No abstract

a larry.j.cliatt@nasa.gov b michael.a.hill-1@nasa.gov c edward.a.haering@nasa.gov d sarah.arnac@nasa.gov Abstract. In support of the ongoing effort by the National Aeronautics and Space Administration (NASA) to bring supersonic commercial travel to the public, NASA, in partnership with other industry organizations, conducted a flight research experiment to analyze acoustic propagation at the lateral edge of the sonic boom carpet. The name of the effort was the Farfield Investigation of No-boom Thresholds (FaINT). The research from FaINT determined an appropriate metric for sonic boom waveforms in the transition and shadow zones called Perceived Sound Exposure Level, established a value of 65 dB as a limit for the acoustic lateral extent of a sonic boom's noise region, analyzed change in sonic boom levels near lateral cutoff, and compared between real sonic boom measurements and numerical predictions.FIGURE 1. Lateral cutoff and transition region. 1

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

Successful execution of the flight phase of the Superboom Caustic Analysis and Measurement Project (SCAMP) required accurate placement of focused sonic booms on an array of prepositioned ground sensors. While the array was spread over a 10,000-ft-long area, this is a relatively small region when considering the speed of a supersonic aircraft and sonic boom ray path variability due to shifting atmospheric conditions and aircraft trajectories. Another requirement of the project was to determine the proper position for a microphone-equipped motorized glider to intercept the sonic boom caustic, adding critical timing to the constraints. Variability in several inputs to these calculations caused some shifts of the focus away from the optimal location. Reports of the sonic booms heard by persons positioned amongst the array were used to shift the focus closer to the optimal location for subsequent passes. This paper describes the methods and computations used to place the focused sonic boom on the SCAMP array and gives recommendations for their accurate placement by future quiet supersonic aircraft. For the SCAMP flights, 67% of the foci were placed on the ground array with measured positions within a few thousand feet of computed positions. Among those foci with large caustic elevation angles, 96% of foci were placed on the array, and measured positions were within a few hundred feet of computed positions. The motorized glider captured sonic booms on 59% of the passes when the instrumentation was operating properly.

No abstract