Stealth Technology - Acoustics

Acoustics

See also: Aircraft noise and Helicopter noise reduction

Acoustic stealth plays a primary role in submarine stealth as well as for ground vehicles. Submarines use extensive rubber mountings to isolate and avoid mechanical noises that could reveal locations to underwater passive sonar arrays.

Early stealth observation aircraft used slow-turning propellers to avoid being heard by enemy troops below. Stealth aircraft that stay subsonic can avoid being tracked by sonic boom. The presence of supersonic and jet-powered stealth aircraft such as the SR-71 Blackbird indicates that acoustic signature is not always a major driver in aircraft design, although the Blackbird relied more on its extremely high speed and altitude.

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.