Help with one room studio design in Delta BC, Canada

[Gregwor]

Here it is. Here is a diagram showing how based off of something such as your joist spacing, you can determine 2 of your silencer box dimensions. This diagram is assuming you're using 1" MDF for the exterior of the box and 1/2" MDF for the baffles. You can add length to your box if you have the space which would add more impedance mismatches and also decrease your static pressure.

Gregwor's Silencer Box.png

Greg

[Bhudie]

Okay, I checked out my notes here. I messed with it and leaving a total of 1" of MDF on the baffles overlapping one another through the path (1/2" on the end of each baffle) here is it:

That makes sense to me now. I never extended the baffles within the box on my design. I will incorporate that into my design.

First, your speakers are too high up, and they are angled down.

Thanks for the detailed explanation of the reasons why, and yup, I get it now and am on board. I will flatten out my baffle walls and bring down the speakers to ear level again.

Second, you seem to have the acoustic axis of your speakers drawn in the wrong place: it looks like you have it centered on the tweeter, but that isn't correct. Check with your speaker manufacturer to find out where the real acoustic axis is for your specific speaker.

The manufacturer says that the speaker was designed so the acoustic axis is dead center of the 3" mid driver which is where I have it. There is a small tweeter above the mid driver as well.

Third, you have your speakers laying on their sides, which is not such a good idea (unless the specific speaker you are using is designed to ONLY be used like that).

Thanks again for the detailed explanation. I haven't seen documentation that the speaker MUST be on its side, but the manufacturer has definitely built the speaker so that it is intended to be on it's side. My guess is because when the speaker is on its side, the tweeter is directly above the mid driver, and only the bass driver is off to the side, so given the explanation you provided, orienting the speaker vertically might have the same issues as you are suggesting I have while orienting it sideways, but with the more directional high frequencies. Either way, I will likely reach out to them to get a definitive answer.

Then you have your computer monitors and secondary speakers in the wrong place! Move them.

Roger. Moving them.

Desks have a very negative effect on room acoustics. I have developed a design you could use if you want, for a low impact desk that doesn't have too much negative effect on room acoustics.

If you are willing to share, I would love to see your design to see if I can incorporate it. Thanks!

Use whatever surfaces you can at the front of the room to force the early reflections away from the mix position, towards the rear of the room where they can be attenuated. Angle those wings accordingly, with hard, solid, rigid, massive surfaces in the center regions, and do the top and bottom regions similar to the soffits themselves, with bass trapping.

Will do. And as Greg suggested, I'm doing some investigation into ray tracing in Sketchup to get the best angle.

Ummmm.... why won't the sole plate be anchored to the floor?

Reading back, I may have misinterpreted the comment below to mean that I didn't need to anchor the base plate of the soffit to the floor...

Quote:
If the soffit structure just needs to be affixed to the walls firmly, but doesn't need to be bolted to the floor, then I can lay the floor now underneath and build the soffit on top post reno.

It depends on how you design it, but I normally have a sole plate that rests on the slab across the front of the soffit, and the floor butts up against that. Then there's trim on the soffit that covers the floor/soffit junction.

If I did misinterpret that, I'll go back and figure out a way to anchor it in my design. Either via getting the thermal camera to figure out where I can put anchors in, or gluing down if I can't bolt it given the radiant heating.

Try to find legrand: it's the best.... just not cheap.

Thanks for the info!

Here it is. Here is a diagram showing how based off of something such as your joist spacing, you can determine 2 of your silencer box dimensions.

Thanks Greg! This is very helpful.

OK, so with all that I can now eliminate putting the Silencer boxes in the joists as an option because I just don't think I can achieve the required cross sectional area.

I now am looking at 2 options. I will look at seeing if I can utilize the space behind my baffle inside the room to accommodate the inner leaf ones, and maybe mount the outer leaf ones on the east outside wall. Failing that, I think my only option is something like the ones discussed in this thread. (I finally found the thread with the one silencer box through 2 leaves)

This all leads me to a few more questions.

