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

#31

some guidelines for all of this:
1) i like to have 6-8 room changes per hours. so 100m3 = 600m3 air volume per hour = 10m3/min
2) duct = 0.15m2 = 66m/min = ~1.1m/sec velocity

so, you would need to push roughly 1m/sec through the duct to achieve this turnover. now you might calculate a different result if using the L/person/hour approach but net-net you'll come out with the required velocity needed to achieve that turnover.

3) can your fan push 10m3/min? (or whatever you end up with?)

4) fresh air - you need to remove CO2 and improve the oxygen level. i use minimum 35% but some may define this as high as 50%. so your air exchange must support 35% of 10m3/min or 3.5m3/min
5) can your air exchanger support 3.5m3/min (or equiv)?

the device you linked to may support the cooling/heating but it will NOT do air exchange for a sealed space. and depending on temperatures, air quality, and humidity, you actually want to use a proper heat-recovery-ventilator (HRV) or energy-recovery-ventilator (ERV). and preferably a filter as well. and if the outside air is stinky (cars, factories, waste treatment etc), you'll want the filtering to address capturing that, since the goal is clean air...

6) is the 1.1m/sec too fast and will result in noise? (vent register noise, "whooshing" across mics, etc) or does it need to be slower?
7) a "plenum" which expands the volume will reduce the velocity. a speed of 0.5m/sec is desirable for quiet operations. so a plenum which has 2x the volume of the duct should do it. i often make these of duct board w/ liner and hide in the ceiling soffit as it can absorber LF also.

low noise vent registers which allow you to direct the flow

so - summary:

- use an ERV/HRV to provide fresh air -- it may be that depending on your room volume etc above, one of these units could also be your fan. if not, a fan top push the necessary volume is important.
- the system static drag can help reduce speed but could increase duct noises - so balancing that is important.
- speaking of balance -- a cross-supply-return duct is useful to adjusting the pressure balance. slightly positive pressure in the room is good. to maximise this, a return vent fan should be used as well as a supply vent fan.
-- personally - i prefer a proper hvac air blower system for ac, heat, filter, integration with ERV/HRV, and balancing -- and if outside the space and physically decoupled - will generally provide the best overall operation and maintenance access. a small shed on the outside of the garden room would likely suffice.
- build the ducts, silencers, plenums etc to address decreasing velocity and maintaining volume throughout the system - highest off the fan/blower, slowed via duct size and static drag, and plenum volume. reversed on return - large first, smallest last.

#32

That's fascinating to see Glenn approach this puzzle from a different angle. I absolutely agree with his recommendation for at least allowing the possibility of more than the nominal per person airflow I mentioned. In my own build I shot for 6 room changes as "standard" with the ability to go higher (probably 12 or more) or lower depending on how the room is being used. Some fans have wiring for multiple speeds, or you can get a proper variac for more control.

I think Tom used a sophisticated arrangement like Glenn recommends for this place. Saying that, and meaning no disrespect to Glenn, with the caveat I have very little experience of these things, such a system may be over specified for your needs. The climate in Great Britain is temperate and neither gets very hot nor very cold, and my understanding is that the efficiency benefit of HRV units is not so great in that environment, and your initial design with the split unit may be sufficient. Quite happy to be proven wrong though.

Glenn is spot on that the split unit will not provide airflow, just air conditioning. The copper pipe mentioned in the specs is a narrow thing carrying no air. If you do use this, it's worth paying attention to routing the copper pipe through the two leafs in a way that has least effect on your soundproofing. I think I've read about people having the installers bend the pipe so the holes it goes through in the inner and outer leaves are not directly in line with each other. You might want to check this out before leaping.

Before going on I'd like to point out that to maintain sound attenuation of a leaf, the surface density of the wall of a silencer should match the surface density of the leaf it is attached to. So for example if your building's outer leaf is made from 2 layers of 18mm OSB, the silencer for that leaf should also be made form 2 layers of 18mm OSB, or something with the equivalent surface density.

It would seem you have similar space constraints for the silencer's as Gareth's build. Have a look at this post in his build thread and you'll see the Z dimension of his silencers is quite small:

viewtopic.php?p=3390#p3390

From memory, I think Gareth had some difficulty getting sufficient airflow with this arrangement, so there is the opportunity for some learning here. As Glenn says, you can have the cross section on the silencers on the outer leaf a bit on the narrow side, as long as the inner leaf silencers terminate in the room with the correct cross section. Even allowing for that though, I think the possible cross section with the current silencer design is too small.

