Combustion Noise of Auxiliary Power Units

Noise from auxiliary power units (APU) is an important contributor to the overall level of ramp noise. Currently, ramp noise is regulated by international governing bodies as well as by individual airport. A significant component of APU noise is combustion noise. In this study, the unique spectral shape of APU combustion noise is identified. It is found that the spectral shape is the same regardless of engine size, power setting and directivity. Also, it is practically the same as that of open flame combustion noise. The frequency at the peak of the combustion noise spectrum is found to lie in the narrow range between 250 to 350 Hz. The peak sound pressure level of a given APU varies as the square of the fuel consumption rate. In the literature, suggestions have been made concerning a second combustion noise mechanism arising from the passage of hot entropy spots through the exhaust nozzle or constriction. In this investigation, no evidence has been found to indicate the existence of a second APU combustion noise component.

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