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The definition of quiet or silent is very dependent on the environment and the person listening to it.

Personally I refer to quiet as being basically inaudible while standing about 1 or 2 meters from it.

At 2600 RPM the stock fan is definitely audible, but for some people that can still be quiet. It can definitely get way louder at 300W+
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True.
I did read somewhere they had a fan supply issue for a while.
Not sure if the different fans would have different noise level at same speed.

Definitely quieter than my Antminer R4 with APW7 PSU, which were sold as quiet enough for home mining, but man those are loud 
I'm running it outdoor on the balcony it's too annoying loud.

With the Apollo I definitely hear a low fan noise at 1 or 2 meters just like a home ventilator on low.
Not annoying at all