Systems for simulation of aircraft-propeller noise in the laboratory have been developed at LTV Vought Aeronautics. The basic unit of these systems is an electronic propeller-noise generator that was designed and built as a Company-sponsored R and D program. This unit produces a voltage signal proportional to the rotational and vortex sound pressures generated by aircraft propellers. The rotational noise is made up of the fundamental 4P blade-passage frequency plus several significant harmonics. Adjustments are possible on frequency, amplitude, and relative phase of these discrete-frequency components. Vortex noise is random in character and adjustments of over-all level, spectrum shape, and frequency-band limits are possible. In a final stage, the rotational- and vortex-noise components are mixed to produce simulation of the complete propeller-noise signal. This voltage signal is used as an input to a high-intensity progressive-wave sound system. In this system, sound-pressure levels of 150–170 dB are generated for testing of structural panels. Another use for this signal is as an input to a high-fidelity speaker system that operates at SPL's of 70 to 125 dB. This system provides simulated propeller noise for interior-fuselage acoustic investigations and human-factors studies. Equalization of either system operating into various impedances of different panels and enclosures can be accomplished by appropriate adjustment of noise components produced by the propeller-noise generator. Typical results are shown in slides illustrating measured waveforms and spectra. These results are compared to propeller noise measured from aircraft propellers.
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