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This work is based on a current project related to auxiliary power unit (APU) exhaust silencing technology development, specifically an active exhaust noise cancellation technology. The overriding technical challenge is identified as development of a frequency and amplitude controllable noise source that can survive in a hot and corrosive exhaust environment. Installation of an APU enables vehicle platforms to turn off the main engine and still utilize auxiliary equipment that requires electrical power. An APU with a small engine potentially consumes much less fuel than a main engine at low power levels. However, past APU integrations have resulted in unacceptable noise levels, often times louder than the main engine at the equivalent operating point. Exhaust tones created by combustion pose a unique challenge to external acoustic reduction because they generally occur at low frequency, often less than 100 Hz.