Hmmm. Should I avoid Ted White's dead vent design because it has the fan inside the silencer box
I may be wrong, but I think his entire point with that design is for situations where the
fan needs to be silenced, as it has to go inside the structure, and there are no alternatives. I wouldn't suggest that as the design for a silencer box where the fan can be located remotely, outside the studio.
I'm confused. The duct in Ted White's design is within the silencer box, right? So in that design, the silencer box doesn't feed the duct, right?
The way I see it, he is just putting the entire fan inside a box to silence it, while also curving the air path slightly to provide some additional isolation. That's fine, for the purpose it is designed for, but as you can see in the links I posted for you, that's not the way silencer boxes are normally built. Ted's design seems to be specific for the situation where you have no choice but to put the fan inside the studio. You don't seem to have that problem, so you don't need that solution!
Also, when you say "cross sectional area", do you mean the surface area?
The cross sectional area is just taking a cut across the duct itself, and seeing how much "empty space" there is for air to move through. For a round duct, the equation is Pi x r^2. So multiply the radius by the radius, and then by 3.141. So a 6" round duct has a cross sectional area of about 28 square inches (the radius of a 6" circle is 3": 3 x 3 x 3.141 = 28.26). The area of an 8" duct is about 50 square inches. And for rectangular ducts, it is simply the internal height x the internal width. So for a 10" x 12" duct, the cross sectional area is 120 square inches.
The general rule of thumb is that the cross section of the air path inside your silencer box should be at least twice as much as the cross section of the duct that supplies it with air. So if you have a 6" duct coming in, then the air path inside the silencer needs to be at least 56". In other words, the open space between the tip of each baffle and the silencer wall needs to measure about 7" x 8" or greater. The reason for this is very simple: if you give the air twice as much cross section to move through, then it must slow down to half the speed. That also creates an "impedance mismatch", which greatly reduces the level of sounds traveling through the duct.
If I want to come up with appropriate dimensions and still use Ted White's concept, should I measure the surface area of the inside of the rectangular silencer box and make sure it's at least twice as large as the surface area of the full length of 6" round flex duct?
You cannot use this concept at all with that design, since the air is not moving through the box! It is inside the duct, and since the duct is always the same size, there is no change in cross section, and no change in air flow speed, and no change in impedance. Ted's design is to stop the FAN NOISE getting into the studio, not to act as a true silencer box for the air-borne noise itself.
I've been planning to use this "dead vent" design because my room is already constructed and the walls and ceiling are closed.
I don't understand why you are so insistent on using that dead vent when it is not applicable to your situation! You need a true silencer box, which can actually be smaller than the "dead vent", if that's what you need, since you can make it flatter than the duct could ever be, provided that you also make it wider. With a true silencer box, you can adjust all the dimensions as needed to get the best fit, without being restricted to the size of the fan motor or the duct. Just use an outdoor fan unit at the far end of the duct, and size the silencer box as small as you need to make it.
There aren't any ducts in my walls or ceiling yet, and I was just planning to cut the 6" hole for the PVC pipe supply of fresh air, and another 6" hole for the exhaust.
PVC pipe???
That's not the right way to make ducts! Use only proper HVAC duct. Either "flex duct" or normal round sheet metal duct. Don't substitute for products that are not meant to do the job. That is not PVC pipe in Ted's design either: it is flex duct, which is correct.
What I like about this dead vent design is that the whole mechanism seems to be outside of my room.
so it is no different than the normal way of doing a silencer box! In what way do you see it as being different?
If I can stick with this concept it would be great (it seems simple enough), but I'm unsure if I can get away with using different dimensions.
If you insist on following that design, then you are stuck with those dimensions. The limiting factor is the size of the fan motor itself: you cannot make the box any narrower than that, in either dimension (but you could with a normal silencer). And you cannot make the ducts shorter, since you need at least that length to smooth out the turbulence (and resulting high air noise levels) created by the fan itself. The genreal rule is 6 rimes the duct diameter in both directions, so 36 inches up and another 36 inches down (72 inches) plus the fan length (6"?) plus the ducts themselves (8"), plus a few inches clearance top and bottom. Hence, 90". So if you have your heart set on that design, then you have no choice but to follow the dimensions exactly.
How would I calculate the cross sectional area in Ted White's dead vent design?
You don't, because it is fixed. There's no need to calculate anything, since the air never leaves the flex duct, and never enters the box itself. The box is only there to silence the FAN, not the AIRFLOW, since the air never flows through the box, outside of the duct. The box has to be dimensioned at least that big, since tie OD of 6" flex duct is more like 8", plus the radius of the curves: you cannot bend flex duct in a tight corner: it has to be curved gently, hence the need for a box 24" deep, and also wide enough to accommodate the fan (12"?).
So with that design, you have no choice but to use the same dimensions that Ted gives. The only way you can use a smaller box, is if you do a normal baffled silencer box, such as the ones I linked you to in my last post, without the fan inside it. Just put the fan at the far end of the exhaust duct, wherever that is, hopefully a long way from the studio.
- Stuart -
The foundation wall plus the new outer-leaf walls can all be tied together as one unit, no sway braces or isolation or decoupling needed. The only decoupling that you need is between the inner-leaf and the outer leaf: they must be kept apart from each other. But the individual parts of each leaf do not need to be separated from the other parts of that leaf.
So it can take a while to get a response.
. I just want to 
