John Sayers' Recording Studio Design Forum Archive

I can read on the forum that the silencer boxes for the ventilation need to be at least 2X bigger than the duct for lowering the static pressure and keep the velocity low.

In my case the back wall's ventilation contact points had to be extended so I can use this back wall for the hangers.
I mean if I 've installed the silencer box directly on back wall would have taken a lot of space.
So I made this vent extension box which is sealed and will allow me to attach the silencer box outside the false "hangers" wall.
FYI the ventilation motor which will be outside, placed after the silencer has a diameter of 250mm same as the duct on the extension box and the section inside the wall. Is there any point on making the silencer twice bigger than the duct now that there is this extension box on back wall?
Or it is of no use since the velocity will already be high due to this middle section (which is about a meter long)?

Attaching some more pics so you can understand what I am talking about.

The outside: Inside: inside vent extension:

I can read on the forum that the silencer boxes for the ventilation need to be at least 2X bigger than the duct for lowering the static pressure and keep the velocity low.

It does more than lower the static pressure and keep the velocity low. It actually creates an impedance mismatch which helps to attenuate low frequencies by reflecting sound energy back towards the source.

So I made this vent extension box which is sealed and will allow me to attach the silencer box outside the false "hangers" wall.

How thick is the MDF you sealed the extension box with? It should have the same surface density as your silencer boxes to realize the maximum insertion loss. For the typical isolated home studio, people use two layers of 5/8" drywall (and often a layer of 1/2" or thicker OSB). To match two layers of 5/8" drywall, you'd need 1" thick MDF.

Is there any point on making the silencer twice bigger than the duct now that there is this extension box on back wall?

Yes. See the first point in my post. Actually, introducing several different cross sectional changes (of different ratios) will improve the insertion loss across the audible spectrumgfg xqdscx den bntttt 6ttfftntfxa Q.

Or it is of no use since the velocity will already be high due to this middle section (which is about a meter long)?

No. Having high velocity sections is fine as long as your fan can handle the added static pressure. If you didn't use flex duct, you could have decreased static pressure as well.

Attaching some more pics so you can understand what I am talking about.

My major concern with this design is how much space you left between your supply and return duct work. Let's assume you're using 1" MDF with 1" duct liner (the thicker the duct liner, the lower the frequencies that will be attenuated). That means you need over 4" between the inlet and outlet on your outside wall and your extension box. You didn't include any measurements and I can't really tell from pictures how much you've left between them. Worse case scenario, you could build MORE extension sleeves that feed your silencer boxes out of 1" MDF. You would need just over 2" of space which it looks like you MIGHT have. It just means more material, design, and work. I'd suggest you work out all of these design details in SketchUp Make before you continue. And before you physically build anything, post your design pictures with calculations and dimensions on here. It will prevent any screw ups.

If you DON'T have enough surface density on your extension box, that will need to be addressed. Also, if you DON'T have 2"+ space between your duct work, you'll have to rip apart your extension box and adjust it. Fingers crossed for you!

Greg

Gregwor wrote:It actually creates an impedance mismatch which helps to attenuate low frequencies by reflecting sound energy back towards the source.

Hi Greg, thank you for your reply, it is much of help for me since I read a ton of posts about hvac and I got confused at some point.
I thought that it is the baffles inside the silencers that causing the impedance mismatch and causes the cancellations.
Good to know that the difference on sizes help too.

How thick is the MDF you sealed the extension box with?

The extension box is made out of one layer of 5/8" drywall.
I was thinking of making the silencers out of a single 5/8" OSB.
You think that's not enough?
I can double the drywall layers for the extension and the OSB for the silencers if it is critical for the overall performance of the system.
Actually I will do that.

My major concern with this design is how much space you left between your supply and return duct work.
Worse case scenario, you could build MORE extension sleeves that feed your silencer boxes out of 1" MDF.
You would need just over 2" of space which it looks like you MIGHT have.
It just means more material, design, and work.

I was indeed planning to build two more short extension sleeves out of OSB -using flex duct inside- (for the outside wall and the inside wall).
I can not place them directly where they need to be due to their size, they are really big for the 250mm ducts...
For having twice the area of the duct the square opening need to be like 23x43cm + 5cm for the duct liner each size + plus 2layers 15mm OSB each size...you can see where it's going...

I'd suggest you work out all of these design details in SketchUp Make before you continue.

