Optimizing the Sound Quality of Air Cooled Engine Fans Using Synthesized Noise Sources 951313

The cooling fans for small air cooled engines can be an important contributor to overall noise levels. Frequently, the noise spectrums contain strong pure tone components which degrade sound quality. For fifteen years, Briggs & Stratton has been spacing the fan blades unevenly to reduce the pure tone noise caused by equally spaced fan blades. This phase modulation technique has been very successful, however the modulated fin spacing produces an inherently unbalanced part and it is desirable to minimize the amount of modulation required. Unfortunately, building experimental fans with varying amounts of modulation is expensive and time consuming.
This paper describes a system for electronically simulating fan noise to be used for subjective testing. A multifunction synthesizer was used to generate signals with varying amounts of phase modulation. The modulated signals were mixed with filtered pink noise to closely approximate the total fan noise. Later, a software package which can produce arbitrary waveforms was used to produce a signal with harmonic content closely matching that produced by a real fan.
The results of subjective jury tests showing the ability to mask phase modulated sine waves with shaped pink noise is given.
Due to manufacturing constraints, it may not be possible to completely mask the tonal portion of fan noise. Results are presented for a second series of tests where jurors evaluated varying amounts of phase modulation when the tonal noise is not masked by the random noise signal.


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