1) If I am unable to achieve 2x the cross sectional area of my register in any of my designs, is there a way to design around this? I'm thinking maybe Greg's suggestion of making the silencer boxes longer with more impedance mismatches could help compensate. Is this a thing? Or is there a requirement that the impedance mismatch must be at least 2x to have any effect?

2) If I am able to fit the boxes behind my baffles on the ceiling (haven't had a chance to model this in sketchup yet, but it will be my first task when I get home tonight), should those boxes be incorporated into my inner leaf, i.e. drywalled around them as if they were soffits? Or should I build my inner leaf and then punch a hole through it and mount the silencer boxes inside the inner leaf?

3) If the above fails, is doing what saemola did in this thread. a viable option using the pool equipment room next door?

4) If #3 is an option, how would I calculate how much I would potentially lose in isolation from only having 2 vs 4 silencer boxes?

5) I know I am going to have to build a cloud at some point, but I honestly haven't spent much time at all thinking about this. Can you suggest some good resources that I could use to get up to speed on what I need to consider in a cloud design? I'd like to get to a rough size and position pretty quickly so I can finalize where my track lights need to go. Or is this just the same concept as the baffle wings, deflect reflections from the speaker to the back of the room?

Thanks again to you both. I really appreciate all the help.

-Aaron

[Gregwor]

If you are willing to share, I would love to see your design to see if I can incorporate it. Thanks!

Basically just make the desk as small as possible, maybe angle the top of it a bit to prevent reflections from bouncing off of the desk into your ears. Also, cut as much material out of the sides and back of the desk as you can and put insulation in those holes. You can use a perforated material to cover it up if you'd like. Put as much of the equipment down low in the front of the desk as possible. Other than that, customize it for YOUR needs and YOUR room. Personally, I hate having my hands up high so my plan is to have a low drawer type spot for my keyboard and mouse. I'll also embed my screens into my desk. I don't mind looking down at my screens and this will also keep my view into my ISO room nice and open. The top of my desk where there is no need for a hard surface to hold things like drinks, it will be either perforated or just fabric covered insulation. Basically making the desk as "invisible" acoustically as possible.

If I did misinterpret that, I'll go back and figure out a way to anchor it in my design. Either via getting the thermal camera to figure out where I can put anchors in, or gluing down if I can't bolt it given the radiant heating.

Construction grade glue (think PL Premium) is pretty insane stuff. It will literally rip the wood apart as you try to remove it from the concrete. For anything other than walls that have door frames in them, you're probably safe with this glue. If you're renting a thermal camera, it won't hurt to throw some anchors in here and there.

1) If I am unable to achieve 2x the cross sectional area of my register in any of my designs, is there a way to design around this? I'm thinking maybe Greg's suggestion of making the silencer boxes longer with more impedance mismatches could help compensate. Is this a thing? Or is there a requirement that the impedance mismatch must be at least 2x to have any effect?

Here's an image showing the effect of increasing or decreasing your cross sectional area. As you can see, even doubling the cross sectional area doesn't yield amazing results. The bigger the difference, that greater the insertion loss!

Insertion Loss Graph from Engineering Acoustics 2009.png

2) If I am able to fit the boxes behind my baffles on the ceiling (haven't had a chance to model this in sketchup yet, but it will be my first task when I get home tonight), should those boxes be incorporated into my inner leaf, i.e. drywalled around them as if they were soffits? Or should I build my inner leaf and then punch a hole through it and mount the silencer boxes inside the inner leaf?

Whichever way you can fit them. It's best to fit them in between your inner and outer leaf so that they aren't eating into your inner leaf space, but if your inner leaf box has to be inside your inner leaf, no problem. The biggest compromise is that they typically end up fitting into a tri corner that will then hinder your bass trapping.

4) If #3 is an option, how would I calculate how much I would potentially lose in isolation from only having 2 vs 4 silencer boxes?

I can tell you from personal experience that the difference from having 1 silencer box to 2 on a run is crazy. I would NEVER suggest using a single box after hearing the difference. The bigger the box, the better. It makes sense if you think about it. The further the sound has to travel through a large box, the more insulation there is. The quieter it gets.

