Pressure Level Spectra Research Articles

A proper framework for studying noise from jets with non-compact sources

A quantitative assessment of the acoustic source field produced by a laboratory-scale heated jet with a gas dynamic Mach number of 1.55 and an acoustic Mach number of 2.41 is performed using arrays of microphones that are traversed across the axial and radial plane of the jet's acoustic field. The nozzle contour comprises a method of characteristics shape so that shock-related noise is minimal and the dominant sound production mechanism is from Mach waves. The spatial topography of the overall sound pressure level is shown to be dominated by a distinct lobe residing on the principal acoustic emission path, which is expected from flows of this kind with supersonic convective acoustic Mach numbers. The sound field is then analysed on a per-frequency basis in order to identify the location, strength, convection velocity and propagation angle of the various axially distributed noise sources. The analysis reveals a collection of unique data-informed polar patterns of the sound intensity for each frequency. It is shown how these polar patterns can be propagated to any point in the far field with extreme accuracy using the inverse square law. Doing so allows one to gauge the kinds of errors that are encountered using a nozzle-centred source to calculate sound pressure spectrum levels and acoustic power. It is proposed that the measurement strategy described here be used for situations where measurements are being used to compare different facilities, for extrapolating measurements to different geometric scales, for model validation or for developing noise control strategies.

DGEN Aeropropulsion Research Turbofan Core/Combustor-Noise Measurements—Experiment and Modal Structure at Core-Nozzle Exit1

Abstract Data from a recent core/combustor-noise source-diagnostic test (SDT) utilizing a small turbofan engine are analyzed. The campaign continued the exploration begun in a baseline test, but with more extensive acoustic instrumentation. Both tests were aimed at developing a better understanding of propulsion-noise sources and their impact on the farfield noise signature, in order to enable improved turbofan noise-prediction methods and noise-mitigation techniques. Simultaneous high-data-rate acoustic measurements (93 channels in total) were obtained using a circumferential sensor array at the core-nozzle exit in conjunction with sideline and farfield microphone arrays for several relevant engine operational points. Measurements were repeated for different circumferential and sideline array configurations, as well as for redundancy. The unsteady pressure field at the core-nozzle exit is documented in detail. Previous work suggested that the ±1 azimuthal duct mode could be cut on at this location, which would have implications for combustor-noise modeling and prediction. The modal decomposition of the combustor noise at the core-nozzle exit verifies this observation. Select farfield sound pressure level (SPL) spectra are also presented.

Thrust and Acoustic Performance of Small-Scale, Coaxial, Corotating Rotors in Hover

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.

Analysis of pressure fluctuation in transonic cavity flows using modal decomposition

Turbulent flows at a free stream Mach number of 1.19 over an open cavity with L/D ratio of 5.0 are numerically investigated using improved delayed detached-eddy simulation based on two-equation shear stress transport model. Modal decompositions including proper orthogonal decomposition (POD) and dynamic mode decomposition (DMD) are applied to analyze the pressure fluctuations. The extracted first six POD modes possess more than 75% of the total energy and contain multiple frequencies. Their spatial structures exhibit well regular and periodic behaviors along the cavity lip line. The DMD algorithm identifies the flow structures associated with single frequencies. Major distribution areas of high intensity pressure fluctuations move upstream as the mode frequency increases, and the structures with high frequencies are prone to break down near the trailing edge. The alternating pressure patterns convection process is clearly presented, and the propagation of acoustic waves validates that the acoustic waves share the same sound source but are radiated following two different paths, consistent with the feedback mechanism. In addition, the effects of free stream Reynolds number on sound pressure spectrum levels are investigated at a fixed pressure and temperature. Results show that free stream Reynolds number has no effect on the non-dimensional frequencies of the dominant modes, while the main recirculation area shrinks as Reynolds number increases. Furthermore, variations of sound pressure levels as a function of Reynolds numbers exhibit significant discrepancies for a fixed free stream pressure or temperature, indicating that the Reynolds number is not critical to the feedback tone amplitude. The enhancement of sound pressure levels mainly attributes to the increment of free flow pressure or the impingement of enlarged fluid velocity due to higher free stream temperature.

Gulf of Mexico low-frequency ocean soundscape impacted by airguns.

The ocean soundscape of the Gulf of Mexico (GOM) has not been well-studied, although it is an important habitat for marine mammals, including sperm and beaked whales, many dolphin species, and a potentially endangered baleen whale species. The GOM is also home to high levels of hydrocarbon exploration and extraction, heavily used commercial shipping ports, and significant fishery industry activity, all of which are known contributors to oceanic noise. From 2010-2013, the soundscape of three deep and two shallow water sites in the GOM were monitored over 10 - 1000 Hz. Average sound pressure spectrum levels were high, >90 dB re 1 μPa(2)/Hz at <40 Hz for the deep water sites and were associated with noise from seismic exploration airguns. More moderate sound pressure levels, <55 dB re 1 μPa(2)/Hz at >700 Hz, were present at a shallow water site in the northeastern Gulf, removed from the zone of industrial development and bathymetrically shielded from deep water anthropogenic sound sources. During passage of a high wind event (Hurricane Isaac, 2012), sound pressure levels above 200 Hz increased with wind speed, but at low frequencies (<100 Hz) sound pressure levels decreased owing to absence of noise from airguns.

