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So, that begs the question: with 4 (for example) registers in a control room, roughly 20x30, would it make sense to have the same size ductwork (or close to it) feeding four legs of supply? And thus acheive enough of a 'slow down' that way?
Think of it this way: there is a direct, unbreakable relationship between the air flow velocity, the air flow rate, and the duct cross section. Since the AHU fan will be moving air at a fixed RATE and VELOCITY as it comes out of the unit, you can control the VELOCITY in the rest of the system ONLY by varying the cross sectional area of the duct. If you increase the cross-sectional area, then the VELOCITY goes down, but the RATE stays the same. If you decrease the cross sectional area, then the velocity goes up, but the rate stays the same. However, decreasing the cross-sectional area also decreases the AIR PRESSURE (Bournelli's theorum), but increases the STATIC pressure, and considering that your AHU will have certain limits on how much pressure it can handle, you do have to be careful (just making the duct longer also increases the pressure, even if there is no change in cross-sectional area).
Think of it like water flowing through your garden hose pipe: It is moving at a constant rate. If you put your thumb part way across the end of the hose, then that changes the pressure and velocity, but not the rate. To increase the rate, you have to turn the tap on harder.
So there's this relationship between velocity, rate, pressure, and cross-sectional area. To be more precise, the TOTAL energy in the moving air is the sum of the KINETIC energy and the POTENTIAL, and it is fixed: you cannot increase or decrease the total energy of the moving air, except by adjusting the AHU controls. If the kinetic energy goes up at some point in the the system, then the potential energy MUST go down, such that the total stays the same. And vice-versa: if the potential energy goes up, then the kinetic energy goes down by the same amount. If this were not true, then airplanes and birds would not be able to fly.
Therefore, changing the cross-sectional area at various points throughout the system is the ONLY parameter that you have control over in the design. Since you can easily check the manufacture's specifications for the AHU to find out what air flow rates and air flow velocity it produces at various settings, you can easily determine what the total cross sectional area is that you will need at your register, in order to ensure that the velocity NEVER goes over 300 FPM, even when the AHU is running at the highest setting.
So that's your starting point: You will do the math, and figure out that you need "X" square inches of cross sectional area at your register.
Now go to your local HVAC supply store, and look at registers: If the ones you are looking at that have the correct area just seem to be too big for your room, then you will probably have to split the air-stream at some point and use two smaller registers, that STILL ADD UP TO THE THE SAME TOTAL AREA. And if those smaller ones still seem too big, then you will need to split the airflow again, into FOUR registers, once again where the individual areas of those sum to the correct total area.
There's another factor to take into account here: "open area". Registers have frames around them, and vanes on them that direct the air flow into the room. Those block some of the total surface area. So for example a 10" x 10" register would seem to have 100 square inches of area, but by the time you allow for the frame and vanes, there might only be an actual cross-sectional area of maybe 70 square inches. IF the vanes are adjustable, the more you angle them, the less area is left. So take a very close look at the specs for the register, to determine how much REAL area is left over. And also look at the "smoothness" of the design: registers that have lots of protrusions and sudden sharp edges will create turbulent air flow which is noisy. With better designs, the vanes are smooth and "aerodynamic", which creates a lot less turbulence.
OK, so now you have your registers figured out: that automatically defines the duct that leads to the register! You have no options here. If the duct is 8" x 6", then the duct has to be AT LEAST 10" by 8", because it is going to be lined with 1" of duct liner on the inside, all around. Or if you are using round duct, then it needs to have the same "equivalent cross-sectional area" as the rectangle of the register.
So now you know the correct size of your registers, and the correct size of the ducts that lead to the registers, and you know that you need a long straight section of duct right before the register, to reduce turbulence. But how long does that straight part have to be? There are various methods for calculating that, but a good rule of thumb is that it should be at least three times the smallest dimension of the duct. So if you are using an 8" x 6" duct, then you need at least 3x6"=18" of straight duct before the register. That's the ideal, of course, but frequently that just isn't possible in a studio. So you do your best to make it as long as possible, and aim for an even lower velocity at the register, to reduce the noise from turbulence even more.
