequency assumption is
made in the analysis, but comparison with calculations not making this
assumption indicates that the present method is accurate for frequencies
above the first few rotor harmonics. Given the spectral characteristics
of the turbulence, the analysis is of an absolute nature containing no
adjustable constants, and gives a prediction of both the frequency
spectrum and directivity of the far-field noise.