Quantitative analysis of a frequency-domain nonlinearity indicator.

In this paper, quantitative understanding of a frequency-domain nonlinearity indicator is developed. The indicator is derived from an ensemble-averaged, frequency-domain version of the generalized Burgers equation, which can be rearranged in order to directly compare the effects of nonlinearity, absorption, and geometric spreading on the pressure spectrum level with frequency and distance. The nonlinear effect is calculated using pressure-squared-pressure quadspectrum. Further theoretical development has given an expression for the role of the normalized quadspectrum, referred to as Q/S by Morfey and Howell [AIAA J. 19, 986-992 (1981)], in the spatial rate of change of the pressure spectrum level. To explore this finding, an investigation of the change in level for initial sinusoids propagating as plane waves through inviscid and thermoviscous media has been conducted. The decibel change with distance, calculated through Q/S, captures the growth and decay of the harmonics and indicates that the most significant changes in level occur prior to sawtooth formation. At large distances, the inviscid case results in a spatial rate of change that is uniform across all harmonics. For thermoviscous media, large positive nonlinear gains are observed but offset by absorption, which leads to a greater overall negative spatial rate of change for higher harmonics.

Ocean ambient sound south of Bermuda and Panama Canal traffic.

Comparisons of current and historic ocean ambient noise levels are rare, especially in the North Atlantic. Recent (2013-2014) monthly patterns in ocean ambient sound south of Bermuda were compared to those recorded at the same location in 1966. Additionally, trends in ocean traffic, in particular, Panama Canal traffic, over this time were also investigated. One year of ocean ambient noise measurements were collected in 1966 using cabled, omnidirectional hydrophones at the U.S. Navy Tudor Hill Laboratory in Bermuda, and repeat measurements were collected at the same location from June 2013-May 2014 using a High-frequency Acoustic Recording Package. Average monthly pressure spectrum levels at 44 Hz increased 2.8 ± 0.8 dB from 1966 to 2013, indicating an average increase of 0.6 dB/decade. This low level of increase may be due to topographic shielding at this site, limiting it to only southern exposure, and the limit in the number of ship transits through the Panama Canal, which did not change substantially during this time. The impending expansion of the Canal, which will enable the transit of larger ships at twice the current rate, is likely to lead to a substantial increase in ocean ambient sound at this location in the near future.

A Method of Mitigating Pulsation in Ventilated Supercavities

A method for mitigating ventilated supercavity pulsation is presented. The method, which has its roots in parametric oscillators, shifts the supercavity resonance frequency by modulating its gas ventilation rate. When appropriately modulated, the supercavity is driven off resonance by the waves on the gas/water interface (that remain unchanged) and pulsation is, therefore, suppressed. Initial experimental results indicate that the gas ventilation rate modulation frequency must be sufficiently different from the supercavity resonance frequency to mitigate pulsation. If the modulation frequency is not sufficiently different from the supercavity resonance frequency, pulsation is simply shifted in frequency with a corresponding small reduction in the supercavity interior pressure spectrum level and radiated noise.

Numerical evaluation of pile vibration and noise emission during offshore pile driving

When offshore structures like converter platforms or wind turbines are founded by pile driving, high noise level are emitted into the water. In this paper a numerical model will be presented and validated which is able to effectively predict the noise emission during pile driving under offshore conditions. It combines a Finite Element Method in the near field of the pile with a Parabolic Equation technique to compute the pressure spectrum, sound exposure and peak level in a certain distance of the pile. The results will be compared to measurements performed during two full scale offshore tests. Furthermore, a procedure for computing the acoustic properties of the sediment as a function of the frequency, depth and density is presented. This allows to represent the layered sediment at the construction site within the numerical methods.

Optical fiber acoustic sensing multiplexing system based on TDM/SFDM

We propose a system of time-division multiplexing (TDM) and spatial frequency-division multiplexing (SFDM). Extrinsic Fabry-Perot interferometric sensors are applied to detect weak acoustic signals. The broadband source is employed, the light from it is modulated by a pulse signal sequence and is efficiently amplified by semiconductor optical amplifiers. Experimental results show that the equivalent noise pressure spectrum level is -97.2 dB re 1 rad/vHz below 1250 Hz, and the cross talk between two sensors in one TDM channel is -32.7 dB with a cavity length difference of 60 µm. The number of sensors in this multiplexing system can theoretically reach 160.