One way of increasing the cross section is to use the leaf of the building as one of the silencer walls. Starlight did this for one of his silencers, and you can see that in his build thread. The standard arrangement is represented by silencer A. But since this is for the outer leaf, we can just attach it directly to the leaf, meaning one side less is required. This will allow a larger internal cross section.

The downside is that it is more complex to build, and since the outer leaf in your case is the roof, it may not be possible if you've already felted the roof etc. If this is the case, I see a couple of possible workarounds;

1) Use a more powerful pan sufficient to drive your required airflow through the narrower outer leaf silencer path, or;

2) Mount the outer leaf silencer on the outside of the leaf rather than inside. So you'd likely have some sort of box or blister on a wall. Far less constraint on the size this way, but of course it will have to be weatherproofed.

People here and other places have discussed "straight through" designs or silencer boxes without baffles. There's a little bit about the latter in the Everest book, "Master Handbook of Acoustics" p.388 I'm afraid it is beyond my ken though; I've only ever studied, designed and build silencers with baffles in them.

Cheers!
Jennifer

#33

I've had a refresh on your design thread and see another possibility for increasing the Z dimension of your silencer boxes.

Here's a photo from the design thread:

You noted in the thread that the roof rafters (outer leaf) are 170mm deep and the inner roof ceiling joists will be 150mm deep.

In the above drawing, you have the outer leaf silencers above the inner leaf silencers, so they overlap when viewed from above. What if you altered the design so they did not overlap at all? I.e. the outer silencers in the same place as above, but the inner ones start toward the rear of the structure. That way both could make use of the full height of the cavity between ceiling and roof, about 300mm or so. A massive improvement in silencer cross section!

Also, I recall your roof has a slight slope, but the inner room does not. Therefore the cavity will be deeper at one end. This deep end should be where the inner leaf silencers emerge into the the inner room. This will allow the largest cross section of silencer at that point, giving slowest air flow speed, as per Glenn's notes.

Conversely, the shallow part of the cavity should be where the outer leaf silencers vent to outside. Air flow will be faster there, but it won't matter so much.

Cheers,
Jennifer

#34

thanks Jen - otoh - the use of the HRV/ERV is more than simply temperature - they can handle humidity and provide filtering, and even the air movement needed rather than just fans. however, if the humidity is fairly stable, then maybe just a decent filter on the fan units will suffice and the mini-split unit can handle the humidity and final temperatures.

so one more - you only need the inner silencers and since you only have a single room and the isolation is only the two layers. you can build a set of nested sleeves with plywood and drywall with duct liner between to provide a decoupled link between the silencer and the room (or if the silencer in on the inner room, then between the silencer and the exterior. a dual silencer setup (imho) would be overkill.

#35

Yes, good point, for best recommendations it would be useful if Sam could restate how much isolation he'd like to achieve in dB terms, I've forgotten.

I have these notes on the subject from some time ago. This was from Stuart:

the design of those silencer boxes depends on the overall isolation design. You need to know how many decibels of isolation you need, which in HVAC terms is called "insertion loss", so you can build those boxes correctly. You also need that number to determine how many silencer boxes you need: for low to moderate isolation, you can get away with just one silencer in each duct, but for high levels of isolation you need two: one on each leaf penetration.

I think this was too:

So that's the general concept. And the decision of "one box or two", depends mainly on "The Number One Big Question": How much isolation do you need? If you are looking for 40-something dB of isolation then you'll likely be fine with just a single box. But if you are shooting for the rather high goal of 60- or even 70-something dB of isolation, then you really do need boxes on every penetration.

One thing I should mention is that I have only one box per duct in my setup, i.e. two silencers in total, one for return, one for supply. My isolation goals were in the moderate 40 dB type range, and as far as I can tell they achieve that well in terms of sounds getting in and out.

However, despite the noise with the fan switched on meeting strict NR control room standards, it's still louder than I'd like. So in the future I plan on adding another silencer inside the room on the grille where the fan noise is coming from.

It may be that this could be reduced simply by having the fan further away though, and pushing air down a longer insulated pipe before entering the silencer. I really don't know!

Cheers,
Jennifer

#36

one option is to use larger slower fans in a supply plenum which is damped. then route into the duct. this can reduce fan noise and mechanical noise as well because you have room to decouple it better. not really a silencer but more about leveraging the speed to reduce noise.

#37

Sorry for delay; very busy at the mo..

