acoustic treatments for home theatre room

[wilsonj]

Hi all, this is my first post on these forums but It seems like the right place to ask my questions.

Firstly I'm fairly new to room treatments, but I've been reading as much as I can and have just bought some 2" 703 equivalent material to make my first absorbers. These will be more for experimentation at this stage as I am using a temporary room until I can finish the theatre.

The room will be used for 80/20 HT/music. It has a raked ceiling starting at ~7ft and extending to ~9ft at the rear wall.

The room has a concrete floor and plastered brick walls.

The floor will be carpeted and I am contemplating a star ceiling. If I do I will probably use absorbing material, but not sure how effective it will be once covered with velvet material.

OK, on to some questions.

1. Is it worth using a perforated screen and putting 703 behind? As it is I will use absorbers below the screen, and heavy curtains that open to the sides. For music I could close the curtains giving some absorption.

2. Corner bass traps are an issue, due to space. I could possibly use them at the front and have them hidden behind either a second curtain, or use a curved track.

3. As the diagram shows I plan on absorbers at first reflection points on the walls. But what do I do on the back wall? Diffusers absorbers, both or nothing?

4. With the lack of corner traps, am I best to use 4" absorbers where I can? My understanding is that a 2" absorber should be spaced 2" off the wall, but this is not as good as a 4" absorber against the wall.

cheers
Jamie

[Ethan Winer]

Originally Posted by wilsonj

1. Is it worth using a perforated screen and putting 703 behind?

Bass trapping on the front (and rear) walls is always a good idea.

2. Corner bass traps are an issue, due to space. I could possibly use them at the front and have them hidden behind either a second curtain, or use a curved track.

Don't forget that rectangle rooms have 12 corners, not just four.

3. As the diagram shows I plan on absorbers at first reflection points on the walls. But what do I do on the back wall? Diffusers absorbers, both or nothing?

Best is diffusors, second best is absorbers, and a bare wall is worst and unacceptable.

4. With the lack of corner traps, am I best to use 4" absorbers where I can? My understanding is that a 2" absorber should be spaced 2" off the wall, but this is not as good as a 4" absorber against the wall.

Better still is 4-inch absorbers with 2 to 4 inches of air gap if you can manage that.

--Ethan

[SaSi_SiDi]

If you don't have enough width on your front end of the room, consider bass traps above and below the screen. With the 16:9 format, width is at a premium and height is mostly wasted on the front wall.

I am also considering to utilize a double role screen/absorber for the front wall.

A HT setup benefits from absorbption much more than conventional stereo audio setups. Especially if you go for a 5.1 system, the more you kill reflections the better.

[wilsonj]

Hi Ethan, thank you for your reply.

Maths is not my strongest point, but did you mean 8 corners, not 12??

Can you recommend what sort of diffuser would be best on the back wall? And what sort of area I should be aiming to cover? 1/4, 1/2..?

Sasi, thanks for your reply.

So you are also contemplating a perforated screen also? Are your speakers to be placed behind the screen? Or are you going perforated purely to allow the use of absorbers on the wall?

Another thought I had was running carpet halfway up the side walls. Any thoughts on this? Although it might be cheaper to just paint them and use absorbers where needed, if there is no real advantage.

Cheers
Jamie

[Millo 3.1]

Hey! No, he meant 12... 4 wall-to-wall, 4 wall-to-ceiling, 4 wall-to-floor

This point he brought up might give you ideas for "hidden" absorber placement.

[SaSi_SiDi]

I am going to try to use an absorber panel as big as the screen with mineral wool inside and white sound transparent cloth as cover. I know it won't have great reflectivity but it's worth to try.

Running a carpet up the walls is a bit of an overkill.

The carpet (be it on the floor or the walls) will only absorb high frequencies and will do that pretty well.

However, in a small room, it is the low frequencies that are harder to tame. The amount of absorbers required for that exceed the amount of absorbtion required for the high frequecies. That's why bass traps placed anywhere but on first reflection points are recommended to be FRK faced - so that they can reflect some of the high frequencies and not over absorb them.

It is much better to spread the absorbers around. Spacing them with bare wall between causes some sort of diffusion due to the difference in reflectivity between the bare wall and the absorber panels.

