Blade-Vortex Interaction noise mainly occurs during low speed descending flight, low speed
turning descents and during some maneuvering flights [14]. These conditions are conducive to the
rotor blade passing close to a vortex that is trailed from previous blades (Fig 1.4). The induced
velocity of each tip vortex essentially induces a sharp change in effective angle of attack observed
by the interacting blade, leading to a sharp change in surface pressure. This rapid pressure change
radiates out in the form of the impulsive noise known as Blade-Vortex Interaction or BVI noise.
BVI noise is known to be highly directional and quite sensitive to flight conditions. The fact
that most of the acoustic energy is concentrated in the frequency range from 75Hz to 1500Hz (5
rotor harmonics to 100 rotor harmonics) makes it quite annoying as the human ear is very sensitive
to sound at the higher end of the frequency range. The noise generation itself is a complex aeroa
coustic phenomenon and depends on a variety of factors encompassing rotor design, operational