Acoustic Pressure Waveforms Measured in High Speed Jet Noise Experiencing Nonlinear Propagation

An experimental study has been carried out in a controlled environment in a large anechoic chamber at Boeing to clarify certain features associated with the nonlinear distortion of acoustic waves. The test points were chosen to yield both subsonic and supersonic convective Mach numbers. Both spectral and time-domain analyses have been performed to elucidate nonlinear effects. The spectral analysis indicates that there is agglomeration of energy at the higher frequencies as the propagation distance increases. The time-domain analysis shows strong positive peaks in the pressure signals; the skewness values of the acoustic signals jump from ∼0.05 to ∼0.3, when the convective Mach number is increased from low subsonic values to just above unity. An examination of the Morfey-Howell nonlinear indicator reveals that energy is transferred from the spectral peak to the higher frequencies as a consequence of long-distance propagation. It is established that the convective Mach number is a critical parameter that may be used to identify the onset of nonlinear effects. When the value exceeds unity, there is a dramatic difference in the wave characteristics. In addition, a set of high-quality data has been generated that can be used to guide the development and the validation of prediction methods for nonlinear propagation.