Some observations from measuring the inserted-board airflow inlet distortion dynamic pressure field were presented in this paper.When the inserted-board rose above 35%

the inlet airflow disturbance occurred at about 32Hz.The pressure wave equation calculation shows that the Phase1 syntonic frequency is 34Hz.This syntony lies between the engine and inserted-board.Its frequency is steady at different rotor speeds and fluxes.The follow-on analysis indicates that the low frequency pulse pressure of component below 150Hz is powerful

steady and seasonal.The high frequency component is mainly stochastic pressure pulsation with minimum power.The changes of its rate and acceleration are great.When the engine is about to surge

the power of the syntonic frequency increases greatly while the power of other low frequency components and high frequency components decreases rapidly

so a typical syntonic pressure oscillation is formed.