Garden Recording/Mixing Room in Surrey, UK - Build Diary

#16

Day 340 - Screwing the 1st layer of Wall OSB boards in earnest. Only had until lunch time as we had a 50th party to goto, so another early start to get as much done as possible. Boxes of hundreds of screws we disappearing quickly. Screwing the screws in every 6 inches or so on every board is very time-consuming. Managed to get all walls apart from the back wall done before leaving.

Planning to finish back wall and get the house wrap on tomorrow (after Sunday football match) before the rain comes; of which there is torrential downpours predicted for the coming week.

#17

Day 341 - Game Over! - After 70 mins of playing football, I twisted my knee and damaged my ligaments. Only just came back to sport after 2 years out with the same injury. Had to be carried back to the car (luckily it's automatic and my left knee) and drive home. Ice and elevation for the knee, and back on crutches. Unbelievable... Couldn't have come at a worse time. Regret playing now but it felt healed. I guess the knee wasn't ready.

My wife has been fantastic - all we could do was use the old DPC sheet, split it in half and staple it around the sides of walls. With the excess that she cut off, the windows/door holes could also be covered along with all the rest of the exposed walls. Lots of rain predicted this week so hopefully it holds out.

That's the project on hold for a bit but I can start researching/ordering the doors windows and other bits for the next stages... Such a bummer!

#18

bummer! maybe a local handy-person could be employed for a reasonable fee so you get the space enclosed and sealed against the coming winter months.

#19

gullfo wrote:Source of the post bummer! maybe a local handy-person could be employed for a reasonable fee so you get the space enclosed and sealed against the coming winter months.

Thanks Glenn.

We've got it pretty much wrapped up and I plan to be back on my feet (or at least one crutch) by next week, so I should be back on the build soon ish.

#20

Good work! Takes me right back, and I'm glad I'm not the only one with an outer skin that's not exactly true/square!

Gareth

#21

Day 382 - Knee is finally a bit better and so was able to make advances on the roof. The plastic sheeting I had sitting on the bottom of deck, whilst joined as best I could, still managed to let water through and underneath (water always finds a way) and leak through the studio roof, so it felt good to finally get the PIR insulation and top deck of OSB secured and the EPDM (Damn, Gareth was right about that stuff - Insanely heavy) over the top, although not secured yet as I'm waiting for facia board... At least I'm not going to get any more rain through the roof!

#22

Next job: Ventilation boxes. Tom has kindly agreed to help me out with the duct liner as I am really struggling to find it here in the UK. I need to start work on the boxes but have no idea how to calculate how big they should be. I'll be getting the same fan as Gareth and Jennifer TD500 Mixvent - 150mm - TD500.

My plan was to make each ventilation box out of one sheet of 18mm OSB (1.2m x 2.4m) which would be about 2.4m long, 150mm high and 500mm wide. Would this be fine? or can anyone chime in on how to calculate the sizes of the boxes please? The 2 external boxes are going between the roof joists, so I'm limited to a 150mm height/500mm width anyway, I guess I can make them longer but no idea how much that will help... Help please!

Cheers,

#23

given the duct/fan would max at 150mm, you could use the width to create expansion at the end of each to reduce the air velocity and reduce the need for a plenum in the room (and which could be added later if needed). example of how this might look:

#24

The design Gulfo has suggested is very similar to what I ended up with, although mine had about double the twists/turns, which might explain why my airflow is quite poor/slow. I am defo not the right person to guide on this aspect!

#25

I've looked out the old notes from my baffle designs. They're a bit rough and ready so please pardon the untidy workings. What I can say is that to the best of my judgement, the system successfully delivers the volume of air it was designed for.

To my mind the key measurement you are looking for is the cross sectional area of the path through the silencers. The entire design is based on this. This is based on how much airflow you need, and the required air velocity at the outlet. In Rod's book p.135 he says 15 cfm of fresh air per person is required. That's cubic feet per minute. My notes say:

Studio side wall area is 8.8 m2 =>
Volume is 31 cubic metres (1094 cubic feet)

6 changes per hour gives 6564 ft3/hr or 109.4 cfm or 186 m3/h or 51.6667 l/s. From Gervais book, more than enough for five active people to breathe.

So the minimum cross sectional area of register to maintain a velocity of less than 300 ft/m must be no less than 339 cm2 (CSA = CFM/300 or 109.4 divided by 300 = 0.365 ft2).

