John Sayers' Recording Studio Design Forum Archive

A few quick questions.

In building the soffit, I am planning on building a built up panel using MDF, 2 layers of drywall, and a front of bamboo flooring all assembled using green glue.

Questions:
Would OSB be better than MDF?

I was thinking of using 5/8” Fire Code Drywall for more mass, would there be any advantage to using one sheet of 5/8” and one sheet of 1/2” to give different resonant frequencies, or (I am assuming) just go for as much mass as possible since it’s not supposed to resonate at all.

Should there be any 2x4 bracing behind the MDF/ drywall et all soffit or should it be free standing except for the edges?

Should there be a gap between the speaker shelf (3/4” MDF) and the soffit wall, or if the bottom of the speaker is isolated with sorbothane, should the speaker shelf attach to the soffit wall for bracing?

Should the Angled Slat Wall be attached to the infinite baffle, or should there be isolation in between?

Finally, their will be a 14 1/2 x 18 1/2 inch space in the wall corner area between the speaker section and the slat wall. This seems like a great space for a few tall hangers, but should that be open to the bass hanger area under the speaker shelf, or be part of the sealed air space behind the slat wall? And if it is open to the bass hanger area under the speaker shelf, would it be helpful to also put in another shelf above the speaker area that would give an 8 or 10” open area back to the back corner?

Alright, that was more than a few questions, but this stuff is complicated! When I built my last studio, I read everything in this forum, and now, 6 years later I am reading it again and realizing just how much I didn’t understand at all!

Thanks!

In building the soffit, I am planning on building a built up panel using MDF, 2 layers of drywall, and a front of bamboo flooring all assembled using green glue.

Green glue plus what? Green Glue is NOT adhesive, and cannot be used to stick things together.

Also, that's going to be rather thick: How will you balance the speaker inside the soffit so that it does not tip over from the cantilevered weight? Assuming two layers of 5/8 plus 3/4 MDT plus 1/2 flooring, that's 2 1/2" thick witout accounting for the GG and the gap between the rear of the baffle and the front of the enclosure box.... so your speaker will have to stick out nearly 3" beyond whatever is supporting it. Is that going to be feasible?

Would OSB be better than MDF?

MDF is higher density.

would there be any advantage to using one sheet of 5/8” and one sheet of 1/2” to give different resonant frequencies

No.

Should there be any 2x4 bracing behind the MDF/ drywall et all soffit or should it be free standing except for the edges?

Oh yes! DEFINITELY yes! The soffit needs to be massively sturdy and rigid. I often use 2x6, especially if the speaker is large.

Should there be a gap between the speaker shelf (3/4” MDF) and the soffit wall

Definitely.

or if the bottom of the speaker is isolated with sorbothane,

The entire speaker should be isolated with Sorbothane, not just the bottom.

should the speaker shelf attach to the soffit wall for bracing?

The shelf that the enclosure box sits on must be massive, rigid, and veru, very firmly attached to the framing inside the soffit.

Should the Angled Slat Wall be attached to the infinite baffle,

What "Angled Slat Wall"? Where did that come from? I don't see any mention of it here in this thread.

or should there be isolation in between?

What type of isolation, and in between what and what?

Finally, their will be a 14 1/2 x 18 1/2 inch space in the wall corner area between the speaker section and the slat wall.

I'm not following: What slat wall are you talking about? Slat walls are not usually part of a speaker soffit.

- Stuart -

To add to Stuart's reply:

Would OSB be better than MDF?

OSB is stronger but MDF is heavier. Depending on how you are joining your layers of mass, OSB might be better for a first layer so that you have something to screw into.

I was thinking of using 5/8” Fire Code Drywall for more mass, would there be any advantage to using one sheet of 5/8” and one sheet of 1/2” to give different resonant frequencies, or (I am assuming) just go for as much mass as possible since it’s not supposed to resonate at all.

Mass is what matters here. How will you anchor your finish layer to this drywall?