I am trying so hard to learn this program..3D confuses me..Sorry

Greg thanks again

The extension box is made out of one layer of 5/8" drywall.
I was thinking of making the silencers out of a single 5/8" OSB.
You think that's not enough?

I don't think it's enough. I'd use two layers of 5/8" minimum! For the silencers, should use a thickness that will be equivalent to the two layers of 5/8" drywall. I don't have that info handy right now, so you will have to look it up. But I'm guessing it's around 1 1/2" or so.

I was indeed planning to build two more short extension sleeves out of OSB -using flex duct inside- (for the outside wall and the inside wall).

You don't need more flex duct as long as you can separate the two air paths. Flex duct has a maximum recommended length and is famous for introducing static pressure. Furthermore, I'm not sure how you calculated your duct size, but when using flex duct you should always go up a size (example, if you have 10" flex duct, it will only perform as well as a regular 8" tin duct). It's handy, but horrible all at the same time!

Either use proper round duct (boxed in as you must maintain your surface density) or frame some paths with OSB. If you're design shows too much static pressure, use vanes in some corners or round duct.

I am trying so hard to learn this program..3D confuses me..Sorry

Google and watch some videos on:
Inference
Layers
Components

After that you should be off to the races!

Greg

Gregwor wrote:I don't think it's enough. I'd use two layers of 5/8" minimum! For the silencers, should use a thickness that will be equivalent to the two layers of 5/8" drywall. I don't have that info handy right now, so you will have to look it up. But I'm guessing it's around 1 1/2" or so.

I will do that!

You don't need more flex duct as long as you can separate the two air paths. Flex duct has a maximum recommended length and is famous for introducing static pressure.

Ok I will frame the paths with OSB as you suggest.
Yes I remember something like 1.5m maximum length for flex duct.
To be honest the one I used is semi-rigid aluminum duct which you can bend by hand and keeps its form to ensure that the cross sectional area is maintained through the bends.
I dont know if there is any difference in using that over the regular flex duct though..

Furthermore, I'm not sure how you calculated your duct size, but when using flex duct you should always go up a size

I havent really calculated the size. I assumed, from reading the forum and what others used for their rooms, that for my room which has a volume of 153 cubic meters the 250mm duct will be ok.
More I already had the ventilation motor which has a diameter of 250mm so I went with that.
On a previous smaller room (75m3) that I used the same motor was way too much, so I suppose it will be ok...or not

If you're design shows too much static pressure, use vanes in some corners or round duct.

How can I understand if the static pressure is high?

Google and watch some videos on:
Inference
Layers
Components

Great, I will, thanks for the tip!

I havent really calculated the size. I assumed, from reading the forum and what others used for their rooms, that for my room which has a volume of 153 cubic meters the 250mm duct will be ok.
More I already had the ventilation motor which has a diameter of 250mm so I went with that.
On a previous smaller room (75m3) that I used the same motor was way too much, so I suppose it will be ok...or not

Okay, you need to sit down and figure this stuff out. It's very important.
I can't remember what your HVAC system consists of -- what are you using for heating, cooling, and ventilation? I wish I could with you here, but it's too serious of matter for that.

How can I understand if the static pressure is high?

There are a handful of spreadsheets people have posted online that you can type your data into and it will calculate the total static pressure. You'd enter things like how big your duct work is, how many corners of what type, etc.

For real though, stop everything and force yourself to learn SketchUp Make. You should be able to learn it well enough in a few days to do everything you need with your project. Then, take a week and draw everything you've done so far. Calculate your HVAC needs and draw it up. No one here wants you busting out that wall you just built because you didn't take something into consideration.

Greg

Gregwor wrote:I can't remember what your HVAC system consists of -- what are you using for heating, cooling, and ventilation?

I will use a split a/c unit for heating and cooling.
The ventilation will be separated from the a/c.

There are a handful of spreadsheets people have posted online that you can type your data into and it will calculate the total static pressure. You'd enter things like how big your duct work is, how many corners of what type, etc.

I will try to find the spreadsheets, if by any chance you have a link please share it

You should be able to learn it well enough in a few days to do everything you need with your project.

I will try to do that. You see being a father of two boys, 2 and 3 years old, and working two jobs leaves you with almost no free time..but I will try my best!