Some people aim for substantially less than 300 ft/m velocity. I've found it to be perfectly fine though.

This gives you the cross section area of the silencer path and outlet. I should point out that ventilation grilles will reduce that cross section, so if this is relevant you might have to add an expansion box at the end of the silencer (as per Glenn's post) or plenum in the room to achieve this.

Rod's book p.155 points out that the inlet to the silencer should be no less than half this (outlet) area. My notes:

Silencer inlet can be 170 cm2. 150 mm (6") diameter duct gives 177 cm2 which will work well.

I got the basis for the silencer designs and dimensions from Gregwor's silencer design template. It's a bit tricky to figure out at first, but if you persist it eventually clicks.

My notes say:

From Gregwor's silencer design for 3 baffles, with X=20 cm, Z=18 (gives 360 cm2 cross section), 25mm duct lining, 25mm walls, silencer outer dimensions are

36mm walls (2 x 18 OSB) is;
Y = 597mm
X' = 1126mm
Z' = 302mm

Add 75mm to X to get the width of the inner baffles => 275mm

The silencer walls should have an equivalent sound transmission reduction to the room leaf they are mounted on. That wall in the room had one layer of OSB + one layer of plasterboard, so I figured two layers of OSB for the silencer would be about right. For maximum noise reduction, if possible try to avoid having the inlet and outlet in the same plane, i.e. they shouldn't point the same way.

A related observation is that I made another baffle design based on a narrower cross section and slightly lower airflow. The static pressure calculations indicated a significant increase in static pressure, therefore I'd still need the 150mm fan to push the air through it all. The 150mm fan with (essentially) double that for the silencer cross section was optimal in theory and practice.

Cheers!
Jennifer

#26

Hi Guys,

Thanks for the replies!

Glenn, I assume your diagram was for the internal boxes only, and it seems to just have the internal room outlets at 300mm diameter. the design will be 150mm throughout until it would reach that outlet. Understood.

Jennifer - Thanks for taking the time to create such a detailed reply!

To be honest, some of it is over my head (I am only a musician after all, lol), but I do understand the gist of what you are saying. Also my 13 year-old son is a maths wizz so I'll get him to take a look later.

Seems that the important measurement is the diameter/opening of the channel in the boxes to allow the air flow as you mentioned. I don't think I'll ever have more than 4 people in there at a time but makes sense to have the coverage anyway. I' guessing the length of the box would increase the sound reduction as the only important thing is the volume of air through the channels. Unless a longer box enables more flow.. hmmm.

I think what I need to work out now (with my son's assistance) is the size of the channels, baring in mind that I am forced to make a box that is 500mm x 160mm due to them fitting inbetween the roof joists of 170mm thick. This would mean that the channel will no doubt be wider rather than square to allow the air floor within my constraints of the WxH of box.

to reiterate, assuming your 'Z' dimension of 302mm is the height, I will need a wider 'X' due to the height restriction of 150mm imposed by my joist height.

I'll do some calc's later and see what I come up with.

Cheers!

#27

endorka wrote:I've looked out the old notes from my baffle designs. They're a bit rough and ready so please pardon the untidy workings. What I can say is that to the best of my judgement, the system successfully delivers the volume of air it was designed for.

To my mind the key measurement you are looking for is the cross sectional area of the path through the silencers. The entire design is based on this. This is based on how much airflow you need, and the required air velocity at the outlet. In Rod's book p.135 he says 15 cfm of fresh air per person is required. That's cubic feet per minute. My notes say:

Studio side wall area is 8.8 m2 =>
Volume is 31 cubic metres (1094 cubic feet)

6 changes per hour gives 6564 ft3/hr or 109.4 cfm or 186 m3/h or 51.6667 l/s. From Gervais book, more than enough for five active people to breathe.

So the minimum cross sectional area of register to maintain a velocity of less than 300 ft/m must be no less than 339 cm2 (CSA = CFM/300 or 109.4 divided by 300 = 0.365 ft2).

Some people aim for substantially less than 300 ft/m velocity. I've found it to be perfectly fine though.

This gives you the cross section area of the silencer path and outlet. I should point out that ventilation grilles will reduce that cross section, so if this is relevant you might have to add an expansion box at the end of the silencer (as per Glenn's post) or plenum in the room to achieve this.

Rod's book p.155 points out that the inlet to the silencer should be no less than half this (outlet) area. My notes:

Silencer inlet can be 170 cm2. 150 mm (6") diameter duct gives 177 cm2 which will work well.

