Aircraft sound propagation near to the ground: Measurements and calculations

In the framework of acoustic and seismic monitoring of airports for verifyi ng disarmament or peace-keeping agreements the propagation of sound emitted by aircraft close to the ground was investigated. The ground effect due to the porous, grass-covered ground is maximum at around 100 Hz to 200 Hz. Th e attenuation reaches up to c. 20 dB for a distance of 100 m. In addition, the microphones located close to the ground (h = 0.3 m) show a significant attenuation over a large frequency range above 200 Hz. By means of an experimentally determined surface impedance propagation calc ulations were carried out. Transfer functions were calculated which show th e frequency dependent attenuation due to the porous ground for a given geom etry. Using reference signals measured close to the sound source spectra we re calculated corresponding to locations of microphones above the grass-cov ered ground. The calculated spectra were compared with the measured ones. I n the case of transfer function measurements with white noise over a range of 20 m, the calculated spectra agree very well with the actually measured ones. In the case of the jet aircraft, the simulated spectra fit to those o f the measured signals only in general appearance of amplitude, and not in phase. This is probably mainly due to an inaccurate localisation of the sou nd source, its extension, and directivity.