Modeling specular reflections and post boom noise to augment synthesized outdoor sonic boom signatures

Open Access
Author:
Lind, Amanda Blair
Graduate Program:
Acoustics
Degree:
Master of Science
Document Type:
Master Thesis
Date of Defense:
July 14, 2010
Committee Members:
  • Victor Ward Sparrow, Thesis Advisor
Keywords:
  • synthesized sonic boom specular reflections terrai
Abstract:
In order to increase the fidelity and perceived realism of synthesized sonic booms, a model was created which superimposes specular reflections, arising from simple terrain and buildings, on the sonic boom signature that would be recorded in the free field. A finite impulse response (FIR) filter characterizing the listening environment and source/receiver orientation is generated using the Image Source Method (ISM). The generation of an FIR filter predicting specular reflections was automated for a receiver in any planar geometry: reflections from more complicated terrain and structures could be approximated by thoughtfully created planar geometries. The ray based Image Source Method (ISM), common to architectural acoustics, was adapted for outdoor applications and sonic boom excitations. Each reflection originating from a finite surface (i.e. not the ground) was high pass filtered with a roll-off determined by the dimensions of the reflecting surface. Accurately accounting for reflections from the ground and a single structure was shown to dramatically alter subjective loudness metrics. Simulations over a variety of listener positions and incident angles demonstrated that constructive and destructive interference due to inclusion of specular reflections varies the PLdB by −11 to +3 dB. a Post boom noise (PBN), the 1.5 seconds of rumble following the tail shock of a sonic boom, is necessary for synthesized and recorded sonic booms to be perceived as realistic in listening tests. In flat landscapes with a single structure, it was found that simplifying the scattering problem and calculating specular reflections from rigid sources is an insufficient approach to modeling PBN. However, using a data-set known as HouseVIBES 2007, in which sonic boom records were acquired by a 10 meter, 10 element vertical array, ground reflections and potential sources of post boom noise were explored. Analysis of the data set indicated that scaling ground reflected sonic booms with a plane wave reflection coefficient would underestimate the reflected pressure at low frequencies. By contrasting the PBN at the ground with that above the turbulent boundary layer, it was shown that engine and jet noise were not dominant sources of PBN.