John Sayers' Recording Studio Design Forum Archive

I would like to modify my existing rectangular room to flush mount my ATC scm150 passives
I have read plenty of info regarding this but I'm still stuck on some details
The room is 5.8m x 3.5 x 2.7
Bit small for scm150s you may say but that's what I've got to work with. The ATC designer Ben Lilley reassured me that he did not think the size of my room was a problem especially if I had plenty of bass trapping.
I love the monitors and I'm very used to them even working in small rooms.I've had my speakers for 10 years and never had them soffit mounted and always wanted to. Always felt like something left undone.

This is my design so far I've left a lot out of the room treatment sorry about that! I was mainly looking for advice on the flush mount wall design and construction to start with. I planned to have the angled slot resonators from floor to ceiling which are only shown short on the 3D image to extend the useable listening area a little deeper into the room amongst other things; 500mm porous absorber on back wall, also have openings at top and bottom in the front wall which will also be filled with absorption, probably more broadband absorbers towards the back of the room and ceiling mounted super chunks running down the sides of the room, broadband absorbers in angled clouds with hard backing.
First query is - is it a problem that my speakers are off centre on the baffle panels? The acoustic axis is 300mm and 755mm from each side of the panels. The panels are just over 1m wide and I planned to make them 2.4 high. 3 layers 18mm mdf with mlv sheets sandwiched between. The only way I can make the speakers more central on the baffles is to lessen the listening angles from the typical 30 degrees, or to omit the reflective angled walls and extend the baffles perhaps all the way to the side walls (and perhaps replace the reflective walls with movable standalone angled units)
I'm not entirely sure what design criteria I should prioritise
My other query is the details of the actual construction. I plan to place the speakers on neoprene on heavy duty stands (perhaps concrete) completely isolated from the soffit wall except through the concrete subfloor. A design closer to Barefoots than John's and based on the ATC document. Some people insist that the soffit wall itself is also isolated from the studio boundary walls, ceiling and floor with expensive isolation brackets and standoffs. How necessary is this? If at all necessary. I understand that if the speaker has connection with the baffle wall that vibration transmits faster than airborne sound and smears the LF but if there is no connection how necessary is it to add this further isolation? In discussion with some techies at ATC there was no mention of this extra isolation
Here is the ATC document for flush mounting I understand that I have left out a lot as regards treatment but I wanted to clear up these loose ends before I go further down the rabbit hole Can you help

I planned to have the angled slot resonators

Well, you could, but with big speakers like that in a small room. I wouldn't do it that way, to be honest. Your speakers are not really full range: IIRC they roll off at about 60 Hz and 17 kHz, so you already have issues with spectrum coverage. Placing tuned absorbers so close to your ears is not going to help that at all.

500mm porous absorber on back wall,

I would do that in the form of hangers, increasing the depth into the corners.

First query is - is it a problem that my speakers are off centre on the baffle panels?

Not at all a problem. In fact, that's actually good. If the speaker is centered on the panel you run the risk of having a focusing or lobing issue. One good recommendation I have seen and used is to have the acoustic axis at about 2/5 the width of the baffle. It doesn't matter much which way.

The panels are just over 1m wide and I planned to make them 2.4 high. 3 layers 18mm mdf with mlv sheets sandwiched between.

... and plenty of hefty fraewark behind to keep it solid and rigid.

The only way I can make the speakers more central on the baffles is to lessen the listening angles from the typical 30 degrees,

Why did you choose 30°? That doesn't look like the best angle for that room. It's long and narrowish, so I would have thought a slightly smaller angle would have worked better.

or to omit the reflective angled walls and extend the baffles perhaps all the way to the side walls (and perhaps replace the reflective walls with movable standalone angled units)

I would not do that, no. I would keep those sections pretty much like you have them at present, but check the angles! Did you raytrace that? To me, it looks like you haven't quite created a full reflection free zone. By eye, it seems to be that you still have some first reflection issues that could be resolved by better angling.

I'm not entirely sure what design criteria I should prioritise

Ahhh, now that's the 64 thousand dollar question, isn't it!? Like most things in studio design, it's all about compromise and finding the best balance between all of the numerous variables that need to be juggled at the same time.

But first and foremost, this is an RFZ style room, so getting the geometry correct to maximize that is your first priority. If you do that right, it also automatically takes care of the stereo imaging, sound stage, and sweet spot.

Next, in my opinion, comes overall frequency response (and its distant cousin: phase response) of the room. It needs to be as close as you can get it to BS.1116-2 specs.