5) I know I am going to have to build a cloud at some point, but I honestly haven't spent much time at all thinking about this. Can you suggest some good resources that I could use to get up to speed on what I need to consider in a cloud design? I'd like to get to a rough size and position pretty quickly so I can finalize where my track lights need to go. Or is this just the same concept as the baffle wings, deflect reflections from the speaker to the back of the room?

Just like soffit wings! Ray trace it. Like soffit wings, it needs to have a hard backing (think at least 3/4" OSB) and having insulation below the OSB will help. In theory if it is angled correctly, it could just be a hard surface, but it's not really acting as a baffle extension so much as just absorbing and reflecting sound towards the back of your room. Therefore you'd be crazy to not have insulation on the bottom side of it. Filling above the cloud with insulation is a great place for insulation too. Make sure you place your lights in a position so that they won't reflect off of your screens or windows into your eyes.

Greg

[Bhudie]

For anything other than walls that have door frames in them, you're probably safe with this glue. If you're renting a thermal camera, it won't hurt to throw some anchors in here and there.

Thanks Greg!

Here's an image showing the effect of increasing or decreasing your cross sectional area.

Whoa, if I'm reading that right, having 2x the cross sectional area impedance change only gives you about half a db in TL?

If that's correct does that mean that unless you can get much more multiples on the cross sectional area, the majority of the attenuation comes from the turns inside the silencer box?

And if that's correct, am I right to assume just making way longer boxes is a better use of my efforts than trying to wrestle with getting big impedance changes given my lack of usable space?

Same question said a different way...

I can tell you from personal experience that the difference from having 1 silencer box to 2 on a run is crazy

Is the difference from having 2 silencer boxes on a run, vs having 1 that is as long as the two separate boxes combined significant as well?

Use the search feature on the forum to look up ray tracing. It's very very important that you do ray tracing in SketchUp to ensure your design works perfectly.

OK, I took my first punt at ray tracing in Sketchup. Here's what I did. First I ray traced without any soffit wings to see where my problem areas would be... I came up with this: (arrows showing the rays that came close to my listening pos, x showing approximately where I would need to extend my wings to)

RayTraceNoWingsResized.jpg

Then, I tried to extend the wings such that they covered the intersection of those rays, but I believe the new wing angle introduced a new problem ray...

RayTrace1stWingAttemptResized.jpg

so I shortened the wall (increased the angle of the wall) as much as I could to capture the first problem rays, but still deflect the new problem ray... this is what I netted out with.

RayTrace2ndWingAttemptResized.jpg

Question I have is, how do I know when I've cleared enough room around my listening position? ... and if what I have there isn't adequite, I'm a little unsure of how to remedy it... Would love to hear your thoughts.

Thanks,
Aaron

[Soundman2020]

Then, I tried to extend the wings such that they covered the intersection of those rays, but I believe the new wing angle introduced a new problem ray...

... or you could try changing the angle of the soffits themselves, and the position of the speakers within the soffits...

If your speaker toe-in angle is not exactly 30°, I promise you that you will not be arrested by the dreaded Soffit Angle Inspection Police...

Don't get hung up on the famous (or should I say "infamous") equilateral triangle that you see in so many speaker manuals and web-sites: It is not necessarily the best layout for ANY room. Yes, it will work reasonably well in most rooms (hence the reason why you see it everywhere), but that doesn't mean it is the BEST layout. Pretty much always you can find a better layout by changing the angle to something other than 30°. Try to keep it between 25° and 35° if you can, but I do know that John has gone up to 45° on at least one occasion, and the world did not end!

You can also slide your speakers across the soffit face: they do not have to be in the middle of that face, and indeed they should NOT be there! It's best to have them off-center a little to one side or the other. Supposedly 3/5 of the baffle width is a good spot, but I can't find the theoretical justification for that: However, I do know that it works... and that the Baffle Center Bureau of Investigation will not indict you if your speaker isn't centered on the baffle.

Question I have is, how do I know when I've cleared enough room around my listening position?