Should there be any 2x4 bracing behind the MDF/ drywall et all soffit or should it be free standing except for the edges?

Yes, the more bracing there is, the more sturdy the structure will be and therefore it will resonate less which is the entire purpose of the design. Over build it if possible.

Should there be a gap between the speaker shelf (3/4” MDF) and the soffit wall, or if the bottom of the speaker is isolated with sorbothane, should the speaker shelf attach to the soffit wall for bracing?

It depends on your design. If you're isolating the speaker using Sorbothane, the speaker shelf can be attached to everything and used as another bracing component. Some people's designs have the speaker sitting on a pedestal that isn't attached to the rest of the soffit framing.

Should the Angled Slat Wall be attached to the infinite baffle, or should there be isolation in between?

I assume you are referring to the soffit wing when you say "angled slat wall". This wall actually shouldn't be a slat wall. It should be a solid wooden wall like the soffit wall, except it doesn't have to be as rigid. Ultimately, there is no real way to decouple the walls since they're both anchored to the inner leaf frame. So the answer to your question is that it will be attached as you cannot avoid it.

Finally, their will be a 14 1/2 x 18 1/2 inch space in the wall corner area between the speaker section and the slat wall. This seems like a great space for a few tall hangers, but should that be open to the bass hanger area under the speaker shelf, or be part of the sealed air space behind the slat wall? And if it is open to the bass hanger area under the speaker shelf, would it be helpful to also put in another shelf above the speaker area that would give an 8 or 10” open area back to the back corner?

Sorry, there are conflicting descriptions in your question so I do not know what you're referring to. Basically, if there is any free space in the corner, you should put hangers in there. Please draw something up in SketchUp so we know what area you're talking about. It certainly doesn't have to be a correct final design, just something to show where you're describing.

Are you going for an RFZ design?

Greg

Here is a sketch up model of the studio.

http://www.johnlsayers.com/phpBB2/downl ... p?id=55163

I am a little confused about the soffit extensions. If I have the right terminolgy, the angled section that I am thinking of is usually a slat wall in John’s designs.

I’ve seen several different solutions to mounting the speaker. The Barefoot model builds an entire box around the speaker out of pretty thick wood that I assume ramps down vibrations from the speaker cabinet along with a strap to hold it down, further reducing vibrations as well as movement. Other designs show just a top plate and the speaker shelf, with sorbothane isolation and some method of clamping the speaker down. (I like the one with threaded rods and bolts. This leaves the back and sides of the speaker open to the area behind the soffit wall. I know that you need to leave space for ventilation, and to fill the entire area with loose insulation, but you said that this area is not air tight. Does it need to be a full shelf and top or, would a partial shelf do the job? I ask because I am trying to decide if I should build a half wall along the wall with a top plate for the shelf to rest on, or if I can run the studs to the ceiling (with a top plate there of course) and then attach a horizontal 2x6 to the studs to support the speaker shelf. I like the idea of having a small gap between the speaker shelf and the soffit wall, but I am having trouble envisioning how that works with 2x6 braces for the soffit wall.

I will use thinner drywall (yes I can add, but I Was only thinking of mass.) They also have thinner engineered wood for the front.

I downloaded your SketchUp file.

- with such a small control room, why are you having 3.5 ft for treatment on your back wall?
- why do you only have a tiny bit of treatment in your 3.5 ft of space at the back of the room?
- you seem to have unnecessary angled treatment panels in front of your rear treatment.
- your current soffit placement shows that WITH the hard face material and insulation behind your speaker, you only have ~7.5". It seems strange to me that you leave so little space up front where you need a lot of lumber to build a super sturdy soffit and utilize the corners for bass trapping yet you have a huge portion of your room designed so that the rear of the room is mostly treatment (not a bad thing if you have a massive room), but then you don't even fill it entirely with hangers.
- I hope that the angled ceiling at the back of your room is just fabric with a ton of insulation up above it because if it is hard backed, you are going to be compressing the low frequencies there which is bad. Why are you making your small room feel even smaller with all of these weird angles at the back of your room?
- why are there walls protruding out of your back wall?

