Mark's Studio Build Diary - The Construction Phase

Post and discuss acoustic topics, Studio design, construction, and soundproofing here

Postby MarkEdmonds » Sun Aug 29, 2004 9:56 am

Hi All,

Hasn't been much time for studio work over the last few days but a few things to report on.

Cutting Rockwool

Been meaning to say this for ages but kept forgetting. This tool should be considered a friend of anyone cutting lots of rockwool:

Image

I have tried a 12" hacksaw, bread knife, stanley knife with saw blade, tenon saw, jigsaw and regular stanley knife and none of them are as effective as the jet saw.

The jet saw has two trumps which the others don't have. First is you can halve or third the depth of entire rockwool slabs with ease so you can turn 100mm into 50mm or whatever - really useful for filling areas.

The other is that you can flex the blade and scoop out areas - details below.

I don't think it is good for the blade to cut rockwool but the time it saves and the clean cuts it makes more than makes up for this. Seriously, I strongly recommend it.

Carving a trihedral corner

Carrying on about flexing the saw blade to scoop out areas, I used this technique to shape this corner. It started out like this:

Image

There was no way I was going to be able to cut the RW3 into the desired shape before mounting it so I put it in as rectangles and then used the curved saw trick to convert it to this:

Image

This still left me with a considerable problem about covering it with the fabric so no exposed fabric ends are visible when all the surrounding covering is done. I eventually settled on pinning in some beading like this:

Image

and then wrapping round that to get this:

Image

When the corner trap covers are in, there will be no visible edges I hope!

Wrapping trihedral corners is a nusiance if your original design is poor - which mine was so big word of advice - always plan your frames with a view to getting seamless fabric covering. I've wasted a lot of time trying to solve this problem because I didn't plan properly in advance.

Final room tuning

I've now settled on the final room sound. The only thing I have changed from all the absorption is to add a bit of diffusion on the rear wall:

Image

Strange thing was that before I put the fabric up, I thought I was going to need much more of diffusion so I ended up buying way too many DST-Rs than I need.

Since the fabric has gone up, the room has changed. I am puzzled by this. The sound is definitely smoother and when I added all the diffusion I was planning to the room with fabric, it became very edgy and uncomfortable - so I had to take all the diffusion out again.

The only thing I can think of to explain this is that the fabric effectively blunts the right angles of all the wood in the frames. Is this possible?

Anyway, lesson to learn from this is that from my experience, putting all the fabric up with smooth the sound out so you must allow for the room to change as you complete it. At least with velcro attached frames, you can take them on and off a lot if you need to add stuff behind them.

Well, that's about it for progress. Bad news though is that I am going to run out of fabric. How the hell did I cock that up? Either I was sold short or I miscalculated. Anyway, the only problem with this is getting about 3 metres (that's all I need!) in the exact same colour and the people I bought it from advise that it is difficult when dying hessian to get exact matches. I will probably end up with a small section at a different shade. Damn :(

Mark
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Postby Bob » Mon Aug 30, 2004 12:58 am

Mike:
But have you tried an electric carving knife?
(BTW, I added your saw to the list of Cutting tools at
http://www.bobgolds.com/AbsorptionCoefficients.htm 95% of the way down)
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Postby Paul Woodlock » Mon Aug 30, 2004 3:03 am

Greetings

I've been using a serrated bread knife, which I got from the kitchen. Works fine.

Paul
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Postby MarkEdmonds » Mon Aug 30, 2004 4:32 pm

Bob - The offer of a loan of an electric carving knife was withdrawn when I said what I was going to be doing with it! :roll: I dunno, sometimes it is better to be economical with the truth!

Paul - when you get to do some awkward shape cuts, try the type of saw I've been using - trust me, it is hugely easier and more accurate than a bread knife. You get twice the cut per stroke due to the length of the blade and due to the height of the blade, it doesn't wander. Also, the tooth density is higher so it doesn't pull the rockwool as much as a breadknife and you get a cleaner cut.
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Postby Paul Woodlock » Mon Aug 30, 2004 11:08 pm

MarkEdmonds wrote:Bob - The offer of a loan of an electric carving knife was withdrawn when I said what I was going to be doing with it! :roll: I dunno, sometimes it is better to be economical with the truth!


Yeah, aren't some people just irritating. There's absolutely no logic in NOT using an electric carving knife to cut rockwool. I mean, have these people forgotton that the cleaning of utensils is commonplace these days?