Noise generated by ventilated supercavities

The noise generated by ventilated supercavities has been explored experimentally in a water tunnel facility. The most prominent acoustical characteristic is the monopole behavior exhibited by a ventilated supercavity in its pulsating closure regime. The interior cavity pressure and near-field radiated sound are monotonic with a frequency that is related to the speed and length of waves propagating on the supercavity gas/water interface. The cavity interior pressure spectrum level is shown to be related to the near-field and far-field noise spectrum level through spherical spreading of the sound waves from the supercavity interface. As a result, the cavity interior pressure can be used as a measure of the radiated noise. The noise radiated by a pulsating supercavity at the pulsation frequency is at least 40 dB above that radiated by comparable re-entrant jet and twin vortex cavities.

Aerodynamic and aero noise of stationary tire

The characteristics of aerodynamic and aero noise of stationary tire are investigated using computational fluid dynamics (CFD) and wind tunnel tests. A positive lift coefficient of almost 0.27 was found at all tested Reynolds numbers, while the drag coefficient was twice as large. The averaged pressure coefficient computed agrees with the measured, while the separation point predicted by numerical simulation is brought forward. The aero-noise numerical results using hybrid method based on large eddy simulation (LES) and FW-H formulations compares well with results from the wind tunnel test for total sound pressure level and noise spectra. Due to the defects of turbulence model and limited computer resources, the agreement of noise spectra for some test points in the frequency above 600 Hz is not the best. Sound energy of sound source and sound energy radiated from sound source are nearly proportional to the fourth power and sixth power of wind speed, respectively. At the simulated speeds, about 2.7% of sound energy comes from quadrupole sources, while 97.3% of sound energy comes from the dipole sources, which is the first time a quantitative description has been given. It should be pointed out that due to the different phases of dipole and quadrupole, the contribution of quadrupole to radiation is different at the receivers.

Near-Field Noise Prediction for Landing Gear Based on Detached Eddy Simulations

A four-wheel rudimentary landing gear is studied numerically by detached eddy simulation (DES) based on the Spalart-Allmaras turbulence model. The surface sound pressure level and sound pressure spectra are calculated using the obtained unsteady flow field. The investigation shows that DES can describe the steady and unsteady properties in the flow around rudimentary landing gear. It can give reasonable results since the flow around the landing gear is a massive separated flow. The results prove the feasibility of DES type methods in massive separated unsteady flow field and aerodynamic noise prediction for landing gear, and can be used in the study of landing gear noise reduction.

Sensitivity analysis of simulation model on railway rolling noise prediction

The railway rolling noise prediction software (RRNPS) is a simulation model for predicting the sound pressure levels during a train passage due to wheel/rail roughness, based on vibration dynamics, contact mechanics and sound radiation modules. Some field validation has been done under European and Australian conditions. Comparisons between simulations and measurements have shown that this software model gives reliable predictions in terms of overall A-weighted sound pressure level and noise spectrum. In this paper, based on a typical railway rolling noise situation in Australia (a series of field measurements were taken at a narrow track gauge testing site), a sensitivity analysis of the RRNPS was carried out to investigate the effect of train speed, normal load, sleeper spacing, rail pad stiffness, track gauge and rail cross receptance factor on railway rolling noise. The results provide some insight into model-based methods to control and mitigate railway noise at its source.

Shape Optimization of High-Speed Train Pantograph Insulators for Low Aerodynamic Noise

With large eddy simulation(LES) and Lighthill-Curle acoustic theory, the aerodynamic noises radiated from pantograph insulators with rectangular, circular, elliptical section were calculated, and the optimal pantograph insulator shape was obtained. The results show that in the same model, the sound pressure level (SPL) spectrum at different monitoring points are basically the same, but the amplitude is different. In different models, the SPL spectrum are different. As for rectangular, circular, elliptical section insulators, the frequency with maximum SPL reduces gradually. For reducing aerodynamic noise, the elliptical section insulator is optimal, and the long elliptical axis should be consistent with air flow. The pantograph with bigger and less components is helpful to reduce the aerodynamic noise.

Experimental Characterisation of the Far-Field Noise in Axial Fans Fitted with Shaped Tip End-Plates

The authors investigate the far-field noise emissions of a datum fan blade fitted with tip end-plate geometries, originally designed to control the leakage vortex swirl level. The end-plate geometries influence the tip-leakage flow, vortex formation, and swirl level. In doing so, the end-plate geometries influence the sound-power levels. After an evaluation of fan rotors' aerodynamic performance, the study compares the rotors' far-field noise signature characterised in terms of sound-power and pressure-level spectra to enable and assess the end-plate acoustic pay-off. The investigation attempts to establish a cause-and-effect relationship between the tip-flow dynamics and the radiated sound fields, exploring the diverse directivity patterns. The authors found a tonal reduction, due to the enhanced blade-tip end-plates and clarified the relevance of the tip features influencing the radial distribution of the noise sources using coherence analysis. The modified multiple-vortex breakdown end-plate design was effective in reducing the broadband noise, giving an improvement in the frequency range of the turbulent noise.