How big is your console/desk? When you are seated normally at your desk, mixing in typical fashion, how far do you need to lean/move left or right of the center-line, for normal mixing? Measure that, and add about 6" or so each way for good measure. Ditto for how far forward/backward you might lean or move over the desk while mixing? For most home studios, that will like be a circle or ellipse of maybe two or three feet in diameter, which actually isn't so easy to achieve. For a large console in a large room, it could be a dozen feet each way, which is REALLY hard to achieve...

EDITED TO ADD: Are you sure your angle of reflection is equal to your angle of incidence for the soffit wings? To me it looks like your surface normal at the point of reflection is not perpendicular to the surface...

- Stuart -

[Paulus87]

For anything other than walls that have door frames in them, you're probably safe with this glue. If you're renting a thermal camera, it won't hurt to throw some anchors in here and there.

Thanks Greg!

Here's an image showing the effect of increasing or decreasing your cross sectional area.

Whoa, if I'm reading that right, having 2x the cross sectional area impedance change only gives you about half a db in TL?

If that's correct does that mean that unless you can get much more multiples on the cross sectional area, the majority of the attenuation comes from the turns inside the silencer box?

And if that's correct, am I right to assume just making way longer boxes is a better use of my efforts than trying to wrestle with getting big impedance changes given my lack of usable space?

Same question said a different way...

I can tell you from personal experience that the difference from having 1 silencer box to 2 on a run is crazy

Is the difference from having 2 silencer boxes on a run, vs having 1 that is as long as the two separate boxes combined significant as well?

Use the search feature on the forum to look up ray tracing. It's very very important that you do ray tracing in SketchUp to ensure your design works perfectly.

OK, I took my first punt at ray tracing in Sketchup. Here's what I did. First I ray traced without any soffit wings to see where my problem areas would be... I came up with this: (arrows showing the rays that came close to my listening pos, x showing approximately where I would need to extend my wings to)

RayTraceNoWingsResized.jpg

Then, I tried to extend the wings such that they covered the intersection of those rays, but I believe the new wing angle introduced a new problem ray...

RayTrace1stWingAttemptResized.jpg

so I shortened the wall (increased the angle of the wall) as much as I could to capture the first problem rays, but still deflect the new problem ray... this is what I netted out with.

RayTrace2ndWingAttemptResized.jpg

Question I have is, how do I know when I've cleared enough room around my listening position? ... and if what I have there isn't adequite, I'm a little unsure of how to remedy it... Would love to hear your thoughts.

Thanks,
Aaron

The ITU recommends a listening area minimum of around 0.7m radius, but as Stuart said you should try to get your RFZ to extend as wide as your console/work station and as far back as you think you'll need while working.

I had a quick attempt at ray tracing your room for you and managed to get this by bringing the front in 1' and angling the soffits and soffit wings by 25 degrees. The speaker axis meet 1' behind your head...

Screenshot 2019-03-23 at 17.38.37.png

But, I'm sure you could get it even better than this if you experiment with other angles and bringing your soffits closer to your listening position. You'll find that how long you extend the wings will determine how far back your RFZ will due to the ray angle coming off of the straight side wall. But, how wide your RFZ will be determined more by the angle made between the wing and the speaker soffit itself, and how far down the wing that angle happens. So my advice is, if you need your RFZ to be wider than longer, move the bottom of the wing up, and vice versa.

This part of my own design is probably what has taken up the most amount of time to get to the point where I'm finally happy with it. a couple of degrees can make such a big difference...

Paul

[Bhudie]

OK, thanks everyone for helping out with the ray tracing, and thanks especially Paul for going the extra mile and modeling out my room! This forum is fantastic. I now think I have enough info to iterate on my ray traces to get something good. I'll report back when I get through it to make sure I haven't made any mistakes.

I am now turning my focus back to HVAC, because I am still struggling to find a solution here.

A couple of questions:

If I'm reading Greg's chart that he put up right, then half the cross sectional area is as good as double the cross sectional area for impedance changes. I imagine that the problem with half of the cross sectional area is that the air speed would get too fast, and then you would need to have enough space for the turbulence to settle before air entered the room otherwise you would create noise at the register. So questions:

1) Is this assumption correct?
2) If so, can I utilize a smaller silencer box for the return air because the air speed won't pick up until it leaves the room, and I would have a long run of joist space to let that air settle before it hits the second silencer on the outer leaf?