Just as annoying is people who go cray when you use a plate or saucer as an ashtray. Particularly one that's just been eaten off. The damn thing's gonna get chucked in the dishwasher or sink anyway!!!



Paul - when you get to do some awkward shape cuts, try the type of saw I've been using - trust me, it is hugely easier and more accurate than a bread knife. You get twice the cut per stroke due to the length of the blade and due to the height of the blade, it doesn't wander. Also, the tooth density is higher so it doesn't pull the rockwool as much as a breadknife and you get a cleaner cut.


To be honest, I'm not bothered about getting a perfect cut. IT's all gonna be covered anyway. The bread knife I use is double sided and has very fine teeth on on one side anyway. :)

HOWEVER, the scooping out scenario looks like one of those saws will do the trick nicely. Good job I've got one! :)

Paul
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Postby MarkEdmonds » Fri Sep 03, 2004 10:28 pm

Evening All,

This job has been outstanding for a long time - I've been putting off doing it due to lack of clear design - the fan box.

I need to re-work this for a few reasons:

1. It is too noisy!

2. I have the duct length at the max spec for the fan which means it is running on minimum rating and isn't powerful enough for my liking. The ducting also goes through a few tight right angles which means it gets compressed which reduces the diameter and further reduces suction power. By redesigning the duct flow, I can shorten the length, make it straighter and therefore, increase overall power.

3. I need a good excuse to reposition the back-draft shutter which I never put on right in the first place.

Anyway, inspired by Paul's very quiet tornado generator system, I psyched myself up to strip out my fan area again and actually do the job properly. I've spent the evening roughing out the plan for construction over the weekend.

Use these photos for reference:

Image

Image

Here is the general idea:

The fan will sit in a double skin 18mm MDF box which will be underneath the triangular "shelf" - the shelf is structural and can't be removed.

The inner skin will be big enough to hold the fan and not much else. The gap between inner and outer skin will be up to 10cm where space permits and this will be filled with RW3.

The exit from the entire box will be a short duct run to the outlet vent with minimal duct silencing - it just isn't necessary - I did a test earlier with the fan right up against the exit vent and although the noise outside is less than ideal, it isn't going to upset my neighbour.

The box will be decoupled from all surfaces by resting on RW3.

The inlet to the fan box will come down through the triangular shelf.

The duct silencer box will sit above the triangular shelf and will also be decoupled by RW3. I am going to lose bass trap space for this but as the bass trap in that corner is pretty huge anyway (bigger and deeper than a Studiotips SuperChunk) , hopefully it will still work. The silencer box will probably by 2' high and 1' wide and deep. I plucked those figures out of the air not knowing the exact ideal ratios.

Now, Paul recommends chicken wire to keep the rw3 out of the air flow. I am not going to be able to get a small length of chicken wire so I am proposing to use the structure of the ducting itself. By this, for the height of the box, I will strip the duct of the plastic to expose the wire spiral. I will also cut the rw3 as well as I can to keep clean faces. Hopefully, the combination will prevent stay rw3 buggering up the flow and above all else, hopefully, keep the noise down.

Finally, the top of the silencer box will go to the inlet which will be in the same position as per original design.

All boxes will be screwed and caulked.

That's hopefully the master plan to solve this little issue - the last remaining "big" job I have to tackle. Once I have this done, I can put in the multicore which then leaves me completely free to finish off the fabric.

I'm leaving it a bit late to discuss this plan but any feedback (especially about the silencer box dimension ratios) would be gratefully appreciated. :)


Mark
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Postby Paul Woodlock » Sat Sep 04, 2004 3:04 pm

Greetings MArk


You know how quiet you thought my vent inlet was?

Well it's still far too loud :)

However, I' ve still got another silencer to put in front of it, so it will be OK ( Silent ! )once that is done.

Some tips and advice....


1] 'Room within a room' Fan box.


a] Make the INNER fan box HEAVY!!! - I did mine by leaving room to pour 4" of concrete in thw bottom. Steel is 3 times as dense as concrete so use that if your tight for space or don't wanna mix a small amount of concrete up.

b] Stuff wool tightly in the voids inside the iner box, and also put wool in teh void between the two skins.

c] When you place the INNER fan box, make sure whatver Elastomer material/foam,etc squahes by about 15 to 20% of it's original thickness.

d] Use a flexible coupleing between inner and out box. I made these out of an old pair of Woman's leather trousers.

