I use the wall cavity in the wall on the right.
On the inside of the enclosure the wall is open at the top. The picture below is before the sheeting was put on the outside of the wall.
So the then the sheeting ( 1/2" plywood, because I had some ), was put on the outside of the wall leaving a space at the bottom.
so the air goes thru the wall to exhaust. This provides more surfaces for the sound to be absorbed but lets the air exhaust. The duct formed by the wall is about 18" x 2 1/2" This provide plenty of capacity for the air to exhaust.
Also the room is not where near air-tight.
Hope this is clear.
Remember, sound reduction is accomplished in 2 ways. Contain the sound in a n air-tight room and/or provide materials that will absorb the sound. So the MDF and 2" hard foam absorb sound. I tried to control the escaping of air ( sound ) as much as possible, but with my open rafters it was impossible.
I would recommend that you use different materials for the inside and outside walls of the enclosure. Like sheetrock and plywood. Different materials will absorb different frequencies of sound.
I will get the db meter again and check.
Again, I repeat, a LOT of the sound is reduced as you add your piping. I used PVC so it has some density. My initial measurements were without any piping. I think this was somewhat misleading.
But non-the-less I am very satisfied with my enclosure.