The exponential decay of underwater acoustic intensity with increasing altitude of low-frequency sound from a Robinson R44 helicopter

A series of underwater acoustic experiments utilizing a Robinson R44 helicopter and an underwater receiver station has been conducted in shallow (16 m) and deep (>100 m) water. The receiver station consisted of an 11-element nested hydrophone array with a 12 m aperture. In the shallow water experiments the array was configured as a horizontal line (HLA) 0.5m above the seabed; whereas in deep water the array was suspended from the surface in a vertical line (VLA). An in-air microphone was located immediately above the surface. In this paper the power spectral density as a function of helicopter altitude will be reported from measurements on a single hydrophone. The main rotor blades of the helicopter produce low-frequency harmonics, the lowest frequency being ~13 Hz. The tail rotor produces a sequence of harmonics approximately six times higher in frequency. Between heights of 30 m to 600 m above the sea surface the underwater intensity was found to decay exponentially with increasing helicopter altitude. ...