The larger the pipe diameter the less airflow restriction. A very common error that causes premature failure is not accounting for this restriction If there is a restriction on the intake air supply, there will be a reduction of pressure which will result in less heat transfer to the air. So, same volume of air at a lower pressure can cause some ASICs to run hotter. There is a similar problem on the output except the increased backpressure will reduce the airflow. If you restrict both sides, the flow and pressure are both reduced.
The area of the circle changes by a squared proportion. Recall, Pi x RadiusSquared = area of a circle.
4" duct = 3.14x2"x2 = 12.28 sqin
6" duct = 3.14x3"x3 = 28.26 sqin
8" duct = 3.14x4"x4 = 50.24 sqin
So where 8" duct is 2x the diameter, it is 4X the area.
Airflow resistance is directly proportional to the length of the duct. So, if you double the length of the duct, you double the resistance. This also means you can run 4X the length of duct by doubling the diameter.
It does sound like you need to sacrifice a cooler. I'd probably just get one of those Styrofoam units, cheap and quick to cut, very good insulator.
If you do what you saw on youtube, I expect you'll get a similar result. I saw one or two there; I do think their duct looked too small.
Ok good answer, now the question is if I get say a 6'' antminer shroud and attach it directly to a cooler (intake) and have the other shroud connected to the antminer on one end directly connected to the 6'' insulated duct would that allow the cooler to get air in and out but be very quiet?
I was thinking about buying a fan that does 390 CFM that is 6 inch and attached directly to the intake fan but would it cause problems if say too much air is being forced into the box? What would be better to just have an open 6'' insulated duct laying in my room sucking in air flow or to have the fan pushing the air into it at same time antminer is sucking in the air?
Also, is it better to have the duct directly connected to the box (intake) or directly to antminer (intake) fan?
I've tried styrofoam and it didn't reduce it more than 5 db and that's why I think I need a larger cooler with possibly even foam on the insides?