Does it need to be a full shelf and top or, would a partial shelf do the job?

It all depends on your design. I know that's a horrible answer, but it's the truth. Everyone designs their soffits differently. There are rough guidelines to follow, and that's it. As long as you follow those rules, you're probably off to the races. Draw up a soffit design, post it here, and we'll critique it for you. As I stated above about how deep your soffits are, you really should look into the depth of the speakers you own, and ones you might want to upgrade to in the future. Try and leave enough room so that any deeper future speakers can be fit into the soffit without rebuilding your entire room.

I like the idea of having a small gap between the speaker shelf and the soffit wall, but I am having trouble envisioning how that works with 2x6 braces for the soffit wall.

You'd want your speaker shelf to be decoupled from the face of the soffit if it was floating on Sorbothane and vibrating. Otherwise, it doesn't matter because the entire soffit frame is one unit. Hell, the entire thing is attached to your entire room. That's why you have to make it as beefy as you can so that it doesn't vibrate. Basically, your speaker and/or speaker shelf needs to be detached from the soffit frame and front panels. Again, it depends on your design.

Greg

Thanks Greg,

why are you having 3.5 ft for treatment on your back wall?

There is a closet at the back of the room. Unfortunately there is also a heating duck running across the top of it. The Weird angle is really a problem. I will have to come up with a solution for it. I will need to do some exploratory surgery to see if there is actually anything behind the angle, if not I can get rid of it and put in some serious absorption in front of the duct. If I can’t get rid of it, I will extend the absorption down over it. I will be filling up the closets with treatment, just have to figure out the best approach. The closet is divided into three sections with an open area in the center (hangers with the back wall covered in Roxul) the two side sections are filled with shelves. My first idea was to put 4” of insulation blocking out the corners to make bass traps, but I welcome any other ideas.


-

your current soffit placement shows that WITH the hard face material and insulation behind your speaker, you only have ~7.5".

I measured out the front by putting the listening point 1/3 of the way back on the room, but that may have been before I decided to take the doors off the closet. I will check that

Draw up a soffit design, post it here, and we'll critique it for you.

I will do that!

I am having trouble with getting sketchup to do what I want. Meanwhile, I have some more questions about the speaker box.

As I understand it, the goal is to eliminate sound radiating from the existing speaker box by encasing the speaker with a “speaker box” isolated from the speaker with sorbothane and clamping it to damp vibrations and prevent movement, and then fill the “enclosure” behind with loose insulation to convert any stray waves to heat. I would think that you would want everything to be as air tight as possible to keep sound from escaping, but of course you need airflow for ventilation of the speaker (my speakers are passive, but I want to make it possible to upgrade the speakers when I win the lottery...... lol) so here are my questions.

For the speaker box- does it matter what I make it out of?
Last time I used 2x12s but would The density of MDF be better than the thickness of the 2x12.

The sorbothane pads I am looking at for the top and bottom are between 3/4 and an inch thick, can I use thinner material for the sides that are not being clamped? Should I fill any gaps between the pads with fiberglass or rock wool?

Most of the plans I’ve looked at show a space behind the speaker of around 2”, with a hole at top and bottom for ventilation, and sometimes a hole for the rear port, (which my speakers, Alesis Monitor 1 passives have). Is this 2” space just for ventilation? With passive monitors does it need to be that big?

Finally, should I make the speaker box as airtight as possible with caulk, etc. or is it more just about damping direct transmission?

