Investigating and quantifying the ground effect using spectrograms of realistic aircraft flyover measurements

This paper proposes a measurement-based methodology for investigating the ground effect observed in real-life aircraft flyover measurements in low-noise regions. An acoustic array is used for obtaining a spectrogram of the sound with ground reflections to a large extent eliminated. By applying a model to account for the ground effect, a semi-synthetic spectrogram is obtained. This spectrogram is then qualitatively compared with the spectrogram obtained from single microphone measurements (Munisense) at two heights to verify whether indeed the semi-synthetic spectrogram agrees with those measured by corresponding results. As a next step, the ground effect for the flyovers measured is quantified by using the array-recorded signals together with the model for the ground effect for all flyovers. By correcting Munisense signals for the estimated ground effect, improved agreement between noise levels at both receiver heights and the array measurements is obtained. Noise differences are evaluated using the proposed alternative metric that only accounts for levels within 4 s duration. For flyovers with higher elevation angles, a greater reduction in noise, up to 2.3 dBA, is observed, despite the larger slant distances. In contrast, flyovers at lower elevation angles show a smaller reduction of approximately 1.8 dBA. This contrasts with the common corrections applied for lateral attenuation in best-practice modeling where the attenuation (accounting for more than only the ground effect) diminishes at higher elevation angles.