Sound Pressure Spectra Research Articles

Using dynamic time warping to measure similarity between spectra

Many situations in acoustics and noise control involve comparing two or more spectra. For example, when tuning a numerical model to match measured data, the measured and predicted sound pressure spectra will be compared. In machine learning applications, it is useful to express the similarity between two spectra as a single number, which can then be incorporated into a cost function. It is common to use the mean square difference between the two spectra as this number. However, the mean square difference does not always reflect an acoustician's intuition of the similarity between two spectra. For example, if the natural frequencies of the two spectra are slightly misaligned, the mean square difference can be quite high, even though the underlying system parameters are similar. In this paper, several different similarity metrics for spectra are explored. Most notably, dynamic time warping (DTW) and its variants are applied to spectra and compared to more traditional metrics. Modifications to the DTW algorithm to handle logarithmic frequency and amplitude spacing as commonly arise when considering spectra are also addressed.

Research on Numerical Simulation Methods of Cavity Wind Noise Based on Different Turbulence Models

Abstract In order to more accurately simulate the acoustic properties of the fluid in the cavity, this paper uses four different turbulence models, LES, Spalart-Allmaras/LES, Realizable/LES, model, to simulate and calculate the simple cavity, combined with the acoustic experimental data carried out on the cavity model, and found that the simulated experimental values of pulsating pressure from the SST-k-/LES model and the Spalart-Allmaras/LES model have a better agreement with the simulated experimental values of the cavity pulsation pressure, and the calculated sound pressure spectra obtained from the SST-k-/LES model are the closest to the experimental values.

Experiment research on the characteristics and mechanism of noise caused by a car door sealing cavity at wind excitation

Due to the various geometric shapes of cavities within car door sealing systems, the resulting cavity noise induced by wind excitation exhibits complex characteristics, leading to unclear noise mechanisms. To address this, a study was conducted to examine the acoustic properties of door sealing cavities located in the B-pillar, C-pillar, and backdoor of an SUV within a full-scale aeroacoustic wind tunnel. The main objective was to explore the relationship between cavity noise and interior noise. The results showed that peak components in the sound pressure level spectrum of the cavities significantly contribute to interior noise, particularly for cavities located close to the car’s interior. To gain further insight, the geometric characteristics of the cavities were extracted and transformed into equivalent regular cavities. These equivalent cavities were subsequently tested for their noise performance in a small-scale aeroacoustic wind tunnel, and the occurrence mechanisms were thoroughly investigated. The results demonstrated that the noise spectra of different cavities, whether they had sealing strips or leakage gaps, exhibited typical multipeak characteristics, with some cases even leading to whistling (e.g. backdoor cavity). The reason for the whistling was a resonance when the self-sustained oscillation frequencies of the cavities coincided with or approached the Helmholtz resonance frequencies or modal frequencies of the cavities. Interestingly, the self-sustained oscillation frequency and cavity modal resonance still persisted even when the two frequencies were somewhat separated, albeit with peaks of lower magnitude in the sound pressure spectrum (e.g. in the sealed cavities of the B-pillar and C-pillar).

Study on tone-to-noise ratio calculation method for tones with frequency and amplitude variation

This paper is to present a method on improving consistency and accuracy of Tone-to-Noise Ratio (TNR) calculation for varying frequency tones in reporting prominent discrete tone according to ECMA-418-1. For given sound pressure spectrum, existing TNR calculation methods require suspect tones to be selected by the analyst, who must also determine the width of the tone, that could introduce significant uncertainty and variation between different analysts. Moreover, tones associated with unstable frequency could cause even more uncertainty in determining tone frequency band. In this study, an approach is proposed to determine the existence of discrete tone, define the tone frequency band, and lastly calculate TNR such that different analysts will be able to obtain consistent and more accurate TNR value. To validate the approach, several different noise spectra were used as mask noise, and thousands of numerically generated tones, with randomly varying frequencies and amplitudes, were modulated to the mask noise to generate synthesized waveform for TNR calculation. These randomly generated synthesized waveform were subjectively evaluated and confirmed that prominent discrete tone criteria are still applicable to tones with varying frequencies and amplitudes. The results show that proposed approach can significantly improve TNR calculation accuracy while ensure consistency between different analysts.

