John Sayers' Recording Studio Design Forum Archive

Hi, I'm building a rectangular gabled roof live room, 18ft x 31ft (5.5m x 9.5m). The ceiling starts at 10ft (3m) and peaks at 15ft (4.6m). The one thing I'm concerned about is that the two slanted ceilings of the gabled roof will create multiple first reflections back on a musician. You can see in the diagrams that a singer slightly off center will encounter ceiling reflections at the specified distances. The walls will have angled diffusers to deal with this.

I see two options:

1. Not worry about it due to the distance (allows maximum flexibility with setups)

2. Create specific reflection free zones with ceiling/wall diffusers where musicians will be positioned (limits flexibility but eliminates first reflections in those zones)

Any thoughts would be appreciated.


Diagrams below:
The two diagrams are identical - one in feet, one in meters. The three intersecting lines represent 3 common heights for instrumentalists (standing, seated and floor) and demonstrate that the double slap back from gable ceiling can not be avoided in any location. The walls will have diffusers to eliminate slapback. Ignore the "N" compass indicator -it's part of the software meant for floor plans.

zepdude wrote:Hi, I'm building a rectangular gabled roof live room, 18ft x 31ft (5.5m x 9.5m). The ceiling starts at 10ft (3m) and peaks at 15ft (4.6m). The one thing I'm concerned about is that the two slanted ceilings of the gabled roof will create multiple first reflections back on a musician. You can see in the diagrams that a singer slightly off center will encounter ceiling reflections at the specified distances. The walls will have angled diffusers to deal with this.

I see two options:

1. Not worry about it due to the distance (allows maximum flexibility with setups)

2. Create specific reflection free zones with ceiling/wall diffusers where musicians will be positioned (limits flexibility but eliminates first reflections in those zones)

Any thoughts would be appreciated.


Diagrams below:
The two diagrams are identical - one in feet, one in meters. The three intersecting lines represent 3 common heights for instrumentalists (standing, seated and floor) and demonstrate that the double slap back from gable ceiling can not be avoided in any location. The walls will have diffusers to eliminate slapback. Ignore the "N" compass indicator -it's part of the software meant for floor plans.

I'm not really sure what you are trying to say: ANY ceiling will ALWAYS create reflections, and they will ALWAYS go to other parts of the room. Musical instruments do not emit sound in one direction only, nor do they emit sound as straight lines. Yes, very high frequencies travel approximately like rays, but low frequencies do not: they travel as spherical wavefronts, so each part of the wave will impact a different point on the ceiling, and be reflected at a different angle. That's the same for flat ceilings and gabled ceilings.

So I don't understand the problem you think you will have.

will create multiple first reflections back on a musician.

True for any ceiling, not just gabled ceilings.

The walls will have angled diffusers to deal with this.

How? What type of "angled diffuser" are you talking about, and how would it prevent reflections form the ceiling? Please provide a link to the type of diffuser you have in mind.

I see two options:
1. Not worry about it due to the distance (allows maximum flexibility with setups)
2. Create specific reflection free zones with ceiling/wall diffusers where musicians will be positioned (limits flexibility but eliminates first reflections in those zones)

Is there any reason why you don't want to do the typical ceiling treatment that is commonly used in the majority of live rooms, tracking rooms, high quality rehearsal rooms, and performance rooms? Is there some limitation that you didn't mention, that would make conventional treatment unusable in your room?

I guess I'm just a little confused at understanding what you are trying to achieve, and what prevents you from doing it in the usual ways.


- Stuart -

Thanks for your reply, Stuart.

Soundman2020 wrote: I'm not really sure what you are trying to say: ANY ceiling will ALWAYS create reflections, and they will ALWAYS go to other parts of the room. Musical instruments do not emit sound in one direction only, nor do they emit sound as straight lines. Yes, very high frequencies travel approximately like rays, but low frequencies do not: they travel as spherical wavefronts, so each part of the wave will impact a different point on the ceiling, and be reflected at a different angle. That's the same for flat ceilings and gabled ceilings.
- Stuart -

You seem to be implying that slapback echo doesn't exist because sound doesn't travel in a straight line from the source. Am I misreading this?