So, at this point you have your register size, duct size, and final duct length. Now you go another step back up the chain: the silencer box. Once again, there's a general rule here, that the cross-sectional area must change suddenly by a factor of at least two, where the air enters the box and where it exists the box. So normally the area inside the box is twice the area of the duct. In other words, if you figured that your duct needs to b 8x6, that works out to 48 square inches, thus you need 96 in2 cross sectional area (minimum) inside your silencer. So you could make it 11" x 9" which is 99 in2m or you could make the box flatter, for example 12" x 8", which is 96, or even 16" x 6" (also =96). OF course, in theory you could make it ultra-flat, at 96" x 1", but in reality that won't work, as the static pressure would be too high from such a system. So keep your dimensions reasonable, and never go below about 6" on the smaller dimension.
However, once again, this is the cross-sectional area that the AIR FLOW sees, not the actual internal dimensions of the wood. Why would that be different? Because the box is lined with duct liner! Just like your rectangular duct there's 1" of true HVAC duct liner on each side. So if your calculations show that you need 12" x 8" internal cross section, then the interior of the wood box needs to be 14" by 10" at all points. Add the thickness of the wood, and you get the actual EXTERNAL dimensions of the box, at that point. Assuming that you are using 1 1/2" thick wood, you'd add 3", to get a final external size of 17" x 13".
But that's just the width and height: how LONG does the box need to be? Well, you need a certain number of "baffles" inside the box: the more isolation you need, the higher the number of baffles. At the VERY least, you need two baffles. Probably three or four (usually). Maybe as many as five or six, for very high isolation needs. Each baffle is probably 1" thick plus 1" of duct liner on each side, so 3" thick. Plus the spacing between them (to get the same cross sectional area as above). So let's say you need three baffles spaced 12" apart, that works out as follows: (4x12) + (3x3) = 57" internal length, plus 3" wood = 60" total length. So for this hypothetical case, your silencer box would be 60" long, 17" wide, and 13" high.
Did I mention that silencers are usually pretty large?
So now we go yet ANOTHER step back up the chain: the duct on the far side of the silencer. How big should that be? Answer: it doesn't matter! As long as it is half the cross sectional area of the silencer interior, that's fine. If it is 1/4 the area of the silencer interior, that's also fine, EVEN THOUGH THE AIR FLOW SPEED WOULD BE MUCH HIGHER. It does not matter, because you'll never hear it! That can flow fast, or slow: it won't affect the noise level inside the studio, because it is on the FAR side of the silencer.
Of course, there are limits: the smaller you make it, the higher the static pressure, and you cannot exceed the total static pressure that your AHU can handle. So you have to be careful when adding up all the ducts, silencers, registers, dampers, and other things, to make sure the total static pressure from all of those is less than the capacity of the AHU. If the static pressure is too high, the AH will not be able to run efficiently, the fan blades will stall, the motor will overspend, the service life will be very much shorter, and you won't get the performance you were planning on, either in terms of ventilation or in terms of cooling.
And so on. You keep on working backwards up the chain, doing the math for each part, to make sure that you have the flow rates, flow velocities, and static pressure within the range that you need.
"Range that you need"? What's that? Well, to dimension the actual AHU itself, you must ensure that it is able to move enough air to replace all of the air inside your room, at least 6 times per hour. In other words, if the volume of your room is 2000 cubic feet, then your AHU must be able to move at least 12000 cubic feet per hour, and if you divide by 60 then you get cubic feet per minute: 12000/60 = 200 CFM. For that room, your HVAC AHU would have to be capable of moving 200 cubic feet per minute (air flow rate) at a velocity that will produce no more than 300 FPM AT THE REGISTERS, on the highest setting. That's just the specs for how it moves air: in addition, you need to calculate the spec of how much heat it must be able to add/remove, and that, in turn, depends on the sensible heat load and the latent heat load. But that's an entirely different subject...
That probably didn't answer your question directly, but there's enough info in there for you to be able to figure it out, I hope...