Investigation of sonic boom waveforms and metrics at elevated receiver locations

Abstract

Microphones used for aircraft noise measurements are sometimes positioned above local ground level, for example to accommodate environmental protection or to comply with certification regulations. Acoustic waves reflected from the ground can interfere with waves propagating directly to elevated receivers. A modeling study was conducted of sonic boom waveforms received at ground level versus at elevated positions, using source characteristics for an early X-59 low-boom aircraft configuration (C609) and a variety of atmospheric conditions. Ground impedance effects from different ground cover types were also considered. Among several acoustic metrics, it was found that perceived level of loudness (PL) and A-weighted sound exposure level were in general the most sensitive to differences due to receiver position. Location within the boom carpet also affected receiver levels based on the angle between the incoming ray and the ground: the difference between metrics at ground and elevated receivers was generally higher at undertrack locations where that angle was largest. Depending on atmospheric conditions and receiver location within the carpet, predicted PL at an elevated receiver was lower than the level at the ground by up to 5.0 dB for a receiver height of 4 ft and 2.2 dB lower for a 1.6-ft receiver.