The point of my original question is that, if in a control room we are creating a RFZ to reduce comb filtering by putting gobo's at the first reflection points of side walls etc or by angling walls, don't we also have to address first reflection points in a live room to reduce comb filtering?

You seem to be implying that slapback echo doesn't exist because sound doesn't travel in a straight line from the source.

No. I'm saying that it always exists, when the conditions are right, regardless of what type of ceiling or wall you have. Which is why it is usually dealt with in the conventional manner. I'm not getting what you think is different about your ceiling, and why you can't use conventional treatment.

The point of my original question is that, if in a control room we are creating a RFZ to reduce comb filtering by putting gobo's at the first reflection points of side walls

You seem to be misunderstanding how RFZ control rooms are designed and built! There are no gobos in RFZ control rooms, there are no first reflection points in a RFZ control rooms (if there were, it would not be RFZ...), and gobos don't reduce comb filtering anyway. Comb filtering is a phase-related artifact, not at all similar to slapback echos, and there's not much a gobo could do to fix it anyway.

don't we also have to address first reflection points in a live room to reduce comb filtering?

Those are two rather different things, and you still seem to have your acoustic concepts a bit muddles. Treatment on first reflection points would not be used in an RFZ control room, since there are no first reflection points. That's the entire reason why true RFZ rooms are shaped the way they are: in order to ensure that first reflection points do not exit at all. And in control rooms that are not based on RFZ concepts, and therefore do have first reflection points, the treatment you put on those points is not there to reduce comb filtering: it s there to damp the reflections, and reduce their intensity. You would not get comb filtering from first reflection points on the side walls or ceiling, for example, and the type of comb filtering you get from the front wall is more correctly called SBIR, not comb filtering. But SBIR from the front wall cannot happen in an RFZ room in any case, since in a true RFZ room the speakers would be soffit mounted, so it would be impossible to have SBIR coming off the front wall.

So I'm still not sure what you are concerned about, as you seem to be mixing up terms. Slapback is not comb filtering, neither of them would happen in a properly treated control room, especially not an RFZ room, there are no first reflection points in an RFZ room, and gobos are only used in live rooms, not control rooms.

If you are concerned about reflections from your gabled ceiling, then why not just use conventional treatment to deal with that? If you are also worried about slapback echo form your ceiling, then why not use conventional treatment for that as well? And if in addition to the above you are also worried about comb filtering artifacts from your ceiling, here too I would simply use conventional treatment for that, or just re-position the mic or instrument slightly so it is no longer at a location that suffers from comb filtering.

- Stuart -

If you re-read my post, I mentioned that RFZ used angled walls as I am familiar with these designs, but was referring to the fact that the common methods of dealing with sidewall first reflections in a control room are to either angle the walls (in a true RFZ design) or put gobos at the reflection location if the walls are not angled. The point of which was to point out that if we are concerned with those reflections in a control room because they interfere with the direct sound (you don't seem to want to use the term "comb filtering" as a negative artifact of two sounds that combine at slightly different times so lets just call it "smearing the sound")

But to get to the meat of my question...

Soundman2020 wrote:I'm not getting what you think is different about your ceiling, and why you can't use conventional treatment.

- Stuart -

Given that I would like to have a wood slat ceiling in the style of Avatar Studios, what would be conventional treatment?

was referring to the fact that the common methods of dealing with sidewall first reflections in a control room are to either angle the walls (in a true RFZ design) or put gobos at the reflection location if the walls are not angled.

Sorry, but that simply isn't the case. I have never, ever seen any professional studio with gobos as part of the actual original design of a single-purpose control room. If you have come across one like that, I'd be very interested in seeing it. Please post a link to their website, or at least some pictures of that.

