John Sayers' Recording Studio Design Forum Archive

In this image, the grey part was going to be flexi duct. It is about a meter only. Do you think lining this will make a difference?

That part is fine for flex-duct, since it is completely outside the studio, beyond the outer leaf, so there's no implications at all there fore the studio. The noise produced by that duct will only be sent out into the garage area, and won't affect the studio. The same applies to teh fan noise, in that design: since the fan is outside the studio but inside the garage, it will be heard inside the garage, and outside the garage too (through the intake duct), but it will not be heard inside the studio, since the silencer will block that noise if designed and built correctly.

Also, I see some fans ship with a fan silencer which apparently helps reduce the fan noise, do these help too?

In this case, it won't make much difference, since the flex duct and the fan are both located in the garage. So any noise that the in-line silenced blocked would be picked up again through the flex duct itself, and the silencer would not stop that noise getting into the garage either. It won't do any harm, but it won't do much good either.

- Stuart -

Soundman2020 wrote:

In this image, the grey part was going to be flexi duct. It is about a meter only. Do you think lining this will make a difference?

That part is fine for flex-duct, since it is completely outside the studio, beyond the outer leaf, so there's no implications at all there fore the studio. The noise produced by that duct will only be sent out into the garage area, and won't affect the studio. The same applies to teh fan noise, in that design: since the fan is outside the studio but inside the garage, it will be heard inside the garage, and outside the garage too (through the intake duct), but it will not be heard inside the studio, since the silencer will block that noise if designed and built correctly.

Also, I see some fans ship with a fan silencer which apparently helps reduce the fan noise, do these help too?

In this case, it won't make much difference, since the flex duct and the fan are both located in the garage. So any noise that the in-line silenced blocked would be picked up again through the flex duct itself, and the silencer would not stop that noise getting into the garage either. It won't do any harm, but it won't do much good either.

- Stuart -

Ok!

With regards to my fresh air needs, I wanted to ask your advice if it is really necessary to have 6 fresh air changes per hour since I am the only one down there and I have no gear (except my thunderbolt interface and my mac laptop). I have read in several other threads on this forum that 6 exchanges per hour is what is recommended.
The fan I am thinking of getting can push 350m3/h on its absolute lowest/quietest setting and my room is 80m3. This will give me 4.4 fresh air changes per hour. Am I doing the right kind of calculation here? And do you think that 4.4 changes is ok?

this is the fan I am looking at

https://www.amazon.co.uk/dp/B009UG4YBQ/ ... 656cff7de9

Gregwor wrote:Remember, when finding a fan that will work, take the static pressure of the entire system into consideration as, like I just said, it's working on both the supply and return line air paths.

Greg

How do I work out the static pressure of the entire system?

How do I work out the static pressure of the entire system?

Great question. One I recently asked myself! HVAC is a beast to conquer! There are several spreadsheets online that you can download and fill out. Things like size of ducts at what lengths with what types of corners. Put all of that info in and it will calculate your total static pressure. Again, it's all hypothetical, but it should get you in the ballpark. After you build the room, you can actually use a meter to measure it and tune your duct work as necessary (adding length, changing duct size, running extra duct work in parallel, etc)

Greg

Gregwor wrote:

How do I work out the static pressure of the entire system?

Great question. One I recently asked myself! HVAC is a beast to conquer! There are several spreadsheets online that you can download and fill out. Things like size of ducts at what lengths with what types of corners. Put all of that info in and it will calculate your total static pressure. Again, it's all hypothetical, but it should get you in the ballpark. After you build the room, you can actually use a meter to measure it and tune your duct work as necessary (adding length, changing duct size, running extra duct work in parallel, etc)

Greg

It sure is, it seems I never stop learning. I just hope that one day I will be able to get back to making music

Regarding using calculators here, I doubt that those calculators take into account the static pressure changes caused by silencer boxes though and wouldn't those be the biggest cause of static pressure changes?

Gregwor wrote:

How do I work out the static pressure of the entire system?

