Identification Of Noise Sources On Air-conditioner Unit And Sound Power Measurement With Air-flow

Identification of noise sources is the premise of air conditioner designoptimization to improve its sound quality. The first work of this paper isaimed at identification of noise sources on the indoor unit of an airconditioner. The noise generated from air conditioner is always the mix ofseveral simple noise sources, such as aerodynamic noise, mechanical vibrationnoise, and electromagnetic noise. Acoustic experiments are performed insemi-anechoic room to measure the sound pressure level under differentoperation conditions. First, the property of symmetry and propogation ofsound field are well studied to gain a general understanding. Then a series ofcomparison measurements are made in order to distinguish each noise source.Results show that there are three main noise sources, blade passagefrequency(BPF) noise, which is generated for the periodic rotation of bladesand its sound poressure level increases as the rotation speed of fan. Secondly,the structure vibration noise is identified which peaks in the region of about100Hz. It is due to changing magnetic forces at twice the power frequency.The third is urbulence broad band noise, which is maily concentrated at therange between 100 to 500Hz.The effect of serrated trailing edge to reducenoise is also presented.It is found that the noise-reducing effect is mostsignificant at the range of high frequency,especially above 1000Hz. However,it has a minor effect on low frequency noise.Another part of work is the investigation of sound power determinationwhen the noise source is an air-moving device. The turbulence fluctuationpressure will impose a non-acoustic sigal on microphones. Two measuementmethods are applied in air flow condition to determine sound power level andits frequency spectrum. one is semi-anechoic room precision method(SARPM)and the other is sound intensity scanning method(SISM). In this experiment,enclosure surfaces of different sizes are selected to measure a sameair-moving equipment. By comparison, SARPM is more steady than SISM.Further experiment is performed to measure the sound pressure, soundintensity and air-flow velocity simultaneously. It can be found that soundintensity is more easily affected by airflow. As the airflow velocityincreases,the affected range is extended to higher frequency range. Resultsalso indicate that SISM is more sensitive to air flow than SARPM. As the flowvelocity is higher than 2m/s, the sound intensiy result will be much moreaffected. To suppress the disturbance of air flow with SISM ,it is advicable toproperly avoid the high wind region or choose a large enough enclosuresurface to make sure the normal airspeed lower than a certain critical velocity.