The point of which was to point out that if we are concerned with those reflections in a control room because they interfere with the direct sound

They don't. Reflections from the side walls and ceiling or ceiling cause problems with the psycho-acoustic perception of sound, if they arrive within the Haas time. That's not comb filtering. There may well also be comb filtering going on, but that's not the main issue with first reflections. They reduce the perception of directionality, and they modify the way the ear and brain interpret frequency response, but that isn't comb filtering.

you don't seem to want to use the term "comb filtering" as a negative artifact of two sounds that combine at slightly different times

Right. Because that's not what comb filtering is! I'm not even sure how you arrived at comb filtering, when you originally said that the problem is slapback echo.... which something else again! It would help if you would keep to one problem at a time, instead of throwing in more and more issues that aren't even related to each other.

Comb filtering is a phasing issue, created by a direct sound that interacts with a delayed reflection of itself (less than the Haas time) that causes phase cancellation. It is a frequency domain issue, and looks like this: And it isn't even much of a problem in most cases, since your brain is very used to dealing with it, all day every day, and simply ignores it. It CAN be a problem in control rooms under certain circumstances, but we treat those.

First reflections are different. They are time-domain problems (not frequency domain), so you'll see them clearly on impulse response graphs, and they look like this: The peak marked by the cursor is a first reflection. It may or may not be causing comb filtering too, but that0s not the point. The point is that it is a first reflection, and causes problems with directionality perception, and frequency perception. Tones can appear to sound at a different frequency, or coming from a different direction than where they really are. That's the problem with first reflections, not comb filtering.

Slap-back echo is strong reflections outside the Hass time, such as these: The spike marked by the cursor there is slapback echo, well outside the Haas window. The two smaller peaks just before that are also slapback, but a bit lower intensity. The peaks further over to the left, close to the impulse itself, are first reflections. You can see why.

Those are three very different acoustic issues, with very different implications, and very different treatments.

Given that I would like to have a wood slat ceiling in the style of Avatar Studios, what would be conventional treatment?

It's right there, in the photo! And that's not a gabled ceiling, either.

You keep changing the description as you go along, so it's hard to follow what it is that you want to do. In acoustics, using the correct terminology is important, and so is providing detailed descriptions. You originally seemed to be concerned about specular reflections from flat surfaces on a gabled ceiling in a live room, but then compared that to first reflection treatment on the walls of control rooms, then moved on to concerns about slap-back echo, and somehow that changed to comb filtering issues at the mix position. You are all over the place! None of those are comparable.

It's as if I started talking about wanting to go to the supermarket in my car, but then started talking about how I'm worried about driving up the hill because of the propulsion system in nuclear submarines, and how that relates to the thrust-drag ratio of business jets, which is why my bicycle would not be useful...

Please try to just describe the problem that you are actually concerned about, using the correct terminology. One problem at a time.

The ceiling at Avatar won't have comb-filtering problems, nor will it have issues with specular reflections (because ti is not a specular surface), or flutter echo, or slap-back echo, because of the acoustic design of the room, and the treatment. The gobos visible in that picture are irrelevant to the all of those points (except maybe flutter echo), and gobos would never be used in a control room, either RFZ or otherwise.

If you stick to the right terminology, and talk about one issue at at time, then we can probably help you resolve your concerns.

- Stuart -

Thanks for your detailed response, Stuart.

I agree, lets stick to one issue -first reflections, in particular from a gable ceiling.

Essentially, I have an 18ft x 31ft rectangular room with gabled ceiling and want to eliminate first reflections wherever possible. I have a preference for the sound of fairly live sounding wood rooms (like Avatar) and therefore would like to go with a slat wood wall/ceiling treatment.

My thoughts for the angles of the slat walls to eliminate first reflections are on the diagram below (please let me know your thoughts). But the ceiling is giving me trouble as there will be two points of direct reflection for almost any position a musician might occupy, thus they are unavoidable.

So I want to get opinions on the potential nature of these ceiling reflections -will they be destructive or nothing to worry about in the real world? What can be done to eliminate them?