And right on the heels of that comes time-domain response, which also needs to be as level and even as you can get it, with the correct number for that size room, and also meeting BS.1116-2 specs (or at least getting close).

In fact, some would put time-domain response in front of frequency response, and justifiably so in many cases. But in this case, given the limited spectrum of your speakers, I would look to maximize that first, without compromising time-domain.

And finally (but certainly not least important!) comes control of the rear end of the room, to create the attenuated diffuse field, take care of possible flutter-echo issues, and eliminate or at least minimize SBIR issues with the rear wall.

I realize you were asking specifically about prioritizing your soffit design parameters, rather the the room parameters, but they all go together. Soffit mounting your speakers will change the frequency response, and also potentially the time domain response, and it will also affect the room's modal response, since you are adding additional walls and angling them. So the priorities for soffit mounting are the same as for the entire room: do what needs to be done to keep the frequency response (and power response!) as flat as possible, while also keeping the reflection free zone as large as possible, and also providing an accurate and precise sound stage with good stereo imaging.

In other words, get your geometry correct first: Speakers and mix position in the optimum locations, and with the correct distances and angles.

I plan to place the speakers on neoprene

Did you do the calculations for hat yet? What durometer? How thick? What deflection? What surface area? What form factor? What resonant frequency are you tuning that for? This is very important: Don't guess! You plan is similar to what I do for large, heavy speakers: you want to make them "float", such that they cannot transmit any vibrations into the surrounding structure. In order to achieve that, you have to do the math to ensure that they really do float! If you don't get the deflection correct, then they won't float at all, or will only float at higher frequencies, not in the bass range. If you have too much deflection, the the rubber "bottoms out", and there's no isolation. If you don't have enough deflection then the rubber "tops out" and there's no isolation.

Think if it this way: the rubber is like the suspension in your care. It is both spring and shock absorber at the same time. If you take the suspension from a Mini and try to use it on a Mack truck, the suspension will be overloaded, and it won't "suspend". And if you try to put the Mini on suspension from the Mack truck, the same things happens: It is way to rigid, does not compress at all, and no "suspension".

Do the math. It would be really sad to go to all that trouble of mounting great speakers in a great soffit, but then find out that they are not floating...

on heavy duty stands (perhaps concrete) completely isolated from the soffit wall except through the concrete subfloor.

You could do that, yes, but the large, heavy, solid, massive, rigid, stand takes up a lot of space under the speaker, which could be better used for bass trapping.

Some people insist that the soffit wall itself is also isolated from the studio boundary walls, ceiling and floor with expensive isolation brackets and standoffs.

Not necessary. Overkill, and no real benefit.

Think of it this way: the baffle is not making any sound. The walls, floor and ceiling are not making any sound. Why would you need to decouple them?

If the room is built correctly, the walls and ceiling are already massive, rigid, solid, and acting as a single inner-leaf, all together, while being fully decoupled from the outer leaf by a very well damped substantial air gap. The soffit baffle is just an extension of that: It's just a large chunk of solid, heavy, rigid mass. If you did not decouple the left wall from the front wall and the ceiling, then why would you need to decouple it from this new "wall", in which you will be mounting your speaker?

There's no benefit.

If you follow Barefoot's philosophy, your speaker does not contact the baffle at any point anyway, so there cannot be any mechanical transfer between the two. The only possible path for energy transfer is through the air, but that will be well damped, and in any case the baffle is so rigid and massive that it just won't pick up much energy like that. It makes more sense to have that mass in firm contact with the rest of the inner-leaf mass, not decoupled from it.

I understand that if the speaker has connection with the baffle wall that vibration transmits faster than airborne sound and smears the LF but if there is no connection how necessary is it to add this further isolation?

Right. You answered your own question: not necessary.

In discussion with some techies at ATC there was no mention of this extra isolation

Yup. Because they just aren't at all concerned about it. There's no reason to be concerned about it.

I understand that I have left out a lot as regards treatment but I wanted to clear up these loose ends before I go further down the rabbit hole. Can you help

I would add one more thing: That's a big, heavy speaker, with some big, heavy moving parts inside. At high power and low frequency, that thing will want to move, shake, vibrate, wriggle, etc. Just sitting it on some bits of rubber is not going to stop it from doing that: It needs to be held a bit more rigidly than just sitting there under it's own weight. Barefoot shows one method for doing that, but that's more suitable for small speakers, not big ones like yours. I would go with a more substantial method of resilient tie-down.

- Stuart -

Thanks Stewart. Your patience and attention to detail are inspiring. I really appreciate it. The internet is an amazing resource but it is so easy to get put off track by people who seem to know what they are talking about. I appreciate coming here for you to put things straight for me.