Direct-Noise of an Ultrahigh-Bypass-Ratio Turbofan: Periodic-Sector vs Full-Annulus Large-Eddy Simulations

The tonal and broadband noise of an ultrahigh-bypass-ratio fan stage, which has been developed at École Centrale de Lyon, is studied using large-eddy simulations (LES). Wall-modeled LES of a periodic sector and a 360-deg full fan stage have been performed at approach conditions, which is a relevant operating point for aircraft noise certifications. The fan noise is directly obtained from the fully compressible LES, using a well-refined unstructured mesh, and compared with state-of-the-art analytical models. The impact of the periodic boundary conditions, which are often used for high-fidelity simulations of turbofan engines, is assessed. The results from the 360-deg and periodic-sector LES are compared from aerodynamic and acoustic perspectives, including an analysis of the mean and turbulent flow quantities and sound-pressure spectra. Aerodynamic parameters show similar results for both configurations. However, the fan blade loading is slightly reduced in the 360-deg- LES near the blade tip. Acoustically, lower sound power levels at the intake and exhaust sections of the fan stage are obtained in the 360-deg LES, when compared to the periodic sector LES, particularly at low and middle frequencies. This can be associated with lower coherence levels in the fan wakes and smaller spanwise correlation lengths at the trailing edge of the blades. The modal content of the acoustic field has also been analyzed in detail and shows that the periodic sector LES cannot correctly simulate the modal content of the fan noise.

An overview of aeroacoustics-related analyses for the Artemis-I mission

To improve noise scaling laws for rockets and heated, supersonic jets, this paper discusses jet aeroacoustics-related scalings for the Space Launch System (SLS). Data from four measurement stations at the Artemis-I launch, located 1.4–1.8 km from the launchpad, are used in the analysis. First, SLS maximum overall sound pressure levels are compared against other rocket noise measurements at the same scaled distance using effective nozzle diameter. Second, Strouhal number frequency-scaling is used to compare the one-third-octave sound pressure and power level spectra with other jets and rockets. Third, the Oertel convective Mach number is used to interpret a maximum directivity angle of 60°–70°. Finally, SLS’s acoustic efficiency is evaluated relative to that of other rockets.

Rotor aeroacoustic response to an axisymmetric turbulent boundary layer

The acoustic response of a five-bladed rotor to an axisymmetric turbulent boundary layer at the tail end of a body of revolution (BOR) is investigated numerically to elucidate the physical sources of acoustics, particularly the role of coherent structures in sound generation. The BOR is at a length-based Reynolds number of $1.9 \times 10^6$ and free-stream Mach number of 0.059. Two rotor advance ratios, $1.44$ and $1.13$ , are considered. The turbulent boundary layer on the nose and midsection of the BOR is computed using wall-modelled large-eddy simulation, whereas that in the acoustically important tail-cone section is wall-resolved. The radiated acoustic field is calculated using the Ffowcs Williams–Hawkings equation. The computed flow statistics and sound pressure spectra agree well with the experimental measurements at Virginia Tech. In addition to broadband turbulence-ingestion noise, spectral humps near multiples of the blade-passing frequency and accompanying valleys are captured. They are shown to be caused by correlated blade unsteady-loading dipole sources and their constructive and destructive interference as a result of successive blades cutting through the same coherent structures. The latter undergo rapid growth in the decelerating tail-cone boundary layer before their interaction with the rotor. The acoustic radiation is dominated by the outer region of the blade owing to a combination of larger blade chord-length, inflow turbulence intensity and blade speed. The numerical results also correctly predict the effect of the rotor advance ratio on the acoustic field. A mixed free-stream/convection Mach-number scaling successfully collapses the sound pressure spectra at the two advance ratios.

Frequency-domain expression of sound pressure field radiated by monopole source with general moving trajectory

Frequency-domain expressions of sound pressure field radiated by moving source were derived successfully only for some special moving trajectories in the literature. In this paper, a frequency-domain expression of sound pressure field radiated by monopole source with general moving trajectory is firstly derived. This expression has a simple mathematical form, where the sound pressure spectrum is written as a Fourier transformation of free-field frequency-domain Green functions along the trajectory. This expression is subsequently verified through theoretical and numerical verification. In theoretical verification, this expression is degenerated into the known analytical solution in each case with special moving trajectories. In numerical verification, the sound pressure spectra calculated via this expression are consistent with those calculated via a time-domain numerical method in cases without known analytical solutions. This expression is finally applied to enable the mode composition beamforming method to localize a rotating source with time-variant rotation speed.