2] Silencing the airflow

Use the whole corner trap as a duct absorber. This is easy, just cut a duct in the rockwool and let the air flow through it. As it's an extraction, any loose fibres will be sucked to the outside world.

The only reason I used chicken wire, was becuase I filled my duct silencers with loft insulation type rockwool, which of course isn't rigid.

Lower air-velocity is the key to quietness. I would arrange a series of 'caves' in side the rockwool corner trap, that allows you multiple outlet vents.

The inlet and outlet vents in my studio will do this. They will have a much bigger cross sectional area than the fan duct, and will both travel through rockwool bass traps to quieten the airflow to zilch.

hth

right I'm off to install drywall :)


Paul
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Postby MarkEdmonds » Sat Sep 04, 2004 4:57 pm

ARRRRRGHHHHH!

It's fecking noisier!!!!!!!!

ARRRRRRRRRRRRRRRRRRRRRRRRRRRGGGGGGGGGGGGGGGGGGGGHHHHHHHHHHHHHH!



SHIT :( :( :(



I'm going to get a bite to eat before working out what the SPAMMER went wrong.

Very pissed off.

Mark

PS: Thanks for the tips Paul - will digest these after food.
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Postby MarkEdmonds » Sun Sep 05, 2004 12:16 pm

Salutations - here is the story of the fan box.

As said in previous post, the reasons for the rework are noise and power so I set out on Saturday to build the fan box and a silencer box.

It was a fantastic day really. Weather was brilliant. Set everything up outside and really enjoyed building the boxes. If I was the singing type, I would have been on top voice. I discovered an extra benefit of studio building - it satisfies the urge to build things. We all need to build things - part of the male instinct to make the home whilst wo-man tends to the young. No apologies to any feminists - that is the way nature works.

Building boxes is especially satisfying. This must stem back to the cave days when man decided to put wo-man in a box once in a while when she wouldn't stop talking.

Anyway, fan box was built first. Here is a pic during construction:

Image

Note the position of the exit hole. I was very tight for space if the ducting was not going to be too forced at a right angle. This also meant I couldn't make the box double skin - there simply wasn't going to be room in the space available given angles of air flow and size of fan. However, I felt certain that by heavily damping out the inside of the box and with layers of RW3 on the outside, it would work.

Wind the clock forward to the big switch on:

Image

Made myself a coffee and rolled a ciggie in anticipation of the good feeling of a job well done. Click the switch and ......... arrrrrrggghhhhhh! Something has gone seriously wrong :( :( :( Not only is it much louder but the dominant frequency has gone down. :( In fact, it is so bad that I felt it driving me mad within a couple of minutes. Aural torture. :( This was not desired.

Unfortunately at this time of year, it gets dark earlier and I still had the silencer box to build so although I didn't know what was wrong with the fan box, I had to get the silencer done with 2 hours daylight left.

Clouds of MDF later, it wsa complete...

Image

The silencer box was an unqualified success :) It really dropped the noise in the ducting but the problem remained with the earthquake effect coming from the fan box.

I discovered that the noise problem was too much RW3 in the box. It was coupling the fan too hard to the box and the whole box was acting as a amplifier. By keeping the walls of the box lined but by having nothing touching the fan, the noise dropped massively. I started to feel good about this again but no further work for the day as I needed to scrape layers of MDF and RW3 fragments off me.

Back to work this morning on tuing the box and I got the magic combination :) Now the whole area is filled in again with the fabric panels in place.

How noisy is it?

Pretty damn fucking quiet even if I say so myself! :D

When you walk into the room, you wouldn't know it is on.

When you sit at the mix position, the hiss from the monitors is more obtrusive. You can make the fan out but it is hardly there.

It is only when you stand in the fan corner that you can hear it.

I am still having trouble believing that it is really working and keep going outside and testing the exit vent to make sure. There is a nice healthy air flow coming out as well - not a force 10 gale by any means but enough to keep the air moving out the room I am sure.

I think it works! :D :D

I need to thank Paul again for showing me (or letting me hear) what is possible and that gave me the incentive to finish this job. Cheers old chap!

Mark
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Postby Bob » Tue Sep 07, 2004 8:57 pm

Mark:

Your box around the fan isn't a "box in a box" the way Paul's is rumored to be, but as long as it works !

When you walk into the room, you wouldn't know it is on.

Someone over at RO once posted that they had a fan control switch in the room, and turning it on or off they couldn't tell that there was a change audibly, although there was airflow (paper taped over vents moved).