Your speakers are not really full range: IIRC they roll off at about 60 Hz and 17 kHz, so you already have issues with spectrum coverage

I'm really surprised to hear you say that. ATC are one of the most highly respected and well used professional monitor manufacturers. They are brutally honest about their specs while some companies like PMC and Adams don't even publish them. Please bear in mind that the amplitude linearity figure is at +/-2db - most companies flatter their speakers by quoting at +/-3db. The 6db LF roll off point point on the SCM150 is 22hz. Thats pretty damn low. Sure there are speakers that are more linear. Anyway I love them - the midrange is the most detailed I have ever heard and PMC used this driver in their early designs and then copied it when ATC held it back from them. Other high end companies also use their drivers.

I would do that in the form of hangers, increasing the depth into the corners.

What exactly do you mean by this? Are hangers more efficient at dealing with LF than porous absorbers? Do you mean use the same 500mm footprint the width of the room but with hangers?

Why did you choose 30°? That doesn't look like the best angle for that room. It's long and narrowish, so I would have thought a slightly smaller angle would have worked better.

This is an example of bad internet advice I have been getting. I originally had 25 degree angles which I thought would be better only to be told that was incorrect and should always stick to the equilateral triangle rule. I will go back to 25 degrees which helps with all kinds of other design issues including maximising the space with all my large format equipment and being easier to increase the RFZ. I'll get onto that.

Placing tuned absorbers so close to your ears is not going to help that at all.

The design I would use is this one http://www.johnlsayers.com/HR/index1.htm Because it is built on a slant does this not ensure a wider q. These would be behind the desired mix position. I wanted to break up flutter and ensure that there is a little HF bounce in the room because I prefer this.

I would not do that, no. I would keep those sections pretty much like you have them at present, but check the angles! Did you raytrace that? To me, it looks like you haven't quite created a full reflection free zone. By eye, it seems to be that you still have some first reflection issues that could be resolved by better angling.

Yes I did ray trace. The area around the mix position represents this. The ATCs have very wide horizontal dispersion thats what they are famous for.

Horizontal Dispersion: ±80°, Coherent

I found this fact to be hard to work around. I experimented with the angles a lot but I know a way of making the RFZ larger now I can change to 25 degree angles. I'll get onto that and get a design up as soon as possible.

Some evolution in terms of RFZ design
The first pic is as the original 30 degree. Second is 25 degree RFZ slightly deeper and larger and the third is 20 degrees RFZ even larger still.
I have messed around for ages on different permutations and I think the third is the best. The narrow width of the room makes it super difficult and my large format equipment to boot (2m wide desk 1m deep) getting everything to fit and the right angles is quite a challenge. I reversed the driver lateral position of the drivers in the third one As i said in my last post I got the slanted slat wall designs from John's modular ideas on his site. I'd definitely like to have slats as I really like the sound. For the client listening at the back of the room I'd like to make it sound as good as is possible within reason and I thought angled slat panels would be good way to break up flutter and direct first reflections to the back of the room. I know that the couch at the back will be compromised but if clients can stand behind the mixing position with a good sound that would be a major bonus. Also I was thinking slats (not slanted) would be good on the back wall covering the basstrap to add a bit more life to the room. I'm not a great fan of really dead studios I find its very tiring to work in them for long periods of time.
As for getting the right type of neoprene for decoupling I have no idea where to begin. Can you recommend some sites or links where I might learn more?

bumpety bump

so lonely lol

Some evolution in terms of RFZ design
The first pic is as the original 30 degree. Second is 25 degree RFZ slightly deeper and larger and the third is 20 degrees RFZ even larger still.

Your diagrams do not show the speaker axes, or any ray-tracing, so it's impossible to say which one is best.

I'm not a great fan of really dead studios I find its very tiring to work in them for long periods of time.

Right! Your studio should not be dead. It should not be live either! It should be neutral. It should meet the ITU BS.1116-2 specification, which defines exactly how the acoustic response and layout of a room should be. It's worth reading!

As for getting the right type of neoprene for decoupling I have no idea where to begin.

I use Sorbothane, not neoprene. Sorbothane has excellent damping characteristics, as well a excellent resilience characteristics. You need to calculate (or measure) the entire weight of the speaker and enclosure box (the part that needs to be floated), and you use that to calculate the thickness, dimensions, deflection, and shore durometer, so you can choose the correct Sorbothane product.

Can you recommend some sites or links where I might learn more?

The Sorbothane company website has a lot of info on their products.


- Stuart -