I got the basis for the silencer designs and dimensions from Gregwor's silencer design template. It's a bit tricky to figure out at first, but if you persist it eventually clicks.Gregwor's Silencer Box.png

My notes say:
From Gregwor's silencer design for 3 baffles, with X=20 cm, Z=18 (gives 360 cm2 cross section), 25mm duct lining, 25mm walls, silencer outer dimensions are

36mm walls (2 x 18 OSB) is;
Y = 597mm
X' = 1126mm
Z' = 302mm

Add 75mm to X to get the width of the inner baffles => 275mm

The silencer walls should have an equivalent sound transmission reduction to the room leaf they are mounted on. That wall in the room had one layer of OSB + one layer of plasterboard, so I figured two layers of OSB for the silencer would be about right. For maximum noise reduction, if possible try to avoid having the inlet and outlet in the same plane, i.e. they shouldn't point the same way.

A related observation is that I made another baffle design based on a narrower cross section and slightly lower airflow. The static pressure calculations indicated a significant increase in static pressure, therefore I'd still need the 150mm fan to push the air through it all. The 150mm fan with (essentially) double that for the silencer cross section was optimal in theory and practice.

Cheers!
Jennifer

OK. to follow up on this, we've sat down for a couple of hours working this out tonight and I'm sorry to say that we've become stumped. Sorry Jennifer but I'm not able to work out how you made your calculations.

We've managed to work out our CSA using your formula (CSA = CFM/300 or 109.4 divided by 300 = 0.365 ft2)

With my room internal dimensions (2128 ft3 / 60.25m3) we ended up with: (CSA = CFM/300 or 212.8 divided by 300 = 0.7093 ft2)

But we were unable to understand how you came to your calculations for the box using the above calculation (CSA = CFM/300 or 109.4 divided by 300 = 0.365 ft2):

36mm walls (2 x 18 OSB) is;
Y = 597mm
X' = 1126mm
Z' = 302mm

Surely 'X' is not the X of the box in Gregwor's Silencer Box diagram??

finally, our other issue is that we cannot increase Y of Gregwor's Silencer Box Design. Assuming 'Y' is the width of the box, I have a fixed with of 500mm because that is the distance between the roof joists between which I am going to place the box. They cannot be moved. As I said before, the height is also pretty much fixed as the height of joists are 170mm. I can go longer but it seems that as X is related to Y in Gregwor's Silencer Box design, making X bigger would increase the dividers and that would REDUCE air flow. I cannot seem to get a solution.

Hopefully that makes sense. Appreciate any help; really stuck on this one...

Cheers,

Sam.

#28

No worries at all, there was a bit of rambling and ruminating going on in my notes. Much of the design and build was done in something of a blur - it all made sense at the time

the key measurement you are looking for is the cross sectional area of the path through the silencers. The entire design is based on this. This is based on how much airflow you need, and the required air velocity at the outlet. In Rod's book p.135 he says 15 cfm of fresh air per person is required. That's cubic feet per minute.

For pure ventilation, i.e. exchange of stale air with fresh air in the room for breathing and other purposes, I went with Rod's figure of 15 cfm per person. Five people would require 75 cfm.

I muddied the waters a bit by then going on about room volume, and worked out that I could get 6 room air changes per hour (109.4 cfm) while keeping the air velocity at the registers within the speed limit. What mattered was that this was more than 75 cfm. I thought the additional airflow might be useful as the room gets hot in summer due to heat from lots of South facing windows; and in the absence of air conditioning, bringing more cool in air from outside would be useful. It is! In fact I can get up to about 12 room changes per hour with the fan at full speed. This means air exceeding the speed limit at the registers, but it's fine for "surge capacity", e.g. you can run it like this during breaks. Or if some uncouth muso breaks wind :shot:

If this is not important to you though, according to Rod, 75 cfm would be enough for 5 people. Adjust according to the maximum amount of people you'll have in your room. You may prefer to base normal airflow on normal use. E.g. one mix engineer, one client, would be 30 cfm. Maybe it only gets busier occasionally, and those occasions would permit exceeding the airflow speed limit at the register. If you let us know what your requirements are it would be useful.

See what I mean about rambling? Anyway!

