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).]
A theoretical treatment of the long range propagation of impulsive signals under strongly ducted nocturnal conditions.
On clear nights, over flat land, a sound duct develops in which sound can carry to great distances. As is the case with all ducted propagation, there is strong dispersion so that a broadband signal undergoes severe distortion as it propagates. The signal received at long ranges from an impulsive source is a wave train, of much greater duration than the initial impulse, consisting of a series of arrivals. The long range ground to ground propagation of an impulsive signal in a typical nocturnal duct is studied and the natures of the various arrivals are explained. A direct connection is drawn between the meteorological and ground conditions and the structure of the propagated signal.
Pedagogy of the “Not”: Negation, Exodus, and Postdigital Temporal Regimes
Postdigital capitalist time is an incessant acceleration that acts to homogenize time and wed us to the present, to which we have to constantly catch up. While the impulse of this is no doubt economic (the realization of value), it is crucially undergirded by a pedagogical logic wherein we have to perpetually learn and re-learn the latest apps, social media configurations, operating systems, and so on. Political strategies of resistance thus need to be bolstered by an alternative mode of educational life, and I propose a pedagogy of the “not” as one possibility. Such a pedagogy is an act of suspension that sustains a detachment from the present, clearing out oppositions and thereby exposing us to a radical indeterminacy and potentiality that is always untimely. Suspension is the praxis of negation, which means that negation operates by keeping sense indeterminate to meaning and signification. Rather than suppressing, disavowing, or annihilating the stated content, negation retains even that which is negated. While this would appear as a form of exopedagogy, which withdraws from the dialectic of the private/public, I instead argue that it redefines the terms of any dialectic, redefining the very categories exopedagogy withdraws from. Before concluding, I spend some time with Sandy Grande’s important critiques of Eurocentrism and progress in Western critical education, demonstrating how negation as suspension circumvents these errors through its accommodation of—or, better, insistence on—variegated temporalities and forms of life.