Thanks for the help guys ! As I said, for the most part, it will be me in the room by myself. Obviously I need the capacity for 2/3 others when they may come in; I don't want to have capacity for that.

@Glenn - some of your post went a bit over my head; more reading/learning required

Not sure what the fan can push, was just copying Gareth/Jennifer as it seems up to the job. Thanks for the advice on the A/C unit. noted. I definitely don't want noise/draught coming in. I feel like the HRV you suggest is a bit overkill for my glorified shed :lol:

and having a separate shed to house external kit is not really an option... Also running out of money very quickly and trying to save costs where possible.

thanks also for the later post about the need for only 2 boxes!

@jeniffer - Thanks for your information. Noted on the A/C pipe, i will do something with that.
Appreciate what you are saying on "surface density of the wall of a silencer" - I was using one skin of OSB to increase cross section size, but can now address (see below)... Thanks for going back and looking at my pic of the boxes on my build, as you say there is a solution!

So I've been thinking, and before I even read your post, I realised that I could increase the vent boxes, as you say, If I go down to the roof of the inner room (300mm height or poss more, as you also say) So, that looks to be the best solution. Could even go a bit lower if I plan it well. I did wonder also about the reason for having 4 vent boxes (why could I not just have 2), I guess I was copying Gareth's design and just assumed that I should do the same... That is often the issue with copying; you're not actually understanding what the needs are. So thanks Glenn for picking up on that!

I'm looking at a moderate 40-45db of reduction reduction, Jennifer. So it looks like 2 boxes (1 inlet, 1 outlet) should be fine? Means I only have to make 2 boxes which is great, and also less liner and materials!

When I get a sec I will go back to the sketch up design and work out what I can do size wise for the maximum size. I will still place the boxes at the front of the room as that is where the max size is due to the slope. Once I do that, and have the max cross section available, I will post here and maybe we can discuss the best design going forward.

Once again thanks for the help! Pleased that at least there is a viable solution now...

Sam.

#38

Nice one.

Just to point out, I think the fan I used was different from what Gareth ended up with. Mine is a TD-SILENT 500/150 :

https://www.justfans.co.uk/silent-500150-p-1906.html

#39

endorka wrote:Source of the post Nice one.

Just to point out, I think the fan I used was different from what Gareth ended up with. Mine is a TD-SILENT 500/150 :

https://www.justfans.co.uk/silent-500150-p-1906.html

Cool! As I'm looking at just the 2 boxes now, I guess it makes sense to go with a quieter fan anyway, so that looks like a good option, thanks.

OK. So i've had a look at the design and, as always, it's going to be as straight forward as I hoped. the baffle boxes (inner and outer) do not line up above each other, and they over lap the stud end walls.

Yes I can extend the height (Z) of the boxes if I put them in the external, warm roof joists but because of the inner stud walls, I can't use the full width. Basically the max they could be (without cutting joists and other scary stuff) is:

310mm width
438mm high
(length - any)

And if I place them in the inner roof going up to the bottoms of the roof joists:

490mm width
267mm height
(length - any)

One thing here is that the width could be more than 490mm as I could move the joist over more as the inner ceiling stud has not been built yet. So, if to get the desired cross section we'd need to increase the width a bit, it is possible.

Bearing in mind that those are the external box dimensions, and you are suggesting I make them double skin, do you think that those new dimensions are enough to get a suitable cross section?

If not, It's back to the drawing board and more creative thinking...

Cheers,

Sam.

#40

Do you have a sketchup file or rendering of the stud structure, both inner & outer, you could post here? It would help visualise what you're describing.

Cheers,
Jennifer

#41

endorka wrote:Source of the post Do you have a sketchup file or rendering of the stud structure, both inner & outer, you could post here? It would help visualise what you're describing.

Cheers,
Jennifer

Hi Jennifer, sorry for the delay; been super busy at my proper job

Hopefully the pics below explain the issue... obviously the inner roof is not built yet but the external roof joists are fixed and have been screwed into now from the top deck.

Would the dimensions I provided for the bigger boxes move enough air, you think?

#42

BeardMusic wrote:Source of the post
And if I place them in the inner roof going up to the bottoms of the roof joists:

490mm width
267mm height
(length - any)

One thing here is that the width could be more than 490mm as I could move the joist over more as the inner ceiling stud has not been built yet. So, if to get the desired cross section we'd need to increase the width a bit, it is possible.

