Ground effects in time‐domain simulations of outdoor sound propagation.
Finite‐difference, time‐domain (FDTD) methods are precise and powerful tools to solve linearized Euler equations. Interaction between the acoustic waves with local wind or temperature gradients as well as a complex topography can be taken into account. Recently, a time‐domain boundary condition has been proposed [Cotte et al., AIAA J. 47, 2391–2403 (2009)] and has been implemented in a FDTD solver using methods developed in the computational aeroacoustics community [Bogey and Bailly, J. Comput. Phys. 194, 194–214 (2004)]. Propagation of an initial pulse over a distance of 500 m in a two‐dimensional geometry is considered in a frequency band up to 1200 Hz. Surface waves which propagate close to and parallel to impedance grounds are exhibited. To validate the obtained results, a comparison is made in the time‐domain with an analytical solution. Ground effects are discussed depending on the impedance of the ground. [This work was performed using HPC resources from GENCI‐IDRIS (Grant 2009‐022203).]
Observations of acoustic surface waves in outdoor sound propagation.
Acoustic surface waves have been detected propagating outdoors under natural conditions. Two critical experimental conditions were employed to ensure the conclusive detection of these waves. First, acoustic pulses rather than a continuous wave source allowed an examination of the waveform shape and avoided the masking of wave arrivals. Second, a snow cover provided favorable ground impedance conditions for surface waves to exist. The acoustic pulses were generated by blank pistol shots fired 1 m above the snow. The resultant waveforms were measured using a vertical array of six microphones located 60 m away from the source at heights between 0.1 and 4.75 m. A strong, low frequency "tail" following the initial arrival was recorded near the snow surface. This tail, and its exponential decay with height (z) above the surface (approximately e(-alpha z)), are diagnostic features of surface waves. The measured attenuation coefficient alpha was 0.28 m(-1). The identification of the surface wave is confirmed by comparing the measured waveforms with waveforms predicted by the theoretical evaluation of the explicit surface wave pole term using residue theory.
Surface waves and leaky waves above a porous layer
Abstract In porous layers saturated by air, the vibrations of the frame induced by an acoustic field in air can often be neglected, due to the large density of the frame. To a good approximation the frame can be considered motionless. The air inside the layer can be replaced by an equivalent fluid in which acoustic waves propagate more slowly than in free air. Similarly, electromagnetic waves propagate more slowly in dielectrics than in vacuum. The close analogy which exists between TM electromagnetic surface waves above grounded dielectrics and acoustic surface waves above a porous layer on a rigid impervious backing is displayed in this work. The velocity of air inside the layer close to the backing is parallel to this impervious and motionless surface, and the electric field close to the perfectly conducting backing is perpendicular to the plane. The correspondence relates perpendicular components of acoustic velocity and electric field. The losses in the equivalent fluid are neglected in a first approach to adapt the description of electromagnetic surface waves for acoustics. Subsequently the damping is taken into account.