In the meantime This is the design of the silencer:
(still working on the dimensions though)

Ok I found some spreadsheets over the net for calculating the static pressure etc.
It really seems impossible for me to do the calculations, I don't even know where to start
And I can't really afford to hire someone to do it for me..

so I think I will just stick to the current plan
-making the silencers twice bigger than the duct size,
-two silencer for each path-one inside the room and another one outside- (total of four) for in and out air
-use rigid round ducts where needed,
-frame paths with double drywall or OSB for the extension boxes,
-double OSB for the silencers,
-2'' duct line
and hopefully will work..

I 've been reading here that the point of the registers should be twice the cross sectional area and with a cone type register so will not introduce any sudden changes.

As you might saw on my first post I am not isolating the room as it is not much of a problem for now.
But still I want to keep the current TL levels of the double brickwall that the room is made of and of course the ventilation noise is a problem too.
Having said that, in my case after the silencer which will be at the back of the room I will use rigid duct to travel at the front of the room so that the fresh and the old air are far away from each other, to create a kind of flow.
The duct will have half the cross sectional area of the silencer and as you told me that's not a problem.
But if I indeed have to make the register twice bigger of the cross sectional area, what would that be, of the duct or the silencer?

I will use a split a/c unit for heating and cooling.
The ventilation will be separated from the a/c.

So your ventilation ducts only need to provide you with 25-30% of your required CFM provided by your split unit. Have you calculated your required CFM yet? What about your heat gain, heat loss, sensible heat and latent heat? Without knowing these values, there's no way to know what unit to buy.

I will try to find the spreadsheets, if by any chance you have a link please share it

Sounds like you found one. If you for some reason can't make yours work, I can PM you mine.

I will try to do that. You see being a father of two boys, 2 and 3 years old, and working two jobs leaves you with almost no free time..but I will try my best!

I feel your pain. I have a boy who is almost 4 and a daughter turning 2 on Friday! I also have 5 jobs and no child care. My studies usually happen when everyone else is asleep, when I'm on the toilet - taking extra long of course , or I have just not worked as I need to prioritize my acoustic studies and designing.

In the meantime This is the design of the silencer:
(still working on the dimensions though)

Looks like a standard silencer. Once you calculate your static pressure, you can determine if you can shrink the straight run parts of the design as that will increase the static pressure, but will provide you with more impedance mismatches. Where you want to make sure you have large cross sectional areas is at the corners as small cross sectional areas at corners will introduce a lot of static pressure. Increasing the cross sectional area at the corners will again provide you with more impedance mismatches (this is a good thing).

Ok I found some spreadsheets over the net for calculating the static pressure etc.
It really seems impossible for me to do the calculations, I don't even know where to start
And I can't really afford to hire someone to do it for me..

You have to find the time to research this and learn it. Here is some info:

BTU = British Thermal Units = the quantity of heat required to raise the temp of one pound of water one degree Fahrenheit

Static Pressure is in Pa units. Your Air Hander Unit must be able to handle more Pa than the system is

Design cooling load (or heat gain) is the amount of heat energy to be removed from a house by the HVAC equipment to maintain the house at indoor design temperature when worst case outdoor design temperature is being experience.
Two types cooling loads:
- sensible cooling load = the dry bulb temp of the building (this is basically the air temperature we feel)
- latent cooling load = the wet bulb temp of the building (this is basically moisture from things like us sweating)

Heat gain = heat from components within or on a building such as people, appliances, sunlight, etc.

Heat loss = heat leaves through crack, windows, etc and cold air comes in

Sensible heat = actual temperature --> amount of energy needed to increase or decrease the temp of some substance

Latent heat = basically, humidity --> heat that results from an increase or decrease in the amount of moisture held by the air. The amount of energy needed to cause a phase change (liquid to gas, or gas to liquid) without changing the actual temperature of a substance.

For your calculator, all you really need to do is enter how many feet of each type of duct work there is. But, as you can imagine, you pretty much have to 3D model the entire system to know how big things are and how long they are.

so I think I will just stick to the current plan
-making the silencers twice bigger than the duct size,
-two silencer for each path-one inside the room and another one outside- (total of four) for in and out air
-use rigid round ducts where needed,
-frame paths with double drywall or OSB for the extension boxes,
-double OSB for the silencers,
-2'' duct line
and hopefully will work..

"hopefully"

I 've been reading here that the point of the registers should be twice the cross sectional area and with a cone type register so will not introduce any sudden changes.

I don't want to say "no". But to make my point, I'll say no, even though that's not an entirely wrong statement.