Bearing in mind that those are the external box dimensions, and you are suggesting I make them double skin, do you think that those new dimensions are enough to get a suitable cross section?

Doing some beer mat calculations here:

Calculate Z: Start with 267mm height. Subtract 2 x 36mm for the double skin silencer box gives 195mm. Subtract 2 x 25mm for the duct liner gives 145mm

Calculate X

In Gregwor's box he uses the formula X = (Y - 7) / 2

The 7 represents 7 inches, and must be deducted to account for the size of the walls & duct liner as above, and also extra space to allow the internal baffles to overlap each other to some extent. Let's convert that to metric for our purposes:

2 x 36 + 2 x 25 + 75 = 197mm

With Y of 490mm that gives:

X = (490 - 197) / 2 = 146mm

Cross section through the silencer can now be calculated by X * Z:

145 x 146 = 21,170 mm2 ~= 211 cm2

A recap of one possible set of requirements:

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

So, 211 cm2 is pretty close to 232 cm2. If you increased Y a bit by widening the gap between two ceiling studs, you'd be able to reach it.

Cheers,
Jennifer

#43

Now, here is another consideration.

If you are using one silencer per duct, you'll have to decide which leaf it is fixed to. This was easy in my case: the outer leaf is a ventilated roof space with soffit vents, roof vents, etc, so already has many "holes" in it. Little point in affixing the silencer directly to this outer leaf.

The inner leaf is a boundary to the house eaves, and other parts of the house also boundary onto the eaves. Rooms below, the studio live room, stairwell, etc. I wanted to prevent as much sound as possible getting into these rooms, so it made sense to fix the silencer directly to this inner leaf.

I'm not entirely sure how this might apply to your situation. In the absence of certainty in this, I'd be tempted to go with a two silencers per duct leaf approach, just in case. However, I think Glenn might have some thoughts on this too?

Cheers.
Jennifer

#44

i would use the inner leaf as the silencer boundary. you're generally going to get noise hitting that boundary anyways as it passes from the external boundary. the main goal of the silencer - protect the opening you make for air from being a noise source inbound or outbound. secondly to minimise any noise from the air handling system.

if somehow the opening is still the main source of inbound noise, then you could add some additional control inside the room - and you might for reducing air velocity (for example) a fully damped plenum to expand and deaden + another path of indirection for the flow.

#45

endorka wrote:Source of the post
BeardMusic wrote:Source of the post
And if I place them in the inner roof going up to the bottoms of the roof joists:

490mm width
267mm height
(length - any)

One thing here is that the width could be more than 490mm as I could move the joist over more as the inner ceiling stud has not been built yet. So, if to get the desired cross section we'd need to increase the width a bit, it is possible.

Bearing in mind that those are the external box dimensions, and you are suggesting I make them double skin, do you think that those new dimensions are enough to get a suitable cross section?

Doing some beer mat calculations here:

Calculate Z: Start with 267mm height. Subtract 2 x 36mm for the double skin silencer box gives 195mm. Subtract 2 x 25mm for the duct liner gives 145mm

Calculate X

In Gregwor's box he uses the formula X = (Y - 7) / 2

The 7 represents 7 inches, and must be deducted to account for the size of the walls & duct liner as above, and also extra space to allow the internal baffles to overlap each other to some extent. Let's convert that to metric for our purposes:

2 x 36 + 2 x 25 + 75 = 197mm

With Y of 490mm that gives:

X = (490 - 197) / 2 = 146mm

Cross section through the silencer can now be calculated by X * Z:

145 x 146 = 21,170 mm2 ~= 211 cm2

A recap of one possible set of requirements:
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

So, 211 cm2 is pretty close to 232 cm2. If you increased Y a bit by widening the gap between two ceiling studs, you'd be able to reach it.

Cheers,
Jennifer

Sorry, Work!

Thanks Jennifer, as always! This is great news! I've been doing a bit of thinking and I may even be able to cut some of the joist away to allow a bigger box. Not ideal but there 2nd joist v close so it will be fine. You can see what I mean in this pic: download/file.php?id=6670&mode=view

Reading what you and Glenn have said re' the leaf to mount it on, fixing to the inner leaf should be fine, and makes sense. I'll put together a plan for the boxes and update here.

I guess the longer the box I can get in there, the better for noise reduction...

Not had a lot of time to do much with the build, mainly due to the constant rain here in the UK. I'm working on completing the facia and roof at the mo, hoping to get it done this weekend.

Will update once I have progress... Thanks for your help.

Sam,

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