Benjamin Cotte

Time-Domain Impedance Boundary Conditions for Simulations of Outdoor Sound Propagation

B = Gaussian half-width, m c0 = speed of sound, m=s dL = porous layer thickness, m f = frequency, Hz Im = imaginary part j = imaginary unit k = complex wave number, m 1 p = pressure, Pa q = tortuosity Re = real part S = number of first-order systems in the impedance approximation sf = coefficient of the selective filter T = number of second-order systems in the impedance approximation t = time, s v = velocity component normal to impedance surface, m=s Z = complex impedance, kg=m=s = ratio of specific heats L = sound pressure level relative to the free field, dB t = time step, s x = spatial mesh size, m 0 = air density, kg=m 0, e = flow resistivity, Pa s=m = porosity ! = angular frequency, rad=s

Time-domain simulations of outdoor sound propagation with suitable impedance boundary conditions

Finite difference time-domain methods are attractive for the study of broadband outdoor noise propagation, because they can accurately take into account both atmospheric and ground effects. Moreover, these methods allow moving sound sources to be modeled, which can be interesting in the context of transportation noise. A recently proposed method to obtain an impedance boundary condition is implemented in a linearized Euler equations solver. A long-range propagation configuration in a two-dimensional geometry is studied in homogeneous conditions and in downward-refracting conditions with an impedance ground over a distance of 500 m. Two impedance models corresponding to a grassy ground and to a snow-covered ground are considered. Numerical results are compared in the time domain to an analytical solution in homogeneous conditions and to results from a ray-tracing code in downward-refracting conditions. Near the ground, surface waves are detected in the two cases and are the dominant arrivals in the homogeneous case.

Time-domain simulations of sound propagation in a stratified atmosphere over an impedance ground

Finite-difference time-domain simulations of broadband sound propagation in a stratified atmosphere are presented. A method recently proposed to obtain an impedance time-domain boundary condition is implemented in a linearized Euler equations solver, which enables to study long range sound propagation over an impedance ground. Some features of the pressure pulse evolution with time are analyzed in both upward-and downward-refracting conditions, and the time-domain simulations are compared to parabolic equation calculations in the frequency domain to show the effectiveness of the proposed impedance boundary condition.

Scintillation index of high frequency acoustic signals forward scattered by the ocean surface

Ocean measurements of the scintillation index (SI) of surface forward-scattered signals made in August 2002 are presented and compared with a model developed by Yang and McDaniel [Waves in Random Media 1, 419–439 (1991)]. The acoustic measurements employed continuous wave (CW) pulses and linear frequency modulated (LFM) sweeps with center frequencies of 20 and 40 kHz. Simultaneously, measurements of wind speed, directional surface wave height spectrum, and ocean sound speed profile were made. The sea state was between 0 and 1 during the four days of the experiment, in part because the location is very much in the lee of San Clemente Island. The measured values of SI are found to agree with Yang and McDaniel model predictions, except for measurements with the largest signal bandwidth and/or the narrowest beamwidths, which violate model assumptions of continuous signals and omnidirectional projectors and hydrophones. In addition, the data show that SI decreases with increasing signal bandwidth (or decreasing temporal extent). An extension to the Yang and McDaniel model is developed that accounts for a reduction in signal temporal extent or ocean surface ensonification. The extended model is in qualitative agreement with the measurements, in that SI is predicted to decrease with increasing signal bandwidth.

Extended source models for long range wind turbine noise propagation

Wind turbine noise can be perceived at distances greater than one kilometer and is characterized by amplitude modulations at the receiver. In order to predict this noise, it is necessary to model the dominant aeroacoustic noise sources as well as the main outdoor propagation effects. In most studies from the literature, the wind turbines are modeled as point sources to simplify the coupling between source and propagation models, but this assumption is not always justified. In this study, two original methods are proposed to couple an aeroacoustic source model based on Amiet’s theory and a Split-Step Pade parabolic equation code for acoustic propagation in an inhomogeneous atmosphere. In the first method, an initial starter is obtained for each segment of the blade using the backpropagation approach. This method enables us to accurately model the directivity of the noise sources but is very computationally intensive. In the second method, the blade segments are viewed as moving monopole sources, and only a...

Effect of atmospheric stability and low level jets on wind turbine noise

Numerous studies from the literature have shown that strong wind shear occurs frequently in stable atmospheres, typically at night. This phenomenon can be associated to low level jets, characterized by a wind speed profile with a maximum at a few hundred meters above the ground. This study investigates the effect of such wind speed profiles on wind turbine noise using a physically-based model. The predictions are obtained by coupling an aeroacoustic source model based on Amiet’s theory and a parabolic equation code for acoustic propagation in an inhomogeneous atmosphere. Two important broadband noise generation mechanisms are considered, namely trailing edge noise and turbulence inflow noise, and the coupling method takes into account the fact that wind turbine blades are moving and extended sources. In order to obtain realistic wind speeds for the simulations, a simple model of nocturnal low-level jet is used, with input parameters based on measurements acquired in Cabauw observatory in the Netherlands. ...

Long range propagation of high speed train noise: Sound level variations before and after the pass‐bys

It has been observed relatively frequently that high speed train (TGV) noise could be heard tens of seconds before or after the actual train pass-bys. This study is aimed at characterizing this phenomenon and the conditions in which it occurs, both experimentally and numerically. Acoustic measurements of TGV passbys have been performed under controlled conditions. A relatively strong wind was blowing from the South during the experiment. The measurements show that the TGV noise can be heard before the pass-bys when the TGV was coming from the South (same wind and train directions), and after the pass-bys when the TGV was coming from the North (opposite wind and train directions). This noise is relatively low in frequency (around 400 Hz), and corresponds to propagation distances that can exceed 1 km. Levels associated with this phenomenon can vary significantly over short time intervals (5-10 minutes), which raises the issue of the representativeness of TGV measurements at long ranges. It will be shown using numerical prediction methods (parabolic equation in the frequency domain, linearized Euler equations in the time domain) that these acoustic variations are mostly due to variations in the meteorological conditions between the pass-bys.

Influence of short‐term variations of meteorological parameters on sound propagation outdoors

Predicting long‐range sound propagation over a nonurban site with complex propagation media requires the knowledge of micrometeorological fields in the lower part of the atmospheric boundary layer. In the framework of road traffic noise characterization there is a need for reliable sound pressure level predictions for specific propagation conditions that must be representative of time and space (small scale and site effects) characteristics of the acoustic situation. Outdoor measurements involving roughly 100 meteorological and acoustic sensors have been carried out during 3 months in 2005 in the southwest of France. This large database enables us to study the variability of meteorological parameters (wind speed, temperature, etc.) on different time scales. Scaling parameters in the atmospheric surface layer (such as the friction velocity and the Monin‐Obukhov length) are estimated every 15 min, which allows looking at relatively short‐term variations. Some longer‐term effects, from days to months, are st...