Microphone Measurements In and Out of Airstream

Abstract

The wind tunnel has become an important research facility for the study of aircraft and automobile noise. In this chapter, the acoustic characteristics of wind tunnels are discussed along with methods for conducting research in such an environment. Microphone measurements require low background noise and minimal reflections for accurate results. Typical sources of wind tunnel background noise are described including noise from the wind tunnel components, apparatus support struts, and microphones installed in the flow. In most cases, proper design of wind tunnel compnents and test apparatus are critical to successful aeroacoustic measurements. And, it is often necessary to add silencers and acoustic treatmet to the facility. Criteria for proper simulation of aeracoustic phenimena are discussed along with necessary data manipulations to correct for propagation effects, scaling to the correct source size, and extrapolating to the desired flight or drive-by situation. Finally, current mothods are discussed for identification and analysis of noise sources using advanced signal analysis techniques.