So, the registers have to be "large" enough to provide you with an air velocity of less than 300 feet per minute. Ideally, you want it even slower than that, but for most recording situations, 300 will provide you with a low enough noise floor to be acceptable. If your velocity is faster than this, recordings of acoustic guitar or soft singing with compression applied could result in the wind noise most high velocity home HVAC systems have. I think we've all watched YouTube videos with horrible background noise. That's typically from noisy HVAC.

Your air velocity can easily be calculated by:

Air velocity in feet per minute = CFM divided by your cross sectional area

So, once you know your CFM which is also a simple calculation of:

CFM = [(# of air changes per hour... which is typically a minimum of 6) multiplied by the volume of your room in cubic feet] divided by 60 minutes

And we already know the air velocity has to be <300 feet per min

Again, so once you know your CFM, you can do some basic algebra and rearrange the first equation to be:

Cross sectional area = CFM/air velocity
so
CSA = CFM/300

That will give you the MINIMUM CSA your register can be. Make it a bit bigger to play it safe.

To comment on the last section of your comment regarding the cone shape to not introduce sudden changes. Well, it's true that a gradual change in CSA like a cone (or wave guide) provides will prevent distortion or turbulence, the true rule of thumb is that you need a certain distance between any direction change or CSA change and the mouth of your grille/register. I've heard two values for this distance. It's either 3 or 5 times the diameter of that which is feeding the grille/register. So let's say your have a 4" round duct that goes to a required 6" round grille, you'd need a distance of between 12 and 20" of straight duct to prevent noise at the mouth.

Having said that, in my case after the silencer which will be at the back of the room I will use rigid duct to travel at the front of the room so that the fresh and the old air are far away from each other, to create a kind of flow.
The duct will have half the cross sectional area of the silencer and as you told me that's not a problem.

You will get higher insertion loss with smaller duct as the ratio of CSA to insulation is greater. That is all. I did not mean to imply that small high velocity duct is not a problem specifically with your design. It may introduce too much static pressure. Until you calculate it, you won't know. Realize that with too much static pressure, your fan will stall and you will get no ventilation. Or best case scenario, you will burn out your fan motor.

But if I indeed have to make the register twice bigger of the cross sectional area, what would that be, of the duct or the silencer?

The answer to this question is above. RE: air velocity needs to be <300 FPM

Greg

wow! thank you so much!

I already have the a/c unit from my previous home studio, just like the ventilation motor. I will check their specs.

Give me some time to read, and read and read again to understand your post.
I will make all the calculations and I will get back to you!

Gregwor wrote:So your ventilation ducts only need to provide you with 25-30% of your required CFM provided by your split unit.

Why? My a/c split unit doesnt provide air from the outside to the inside of the room, it just uses the existing air and heats or cool it. I dont know if there are many types of units but the common ones here in Greece do that.
The one I got has 14.500btu/h cooling capability and 16.500/h heating

Have you calculated your required CFM yet?

yes sir, its 900

For your calculator, all you really need to do is enter how many feet of each type of duct work there is.

Its 118 feet.
that would be 75.5ft of 14" duct and 42.5ft of 10" duct
including 8x90 and 12x180 degrees turns.

CSA = CFM/300
That will give you the MINIMUM CSA your register can be. Make it a bit bigger to play it safe.

I did the math , my Csa is 3. whats the unit? feet??????
and if yes is that area? i.e. square feet?

It's either 3 or 5 times the diameter of that which is feeding the grille/register.

At this point the duct will be equivalent to 14" so I need a distance of 3.5' to 5.8'.

Realize that with too much static pressure, your fan will stall and you will get no ventilation. Or best case scenario, you will burn out your fan motor.

I will keep the same opening as of the silencers once I am out of them, so I can keep the s.p. low.

Greg please do pm me your spreadsheet, everything I found is causing me troubles..or its just me..let me find out

Once again , thanks, you got me thinking and learning new stuff.
Good day today

Why? My a/c split unit doesnt provide air from the outside to the inside of the room, it just uses the existing air and heats or cool it. I dont know if there are many types of units but the common ones here in Greece do that.
The one I got has 14.500btu/h cooling capability and 16.500/h heating

I'm suggesting that you can calculate the amount of CFM your fan needs to supply. And from that, you can calculate your other needs (like duct size and velocity).

yes sir, its 900

Its 118 feet.
that would be 75.5ft of 14" duct and 42.5ft of 10" duct
including 8x90 and 12x180 degrees turns.

