What's wrong with the noise of the DC cooling fan

Some customers have bought a nominal noise-free DC cooling fan, but they still feel noisy when they come back to the test. What's the matter? Is the merchant doing something wrong? First, we can analyze the cooling fan alone The power-on test is noisy, if the unit test is no noise, and the product is noisy when running, it may be related to the environment of the product. We may have to design the air duct and install the anti-resonance design of the product. After that, if it is found that the power-on noise of the single unit is also very large, we must choose some relatively quiet fan blades, the blades are relatively long, so that the friction area between the fan blades and the air is reduced when the fan blades are running. Finally, some customers say silent cooling fans. It is impossible, because as long as the cooling fan is running, it must be noisy, but we may not be able to hear it as far as our people can be. We can think of it as no noise.