BTW, I know it's too late now, but here's an alternative for driving staples:

from: http://www.avsforum.com/avs-vb/showthre ... ost4310828
A compressor rated at 4 gallon 3HP which creates about 8CFM at 90PSI will handle several kinds of tools: hammer drill, sander, cut-off tool, even some spray painters. The requirements for brad nailers and/or staplers are very small

Porter-Cable CFFN250N Finish & Brad Nailer Combo Kit
Porter-Cable US58 22 Gauge, 5/8" Upholstery Stapler Kit
Porter-Cable NS150A 18-Gauge Narrow Crown Stapler Kit
Regards
Bob Golds
"The only thing we regret in life is the love we failed to give."
"Be a rapturist -- the backward of a terrorist. Commit random acts of senseless kindness, whenever possible" - Jake Stonebender
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Postby Paul Woodlock » Wed Sep 08, 2004 1:04 am

Yep, my fan box is a box in a box.

It's installed in the Garage Attic, but even when you crawl into the attic you cannot here it when it's on. ANd it was a FUCKING LOUD FAN in free air.

Mark, I'm assuming your 'duct silencer' ( top thinner MDF box ) has the airflow in direct contact with the rockwool ( Except for cloth maybe? )

It looks a great job mate. I bet your real pleased with the SILENCE!!!

YOu could quieten it even more if you were to take more than one duct from the duct silencer to teh room. This is because you double the cross sectiona; area of the duct and thus, halve the air velocity. This is what I would have done, and in fact doing on my build.

I shall be at least quadrupling the duct area where they enter and exit the room. And although I have HUGE Silencers to take care of Isoaltion issues, I will also be using secondary duct silencers as well. IN fact I shall be cutting TUNNELS in bass traps to accompilsh this, so the WHOLE CORNER TRAP ( Studiotips chunks ) will act as a duct silencer.

You could do that yourself. Simply remove the plastic duct from the vent to your MDF duct silencer,and use a carved out tunnel in the Corner trap.

regarding my fan box, it's not only a box in a box, I also made flexible couplings out of leather between the two boxes from a pair of womans discarded trousers. Maximum isolation :)

The INNER box also has 4" of concrete poured in the bottom to increase mass-spring-mass.

I'll post some piccies so you can get the idea.

Paul :)
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Postby Paul Woodlock » Wed Sep 08, 2004 1:11 am

Paul's DIY Washing Machine, sorry, FAN BOX!!
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Paul Woodlock
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Postby Bob » Wed Sep 08, 2004 1:16 am

Paul:

IN fact I shall be cutting TUNNELS in bass traps to accompilsh this, so the WHOLE CORNER TRAP ( Studiotips chunks ) will act as a duct silencer.

When there's rockwool surrounded by wood/gypsum I can see how it absorbs. But if there's no wood/gypsum, won't sound just go through the studiotips chunk into/outfrom your room?
Regards
Bob Golds
"The only thing we regret in life is the love we failed to give."
"Be a rapturist -- the backward of a terrorist. Commit random acts of senseless kindness, whenever possible" - Jake Stonebender
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Postby Paul Woodlock » Wed Sep 08, 2004 3:30 pm

Bob wrote:Paul:

IN fact I shall be cutting TUNNELS in bass traps to accompilsh this, so the WHOLE CORNER TRAP ( Studiotips chunks ) will act as a duct silencer.

When there's rockwool surrounded by wood/gypsum I can see how it absorbs. But if there's no wood/gypsum, won't sound just go through the studiotips chunk into/outfrom your room?


Interesting point bob. However sound doesn't just 'go through' the CHUNK, otherwise rockwool would be pointless.

So the WHOLE CHUNK is a HUGE duct silencer. Bear in mind the duct is Already silenced with the huge splitter silencer, so the absorption I'll be getting from the chunk is there to catch the last remaining noisy airflow from the splitter silencer to the vent.

And in any case you don't need a gysum/wood boundary for absorption to work.

Try making a 4 sided curtain of absoption in the middle of a room ( from floor to ceiling ) ( Sort of telephone box size ). An amazing amount is absorbed ( as there's an air-gap of many feet around the 'absorbing phone box' ) Such a structure is excellent for recording vocals in, in a normally crappy sounding domestic room.