In addition to that, Stuart I saw this quote on another thread, and it's what I was alluding to above about the ability to have only 1 silencer penetrating 2 leaves with solid wood ducting between leaves:

There's one other way of doing it: Since the issue is partly about noise being picked up by the duct that passes through the cavity, that can be eliminated by NOT using a duct to pass through the cavity! Instead, use a massive extension of the silencer box to pass through the cavity. So for example, you would build your silencer box on the outside of the outer leaf using plywood, then create a "duct" out of the same plywood that goes completely through both leaves and terminates in the register, on the other side of both leaves. That plywood "duct" would still need to be broken in the middle (to prevent sound flanking along it) by simply sawing all the way through it, then wrapping the gap with thick rubber (EPDM, Neoprene or Sorbathane) to seal the gap and make up the missing mass.

If I can use a system like this on my East wall where outer leaf is an exterior wall, then I could get my registers in the front and back of the room and have enough isolation from my silencers to still make this all work. I've mocked up roughly what I mean here, to see if it is plausible. I would love some feedback on this possibility.

PossibleDesignResized.jpg

I would put the inlet at the base of the front of the room, and extend the vent on the inside of the room to the front wall of the baffle (red "Fresh Air" on the drawing), and put the return in the rear of the room in the ceiling (red "Return" on the drawing). Both boxes would have solid wood ducting into the room.

Obviously the devil is in the details, and I haven't built the detailed design yet, but just wanted to know your thoughts on if this is a rough workable solution?

Thanks again for all of your time and help.

-Aaron

[Soundman2020]

I imagine that the problem with half of the cross sectional area is that the air speed would get too fast, and then you would need to have enough space for the turbulence to settle before air entered the room otherwise you would create noise at the register. So questions:

1) Is this assumption correct?

Yes.... but it depends on how fast the flow was before and after the change. It it is still low after the change, then there probably won't be any turbulence issue. For example, if the velocity was 50 fpm before and 100 fpm after, then I would not be worried., but if it was 250 before and 500 after, then yes, you'd have a problem.

2) If so, can I utilize a smaller silencer box for the return air because the air speed won't pick up until it leaves the room, and I would have a long run of joist space to let that air settle before it hits the second silencer on the outer leaf?

A smaller silencer box will increase the static pressure, and probably won't provide the transmission loss you need. The general rule in HVAC is that the return run should have a large cross sectional area than the supply run, or at the very least, the same.

If I can use a system like this on my East wall where outer leaf is an exterior wall, then I could get my registers in the front and back of the room and have enough isolation from my silencers to still make this all work. I've mocked up roughly what I mean here, to see if it is plausible. I would love some feedback on this possibility.

What's on the other side of that wall? You only show one silencer box on the outer leaf, but nothing about the silencer box on the inner leaf, nor what joins them....

More detail needed...

- Stuart -

[Gregwor]

After reading your plan, I thought I'd post a few pictures providing a little more information to think about with your design:

HVAC Noise Per Frequency Range.png

Influence of Air Velocity on Attenuation.png

Back Pressure & LF Attenuation.png

Specifically, regarding this last image pointing out back pressure: Unless I'm understanding it incorrectly, it is stating that with higher constriction there is more LF attenuation. Smaller cross sectional area = higher constriction = more velocity = MF noise = higher static pressure. So, in conclusion, it seems that if your fan can handle the added higher static pressure, it would be ideal to have the duct work joining your inner and outer silencer boxes be very small. This would increase your cross sectional area change ratios, provide back pressure, keep the MF noise/turbulence far from the registers, and ultimately increase the insertion loss at LF.

Please correct me if I'm wrong. Thanks!

