"Cross Flow Fan" noise generation, noise is an unwanted people, unpleasant, disrupting the hearing, the destruction quiet voice. Vibrating object had people unwanted noise. Gas is generally noise propagation medium, so that the ear can hear. Sound waves from the sound source departure spread in the air, the air has a quality and flexibility. Fluctuations in the form of acoustic propagation in the medium is just a vibration of the medium itself only in situ vibration, no forward movement. When the acoustic wave propagation, with increasing distance of propagation, the acoustic energy decreases, resulting in attenuation. The main reason is to absorb sound wave attenuation and diffusion of sound waves. When sound waves radiated in all directions from the sound source, the area in front of the wave propagation with increasing distance growing, so the sound per unit area can be reduced. This is the acoustic attenuation caused by diffusion.