Ongoing analysis of far-field acoustical measurements during the Artemis-I launch

As the global space industry expands, impacts from super heavy-lift launch vehicle noise on payloads, communities, and natural habitats are better understood with improved source models. To support model development, this paper discusses ongoing analyses of far-field acoustical measurements made during the NASA Space Launch System (SLS) Artemis-I mission. Fifteen acoustical measurement stations were deployed prior to the launch, including ten autonomous stations within Kennedy Space Center and five manned stations off-Center, up to 50 km from the pad. This paper presents a brief summary of data analyzed from all 15 stations, including A-weighted and unweighted sound pressure levels and spectra. Additionally, the skewness of the pressure time derivative is shown as a quantifier of the noise’s crackling component. Finally, source directivity and sound power analyses are presented. [Work supported in part by the NSF REU Program and the Utah Space Grant Consortium.]

Aeroacoustic and Aerodynamic Adjoint-Based Shape Optimization of an Axisymmetric Aero-Engine Intake

A continuous adjoint-based aeroacoustic optimization, based on a hybrid model including the Ffowcs Williams–Hawkings (FW–H) acoustic analogy, to account for the multidisciplinary design of aero-engine intakes with an axisymmetric geometry, is presented. To optimize such an intake, the generatrix of its lips is parameterized using B-Splines, and the energy contained in the sound pressure spectrum, at the blade passing frequency at receivers located axisymmetrically around the axis of the engine, is minimized. The engine is not included in the optimization and manifests its presence through an independently computed time-series of static pressure over the annular boundary of the simulation domain that corresponds to the inlet to the fan. Taking advantage of the case axisymmetry, the steady 3D RANS equations are solved in the rotating frame of reference and post-processed to compute the flow quantities’ time-series required by the FW–H analogy. The numerical solution of the unsteady flow equations and the otherwise excessive overall cost of the optimization are, thus, avoided. The objective function gradient is computed using the continuous adjoint method, coupled with the analytical differentiation of the FW–H analogy. The adjoint equations are also solved in the rotating frame via steady solver.

Assessing azimuthal asymmetry in the noise radiation from a three-core launch vehicle during liftoff

Creating accurate rocket noise models is important for assessing impacts on humans, the environment, and payloads. The United Launch Alliance Delta IV Heavy launch vehicle is unique because of the separation of the three cores and their associated RS-68A nozzles. This makes it a good candidate for determining how the asymmetry of nozzle configuration affects noise radiation, which can affect noise models. The NROL-82 and NROL-91 missions both launched from Vandenberg Spaceforce Base using Delta IV Heavy Vehicles. For both of these launches, acoustic data were recorded between ∼0.9 and ∼5.2 km from the vehicle at different azimuths to determine the extent of azimuthal asymmetry in noise radiation. Maximum overall sound pressure level, spectra, and overall power level were determined for each launch. Methods for comparing the datasets and results will be discussed.

Initial findings from Space Launch System liftoff measurements

This presentation documents initial findings from far-field noise measurements at NASA’s Kennedy Space Center during liftoff of the Space Launch System’s Artemis I mission, which occurred on November 16, 2022. The vehicle— the most powerful ever successfully launched into orbit—consists of four liquid-fueled RS-25 engines and two five-segment, solid-fuel rocket boosters (SRBs). Because this was the first launch, the noise radiation characteristics of this vehicle were previously unknown. Overall sound pressure levels, waveform characteristics, and spectra are described at distances ranging from 1.5 to 8.4 km. The levels due to the SRBs’ ignition overpressure are particularly intense in the direction of the flame trench exit. The post-liftoff maximum one-third octave spectrum has a peak at 20 Hz, and maximum overall levels are greater than described in a pre-launch environmental assessment. These and other findings presently submitted to JASA Express Letters further understanding of super heavy-lift rocket acoustics.

Artemis I launch noise spectra: Effects of variable weather profiles over short- and long-range propagation paths

NASA’s Space Launch System (SLS) launched the Artemis I mission at 1:47 am local time on November 16, 2022. The timing of the night launch offered a multitude of interesting weather effects over the propagation paths to measurement locations ranging from 1.5 km to 60 km. This presentation discusses spectra at different distances from Launch Complex 39B, both inside and outside the Kennedy Space Center perimeter. Preliminary analysis of the SLS sound pressure levels and spectra suggests many propagation phenomena are at play. In addition to geometric spreading, atmospheric absorption, and nonlinear propagation, complex weather profiles and ground effects are discussed. Initial results of a single-frequency Greens-function parabolic equation model, with estimated weather profiles along the propagation paths, will be offered with approximate source and receiver height to compare levels at several locations.

