Assessment of aircraft noise sources variability using an acoustic camera

The continuous increase of air traffic and stricter noise regulations have caused that aircraft noise is one of the main current issues which the industry has to currently deal with. In fact, noise restrictions are nowadays the limiting factor for the increasing capacity of many airports. In order to reduce the produced noise levels, it is essential to know which elements (engines, airframe, etc.) of the aircraft contribute to it and their relative importance. Aircraft noise is a function of many parameters such as the aircraft type, the engine settings, aircraft speed and the flight configuration. Therefore, this research consisted in the measurement of 115 flyovers in the Amsterdam Airport Schiphol using a low cost, high resolution acoustic array of 32 microphones, developed in TU Delft. The scope is limited to commercial jet aircraft during the final approach stage due to the lower engine noise levels in this stage, which allows a better distinction of the airframe noise. Moreover, the trajectories tend to be more uniform in the final approach stage than during the take-off. An accurate trajectory estimation is crucial in order to account for the Doppler effect and atmospheric propagation effects. Therefore, a combination of ADS-B (Automatic Dependent Surveillance-Broadcast) data, ground radar data and optical camera measurements was employed for the calculation of the aircraft trajectory, showing a close agreement between them. Once that the flight paths had been calculated, a detailed study of the variation of the noise levels for single aircraft types and engine settings was performed. From the spectrograms, the engine settings can be derived by estimating the Blade Passing Frequency (BPF). Three different methods were employed and it was found that both the tonal and broadband noise levels increase with increasing power settings. Airspeed, however, did not have such a strong influence on the emitted noise, increasing just slightly the noise levels. Finally, different beamforming algorithms (such as the conventional frequency domain) were applied to the measurements in order to better evaluate the location of the noise sources and it was found that the airframe noise is often comparable to the engine noise during the landing. All in all, it was confirmed that acoustic arrays are very useful tools for evaluating noise sources and that airframe noise is getting more important, as engines keep reducing their noise levels.