Investigations were made to find a physical concept for evaluating the pressure level of sound that exists in an airflow by means of a wind screen or probe microphone. In order to provide a sound pressure in the airflow, sound of a known pressure level, which is assumed to be of higher level than the aerodynamically generated noise, was radiated into a duct from an external sound source. The output level of the microphone was measured for various differences between the sound‐pressure level produced by the external sound source and the pressure fluctuation level of turbulence in the airflow. Results obtained agree with the superposed level of output by sound pressure and output produced by pressure fluctuation of turbulence. A new concept for characterizing microphone response to wind‐flow fluctuation is presented. This characteristic is defined by the difference between the magnitude of the pressure fluctuation of turbulence and the output level of the microphone produced by the turbulence‐fluctuated pre...