If your walls are not going to be used for anything (like shelving, racks, etc), then you have ample options on how best to spread the absorbers around.

[Ethan Winer]

Originally Posted by wilsonj

Can you recommend what sort of diffuser would be best on the back wall? And what sort of area I should be aiming to cover? 1/4, 1/2..?

I prefer QRD type diffusors. I'd cover at least 1/3 of the rear wall, with the diffusors at ear height. You can put bass traps above and below to handle the ceiling and floor corners.

Another thought I had was running carpet halfway up the side walls. Any thoughts on this?

Don't do that. Carpet is not good acoustic treatment.

--Ethan

[wilsonj]

Ok thanks guys.

Soooooo no carpet on walls, scratch that silly idea!

12 corners, now I get it! Or course I was only counting corners where three faces joined.

1/3 QRD on back wall, check.

Still a little unclear about the front (behind screen) wall. As a perforated screen will cost considerably more than a normal screen, I want to make sure what I am proposing will be of real benefit. As space at the front is at a premium, only 4" (2" 703 type material, 2" gap) will be used. I plan to do the whole wall. So should this be FRK material? Its not all a first reflection point.

Something else I was thinking about. The room will have a false ceiling, as I have 8" beams along the ceiling as it is now, and plan to build the new ceiling onto the bottom of those. This will leave large sealed cavities between the two ceilings. Is there anything I could build use this? I'm thinking bass traps??

Cheers
Jamie

[SaSi_SiDi]

In general, a live floor / dead ceiling combination is good. It might be beneficial to mount a "cloud" from these beams and add absorbtion there.

Regarding the front wall, what you want to do with a small room (apart from taming the room modes) is to make it appear larger. In order to do that, reflections close to the speakers need to be reduced or better killed. One school of thought is to have the back wall with diffusers create the diffuse field creating adequate delay so that reflections give the impression of a larger space. A larger space is one where sound needs to travel quite a few feet before it hits a boundary (wall/ceiling) and comes back to the ears.

[wilsonj]

Originally Posted by SaSi_SiDi

In general, a live floor / dead ceiling combination is good. It might be beneficial to mount a "cloud" from these beams and add absorbtion there.

Regarding the front wall, what you want to do with a small room (apart from taming the room modes) is to make it appear larger. In order to do that, reflections close to the speakers need to be reduced or better killed. One school of thought is to have the back wall with diffusers create the diffuse field creating adequate delay so that reflections give the impression of a larger space. A larger space is one where sound needs to travel quite a few feet before it hits a boundary (wall/ceiling) and comes back to the ears.

Hi Sasi,

So no FRK on the front wall then? and diffusers on the back wall? This is what I had planned. As well as absorbers on the side walls.

As for the floor, I will probably use a low cut carpet. Actually I was surprised the other day while using REW impulses just how much reflection was coming off the current carpet.

I still plan on making a star ceiling with 703 wrapped in black velvet. I just wondered if I filled a cavity along the wall/ceiling corner with insulation and had a perforated panel covering it if it would work as a bass trap.???

[SaSi_SiDi]

Regarding FRK and the front wall: high frequencies are directional so the output from the speakers will travel the length of the room, bounce back from the back and side walls and return to the front wall either directly from the back wall or bouncing at the side walls. These waves will be attenuated enough and bouncing again on the front wall would perhaps be beneficial to create an ambience.

Furthermore, the FRK on the front wall actually enhances the low frequency absorbtion, so the bottom line is that the front wall if treated with absorbers, they should be faced.

A perforated absorber (perforated panel with a sealed cavity in the back and - optionally - absorber material inside) is a kind of tuned absorber. Obviously, what you have in mind will not be airtight, so the tuning effect will be reduced. But you need to be careful that the perforation and depth combination will not create a tuned absorbtion at an unwanted frequency.

[PaulP]

Originally Posted by SaSi_SiDi

A perforated absorber (perforated panel with a sealed cavity in the back and - optionally - absorber material inside) is a kind of tuned absorber. Obviously, what you have in mind will not be airtight, so the tuning effect will be reduced. But you need to be careful that the perforation and depth combination will not create a tuned absorbtion at an unwanted frequency.

And you may have to do your own perforating since regular pegboard (if
that's what's being considered) doesn't have enough holes in it.

Paul P