Instrumentation Considerations for Accurate Jet Noise Measurements

Accurate measurements of jet noise spectra at model scale over a wide range of frequencies critically depend on the quality of the jet rig as well as the measurement system. The many aspects of the instrumentation system that require careful consideration to ensure the acquisition of good data are discussed. The proper choice of the microphones and their orientation, the dynamic range requirements, and the design of the electronics for minimizing the magnitude of the corrections that must be applied to the raw spectra are among the many issues pertaining to jet noise measurements investigated. The reasons for the upturns in the jet noise spectra at the higher frequencies in recent tests are examined to identify the contributing factors, and some recommendations are provided to avoid this problem. Spectral measurements of simple point sources, simple distributed sources, and model scale jets with normal and grazing incidence microphones on traversing arrays, as well as on fixed poles at different distances from the sources, are used to evaluate these factors. It is also established that a distance of ∼35 nozzle diameters ensures measurement in the true far field for jet noise.