Evaluation of noise emissions of a ducted fan on a test rig

Abstract

This paper presents an experimental research study on the sound excitation of a ducted fan designed for low-noise emissions. In particular, the influence of test rig vibrations as a significant systematic error when evaluating the noise emissions of ducted fans is discussed. A propulsor test rig was used to measure the sound emitted by a ducted fan. Significantly increased sound levels are ascertained at specific frequencies. The corresponding sources are often unexpectedly identified at the casing and structure of the rig using a sound source localization. By comparing sound pressures against vibrations, these sources are found to be excited by simultaneous test rig vibrations. When the rig vibrates, the sound pressure levels increase by up to 15 dB compared to vibration-free operation. The sound excitation caused by the rig vibrations predominates over the aerodynamically induced sound since the ducted fan itself emits little noise. In summary, these rig-dependent vibrations can lead to a considerable systematic measurement error for low-noise ducted fans. This error significantly complicates an accurate sound evaluation of such a ducted fan on an acoustic rig. Therefore, these results emphasize the importance of a vibration-free operating range and the necessity to quantify the sound excitation by rig-specific vibrations.