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Electronic equipment fans are often covered with a lattice guard, especially the fan in the power supply unit of a computer case.

Two variants are typical: either there'a a huge round hole in the power supply unit wall and that hole is covered with a lattice made of wire or there're multiple holes cut directly in the power supply wall.

It is believed that the first variant produces less noise because it's more streamlined. Does anyone has any actual evidence for or against that belief?

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I believe it is more a result of the openness of lattice, not so much the shape. Cut metal tends to be more closed than the wire frame setups. The more open, the less airspeed required to move the same volume of air through that hole. Less speed == less noise.

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The airspeed through the lattice is probably the most important factor in how much nooise a fan cover generates. But, for same airspeed, you'll have more turbulence (and thus noise) from the slightly less aerodynamic shape of holes stamped out of metal than you would with the air stream being diverted around a round wire.

I can't say I have any hard data on this, though, I am reasoning from first principles, but I don't have a problem believing holes stamped out of sheet metal (or thick foil) producing more noise than a wire mesh, from both edge effects and from actual air speed through the holes.

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I doubt it. Fan noise is a factor of RPMs, blade design and the motor used.

The amount of air moved by a fan (CFM) increases linearly based on speed. Different fans are more or less efficient at moving air at various speeds. With the amount of air a PC fan is moving, a stamped vs. rounded lattice is meaningless.

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