Great question. One I recently asked myself! HVAC is a beast to conquer! There are several spreadsheets online that you can download and fill out. Things like size of ducts at what lengths with what types of corners. Put all of that info in and it will calculate your total static pressure. Again, it's all hypothetical, but it should get you in the ballpark. After you build the room, you can actually use a meter to measure it and tune your duct work as necessary (adding length, changing duct size, running extra duct work in parallel, etc)

Greg

One other question on the math side of things: From reading this forum, I know I need a velocity of 300cfm or less at my registers. If my room needs 212CFM of fresh air and my 6" duct has a cross sectional area of 0,2 square feet, does this mean that my velocity will be 1060FM ?

The calculators allow you to type in all the values (like the size of your inner dimensions of your silencers and how many 180 degree turns you have to account for your baffles) so you can add in all of your system parts.

Regarding your air velocity, your math seems about right!

Greg

Gregwor wrote: Regarding your air velocity, your math seems about right!

Oh ok but of course 1060FM is much too fast.
So if I use a silencer, which uses a cross section that is 2 X the size of the 6" duct, does that means that the speed will be cut in half to 530FM? Or will the baffles further reduce the speed because of the 180 degree turns as well?

Oh ok but of course 1060FM is much too fast.

Yes it is! You need to bring it down to about one quarter of that.

So if I use a silencer, which uses a cross section that is 2 X the size of the 6" duct, does that means that the speed will be cut in half to 530FM?

If the cross sectional area is twice as large, then the speed will be half. Yes. This is because you cannot create or destroy energy. A basic rule of science. Energy can only be converted from one form to another form, but it cannot be destroyed, or created. So the TOTAL energy of your moving air remains the same at every single point along the path, and cannot change. That total energy is made up from several components, but you can think of the most important two being the flow rate and the flow velocity. The only relationship between those two is the cross sectional area of the duct. You can see this by looking at the units of each. The flow RATE is measured in units of "cubic feet per minute" or "f^^3 / m". The flow VELOCITY is measured on units of "feet per minute" or "f / m". If you divide one by the other, you end up with units if "f^^2", or "square feet", which is the measurement of units for the cross sectional area. And because the flow RATE is fixed (that's the maximum amount of air being moved by the fan, and pushed into the ducts: that does not change), that means that the other two are related by a simple, linear, and directly proportional relationship. As cross-sectional area goes up, flow VELOCITY goes down by the same amount. If you increase the area by 10%, the velocity goes down by 10%. If you increase the area by 200%, the velocity goes down by 200%.

Or will the baffles further reduce the speed because of the 180 degree turns as well?

No. Once again, the flow RATE is fixed: it does not change. It is fixed by the fan, and the static pressure of your system. The fan can only move a certain volume of air, and that amount is set by the manufacturer, and limited by the static pressure. When the manufacturer design the fan, they designed it to work with a a certain range of possible static pressures, and a certain set of speeds. With some fans, you can change the speed by using a variable speed controller, and with most mini-split system, that is something you can set via the remote control unit: you often have a choice of three speeds: low, medium, or high. The "rating" published by the manufacturer refers to the "high" speed, since that's the maximum performance that the fan can produce. So when the fan is running on "high", then it will produce the flow rate stated in the manual. Normally there is a table or a graph that shows what the rate will be vs. the static pressure. The higher the static pressure, the lower the maximum possible rate. The static pressure of your system is fixed: it does not change. Once you have built it, it won't change any more (unless you have variable dampers in the system, but that's an entirely different subject). So no, the baffles in your silencers do not "slow down" the air. They have already been accounted for when you calculated or measured the total static pressure for your system. The "static pressure" refers to the entire system of your ducts, filters, silencer boxes, dampers, registers, the room itself, etc. The "static pressure" is one single number that takes into account all of the "resistance" offered by every part of the system, from beginning to end. That "total resistance" or "static pressure" is what the fan "sees" as it tries to move air through your system, and it does not change. That single number is what sets your flow rate for every given fan setting, and therefore also sets the air flow speed at any given point along that path, based entirely on the cross sectional area.


- Stuart -

Soundman2020 wrote:

Oh ok but of course 1060FM is much too fast.

Yes it is! You need to bring it down to about one quarter of that.

