Abstract: There is an obvious buzz in the outdoor unit for a running air conditioning system with a fixed speed compressor, which seriously affects the sound quality of air conditioner. The noise signals of air conditioner are collected and further analyzed by a band-stop filter. It is found that the buzzing noises are beat frequency signals induced by the modulation of two similar frequencies of 174 Hz and 178 Hz. Accordingly, the excitation source has two kinds of vibrations with two different frequencies. The first frequency corresponds to the three-fold rotational frequency of the compressor, while the second one is the two-way slip frequency of the asynchronous motor. As such, the vibrations of sheet metal are amplified by the resonance of the exhaust pipe, leading to abnormal noises. Finally, the structure of the exhaust pipe is optimized by establishing constrained simulation model. The results show that the buzzing noises are eliminated, and the sound quality of the air conditioner is greatly improved.

Key words: Constant speed rotary compressor, Slip ratio, Beat frequency vibration, Constrained mode, Buzz