helicopter attempted many stealth mechanisms, including attempts to quiet the rotor. One possible technique for reducing helicopter rotor noise is "modulated blade spacing". Standard rotor blades are evenly spaced, and produce greater noise at a particular frequency and its harmonics. Using varying degrees of spacing between the blades spreads the noise or acoustic signature of the rotor over a greater range of frequencies. See also Aircraft noise Aviation and the environment Acoustic quieting Aeronautical engineering BERP rotor Helicopter rotor Noise pollution Noise regulation Stealth helicopter References J. C. Hardin and S. L. Lamkin. Concepts for reduction of blade/vortex interaction noise. Journal of Aircraft, 24(2):120–125, 1987. B. Malovrh and F. Gandhi. Sensitivity of helicopter blade-vortex-interaction noise and vibration to interaction parameters. Journal of Aircraft, 42(3):685–697, May–June 2005. Helicopters Aircraft noise reduction