Another way to look at it, from a ventilation POV is to take a regular air vent and put some slabs of rockwool around it to form a rockwool ONLY tunnel ( no wooden case around it ), and see if it quitens it considerably. My money is on it doing so :)

I'm only 95% certain of this theory mind you ;), and I'm all up for experimentation. And if it works, then it's another tip and trick to put in the FAQ :)


Paul
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Postby Bob » Wed Sep 08, 2004 4:38 pm

Paul:

Sound drops (TL) through a layer of gypsum by several orders of magnitude in energy.
Sound drops (TL) through a layer of rockwool by a half an order of magnitude.

I think your corner trap ducting idea is an 'experiment', and that it'll leak sound near the out-of-the-room end.

You're the king of box-in-a-box. How loud was your fan when you just wrapped rockwool around it (without wood)?
Regards
Bob Golds
"The only thing we regret in life is the love we failed to give."
"Be a rapturist -- the backward of a terrorist. Commit random acts of senseless kindness, whenever possible" - Jake Stonebender
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Postby MarkEdmonds » Wed Sep 08, 2004 6:10 pm

Bob wrote:Mark:

Your box around the fan isn't a "box in a box" the way Paul's is rumored to be, but as long as it works !

When you walk into the room, you wouldn't know it is on.

Someone over at RO once posted that they had a fan control switch in the room, and turning it on or off they couldn't tell that there was a change audibly, although there was airflow (paper taped over vents moved).


BTW, I know it's too late now, but here's an alternative for driving staples:

Wrom: VTLBXFGGMEPYOQKEDOTWFAOBUZXUWLSZLKBRNVWWCUFPEGAUTFJMVRESKPNKMBIPBARHDMNN
A compressor rated at 4 gallon 3HP which creates about 8CFM at 90PSI will handle several kinds of tools: hammer drill, sander, cut-off tool, even some spray painters. The requirements for brad nailers and/or staplers are very small

Porter-Cable CFFN250N Finish & Brad Nailer Combo Kit
Porter-Cable US58 22 Gauge, 5/8" Upholstery Stapler Kit
Porter-Cable NS150A 18-Gauge Narrow Crown Stapler Kit


Bib - no, not a box in a box. I was running into a space and angles of duct problem so resorted to single skin. I wish I had seen Paul's design beforehand though because, *slaps forehead in annoyance*, it never occured to me to build the inner box directly round the fan. I was trying to fit it all in by having the fan and duct routing in the inner box and then the outer box round this. It simply wouldn't fit.

That said, it has been pretty successful. I am going to run a simple test on all people visiting: Sit them in the mix position with the fan in one state but all other equipment on (no music). Give them 20 seconds to listen then eject them from the room, toggle the fan state and repeat the test. I am pretty confident that in 80% of cases, they won't be able to tell. I actually check the vent once in a while just to confirm it is still running.

I have to say that I am bloody chuffed with this. I hummed and harred over putting in a fan in the first place but went for it. I think it cost about 150GBP to install so now that it is working, I do feel pumped up about the success! :)

Concerning the stapling links - yes it is too late for me - but I hope you are keeping the links for future reference so someone will benefit from your research.

Mark
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Postby MarkEdmonds » Wed Sep 08, 2004 6:18 pm

Paul Woodlock wrote:Mark, I'm assuming your 'duct silencer' ( top thinner MDF box ) has the airflow in direct contact with the rockwool ( Except for cloth maybe? )

It looks a great job mate. I bet your real pleased with the SILENCE!!!

YOu could quieten it even more if you were to take more than one duct from the duct silencer to teh room. This is because you double the cross sectiona; area of the duct and thus, halve the air velocity. This is what I would have done, and in fact doing on my build.

I shall be at least quadrupling the duct area where they enter and exit the room. And although I have HUGE Silencers to take care of Isoaltion issues, I will also be using secondary duct silencers as well. IN fact I shall be cutting TUNNELS in bass traps to accompilsh this, so the WHOLE CORNER TRAP ( Studiotips chunks ) will act as a duct silencer.

You could do that yourself. Simply remove the plastic duct from the vent to your MDF duct silencer,and use a carved out tunnel in the Corner trap.

regarding my fan box, it's not only a box in a box, I also made flexible couplings out of leather between the two boxes from a pair of womans discarded trousers. Maximum isolation :)

The INNER box also has 4" of concrete poured in the bottom to increase mass-spring-mass.

I'll post some piccies so you can get the idea.

