Initial Results from the Variable Intensity Sonic Boom Propagation Database

Abstract

An extensive sonic boom propagation database with low-to normal-intensity booms (overpressures of 0.08 lbf/ft 2 to 2.20 lbf/ft 2 ) 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 groundreflected shock waves interacting. = uncorrected acoustic level of the sonic boom, dB re 20 µPa dB c = corrected acoustic level, dB re 20 µPa dB n = uncorrected acoustic level of noise 1 s before the sonic boom, dB re 20 µPa el B = shock wave propagation elevation angle at the BADS toward the source, deg above horizontal el s = shock wave propagation elevation angle at the sailplane toward the source, deg above horizontal el t = shock wave propagation elevation angle at the tower toward the source, deg above horizontal N # , E # , D # = north, east, and downward locations of microphones, m or ft R air = specific gas constant of air T = temperature t = time, s after midnight UTC tac s = time the shock wave left the F-18 airplane for the sailplane, from PCBoom4, s after midnight UTC tac t = time the shock wave left the F-18 airplane for the tower, from PCBoom4, s after midnight UTC tg s = time the bow shock wave hit the sailplane, from PCBoom4, s after midnight UTC tg t = time the bow shock wave hit the tower, from PCBoom4, s after midnight UTC t bow = time the bow shock is measured, s after midnight UTC t L23 = measured IRIG-B time on the sailplane, s after midnight UTC t so = stationary observer time, s after midnight UTC V N , V E , V D = shock wavefront ground-relative velocities in the north, east, and downward directions, ft/s Nomenclature

Mach Cutoff Analysis and Results from NASA's Farfield Investigation of No-boom Thresholds

“…The calibration of the airplane production air data system in the supersonic region has known errors on the order of 0.045 Mach, which can result in up to a 1.7 nmi error in sonic boom location. 10 For the range of Mach numbers flown during FaINT, this error has been shown to be approximately +0.04 Mach, 11 and has been corrected appropriately in the data presented here.…”

A Flight Research Overview of WSPR, a Pilot Project for Sonic Boom Community Response

“…In 2007, the NASA House Variable Intensity Boom Effect on Structures (House VIBES) project performed a similar comparison between microphone data and PCBoom predictions. 38 In that study, nearly 90 percent of the microphone data were within ±0.20 lb/ft 2 of the PCBoom predictions; however, House VIBES compared "idealized maximum overpressures," which is the theoretical overpressure of a sonic boom without the effect of atmospheric irregularities and aircraft influences such as atmospheric perturbations, turbulence, and variations in flight parameters. Such an approach for the WSPR test resulted in inconsistent, unreliable results.…”

SCAMP: Application of PCBoom to Measured Sonic Boom Focus Analysis

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