Investigations were made of the relation between different physical properties of a microphone in an air‐stream and the sound pressure and pressure fluctuations caused by turbulent flow. The theoretical formula for the turbulent fluctuated pressure is deduced by considering small variations of flow in a quasistationary flow tube under a low magnitude condition for percentage of turbulence. Measurements were made in a wind tunnel using condenser microphones and probe microphones of various diameters. One of the probe microphones produced an output level in good agreement with the theoretical value. Another probe microphone, which has four tiny holes located on the side wall of the tube, produced an output level 24 dB below the theoretical value. A convenient theoretical expression of the microphone output level in the airflow is obtained by assuming random incidence of the pressure fluctuation on the microphone diaphragm by an analogy with sound waves. The results obtained are in reasonable agreement with ...