Lance L. Locey

Initial Results from the Variable Intensity Sonic Boom Propagation Database

An extensive sonic boom propagation database with low- to normal-intensity booms (overpressures of 0.08 lbf/sq ft to 2.20 lbf/sq ft) was collected for propagation code validation, and initial results and flight research techniques are presented. Several arrays of microphones were used, including a 10 m tall tower to measure shock wave directionality and the effect of height above ground on acoustic level. A sailplane was employed to measure sonic booms above and within the atmospheric turbulent boundary layer, and the sailplane was positioned to intercept the shock waves between the supersonic airplane and the ground sensors. Sailplane and ground-level sonic boom recordings were used to generate atmospheric turbulence filter functions showing excellent agreement with ground measurements. The sonic boom prediction software PCBoom4 was employed as a preflight planning tool using preflight weather data. The measured data of shock wave directionality, arrival time, and overpressure gave excellent agreement with the PCBoom4-calculated results using the measured aircraft and atmospheric data as inputs. C-weighted acoustic levels generally decreased with increasing height above the ground. A-weighted and perceived levels usually were at a minimum for a height where the elevated microphone pressure rise time history was the straightest, which is a result of incident and ground-reflected shock waves interacting.

Modeling atmospheric turbulence as a filter for sonic boom propagation

Atmospheric turbulence is known to alter the pressure waveform created by supersonic flight. It is challenging to predict how a particular sonic boom waveform will be affected by a particular atmospheric realization without performing intense atmospheric modeling calculations. This paper attempts to approximate the effects of turbulence as a linear FIR filter. Such a filter is estimated from real sonic boom data measured on the ground and at altitude.

FIR Models for Sonic Boom Propagation Through Turbulence

It is challenging to develop a physics‐based model for propagation through atmospheric turbulence. A number of models are currently available but are typically not based on experimental results. A fresh approach might be to construct some type of “black box” filter function for producing typical distorted sonic boom waveforms on the ground. The input to the filter could be arbitrary, non‐distorted sonic boom waveforms above the turbulence. As an initial model, a single input‐output system was assumed. Ground‐recorded waveforms were used as outputs of the system. The transfer function between the two signals was estimated using Welch’s average periodogram method. Data obtained from the 2004 Shaped Sonic Boom Experiment (SSBE) was analyzed and corresponding transfer functions were obtained. With such transfer functions established, FIR filters were determined and convolved with an appropriate input waveform. Such FIR filters allow for the production of “typical” atmospherically distorted waveforms given arbitrary input waveforms.

Atmospheric turbulence filter functions derived from high‐fidelity measurements

Efforts have been underway to develop filter functions suitable for adding turbulent atmospheric effects to theoretical low‐boom waveforms. Filter functions have been created based on a measured input‐output relationship. The input is a relatively clean sonic boom waveform measured at altitude by a glider, and the outputs are turbulized’ waveforms measured on the ground. One input waveform and multiple output waveforms are used to represent multiple realizations of the atmosphere. Work presented in 2005 [Locey and Sparrow, Innovations in Nonlinear Acoustics, 17th International Symposium on Nonlinear Acoustics (American Institute of Physics, Melville, NY, 2006)] yielded an initial set of filter functions using one particular algorithm and data collected during the Shaped Sonic Boom Experiment (SSBE) in January of 2004. In this talk new results will be presented based on high‐fidelity measurements made at NASA Dryden Flight Research Center in June of 2006 [T. Gabrielson et al., Proc. Internoise (2006)]. Tim...

Time-domain modeling of atmospheric turbulence effects on sonic boom propagation

A 2-D nonlinear time domain computational model of sonic boom propagation has been modified to incorporate the e!ects of atmospheric turbulence. This model, based on the Nonlinear Progressive wave Equation (NPE) of McDonald and Kuperman, is used to propagate N-waves from the upper turbulent boundary layer (TBL) to the ground through di!erent turbulence realizations. The output of the model provides detailed images of the full wave field at arbitrary heights above the ground, as well as shock profiles at specified locations along the wavefront. Preliminary results show multiple scales of spiking and rounding of shock profiles that are clearly correlated with wavefront focusing and defocusing, respectively. These waveform distortions, as well as their spatial variation along the wavefront, qualitatively match those seen in sonic booms recorded at the ground. These results are also consistent with earlier studies that applied the NPE to weak shocks with rippled wavefronts propagating through a homogeneous medium; these studies demonstrated that the combination of nonlinear and geometric e!ects arising from focusing wavefronts can produce the variety of distorted sonic boom waveforms observed in test flights. This paper describes the details of the sonic boom propagation code, including the implementation of turbulence e!ects, and discusses its performance on benchmark problems.