As for the enlosure, ie, the area behind the soffit wall, does it need to be actually enclosed?
John’s designs generally show a panels built to enclose the are and separate it from the bass trap below, and the ventilation area above. I always assumed that this should be air tight, but I am reading that it is not airtight, with a gap between the speaker shelf and the soffit wall, and of course the gap between the speaker edge and the soffit. Other places I read that it doesn’t need to be enclosed separately at all. This all contrasts with the concept that the soffit wall, when properly built becomes the new front wall of the studio, and the space behind it is no longer part of the room. My question is, what is the purpose of this enclosure? Understanding this will help me better in knowing how to proceed.

I would think that you would want everything to be as air tight as possible to keep sound from escaping,

That would be ideal, yes, but isn't possible for the reasons you mentioned (among others), plus the wavelengths that are really at issue here are very long: a foot or so up to dozens of feet.

For the speaker box- does it matter what I make it out of?

Make it as massive (dense) as possible. The more mass you have, the harder to is to move, and the easier it is to decouple and damp with Sorbothane. Some place recommend making the surround from solid concrete! I have never gone to that extreme, but I have done concrete baffles...

The sorbothane pads I am looking at for the top and bottom are between 3/4 and an inch thick, can I use thinner material for the sides that are not being clamped?

Do the math! Speakers vibrate in all 3 dimensions, to a greater or lesser degree. It's not just weight you are dealing with, so this is not a simple ODOF issue. You need rubber with the correct static deflection acting in all directions.

Also, why do you thing that the sides would not be clamped?

Should I fill any gaps between the pads with fiberglass or rock wool?

You can, yes, but make it very light stuff, and don't fill 100% there? leave some space for air circulation around the sides, bottom, and top of the original speaker cabinet.

Most of the plans I’ve looked at show a space behind the speaker of around 2”, with a hole at top and bottom for ventilation, and sometimes a hole for the rear port, (which my speakers, Alesis Monitor 1 passives have). Is this 2” space just for ventilation? With passive monitors does it need to be that big?

The gap at the rear is for several reasons, but mostly because that's what pretty much all speaker manufacturers recommend in their manuals! Read your speaker manual to see what it says. But regardless of that, you do need the gap for ventilation, and also for acoustic reasons... and also to connect your cables! For a passive speaker, maybe it's not too much of an issue, but for an active speaker you do need enough space inside the rear of the enclosure box to fit in the XLR connectors, and the power connector...

Finally, should I make the speaker box as airtight as possible with caulk, etc. or is it more just about damping direct transmission?

The spekaer box has a huge hole at the front: the speaker fits in there! And huge holes at the top and bottom for ventilation, and gaps all around for the sorbothane pads.... So no, there's no need to seal it at all, since sealing it is impossible. Just use good quality wood glue and screws, and clamp it tightly while the glue dries, taking care to keep all the parts square, plumb, and level.

As for the enlosure, ie, the area behind the soffit wall, does it need to be actually enclosed?

No, it isn't necessary.

John’s designs generally show a panels built to enclose the are and separate it from the bass trap below, and the ventilation area above.

... and those panels have huge holes in them, for ventilation...

I always assumed that this should be air tight, but I am reading that it is not airtight,

Correct: not air tight. The areas above and below the actual soffit area are almost always used for bass tapping, and that can be just as beneficial to the speaker itself as it can be to the room. Having those areas open to the speaker, through the ventilation slots, is a good thing.

This all contrasts with the concept that the soffit wall, when properly built becomes the new front wall of the studio,

I'm not sure where you saw that, but it isn't correct. The front baffle of the soffit becomes the new front baffle OF THE SPEAKER, not the wall of the room. The entire purpose of soffit mounting is to extend the front baffle of the speaker, making it appear as large as possible (within reason), in order to correct the power imbalance, solve the front wall SBIR issue, eliminate the edge diffraction issue, eliminate the reflection and comb-filtering issue, and a few other things. It does all of those regardless of whether or not it is sealed.

and the space behind it is no longer part of the room.

The space behind can still be open to the room, provided that it is damped correctly. If that space behind is not open to the room, how would the ventilation system work? You would have to run HVAC into the soffit independently from the room if you seal up the soffit interior air-tight, and separate it from the room...