Greg

[Gregwor]

Aaron,

A few other things:
- It appears that your inner leaf ceiling is designed traditionally, not inside out. I strongly urge you to build it inside out.
- To achieve the insertion loss you desire, I think you need both inner and outer leaf silencer boxes.
- Can you not chew into your mechanical room a little bit to fit an inner leaf silencer box? Your outer leaf wall could jut into that room where the box would be.
- If you need larger cross sectional area for the silencer boxes in your ceiling joists, why not just duplicate what you have there now and have 2 or 3 joist cavities filled with silencer boxes? I like the fact that you've made them nice and long.
- Where were you planning to have your diffusers/registers/grilles come out in your room? Remember, you need supply and return to be as far away from one another as possible.

Filling your joist cavities decreases the gap in your MSM system, but for many of us, there's no other choice. I haven't done the math to see if this is feasible, but with that in mind, did you ever consider running 2 or 3 sets of your long joist cavity style silencers down each side of your room? One side being supply, the other being return. You could have your outer leaf silencers mounted on the outside of your room (like the supply silencer in your recent picture). They could have 2 or 3 sleeves on them that would connect to the inner leaf silencers (these would be the long skinny joist cavity style).

Greg

[Bhudie]

Thanks everyone again for all the info and insights.


This got me thinking:

Filling your joist cavities decreases the gap in your MSM system, but for many of us, there's no other choice. I haven't done the math to see if this is feasible, but with that in mind, did you ever consider running 2 or 3 sets of your long joist cavity style silencers down each side of your room? One side being supply, the other being return. You could have your outer leaf silencers mounted on the outside of your room (like the supply silencer in your recent picture). They could have 2 or 3 sleeves on them that would connect to the inner leaf silencers (these would be the long skinny joist cavity style).

I'm wondering if I can do a combination now... running my small silencer boxes for the return on each leaf, 6 runs of them... 3 along each side of the room (6x36" cross sectional area = 216"... I know that's not the right number, but I was thinking I could get away with 5 changes per hour instead of 6, in which case this would be large enough of a cross sectional area, and a low enough velocity), and have them vent out on the east wall. I spoke with my contractor and he doesn't think the large boxes on the East Wall (the wall with my supply boxes in my previous post) will fly with the inspector due to setback requirements. Instead I could utilize the pool room next door for the supply boxes on both leaves as you suggested... bumping out my outer leaf wall to accommodate the larger boxes. This would put my supply register in the front right corner of the room, but I could duct it to the front central bottom of my baffle, and then have multiple return registers on the ceiling in the rear of the room.

I feel like this works for HVAC, but I can't help but think it might not work well for preserving my isolation as I'll be taking out a lot of my spring volume in my MSM system to accommodate 12 silencer boxes. I might consider shortening the boxes to allow for more spring area of just insulation... There would still be 6 inches of spring from the insulation on the inner leaf with this design though. Also, cutting that many openings for registers in my double layers of drywall seems mental...

Something like this is what I was thinking:

AnotherPossibleDesign.jpg

Interested in if anyone has any thoughts on if this is indeed a bad idea.

Failing that, at some point I might just have to back off of my isolation requirements as a compromise and work around noise levels at different times of the day... and resort to one silencer per opening that penetrates both leaves via a massive wooden duct.

Thanks,
Aaron

[Gregwor]

Three quick questions (sorry if you've answered both, but I don't have time to read through your entire thread again to find out. I skimmed it but didn't see the answer):

1. Are you doing forced air or a ductless minisplit? Can you share any HVAC calculations you've done?

2. What is the room opposite of your pool mechanical room being used for? Can you sacrifice a corner of it (up high or a side wall corner) for a silencer box? The reason I suggest this is because it would allow you to easily have supply and return on opposite sides of your room!

3. Are you doing an inside out ceiling?

Also, cutting that many openings for registers in my double layers of drywall seems mental...

With silencer boxes, it is 100% okay!

I spoke with my contractor and he doesn't think the large boxes on the East Wall (the wall with my supply boxes in my previous post) will fly with the inspector due to setback requirements.

What about putting the silencer boxes on the front side of the building? You could hide them with a nice design so no one would really be able to tell what they are.

Greg

[Soundman2020]

I feel like this works for HVAC, but I can't help but think it might not work well for preserving my isolation ...

Well....

Silencer box being raised into place between engineered joists...