Sonic boom post processing to include atmospheric turbulent effects

The Nonlinear Progressive wave Equation (NPE) is modified to include a turbulence representation based upon temperature fluctuations. The turbulence is composed of the sum of 600 Fourier modes based on a modified von Karmspectrum with an outer length scale of 100 m and an inner length scale of .001 m. Variations in temperature induce variations in the local speed of sound. The NPE propagates for several seconds with a propagation time step of 1e-5 s. The resultant 2D waveform is then used to generate filter functions which approximate the eects of propagation through the turbulence. The filter functions can then be used to represent turbulent propagation by convolving the filter with any waveform, assuming it has sucient high frequency content.

Numerical simulation of sonic boom propagation through atmospheric turbulence.

To better understand the mechanisms by which atmospheric turbulence alters sonic boom rise times and peak overpressures, numerical calculations of sonic boom propagation through atmospheric turbulence have been performed using the NPE time domain model. Turbulence is incorporated into the model as a perturbation of the ambient sound speed that has a random spatial distribution determined by a von Karman energy spectrum. Each simulation employs a new realization of the turbulent field. Output from the finite difference solution includes high resolution movies showing the evolution of a full two dimensional wave field as it propagates from the upper region of the turbulent boundary layer to the ground. The evolution of wave forms corresponding to specific locations along the wave front is also obtained. Results are presented that illustrate how the magnitude, complexity, and spatial and temporal variabilities of the sonic boom wave field depend on turbulence spectrum parameters that represent atmospheric co...

Error analysis and implementation issues for energy density probe

Previous research has demonstrated the utility of acoustic energy density measurements as a means to gain a greater understanding of acoustic fields. Three spherical energy density probe designs are under development. The first probe design has three orthogonal pairs of surface mounted microphones. The second probe design utilizes a similarly sized sphere with four surface mounted microphones. The four microphones are located at the origin and unit vectors of a Cartesian coordinate system, where the origin and the tips of the three unit vectors all lie on the surface of the sphere. The third probe design consists of a similarly sized sphere, again with four surface microphones, each placed at the vertices of a regular tetrahedron. The sensing elements of all three probes are Panasonic electret microphones. The work presented here will expand on previously reported work, and address bias errors, spherical scattering effects, and practical implementation issues. [Work supported by NASA.]

Analysis and comparison of three energy density probe designs

Previous research has demonstrated the utility of acoustic energy density measurements as a means to gain a greater understanding of acoustic fields. Three energy density probes are under development. The first probe has three orthogonal pairs of microphones embedded on the surface of a sphere. The second design is a similarly sized sphere with four surface mounted microphones, equidistant from the center of the sphere. The four microphones are arranged to correspond with the origin and unit vectors of a Cartesian coordinate system, where the origin and the tips of the three unit vectors are on the surface of the sphere. As a result, all four microphones lie on the surface of the top hemisphere. The third design consists of a similarly sized sphere with four surface microphones arranged in a tetrahedron configuration. The author will discuss some of the errors and limitations associated with the four microphone designs as compared to the six microphone design. [Work supported by NASA.]

Comparison of echo criteria for a large fan‐shaped auditorium

Complaints of perceived echoes at specific locations in a 21,000 seat fan‐shaped auditorium have prompted the measurement and analysis of numerous impulse responses. The responses were first processed using the echo criterion of Niese, then using the criterion of Dietsch and Kraak. This paper compares the two criteria and explores their abilities to assess whether peaks and anomalies of the measured responses were likely to produce audible echoes in the hall. The Niese criterion was found to better predict the perception of echoes produced by broad irregular decay trends. The Dietsch and Kraak criterion was able to better predict echoes produced by sufficiently strong specular reflections. Neither criterion alone was able to fully characterize these perceptions. Results obtained for both criteria at various seat locations will be presented and compared with subjective findings.