Aircraft sound levels measured at specific monitoring locations may depend on microphone height. Well-controlled measurements at 1.2 m and at 10 m above porous ground illustrate how the low frequency components of the spectrum vary with microphone height. The overall influence of microphone height on the A-weighted levels is small for aircraft with jet engines in a flyover situation and for distances shorter than 1000 m. However, the microphone at 1.2 m measures lower levels than a higher microphone in cases where the measurement site is far to the side of the flight path i.e. at low elevation angles of sound incidence, and when the measured spectrum is dominated by low frequencies, as this is often the case with propeller driven aircraft and propagation over long distances.