BRAUS-ISO-silencer-box--01--going-in-SML.jpg

Silencer box in place (yeah, it's a tight fit, but there are air gaps all around that: no contact with the joists):

BRAUS-ISO-silencer-box--02--in-place-ENH-SML.jpg

View from directly below: camera is on its back on the floor, with the lens pointing up at the ceiling. The silencer box is in place, part of the inside-out ceiling is in place, and the flexduct is visible through the part where the ceiling module has not yet been put in:

BRAUS-ISO-silencer-box--03--view-up-from-floor-SML-ENH.jpg

Completed inside-out ceiling, with the silencer box sleeve extending down through it:

BRAUS-ISO-silencer-box--04--ceiling-in-other-end-SML-ENH.jpg

That's the ceiling of an iso booth, big enough to fit in a drum kit at a pinch. It gets a little more than 50 dB isolation, probably around 55. Drums are barely audible in the room upstairs. Vocals and instruments are dead silent.

Hopefully that answers your doubts!

Also, cutting that many openings for registers in my double layers of drywall seems mental...

Drilling one single 1/2" hole through your leaf is already mental! ... if it isn't done correctly, with excellent seals and enough mass to get back to the right situation. That's why you need silencer boxes on every penetration...

- Stuart -

[Bhudie]

1. Are you doing forced air or a ductless minisplit? Can you share any HVAC calculations you've done?

OK, first off Greg, thanks for asking this question... looks like somewhere along the line I got turned around and doubled my cross sectional requirements for my silencer boxes. In copying my calculations below I realized that 6 runs of these return silencer boxes is more than enough cross sectional area... In fact I only need 4 runs.

Having said all that... I am planning on a fan at the supply duct just to bring in fresh air for the near term, and likely end up putting in a forced air heat pump down the line.

As for calculations here's what I have up to now:

- My room volume = 9'9 1/4" x 14'1 1/2" x 8'11 7/8" = roughly 1240 cf
- my cubic feet per minute should = Refresh rate * Volume / 60 = 6*1240/60 = ~124 cfm
- I need to try to get to sub 300fpm at the register, so I used a calculator I found on this site. It suggested I need a 9" diameter opening at the register to get to ~280 f/m air speed.
- 9" diameter is 63.6 square inches.
- Which means my cross sectional area of my silencers needs to be ~127 square inches
- My silencer boxes in my ceiling joists for the return air have a cross sectional area of 4.5"x8" = 36"
- If I run 4 of them along the joists that would give me a total of 144 square inches of cross sectional area on my return silencers.
- For my supply boxes, I used your formula (width-7)/2=x. I made the depth of my boxes 15" which would give me 11" of air space. I need ~13" to get to 144 square inches, so my width=(2x)+7 = 33" wide. so my Supply boxes will be 2'9" x 15"

2. What is the room opposite of your pool mechanical room being used for? Can you sacrifice a corner of it (up high or a side wall corner) for a silencer box? The reason I suggest this is because it would allow you to easily have supply and return on opposite sides of your room!

Unfortunately not, that'll be the family TV room, and there's going to be a built in media center right across that wall. However, with my current design, I don't think I'll need to do that, as I will have the supply register vented to the front bottom of my speaker baffle wall, and have the 4 return registers in the ceiling in the rear of my room. Am I missing something?

3. Are you doing an inside out ceiling?

I wasn't planning on doing an inside out ceiling. I read that you recommended it above. I'm guessing the main benefit is the extra few inches of effective ceiling height? I am wondering if I'm better off not doing that and using the 6 inches of the inner leaf ceiling joists as spring, since I'm now eating up a lot of space for the silencer boxes.

What about putting the silencer boxes on the front side of the building? You could hide them with a nice design so no one would really be able to tell what they are.

This is a no go as far as the wife approval factor... That wall faces our pool, and she's just not having it. But unless I am missing something, I think using the pool equipment room works doesn't it?


So here's an updated look at what I'm thinking.