My question is, what is the purpose of this enclosure? Understanding this will help me better in knowing how to proceed.

The enclosed space behind the soffit baffle doesn't really have a "purpose", other than it simply got trapped behind the baffle! If there was a method to magically make that enclosed area "disappear" into another dimension, the speaker and soffit would still perform just fine. But seeing that it IS there, and you CAN'T get rid of it, then you have to treat it. Because there are acoustically reflective surfaces all around that, and it is basically a cavity too, there's a whole lot of stuff going on inside there with low frequency sound that emanates from the speaker box itself, and needs to be dealt with to prevent it from doing nasty things and getting back into the room. That's why the general recommendation is to fill that entire space with insulation, except for a "chimney" that allows the ventilation air to circulate. You can fashion that chimney from chicken wire or something similar. I usually suggest that my customers save all of their insulation scraps and off-cuts that are left over from building the rest of the studio, and use those to fill the speaker soffit interior. Insulation isn't cheap, and that's a large volume that needs filling, so to makes sense to NOT throw away those scraps and instead use them for something important!

- Stuart -

For the speaker box, do I have it right that there are two issues we are trying to deal with. First to make the walls of the speaker as rigid as possible, and second, to eliminate the direct transfer of vibrations from the speaker to the support structure. If so, would it be better to have the entire sides, top and bottom of the speaker in direct contact with sorbothane being pressed between the new MDF speaker box, and then put thicker sorbothane pads between the bottom of the new speaker box and the shelf that it is strapped to?

For the speaker box, do I have it right that there are two issues we are trying to deal with. First to make the walls of the speaker as rigid as possible, and second, to eliminate the direct transfer of vibrations from the speaker to the support structure.

Those two are very closely related, actually. Pretty much the same thing. The goal here is to prevent the speaker from transmitting vibration into the soffit structure. But there are several ways of accomplishing that. One is to add a lot of mass to the speaker, so it is harder to move it. Another is to add a lot of rigidity around the speaker so that it is harder to move it. Another is to decouple it, so that no energy can transfer. Another is to damp it, so that no energy can transfer. There are pro's and con's to each method, and also limitations to what each can achieve. Decoupling, for example, is great at first glance, but to fully decouple takes up space, and by allows the speaker to move a bit. Ideally, you should make the speaker levitate in nothing but air, but then it could swing around all over the place... So you use something that limits how much it can move (eg, rubber), but that reduces the decoupling. And damping is great! You'd think.... It "softens" the transfer of energy, so that even at resonance, the resonant peak is much lower.... but that also means that the isolation peak ABOVE resonance is also much lower. Etc. Each one has its advantages, disadvantages, and limitations. But using them all together, in a balanced approach, gets the maximum effect from each one, while minimizing the drawbacks.

If so, would it be better to have the entire sides, top and bottom of the speaker in direct contact with sorbothane being pressed between the new MDF speaker box, and then put thicker sorbothane pads between the bottom of the new speaker box and the shelf that it is strapped to?

You could do that, yes, but it's the same situation as above: pro's, con's, limitations... By doing what you describe, you would indeed get greater overall isolation, BUT you also create the equivalent of a three-leaf system! You have the speaker floated in the box, and the box floated on the shelf. This is exactly the same, acoustically, as a 3-leaf wall system. And because it is "3-leaf", the overall resonant frequency will be HIGHER than that of the equivalent 2-leaf system. So you would need to compensate for that, by tuning BOTH of those floating systems to much lower frequencies, in order that the final F+ frequency is lower than what the F0 would have been for a two-leaf...

It's complicated! There's no such thing as a free lunch in acoustics. What you gain by doing something really well also implies losing in some other aspect. So it's a compromise. You are always balancing the good and the bad, trying to optimize the good and reduce the bad as far as possible.


- Stuart -

Thanks. That makes sense now!