And what about the insides of your silencer boxes? Sounds like you have almost enough to go on!

I did the math , my Csa is 3. whats the unit? feet??????
and if yes is that area? i.e. square feet?

Correct. So if you need 30% of your entire room's CFM of 900, that puts you at 270 CFM. That puts you at 0.9 square feet which is 129.6 square inches.

At this point the duct will be equivalent to 14" so I need a distance of 3.5' to 5.8'.

You could design it such that the before your last change is smaller.

I will keep the same opening as of the silencers once I am out of them, so I can keep the s.p. low.

If your static pressure says you need to, cool, but if not, then you don't need to.

Greg please do pm me your spreadsheet, everything I found is causing me troubles..or its just me..let me find out

PM me to remind me. I'm unable to send it this moment

Greg

Gregwor wrote:I'm suggesting that you can calculate the amount of CFM your fan needs to supply. And from that, you can calculate your other needs (like duct size and velocity).

Yes I got that and I 've seen the number 25-30% again but I can't understand it..
I thought my fan had to supply 100% of the CFM since the a/c unit is not supplying any fresh air at all. It just changes the temperature of the air inside the room. I am missing something
I am trying to find more info but: 25, 30, %, are not accepted on search function and that makes it difficult..

And what about the insides of your silencer boxes?

Yes, including the silencers. I stayed up late last night..

So if you need 30% of your entire room's CFM of 900, that puts you at 270 CFM.

You are talking about if my fan needs to provide 30% of the CFM right? But see my 1st point.

You could design it such that the before your last change is smaller.

Well to be honest it is good like that because my silencer will be on the ceiling so I need to have the fresh cold air coming lower.
And its good for the exhaust too because of the extension duct going on the other side of the room.
I guess it is not a problem if its bigger than 5 times the duct..lets say 13' for 14'' duct.

If your static pressure says you need to, cool, but if not, then you don't need to.

My motor is capable of 590PA (air flow of 1600m3/h if that matters..)
So I am aiming for static pressure less than 590PA.

Yes I got that and I 've seen the number 25-30% again but I can't understand it..
I thought my fan had to supply 100% of the CFM since the a/c unit is not supplying any fresh air at all. It just changes the temperature of the air inside the room. I am missing something
I am trying to find more info but: 25, 30, %, are not accepted on search function and that makes it difficult..

In a normal home, the furnace moves the full required/calculated CFM. For example, say a home needs 2000 CFM. In the winter, the furnace isn't going to dump 2000 CFM out the exterior wall, bring in 2000 CFM and heat it up. This would essentially be replacing the air 100%. This is very inefficient. Realistically, you only need a certain percentage of FRESH air. So, systems usually have return air routed back to the furnace to heat up again and some small duct to pull in some fresh air to mix with the stale air.

So, in your case, with your mini-split, there isn't duct work and without the ventilation, it would just be heating up or cooling down stale air. You need some fresh air in your room, but as my example above shows, it would be silly to 100% replace the air. That's why you only need ~30% of your air replaced with fresh air. One fan on your exhaust side will naturally pull in the air in your supply side. But as you can probably picture in your head, you need your supply and return air quite far from one another otherwise you'll just be pulling in stale air!

HRV units are expensive but very awesome in that they exchange a very high percentage of your hot/cold energy.

My motor is capable of 590PA (air flow of 1600m3/h if that matters..)
So I am aiming for static pressure less than 590PA.

Once you calculate your required CFM and the static pressure of your system, just make sure that your fan matches that. The CFM will drop as more static pressure is placed on the fan.

Greg

Gregwor wrote:That's why you only 6need ~30% of your air replaced with fresh air.

That pretty much clears it up, thank you!

But as you can probably picture in your head, you need your supply and return air quite far from one another otherwise you'll just be pulling in stale air!

Yeah they will be like 8 meters apart from each other, the exhaust register will be near the front wall and close to the ceiling while the fresh air will be near the back wall close to the floor.


So from now on I am making all the calculations in order to find the static pressure , register size, etc, using the 30% of room's CFM (which is 270 as you said)

Let me ask one more thing.
what if the duct just before the register is bigger than the supposed 0.9 ft2 (that is calculated for the 270CFM)?
Then there is no need for a register at all, I can just use the rigid duct as is, right?