So if I use a silencer, which uses a cross section that is 2 X the size of the 6" duct, does that means that the speed will be cut in half to 530FM?

If the cross sectional area is twice as large, then the speed will be half. Yes. This is because you cannot create or destroy energy. A basic rule of science. Energy can only be converted from one form to another form, but it cannot be destroyed, or created. So the TOTAL energy of your moving air remains the same at every single point along the path, and cannot change. That total energy is made up from several components, but you can think of the most important two being the flow rate and the flow velocity. The only relationship between those two is the cross sectional area of the duct. You can see this by looking at the units of each. The flow RATE is measured in units of "cubic feet per minute" or "f^^3 / m". The flow VELOCITY is measured on units of "feet per minute" or "f / m". If you divide one by the other, you end up with units if "f^^2", or "square feet", which is the measurement of units for the cross sectional area. And because the flow RATE is fixed (that's the maximum amount of air being moved by the fan, and pushed into the ducts: that does not change), that means that the other two are related by a simple, linear, and directly proportional relationship. As cross-sectional area goes up, flow VELOCITY goes down by the same amount. If you increase the area by 10%, the velocity goes down by 10%. If you increase the area by 200%, the velocity goes down by 200%.

Or will the baffles further reduce the speed because of the 180 degree turns as well?

No. Once again, the flow RATE is fixed: it does not change. It is fixed by the fan, and the static pressure of your system. The fan can only move a certain volume of air, and that amount is set by the manufacturer, and limited by the static pressure. When the manufacturer design the fan, they designed it to work with a a certain range of possible static pressures, and a certain set of speeds. With some fans, you can change the speed by using a variable speed controller, and with most mini-split system, that is something you can set via the remote control unit: you often have a choice of three speeds: low, medium, or high. The "rating" published by the manufacturer refers to the "high" speed, since that's the maximum performance that the fan can produce. So when the fan is running on "high", then it will produce the flow rate stated in the manual. Normally there is a table or a graph that shows what the rate will be vs. the static pressure. The higher the static pressure, the lower the maximum possible rate. The static pressure of your system is fixed: it does not change. Once you have built it, it won't change any more (unless you have variable dampers in the system, but that's an entirely different subject). So no, the baffles in your silencers do not "slow down" the air. They have already been accounted for when you calculated or measured the total static pressure for your system. The "static pressure" refers to the entire system of your ducts, filters, silencer boxes, dampers, registers, the room itself, etc. The "static pressure" is one single number that takes into account all of the "resistance" offered by every part of the system, from beginning to end. That "total resistance" or "static pressure" is what the fan "sees" as it tries to move air through your system, and it does not change. That single number is what sets your flow rate for every given fan setting, and therefore also sets the air flow speed at any given point along that path, based entirely on the cross sectional area.


- Stuart -

This is a very clear explanation. You should write a book! The ones I am reading don't have it explained quite as well.

So how else can I take the speed of the air down further. If my cross sectional area in my baffle is already twice the size and I am still having speeds in excess of 500FM, what else can I do? Do I have to make the cross sectional area even bigger? Or could I double the cross section size again after the sleeve penetrates the inner leaf?

Or could I double the cross section size again after the sleeve penetrates the inner leaf?

You could do that, yes. What matters is the velocity at the register, so if you double the area inside the final sleeve, you will achieve that goal.

Another way to do this, that I have developed, is to feed the airflow into the MIDDLE of the silencer box, and allow it to split off in two directions, each of which has the same cross section as you have now. That means the silencer box has to be nearly twice as long, because you need baffles down both "branches", but it does get you double the area / half the velocity. You could then combine that with your idea, of making the final sleeves double the size again, so you'd be getting 1/4 the velocity at the registers. This method also has the advantage of REDUCING the total static pressure, because you have two paths in parallel: that acts something like electrical resistors in parallel: the total resistance of both together is LESS than each one by itself. n interesting and useful property...

This is a very clear explanation. You should write a book! The ones I am reading don't have it explained quite as well.

Thanks for the kind words! I've sort of been thinking of writing a book about studio design.... maybe I will, one day!


