Acoustic Treatment Help!

[Simont85]

I'm putting together a home cinema with the initial building work now complete and I am now looking to design the acoustic treatment for the room.

Background:
Room - 4.6 * 3.9 * 2.4m with independent stud walls, 2 layers of green glue, door stops etc.
Speakers: 7.2.4 setup. In-wall speakers (base level sealed speakers and ceiling speakers have back boxes). - Monitor Audio IW460X for LCR, Monitor Audio CP-WT260 for rear/surrounds and Monitor Audio C265-IDC for the 4 atmos. Subwoofers are SVS SB3000.

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I've come up with a couple of different options:

Option 1:

• Build a 30cm baffle wall at the front for the screen/speakers stuffed behind with insulation and covered with acoustic wood panels (or fabric)
• Angled rear corners around 50cm sides stuffed with insulation and covered with acoustic wood panels (or acoustic fabric)
• 30 by 30cm perimeter soffit stuffed with insulation and covered on the bottom with fabric

My concerns about this are: The room gets smaller, it's a more complicated build and will bringing in-wall speakers introduce SIBR? I know you would face the area around the speaker with MDF etc, but still can't be as good as a solid wall.


Option 2:

• Build 50 by 50cm square pillars in each of the 4 corners stuffed with insulation and covered with acoustic panels on the front and acoustic fabric on side
• 30 by 30cm perimeter soffit stuffed with insulation and covered on the bottom with fabric

My concerns with this are whether there is enough bass trapping? Also, due to the space loss at the sides I wouldn't be utilising the full potential projector throw distance.

Option 3:

• Angled all the corners and stuff with insulation
• 30 by 30cm perimeter soffit stuffed with insulation and covered on the bottom side with acoustic fabric

Is this enough bass trapping? Obviously this might be the best solution in terms of room space/aesthetics.

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General questions:

• Are acoustic panels the right product both in terms of how it'll interact with the room and in terms of light reflectivity from the screen?
• I'm thinking I should cover the insulation with something to keep the high frequencies alive? What would that be?

I know the best thing to do is measure first, but that means you need to put all the soft furnishings in the room (carpet, sofa etc) before the carpentry etc, so hoping to get some initial thoughts from people with experience.

If anyone could offer any advice that'd be amazing.

 

[Keith_W]

Option 1:

• Build a 30cm baffle wall at the front for the screen/speakers stuffed behind with insulation and covered with acoustic wood panels (or fabric)
• Angled rear corners around 50cm sides stuffed with insulation and covered with acoustic wood panels (or acoustic fabric)
• 30 by 30cm perimeter soffit stuffed with insulation and covered on the bottom with fabric

My concerns about this are: The room gets smaller, it's a more complicated build and will bringing in-wall speakers introduce SIBR? I know you would face the area around the speaker with MDF etc, but still can't be as good as a solid wall.

In-wall speakers eliminate SBIR, it does not introduce it. It also eliminates edge diffraction.

Option 2:

• Build 50 by 50cm square pillars in each of the 4 corners stuffed with insulation and covered with acoustic panels on the front and acoustic fabric on side
• 30 by 30cm perimeter soffit stuffed with insulation and covered on the bottom with fabric

My concerns with this are whether there is enough bass trapping? Also, due to the space loss at the sides I wouldn't be utilising the full potential projector throw distance.

Option 3:

• Angled all the corners and stuff with insulation
• 30 by 30cm perimeter soffit stuffed with insulation and covered on the bottom side with acoustic fabric

Is this enough bass trapping? Obviously this might be the best solution in terms of room space/aesthetics.

With a small room like yours, any frequency with a half wavelength longer than your longest room dimension will be in pressure mode (assuming your room is sealed). This means that bass waves pressurize the room evenly and do not form room modes. 4.9m is half wavelength of 9.8m, which corresponds to 35Hz. Your room will form modes up to the Schroder frequency, which I estimate to be about 170Hz assuming your T30 is 0.3s. So if you want bass trapping, you need to deal with freqs between 35Hz - 170Hz or so.

