Assessment of Noise Level Variations of Aircraft Flyovers Using Acoustic Arrays

Variability in noise levels for flyovers of the same aircraft type can be as large as 12dB, hampering noise assessment around airports. The variable atmosphere (affecting the acoustic propagation) and variations in the aircraft emitted noise are considered as the two main contributors to the noise level variability. This paper presents two experiments aimed at quantifying these contributions. First, the atmospheric contribution was determined with a loudspeaker (100m height) sending signals to microphones on the ground, indicating a sound level variability of less than 2dB. Second, noise levels from Boeing 737 flyovers (landings) were measured with an acoustic camera. The observed noise level variability was 6-8dB. The acoustic camera imaging capabilities eliminated variations due to ground reflections and ambient noise, and identified the turbofan engines as the dominant noise source. Assuming the two contributions to be independent statistical processes, with the atmosphere contributing 2dB maximally, it is concluded that the total noise level variability (6-8dB) as measured for the flyovers was entirely due to the source. Correlating the engine noise levels to the fan rotational speed (from the spectrograms) shows that variations in engine setting explain over 70% of the observed total noise level variation.