Paul :)


Paul - yep, silencer box is 1' x 3' lined with RW3 and the duct attached to top and bottom. Works really well. It was modelled on the diagram you sketched out on that sheet of MDF :)

I wanted to use the entire corner trap for the silencer duct but couldn't come up with a neat way of drilling the tunnel. The biggest hole saw I have is only 65mm and this would need to be 120mm and then there is the extension bar length. How are you going to drill through many feet depth of RW3 to get the tunnel perfectly in line? I can only assume you will do it slab by slab which means you have to get them all aligned perfectly once bored. With the power of fan I am using, any unevenness in the tunnel would reduce air flow - perhaps with your fan (which looks like the back of a jet engine!), this wont matter?

Mark
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Postby Paul Woodlock » Wed Sep 08, 2004 6:49 pm

Bob wrote:Paul:

Sound drops (TL) through a layer of gypsum by several orders of magnitude in energy.
Sound drops (TL) through a layer of rockwool by a half an order of magnitude.


Yes I appreciate that. Prrof of my appreciation is on teh bomb shetler room design :) :)

I think your corner trap ducting idea is an 'experiment', and that it'll leak sound near the out-of-the-room end.


could you explain further "out of the room end"? I'm not exactly srue what you mean by this.

You're the king of box-in-a-box. How loud was your fan when you just wrapped rockwool around it (without wood)?


Point taken, but we're not talking HUGE LOUDNESS here ( Like the FAN in free air ), just some residual duct noise that I'm hoping the CHUNK will sort out.

I can appreciate your concern with this 'experiment', but the difference between running the duct silencing wool in a box, and NOT is the difference between having,say 1 foot of absoprtion around the duct ( with the box as the solid boundary ), or 8 feet of absorption around the duct ( With the walls. floor and ceiling as the boundary.


In any case if it works, then we have a new technique ;)


Paul
Last edited by Paul Woodlock on Wed Sep 08, 2004 7:02 pm, edited 1 time in total.
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Postby Paul Woodlock » Wed Sep 08, 2004 6:58 pm

MarkEdmonds wrote:
Paul - yep, silencer box is 1' x 3' lined with RW3 and the duct attached to top and bottom. Works really well. It was modelled on the diagram you sketched out on that sheet of MDF :)


Cool! :)

I wanted to use the entire corner trap for the silencer duct but couldn't come up with a neat way of drilling the tunnel. The biggest hole saw I have is only 65mm and this would need to be 120mm and then there is the extension bar length. How are you going to drill through many feet depth of RW3 to get the tunnel perfectly in line?


I'm nto going to try and get the tunnels perfect. There's no need IMHO. see below...


I can only assume you will do it slab by slab which means you have to get them all aligned perfectly once bored. With the power of fan I am using, any unevenness in the tunnel would reduce air flow - perhaps with your fan (which looks like the back of a jet engine!), this wont matter?

Mark


Yup,. I'm proposing to form the tunnels ala Pyramid construction. IOW, build them in slab by slab.

The slight reduction in airflow won't matter. After all people run ducts in all sorts of non-straight runs to try and cut duct noise, which also reduces airflow. Bear in mind the original duct area is 200m diameter, but the eventual vent to the room will be a much larger area than that, to reduce air velocity.

Having said that, I will try and get the tunnels as smooth possible though, I'm just not gonna waste time getting them perfect to get a 0.0001% increase in airflow :)

Anyway, it's a long way off, I gotta finish the Inner room first.

My money's on it working. It's a gut feeling thing, and sometimes you can't ignore those guts. :)

Paul
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Postby Bob » Wed Sep 08, 2004 9:33 pm

Paul

could you explain further "out of the room end"? I'm not exactly srue what you mean by this.


It's a duct. One end is in the room. The other end is out of the room. (This is true for both the forced air duct and the return air duct.)

In this case we're talking about a part of a duct that runs the length of the corner trap. I'll call this run the 'tunnel'. One end will be closer to the room, and one end will be closer to out-of-the-room.

There will be noise traveling the length of the tunnel through open air (say 8'). There will be noise from the room's useful workspace (you) through the rockwool and into the tunnel (say 1.5'). Any noise that takes this second route will skip the efffectiveness of the tunnel's absorbtion, which may matter or not..

I suspect we're picking nits. :)
My inexperienced bet is that you're exposing yourself to more rockwool air contaimation without benefit.
Regards
Bob Golds
"The only thing we regret in life is the love we failed to give."
"Be a rapturist -- the backward of a terrorist. Commit random acts of senseless kindness, whenever possible" - Jake Stonebender
Bob
 
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