Analysis of unsteady flow field and near-field aerodynamic noise of scale high-speed trains in long tunnel

The hybrid LES/APE method is used to simulate the unsteady flow field and the near-field aerodynamic noise of a 1/25 scale eight-coach high-speed train in long tunnel, validated by time-averaged pressure coefficients and Power Spectral Density of surface pressure from the aerodynamic wind tunnel test of a same scale two-coach train model. Train induced wind with the maximum velocity of 28 m/s in the opposite direction of train operating is formed in tunnel. Blocking effect leads to larger turbulent pressure and streamwise increment of turbulent power level from 133.7 dB to 138.2 dB, which is from130.3 dB to 133.4 dB in open air. With APE solver, three cases including tunnel with fully reflective walls, tunnel with fully absorptive walls and open air, are conducted to compare the influence of blocking effect and acoustic reflection on sound pressure. Total sound pressure level on the train body in tunnel is overall stronger than that in open air. The difference of sound power level between tunnel and open air is mostly contributed by acoustic reflection: 84.7 % at Coach 1, 84.3 % at Coach 8 and 66.0 ∼ 77.5 % from Coach 2 to Coach 7. Five peak frequencies: 520 Hz, 800 Hz, 1440 Hz, 2600 Hz and 3880 Hz are found from the frequency spectra of sound pressure of the probes in bogie cavities and pantograph grooves, in tunnel with fully reflective walls. Sound pressure modes help to clarify the generation mechanisms of peak frequencies: 520 Hz and 800 Hz are caused by the tunnel cavity resonance; 1440 Hz is caused by the first-order vertical mode from the bottom of the pantograph groove to the top of tunnel walls; 2600 Hz is caused by the first-order vertical mode from the ground to the top of bogie cavity, but 3880 Hz is not generated by simple in-phase cavity resonance.

Space Launch System acoustics: Far-field noise measurements of the Artemis-I launch.

To improve understanding of super heavy-lift rocket acoustics, this letter documents initial findings from noise measurements during liftoff of the Space Launch System's Artemis-I mission. Overall sound pressure levels, waveform characteristics, and spectra are described at distances ranging from 1.5 to 5.2 km. Significant results include: (a) the solid rocket boosters' ignition overpressure is particularly intense in the direction of the pad flame trench exit; (b) post-liftoff maximum overall levels range from 127 to 136 dB, greater than pre-launch predictions; and (c) the average maximum one-third-octave spectral peak occurred at 20 Hz, causing significant deviation between flat and A-weighted levels.

Aerodynamic noise due to complex internal flow through cordless vacuum cleaner and suction tower station

Although the demands and sales of cordless vacuum cleaner are on the rise in the global home appliance markets, consumers face the inconvenience of short battery life and frequent emptying of the dust box. To solve this inconvenience, a station tower that can automatically charge the battery and empty the dust box has been developed. However, aerodynamic noise generated when the station tower sucks dust causes another inconvenience. The purpose of this study is to identify the major aerodynamic noise sources that occur when such a station tower is connected to a wireless vacuum cleaner and operate, and to develop a low-noise design based on this. First, the entire flow path of a cordless vacuum cleaner and a station tower was simulated using the unsteady compressible RANS equations. Then, the main aerodynamic noise sources were identified using the vortex sound sources. From these results, the flow region through the head throat of the cordless vacuum cleaner was identified one of the most significant aerodynamic noise sources. Based on the identified noise source mechanism, new throat design was proposed, and its validity was investigated using a high-resolution Large Eddy Simulation technique. It was found that the strength of vortex sound source of pipe flow through the new head throat was much less than the original one. Finally, the new design geometry was applied to create a prototype, and the sound pressure spectrum measured for the prototype was compared to the original, which showed that the radiated noise level of the new one was less than the original one.

