Helicopter flight noise tests about the influence of rotor-rotational and forward speed changes on the characteristics of the immitted sound

The noise characteristics of three modern medium-weight twin-turbo engine helicopters were determined using flight tests which collected straight-level overflight noise data. The data were analyzed spectrally and in terms of several commonly applied noise-metrics, such as the maximum. A-weighted Sound Pressure Level L sub A and the maximum Overall Sound Pressure Level OASPL. The results indicate the decisive effect of the advancing blade tip Mach-number on the emitted sound, which shows the growing influence of high-speed impulsive noise components on the noise signature at blade Mach-numbers beyond approximately 0.8. It is found that both the maximum tone-corrected and the Effective Perceived Noise Level observed on the ground for a helicopter in horizontal overflight depend strongly on the forward flight and the main rotor rotational speed.