Case history: Noise control approaches for an air-compressor in a fuel-cell auxiliary power unit

This case history examined the application of passive noise control measures to a fuel-cell air compressor. The air compressor was one of the dominant noise sources in a prototype auxiliary power unit developed for heavy-duty truck applications. The objective was to experimentally evaluate and compare the effectiveness of several practical noise control approaches. A test system was developed so that the supply voltage and the compressor output flow conditions could be simultaneously adjusted. By this approach, a wide array of compressor operating conditions was investigated. Specific noise control approaches included wrapping with fiberglass insulation, surrounding the compressor with an enclosure containing sound-absorbing material on the interior surfaces, identifying and isolating sources of sound on the suction or low-pressure side of the compressor, applying viscoelastic damping material to parts of the structure, and installing a muffler on the compressor suction inlet. The design of reactive and dissipative mufflers for suppressing flow noise is discussed. The experimental results demonstrated that by implementing the selected noise-control approaches the A-weighted sound level at a representative location could be reduced by up to 4 dB with a reduction of up to 10 dB in the A-weighted one-third-octave-band sound pressure level in the 800-Hz frequency band containing the principal tonal component of the spectrum.