You have two options when it comes to bass trapping - pressure absorbers or velocity absorbers. Velocity absorbers are thick, and need to be 1/8 as thick as the lowest wavelength you wish to attenuate. If this is 35Hz, it needs to be 1/8 of 9.8m, or 1.2m thick. Pressure absorbers like membranes can be thinner, but are narrowband and need to be tuned to the specific frequencies you wish to attenuate.

So: forget bass trapping. Use DSP instead. It gets you 90% of the way there without the room intrusion.

General questions:

• Are acoustic panels the right product both in terms of how it'll interact with the room and in terms of light reflectivity from the screen?
• I'm thinking I should cover the insulation with something to keep the high frequencies alive? What would that be?

I know the best thing to do is measure first, but that means you need to put all the soft furnishings in the room (carpet, sofa etc) before the carpentry etc, so hoping to get some initial thoughts from people with experience.

If anyone could offer any advice that'd be amazing.

Your system is for enjoyment. You are not designing a recording studio. So - acoustic panels should be the last resort. Put your system in your room, place all the furniture, and measure. You will probably find you do not need any treatment at all.

 

[er|κzvio1in]

Your system is for enjoyment. You are not designing a recording studio. So - acoustic panels should be the last resort.

You're implying a logical argument here, but I fail to see it. I'd say most home theaters will benefit from broadband absorber panels. If course you don't want to deaden the room, but reasonably reduce reflections.

Also even though it is pointless to attempt to absorb all the way down to 35 hz, it's still a good idea to fill the corners like OP suggested if it's aesthetically acceptable.

 

[312elements]

I think it’s smart that you’re recognizing that everything will act as a system from the speakers to how they interact with the room to as @Keith_W pointed out, thr dsp.

I question whether the 1.2m rockwool is really required as these waves bounce they could potentially be going through the material twice. How much bass are you putting into the room and where are you putting it?

I’ve got about 18” or rockwool in my ceiling and a lot of diffusers in my room. My understanding about treating first reflections is that depending on how well your speaker measures (off axis response) it may be more beneficial in some cases than others.

So have any decisions been made yet or is everything up in the air? If you’re starting with the room and then going from there do you have any preferences regarding the speakers? Are you doing Atmos? Did you figure out a processor/dac yet?

 

[Simont85]

I think it’s smart that you’re recognizing that everything will act as a system from the speakers to how they interact with the room to as @Keith_W pointed out, thr dsp.

I question whether the 1.2m rockwool is really required as these waves bounce they could potentially be going through the material twice. How much bass are you putting into the room and where are you putting it?

I’ve got about 18” or rockwool in my ceiling and a lot of diffusers in my room. My understanding about treating first reflections is that depending on how well your speaker measures (off axis response) it may be more beneficial in some cases than others.

So have any decisions been made yet or is everything up in the air? If you’re starting with the room and then going from there do you have any preferences regarding the speakers? Are you doing Atmos? Did you figure out a processor/dac yet?

I've just got the bare construction done at the moment with all the speakers cut out apart from the front so I can have flexibility with regards to the treatments above.

Speakers I already have outlined in first post. The receiver will be the anthem 1140 and the subs dual SVS SB3000 in opposite corners (at least seems to be best from REW simulation). However, I've wired all corner locations.

 

[312elements]

I would add the two more subs for the opposite corners, especially if the current plan is for back of the room. I strongly prefer bass coming from the front if it has to come from one direction. If you csn, stuff your floor/ceiling joists with rockwool, I’d skip the broadband panels and just treat the corners with pillars. The corner treatments will be enough to reduce rt60 without sucking all of the life out of the room and let your anthem arc processor handle the room correction.

 

[Simont85]

I would add the two more subs for the opposite corners, especially if the current plan is for back of the room. I strongly prefer bass coming from the front if it has to come from one direction. If you csn, stuff your floor/ceiling joists with rockwool, I’d skip the broadband panels and just treat the corners with pillars. The corner treatments will be enough to reduce rt60 without sucking all of the life out of the room and let your anthem arc processor handle the room correction.

Sorry my bad it's opposite front and rear corners.

The ceiling is already up and has rockwool, but is covered with two layers of plasterboard. I could put up a perimeter soffit and put maybe 30*30cm of rockwool with fabric on the bottom side.

