Turbofan and turbojet engines are major sources of intense aircraft noise. Although turboprop-powered aircraft also are used by the . Air Force, their contribution to the overall noise environment is of relatively minor importance compared to jet-powered aircraft. Jet engines are generally more powerful and produce noise of higher magnitude than turboprop or piston aircraft engines. Also, jet engines produce a greater amount of noise in the high-frequency range, thus increasing their relative annoyance factor.