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Measurements were made in order to evaluate the pressure level of sound generated by a grille in an airflow. A streamlined probe microphone, which is covered by a solidified glass fiber of high density, was used for detection of noise existing in the airflow. Distributions in a flow duct of the turbulence-fluctuated pressure of the airflow and of the output level of the microphone were measured and then compared with sound transmission characteristics specified in the duct. It was found that the distribution of the microphone output agrees with the sound transmission characteristics, but not with the fluctuating pressure of the airflow. As a result, the levels of the sound pressure generated by a grille were about 50 dB (re 2 × 10-4 μbar) at frequencies greater than 200 Hz, with a maximum value of 65 dB at 1250 Hz, for a flow velocity of 8.4 m/sec. It was also found that the output of the probe microphone produced by the turbulence-fluctuated pressure of the airflow can be reduced to 40 dB below the turbulent pressure level by a covering of this type. [ABSTRACT FROM AUTHOR]

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 pressure, if aerodynamically generated noise is negligible. It provides a lower limit of sound pressure which can be detected by a microphone in an airflow. Moreover, the detection of noise generated by a grille in a duct is discussed by taking account of the characteristics of the microphone response to wind-flow fluctuation. [ABSTRACT FROM AUTHOR]

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 the experimental values. [ABSTRACT FROM AUTHOR]