4RunCeilingDesignResized.jpg

And an open look at the silencer boxes. First the inner leaf:

SilencerBoxInteriorResized.jpg

Stuart, thanks for the reference, I added the duct from the silencer into the room on the bottom:

SilencerBoxVentDuctResized.jpg

And the outer leaf:

SilencerBoxOuterLeafInteriorResized.jpg

I am unsure of how I should cut out the holes on these boxes for the ducting between them, do I just cut out a circle on the end where I would attach the flex duct (appropriate size flex duct still to be calculated...)

Also, I haven't designed the supply boxes yet, as I think I have ample room to make sure that they're right.

Unless, anyone tells me I've missed something, I'm thinking that this is the design that I'm going to move forward with. Interested to hear your thoughts.

Thanks,
Aaron

[Gregwor]

Hey Aaron,

I have to ask this question again because you kind of failed to directly answer it:

Are you doing forced air or a ductless minisplit?

A few things:

1. I don't see a designated air handler in your space.
2. Due to your back wall architectural guidelines, there is no room for your returns to get back to an ducted air handler.

Therefore, I'm guessing that you're using a ductless mini split?

If so, you could get away with WAY less cross sectional area than you're currently calculating. This means smaller silencer boxes.

Okay so, if you have 1240 cubic feet, yes 6 air changes is 124 CFM. Now, that's needed if you're rocking a ducted system. For fresh air, which your silencer boxes are passing, you only need ~ 30% of that amount. So, 124CFM x 30% = 37.2 CFM. Much less, right? You might have troubles finding an inline fan that will move only that amount of air. You might have to get crafty. Also, before I forget, I need to point out that due to your current design, you can't fit an HRV in there which as I'm sure you know would be really really smart to have in BC climate. If are aren't cool dumping all of your conditioned (heated) air out into the cold winter and then have your fan blowing really cold air in on your feet (under your speakers), you certainly should consider an HRV unit.

Anyway, the smallest round duct you're going to find around your area might be 5" flex duct. Unfortunately, this crap isn't duct LINED, it's insulated. So, we have to rely on the low frequency attenuation in any flex duct runs and rely on the silencer boxes for the mid/high frequency attenuation.

5” RD duct = 19.63 sq in. Double = 39.26 sq in.

That means that the inside total cross sectional area of your silencers only need to be 39.26 sq in. in order to achieve doubling of your cross sectional area. You wrote that each silencer box in your joists has a cross sectional area of 36". So you're ~91% there with a SINGLE joist cavity of boxes! Let's say you have two joist cavities of boxes. That gives you 72 sq in. Now, going from 5" inlets on the box to 72 sq in inside, you've increased your area by 3.67 times! Now THAT's an impedance mismatch. Kick ass. Also, regarding velocity:

Air velocity in ft/min = CFM Flow rate in ( ft3/min) / CSA Cross sectional area in ft2
Air velocity in ft/min = 37.2 CFM / 72 sq in
Air velocity in ft/min = 37.2 CFM / 0.5 ft2
Air velocity in ft/min = 74.4 feet per minute. Now THAT is slow

Remember, you might have more CFM happening though depending on the fan or HRV unit you find to drive the system. Either way, as you can see, you're in very very good shape. With an HRV you could run it on low (as long as the CFM falls into your equations well) and if it is exchanging too much air, you could set it for 40 min per hour instead of being on steady. Or 20 min per hour even if it's just you in there and your chilling out not working hard.

I wasn't planning on doing an inside out ceiling. I read that you recommended it above. I'm guessing the main benefit is the extra few inches of effective ceiling height? I am wondering if I'm better off not doing that and using the 6 inches of the inner leaf ceiling joists as spring, since I'm now eating up a lot of space for the silencer boxes.

Do inside out.

Okay, so, here are some thoughts:

1. show us where you're putting the mini split.
2. you should have your fresh air blowing in right ABOVE your indoor mini split head unit, not the floor under your speakers.
3. try to implement an HRV into your design and put it in the pool mechanical room (I have one really bizarre idea of how to get your duct work on your ceiling joist boxes into the pool room)
3b. since it's a mechanical type room, I'm guessing there's a floor drain in there you could use as a drain for your mini split yeah?
4. you're making insanely good progress here so please stay with us and as you can imagine, once this HVAC crap is behind you, you'll be off to the races!

Greg