Specific features of military low-altitude flight noise: criteria for risk of damage and physiological effects.

The impact of sound from military low-altitude flying (75 m) is characterized by high maximum levels of up to 125 dB, a rapid rise in sound pressure level (steepest 10-dB slope up to 111 dB/s, mean value 36 dB/s), and occasional (30%) follow-up oscillations of up to around 100 dB. The energy within this broadband sound is very often concentrated primarily in the 0.8 to 4 kHz range. A criterion for the risk of damage can be applied using constant-frequency spectral dose analysis in order to establish, for example, how many overflight events might be regarded as tolerable. We can also draw on findings from animal experiments revealing damage to the extremely sensitive stereocilia of the hair cells in the inner ear to derive a risk assessment which shows that slight impairment to hearing is possible in low-altitude flight zones given either the recorded mean frequency of 17 direct overflights a day (with maximum levels over 100 dB) or else a few extreme isolated events. In laboratory experiments involving exposure to this type of highly dynamic low-altitude flight noise, short-term increases in heart rate (up to 21 beats/min) were found to be much greater than those provoked by other forms of environmental noise with comparable maximum levels (pile driving, gunfire).