Aerodynamic and aeroacoustic performance assessment of a vertical axis wind turbine by synergistic effect of blowing and suction

The power efficiency of Darrieus wind turbines significantly deteriorates during rotation caused by periodic dynamic stall at low tip speed ratios. This leads to a strong fluctuation in torque and a reduction in energy acquisition. This paper aims to improve the aerodynamic performance of an H-type Darrieus wind turbine using an innovative fluidic flow control technique based on the synergistic effect of blowing and suction. The aeroacoustic noise emissions accomplished with this enhancement are evaluated. The Improved Delayed Detached Eddy Simulation turbulence model and the Ffowcs Williams-Hawkings acoustic analogy method are adopted to simulate the instantaneous flow field and predict the far field noise. Following validation of the numerical approach using wind tunnel experimental data on a static airfoil, an orthogonal experimental design method was used to analyze and optimize the operating factors. The factors include suction position relative to leading-edge (Ls), blowing position relative to trailing-edge (Lb) and jet coefficient (Cμ). The results indicate that Cμ plays crucial role in determining the airfoil performance, while the role of Lb is almost negligible. Furthermore, the impact of optimal combination of these factors was analyzed based on three different control strategies. It is found that appropriate application of the active control solution can eliminate the wind turbine’s negative torque, avoid excessive alternating load on the rotor and improve the energy extraction efficiency. The aeroacoustic noise estimation shows that the active device can reduce the noise emission by moderating pressure fluctuation, stabilizing the flow field and influencing the vortex shedding. Similarly, the proposed active control solution can reduce the wind turbine noise level by up to 6.56 dB by modifying the sound pressure spectra at frequencies between 100 and 1000 Hz.

Numerical investigation of whistling sound in narrow-gap flow of automobile side mirror

It is frequently reported that a high-frequency whistling noise is generated by airflow through the narrow gap of a vehicle’s side mirror under certain driving conditions. The aim of the present study is to identify the generation mechanism of the narrow gap whistling of a side mirror, which is considered one of the most challenging problems due to too the small dimensions and complex geometry of the gap. A wind tunnel test was performed to find the flow conditions where the whistling sound is generated. The relative directions and speeds of the airflow were identified for the onset of the whistling sound and its frequency. A numerical analysis of the external and internal flows of the side mirror was then conducted using compressible Large Eddy Simulation (LES) techniques with high-resolution grids to capture the generation mechanism of the whistling sound, which is generally based on a feedback mechanism between the vortex and sound waves. The numerical result was validated through a comparison of the predicted sound pressure spectrum with the measured ones. The main aerodynamic sources were identified using the vortex sound sources, and a generation mechanism of the whistling sound based on the feedback mechanism was revealed. This was done by visualizing the standing wave pattern in the narrow gap, which was coupled with the identified vortex sound sources.

Flow Characteristic Study of High-speed Cavity Based on Detached-eddy Simulations

The cavity is widely used in the advanced flight vehicle. However, there are also some new aerodynamic characteristic when cavity is exposed in high-speed flow. The flow characteristic of high-speed cavity is studied in this paper based on detached-eddy simulation method. The M219 cavity benchmark under transonic flow is used as the validation case for numerical methods and grid strategy. Comparison of predicted sound pressure levels and spectra against the wind tunnel experiment has proved that the numerical methods capture the major oscillation dynamics inside the cavity. The aerodynamic characteristic of cavity with a store is investigated in this paper, there are three vortexes in the cavity and the main vortex in the middle rotates around the store. The shear layer breaks earlier compared with clean cavity due to the turbulent flow around the store. However, the small store has little influence to the sound pressure level of cavity.

Cross validation of the aerodynamic and acoustic measurements in two Kevlar-walled wind tunnels

The wind tunnel with Kevlar walls is becoming more and more popular for aeroacoustic measurements, because the necessary corrections to convert aerodynamic coefficients to free field values are moderate and at the same time the signal to noise ratio of acoustic measurements are in the same order as in an open jet configuration. Yet, the technology is new and the correction methods for aerodynamic and acoustic coefficients are not well established. This paper aims on bench-marking two of the largest Kevlar-walled wind tunnels by cross validation of the measurements performed on a NACA633018 aerofoil model. The measured aerodynamic coefficients showed and excellent agreement in the attached flow regime. Maximum lift was predicted differently in the two facilities. The differences might be due to three-dimensional effects for separated flow and the fact that the aspect ration was different in the two facilities. The difference of the measured overall sound pressure levels was less ± 1 dB for attached flow conditions. However, we observed a difference in the slope of the sound pressure spectra and recommend to further investigate the acoustic correction methods.