- Stuart -

Soundman2020 wrote:

Or could I double the cross section size again after the sleeve penetrates the inner leaf?

You could do that, yes. What matters is the velocity at the register, so if you double the area inside the final sleeve, you will achieve that goal.

I think I am going to do it this way although I would imagine then that the silencer box would have to be pretty big. Is it even possible to have my silencer with a double cross sectional area and a further double cross sectional area only at the sleeve? Wouldn't that sleeve be too big for the silencer box?

What about this option: Since the velocity needs to be under 300FM only at the register, what if I had my register toward themiddle of the room instead of right where the sleeve penetrates the leaves as I was going to do in my designs above. I could then add a duct from the sleeve to the middle of the room and I could increase that duct somewhere along the way before it hits the register. So basically, starting from 28 squared inches at the point where it is connected to the fan. Then it hits the silencer box and the cross section doubles to 56 squared inches. The sleeve of the silencer penetrates the leaves and a duct continues around the interior of the room where it doubles to 112 squared inches until it gets to the register. I would need some kind of duct "expander" but not sure if that exists or even if this will work.

This is a very clear explanation. You should write a book! The ones I am reading don't have it explained quite as well.

Thanks for the kind words! I've sort of been thinking of writing a book about studio design.... maybe I will, one day!

You must. You've basically already written one. The pieces are all over the site. I read Rod's book and it is ok but it doesn't explain these things in the detail I need. I would buy your book for sure as i prefer reading on paper than on screen


- Stuart -[/quote]

What about this option: Since the velocity needs to be under 300FM only at the register, what if I had my register toward themiddle of the room instead of right where the sleeve penetrates the leaves as I was going to do in my designs above. I could then add a duct from the sleeve to the middle of the room and I could increase that duct somewhere along the way before it hits the register. So basically, starting from 28 squared inches at the point where it is connected to the fan. Then it hits the silencer box and the cross section doubles to 56 squared inches. The sleeve of the silencer penetrates the leaves and a duct continues around the interior of the room where it doubles to 112 squared inches until it gets to the register. I would need some kind of duct "expander" but not sure if that exists or even if this will work.

If I understand what you're saying correctly, yes, this would work. Basically, as long as your surface density is maintained through your leaf penetration to your silencer box, you're set. Leaving your silencer box you can then use normal lined duct to move the air to your registers wherever you want them. As Stuart stated, you could increase your cross sectional area right before your register enough to slow down the air velocity.

Greg

Gregwor wrote:

What about this option: Since the velocity needs to be under 300FM only at the register, what if I had my register toward themiddle of the room instead of right where the sleeve penetrates the leaves as I was going to do in my designs above. I could then add a duct from the sleeve to the middle of the room and I could increase that duct somewhere along the way before it hits the register. So basically, starting from 28 squared inches at the point where it is connected to the fan. Then it hits the silencer box and the cross section doubles to 56 squared inches. The sleeve of the silencer penetrates the leaves and a duct continues around the interior of the room where it doubles to 112 squared inches until it gets to the register. I would need some kind of duct "expander" but not sure if that exists or even if this will work.

If I understand what you're saying correctly, yes, this would work. Basically, as long as your surface density is maintained through your leaf penetration to your silencer box, you're set. Leaving your silencer box you can then use normal lined duct to move the air to your registers wherever you want them. As Stuart stated, you could increase your cross sectional area right before your register enough to slow down the air velocity.

Greg

Ok great. I am on the fence about it because I would like my registers closer to the penetration. After doing calculations I have concluded that I will need a 100 square inch duct size to get to a speed of 300FM. If I make my silencer really big then I could make it so that the baffles have a cross sectional area of 10 X 10inches (100 square inches). So I think this is what I am going to shoot for and get on with it. Greg does everything here look OK to you? I am thinking of starting building this thing soon.

Since my silencer sleeve will penetrate both leaves at once will it not create a flanking path?

That's why you have to deouple it at both ends! Make the holes through the leaves a bit larger than the size of the sleeve, by a few mm, so there's a gap all around, then fill the gap with backer rod of the right size, and seal it with soft, flexible caulk.


- Stuart -