Assuming 5 people and 75 cfm, the cross section area of the silencer can be calculated:

CSA = CFM/300 = 75/300 = 0.25 ft2 ~= 232 cm2

From Gregwor's silencer design for 3 baffles, with X=20 cm, Z=18 (gives 360 cm2 cross section), 25mm duct lining, 25mm walls, silencer outer dimensions are

36mm walls (2 x 18 OSB) is;
Y = 597mm
X' = 1126mm
Z' = 302mm

These were the external dimensions of the entire silencer box design. X' reads X dashed and is something I've retained from my old high school mathematics days. From what I recall, X' is like another version of X that is related to but not the same as X by a formula or transformation.

You're correct that in Gregwor's box design, Y is related to X. This is to preserve the same cross section area throughout as much as possible.

What you have to do next is establish how much airflow you need, then design the silencer in reverse as it were, and try to make it match the required airflow. It seems you have to limits on the external size of the box by the surrounding cavity which is:

Y = 500mm
Z' = 170mm

You'll have to allow fit tolerances for those though, so allowing 20mm on each side, in practice you could end up with;

Y = 460mm
Z' = 150mm (I'm assuming either the bottom or top can be in contact with the leaf it is part of, so only deduct 20mm)

Let's go into imperial units to make it easier to work with Gregwor's box design;

Y = 460mm = 18"

X = (Y - 7) / 2
= (18 - 7) / 2
= 5.5"

Z is based on the height, 150mm = 6 inches. The sides of the box will be 2" thick in total, the duct liner will be another 2" thick in total. This leaves 2" height within the box.

This gives the path through the silencer a cross sectional area of 5.5 x 2", i.e. 11 square inches.

This is equivalent to 71 cm2.

But to get the required airflow at less than 300 ft/min at the register, the cross section should be 232 cm2.

So the cross section through the silencer is not big enough for sufficient airflow for 5 people at less than 300 ft/min. It's not even enough for two.

With the above design, your only option would be to have the fan move the air through the silencer quicker than our specification allows. This will increase noise, drafts in the room, and increase the static pressure of the system, requiring the fan to work harder, which will make it noisier....

A small optimisation might be to have the top or bottom of the silencer made from e.g. the actual ceiling leaf itself. This would gain you an additional inch of Z, so cross section is now 5.5 x 3", i.e. 16.5 square inches. Still not enough!

As a last resort you can mitigate some of these problems to some extent with a larger end chamber as per Glenn's diagram, or a plenum inside the actual room. But you'll still be left with the higher static pressure.

Workarounds may be possible, but before that, are you with me so far?

Cheers!
Jennifer

#29

BeardMusic wrote:Source of the post I'm guessing the length of the box would increase the sound reduction as the only important thing is the volume of air through the channels. Unless a longer box enables more flow..

I missed this when writing the previous post, sorry. A longer box with more internal baffles will increase the sound reduction. It won't enable more flow though. The opposite is the case as it will increase static pressure meaning less airflow from a fan of a constant power.

I also hadn't notice these particular boxes are for your outer leaf. I had assumed the inner leaf. The general principles are the same though.

What is your outer leaf made from?

Cheers!
Jennifer

#30

Thanks for your reply, Jennifer!

Thanks for explaining, it is making more sense now. I am with you but don't like what I hear

(quickly, to answer your last post - thanks, that's what I thought but increasing static pressure is obvs bad. OSB for the boxes)

Basically, the important number is the CSA of 232cm2 as you say. Work arounds are needed because there is no way back now; have to find a solution. Can we talk workarounds please?

I don't like the idea of bringing more draught into the room or noise from over-working fans, so I'm interested to hear what you would suggest.

If you let us know what your requirements are it would be useful.

Mainly it will be myself working in there. that would increase to 2-3 with my family dropping in from time to time. I would like to have the min flow for up to 4 adults if possible. But again, this would be less common.

Things of note:

• I am planning to put in a A/C split system, which would, I assume bring in fresh air from outside. It's not clear on specs how much it would bring in. This one: https://www.aircondirect.co.uk/p/156135 ... s-pipe-kit or similar

• I am happy to use one skin of the OSB (18mm) for the internal/external boxes. That would give a slight size increase.

• Anyway to change the baffle boxes to have a different design that allows more air flow but at an increased noise level? Straight through for example?

• Having no idea what a plenum was before, a quick google reveals that they seem quite big. Not sure I can fit that in somewhere...

Looking forward to hearing any suggestions, let me know if you need any other info.

Cheers,

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