Tactical speed is the maximum speed of a submarine at which it is possible to remain undetected, whilst still having the capability of detecting other ships. Tactical speed is limited by the acoustic noise generated by the ship itself, which affects the ability to detect external sources of sound. One of the main contributors to internal sources of noise is the propulsion system. Tactical speed for modern submarines with nuclear propulsion is at the level of 8–13 knots, but the newest ships of IV generation can achieve tactical speeds over 20 knots.