besides the tuyere noise, the body noise and solid-state sound transmission of vibration of the fan should also be paid attention to according to the field situation. Because the axial flow fan makes a lot of noise when working, before noise control is usually carried out, we should first analyze the noise characteristics of the oil fume exhaust fan. The revolutions of this kind of fan are usually 711～1 111 r/ min, and the frequency characteristics are obviously low if, and the noise is usually 64 ~ 85 dB, with the air outlet as the maximum. After the air outlet is connected to the pipeline, the noise usually drops to 61 ~ 71 dB. This greatly reduces the impact on the environment. However, when the environmental requirements are strict or other special circumstances, it still can't meet the environmental requirements. The Lp2415 A centrifugal fan we encountered has a speed of n = 961 r/ min, a flow rate of Q = 15 811 m/ h, and a power of 4 kW·. There is no access pipe at the air outlet (it can't be discharged at high altitude due to site restrictions, and the imported electrostatic range hood is used for smoke removal and purification). The noise of the fan is relatively high. The measured noise is 83dB at a distance of 1 m from the fan, and the frequency characteristics are low-if characteristics, as shown in Figure 1. The measured noise is 66dB at a distance of 1 m from the residential window about 21 m from the fan, which exceeds the daytime environmental standards of Class II areas. According to the attenuation characteristics of the noise, the low-if noise attenuates slowly, and if it is attenuated according to the distance from the point sound source, it should be 21 lg21 = 26 dB. However, the actual attenuation is 17 dB, with a difference of 9 dB·. Therefore, the impact of low-frequency noise fans on the surrounding environment is greater than that of medium-high frequency noise fans. In addition, the sound insulation and noise elimination equipment is a weak link for low-frequency noise, that is, the sound insulation and noise elimination effect for low-frequency sound is poor. They not only bring some difficulties to noise control, but also significantly increase the cost of control.

There are several ways to solve the problem of fan noise:

1. The noise of the casing and motor of the range hood can be solved by installing a sound insulation cover. The fan is placed in an independent fan sound insulation room, and sound absorption and sound insulation are carried out in the fan room.

2. A silencer is installed outside the air outlet of the fan, and a muffler insert is built in, so that the noise can be continuously reduced through a special structure. Silencer is an effective measure to reduce the radiation at the inlet and outlet of aerodynamic equipment or to transmit noise along the pipe.

3. The louvers outside the ground floor shall be silenced as far as possible.

4. The fan is hung with a damper spring hanger.

2. carry out overall vibration isolation treatment on the fan foundation.