What RT60 do you think would be right for a cinema room of this size?

 

[312elements]

Sorry my bad it's opposite front and rear corners.

The ceiling is already up and has rockwool, but is covered with two layers of plasterboard. I could put up a perimeter soffit and put maybe 30*30cm of rockwool with fabric on the bottom side.

What RT60 do you think would be right for a cinema room of this size?

.25-.50 I think. As long as the room is carpeted and full of furniture you’re not likely to have big issues there which is why I suggested skipping the broadband. No need to expose the rockwool, the bass will blast right through. Perhaps out of an abundance of caution design the room to accommodate 6” deep broadband wall absorbers, but only add them if you feel you need them. They can get expensive fast and can absolutely be overdone. Focus on the bass (which you’re already doing, and let the room correction take care of the rest.

BtW it seems like you’re putting together a fantastic room with quality gear and attention to detail. Keep us posted on your progress.

 

[Keith_W]

What RT60 do you think would be right for a cinema room of this size?

RT60 is frequency dependent, application dependent, room volume dependent, and taste dependent. This is the DIN 18041 RT60 target for music. The upper and lower dotted lines represent the upper and lower limits of the standard:

(EDIT) I have replaced the graph that I originally posted with a graph that I generated using my measurements but with your listening room dimensions. The RT60 is within target for my room dimensions, but not for yours. My room volume is almost 3x that of yours, so longer reverbs are tolerated.

I have forgotten that HT guys seem to have a different RT60 preference. They seem to prefer a lower target. I am told it has something to do with multiple speakers.

 

[Tom C]

I am planning a similar project, and found the following resource quite helpful:
CEDIA Immersive Audio White Paper

To get the PDF file, you have to register at the site with email and password, then add the file to your cart and check out. It’s free of charge though. Other items you have to pay for, but this particular paper they offer free of charge. The paper discusses level 1, 2, 3, 4 builds, with level 1 being entry level minimum, and level 4 full-on luxury. There is much information on speaker placement, equipment selection, room construction, acoustic treatments, bass management, etc, and considers Dolby, DTS, and Auro. It’s the most comprehensive paper I’ve found, and published in 2023, so pretty recent. There are answers to the question of RT-60, that are too in depth to fully capture here, but their recommendations differ according to which level of quality/complexity you’re aiming for.

 

[Timcognito]

FYI

Soundproof Drywall: What is It and Does It Work?

Is soundproof drywall worth the extra cost? After all, soundproof drywall is much more expensive than regular drywall.

www.homedit.com

 

[Simont85]

I am planning a similar project, and found the following resource quite helpful:
CEDIA Immersive Audio White Paper

To get the PDF file, you have to register at the site with email and password, then add the file to your cart and check out. It’s free of charge though. Other items you have to pay for, but this particular paper they offer free of charge. The paper discusses level 1, 2, 3, 4 builds, with level 1 being entry level minimum, and level 4 full-on luxury. There is much information on speaker placement, equipment selection, room construction, acoustic treatments, bass management, etc, and considers Dolby, DTS, and Auro. It’s the most comprehensive paper I’ve found, and published in 2023, so pretty recent. There are answers to the question of RT-60, that are too in depth to fully capture here, but their recommendations differ according to which level of quality/complexity you’re aiming for.

Thanks. Seems to be saying it's $99.

 

[Tom C]

Thanks. Seems to be saying it's $99.

Sorry about that. Not quite right the first go at it, but I think I have it sorted now. I have edited the link, so you can try again if you like. Notice that only the first name, email and accept terms and conditions are required, although they ask for more.

CEDIA/CTA-RP22

 

[Simont85]

Thx. Very helpful.

Can I ask a question. I had the floor poured in the room and it was smelling, so the builder has now put a SBR sealer down on the floor. Since that has gone down the room is way more reflective. Is this something to worry about, or will it all be fixed when the carpet goes down and make no difference?

 

[Doodski]

Thx. Very helpful.

Can I ask a question. I had the floor poured in the room and it was smelling, so the builder has now put a SBR sealer down on the floor. Since that has gone down the room is way more reflective. Is this something to worry about, or will it all be fixed when the carpet goes down and make no difference?