On the Lombard Effect Induced by Vehicle Interior Driving Noises, Regarding Sound Pressure Level and Long-Term Average Speech Spectrum

In this paper, the influence of typically appearing interior driving noises on the production of speech inside vehicles is described. The automatic reflex of raising one’s voice in noisy environments, known as the Lombard Effect [1], is quantified both in the sound pressure level and in the long-term spectral distribution of read speech. For this purpose, a laboratory experiment with 30 participants was performed. In addition to a noise-free reference condition, driving noises at various sound pressure levels as well as noises with white and speech-like spectra were presented to the test attendees while reading out loud a written text. The effects of these different noises on the speech’s sound pressure level and spectral distribution were quantified. In conclusion, there is no significant impact of the noise shape on the spectral content of the respective speech, while the noise-affected speech is significantly different to speech under the quiet condition. In regard to the speech’s sound pressure level, an octave-band analysis of the background noises with a band weighting commonly used for speech intelligibility measurements is proposed to predict speech levels in the tested noise environments.

Passive acoustic monitoring near Pt. Sur, California, in 2008–2009.

How sounds from human activities affect behavior of marine mammals is in focus of many ongoing research projects. An important step in this direction is to develop regional databases of marine mammal vocalizations and sounds generated by human activities and natural sources. Analysis of passive acoustic recordings collected on top of Sur Ridge, near the Point Sur Ocean Acoustic Observatory (OAO), in 2008–2009 is presented. The data have been acquired with high‐frequency acoustic recording package in the 10 Hz–100 kHz frequency band at a 200 kHz sampling frequency. These recordings have been scanned to detect and identify signals from various underwater acoustic sources. Pressure spectrum level has been analyzed in time/frequency space to reveal interannual, seasonal, and diel variabilities, as well as possible correlations of different sounds, with primary focus on anthropogenic sounds and marine mammal vocalizations. [Research supported by US Navy CNO(N45).]

An integrated performance analysis for a small axial-flow fan

Owing to the high system resistance and space limitations on computer devices, many researchers have begun to pay more attention to developing high-performance axial-flow fans. Evidently, evaluating the fan performance under different operating conditions is essential for both designer and practical engineering applications. However, previous studies do not provide a detailed flow-field analysis, torque prediction, efficiency estimation at various operating points, and qualitative numerical prediction of sound generation. Thus, this comprehensive study was performed with the aim to offer the aforementioned technical information and completely evaluate the fan performance. In this study, computational fluid dynamics (CFD) simulations and experimental measurements are utilized to perform flow visualization, torque calculation, efficiency estimation, and noise analysis. For demonstration purposes, a 120 mm-diameter axial-flow fan is designed and fabricated via computer numerical control (CNC) to serve as the research subject. The result indicates that the P— Q curve and the sound pressure level (SPL) spectrum of the experiment are in agreement with those of numerical simulations. The numerical deviations in maximum volumetric flowrate and static pressure are approximately 7 per cent and 13 per cent, respectively. Regarding the acoustic characteristics, the overall SPLs for measured spectra and large eddy simulation (LES) calculation are 51.3 dB and 48.1 dB, respectively. Consequently, this study establishes an integrated aerodynamic, acoustic, and electro-mechanical evaluation approach that can be used as an important tool for fan designers.

Blue and fin whale acoustic presence around Antarctica during 2003 and 2004

AbstractSeasonal and spatial variations of blue (Balaenoptera musculus) and fin whale (B. physalus) calls were analyzed from recordings collected with Acoustic Recording Packages (ARPs) deployed between January 2003 and July 2004 at four circumpolar locations: the Western Antarctic Peninsula (WAP), the Scotia Sea (SS), Eastern Antarctica (EA), and the Ross Sea (RS). Call characteristics were compared among sites using the average pressure spectrum levels from 1 month of data at each location. Presence of calls was analyzed using automatic call detection and acoustic power analysis methods. Blue whale calls were recorded year‐round, with the highest detections in February–May and November. This suggests that the blue whale population may not migrate synchronously, and may indicate long duration calls are more common during migrations. Fin whale calls were detected only during February–July. Two distinct fin whale call types were recorded, suggesting a possible separation into two populations. The calls at the EA site had a secondary frequency peak in the pressure spectrum at 99 Hz and the calls at the WAP and the SS sites had a peak at 89 Hz. No fin whale calls were detected at the RS site. Acoustics are a good tool to monitor large whales in the Southern Ocean.