Have you used the method the TDN guide gave? It basically goes like this for those who don't know.
First, you have your 4 plants. Put all of them to the top or bottom. Here's an example (.=free space != flower and pot):
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Now, you would activate the fan to see the how much decibels it is at currently. For this, let's say it's at 15 decibels.
Let's move the 1st plant to the left up one space. It would look like this:
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Check at how many decibels it is at now. If it is at 14 decibels, keep going up. If it increases, go back to the same space. Keep it in that space if the decibel increases. (Dang look at how many guests there are!)