Concrete is normally reflective with or without SBR sealer treatment. What makes you think the reflectivity has increased since the sealer was used?

 

[Simont85]

Concrete is normally reflective with or without SBR sealer treatment. What makes you think the reflectivity has increased since the sealer was used?

I can hear the difference. It's quite noticeable.

 

[Doodski]

I don't want to appear as judgmental and stuff but is it possible that you think you are hearing things that are not actually occurring? A listening bias. It's happened many times before for many peeps. They buy a new cable, speaker wire, CD player and DAC and then they think everything changed when anything that changed was not detectable to the human ear.

The best thing is to start using a calibrated microphone and REW (Room EQ Wizard.) to measure your room and detect any changes and to determine what the room needs and how it is working with sound. The software is free but the calibrated USB microphone costs a small amount.

REW room acoustics and audio device measurement and analysis software

REW is free software for room acoustic measurement, loudspeaker measurement and audio device measurement and analysis

www.roomeqwizard.com

UMIK-1

The UMIK-1 is an omni-directional USB measurement calibrated microphone providing Plug & Play acoustic measurement. From speaker & room acoustic measurement to recording, this microphone provides low noise and accurate results you can rely on.

www.minidsp.com

 

[Simont85]

I don't want to appear as judgmental and stuff but is it possible that you think you are hearing things that are not actually occurring? A listening bias. It's happened many times before for many peeps. They buy a new cable, speaker wire, CD player and DAC and then they think everything changed when anything that changed was not detectable to the human ear.

The best thing is to start using a calibrated microphone and REW (Room EQ Wizard.) to measure your room and detect any changes and to determine what the room needs and how it is working with sound. The software is free but the calibrated USB microphone costs a small amount.

REW room acoustics and audio device measurement and analysis software

REW is free software for room acoustic measurement, loudspeaker measurement and audio device measurement and analysis

www.roomeqwizard.com

UMIK-1

The UMIK-1 is an omni-directional USB measurement calibrated microphone providing Plug & Play acoustic measurement. From speaker & room acoustic measurement to recording, this microphone provides low noise and accurate results you can rely on.

www.minidsp.com

Thanks.

However, the room is not yet carpet/furnished etc. So it was always sounding like a lot of echo. But since he did the sealer today it is noticeably more echoey (I can tell from talking in the room and when you open the door handle it now even echoes). This is not a bias.

 

[Doodski]

Thanks.

However, the room is not yet carpet/furnished etc. So it was always sounding like a lot of echo. But since he did the sealer today it is noticeably more echoey (I can tell from talking in the room and when you open the door handle it now even echoes). This is not a bias.

For purposes of being analytical about this stuff I think measurements need to start as soon as the room has been dampened with carpet, underlay, window coverings etc etc. I will take your claim at face value and accept that you can hear more echo because it is possible you do but let's keep in mind it is also possible you don't without having actual measured data using REW and the calibrated microphone.

Any frequency response changes of the room's acoustics will all be done and measured after the room is ready for gear or after the gear has been installed for maximum accuracy. So don't sweat out some additional echo induced by concrete sealant at this time.

 

[Simont85]

For purposes of being analytical about this stuff I think measurements need to start as soon as the room has been dampened with carpet, underlay, window coverings etc etc. I will take your claim at face value and accept that you can hear more echo because it is possible you do but let's keep in mind it is also possible you don't without having actual measured data using REW and the calibrated microphone.

Any frequency response changes of the room's acoustics will all be done and measured after the room is ready for gear or after the gear has been installed for maximum accuracy. So don't sweat out some additional echo induced by concrete sealant at this time.

Okay. I found this

Concrete (unpainted, rough finish)	0.01	0.02	0.04	0.06	0.08	0.1
Concrete (sealed or painted)	0.01	0.01	0.02	0.02	0.02	0.02

Sound Absorption Coefficient Chart | JCW Acoustic Supplies

Absorption coefficients of common building materials and finishes. Take a look at the JCW soundproofing products and options available.

www.acoustic-supplies.com