Ideal control room shape- II continuing

[Chris Kress]

As not to hijack a previous thread, I am continuing from this one.
http://www.gearslutz.com/board/studi...ing-acoustics/

I am adding a mixing/mastering control room in my new garage space which has 14' ceilings. I am also adding another un-related room above the garage. As soon as I commit to my room dimensions and ratios, I'll be moving forward with reverb time calculations. You acoustics guys can get a good laugh and be "wow'd" by my hobbyist acoustician prowess. : )

Originally Posted by avare

1.) We have no knowledge of your budget.

2.) What we do know is that you are asking questions that some people would just go and hire a designer for. If it is an enjoyment of designing it yourself, or lack of money for that we do not know.

3.) If I were you with what I know of your situation, I use the 1:125:1.6 ratio.

4.) You are starting from around 14' and we discussing the 11'4" ceiling cavity. Correct? Does the 14' height take into account sound isolation?

5.) Yes you can use that space as a bass trap. A fantastic bass trap it would be with that depth.

Keep asking questions! Andre

Thanks very much. Here are my answers:

1.) I am still working on my budget.

2.) I have designed 3 rooms to date and I very much enjoy it. I am an acoustician hobbyist. This is the first time I have had this much flexibility with my room dimensions. As soon as I consider the proper ratios (thanks to help from nice people like you), I'll be ready to run the calculations I learned in this book

"How to build a small budget recording studio from scratch", Everest and Shea.

I have a spreadsheet to calculate reverb time, while adding materials such as 703 and bass traps, also considering NRC of all surface materials..

3.) I am, thanks and it is in my current drawings.

4.) I am not worried about sound isolation. I'll be adding 5 insulated walls and this will be more than plenty. I am paying attention to placement of any mechanical things such as washer/dryer, hvac, etc.

5.) The distance between the internal ceiling and the exiting space 14'- 11' 5" is approximately 2' 7". The dimensions of the ceiling bass trap will be 2' 7" x 18' x 14'5"

A question:
At what point does cubic volume create diminishing returns against cost? I have seen 1500 cubic feet as a minimum. My maximum dimensions of the outer room I'll be framing new walls into will be 15' wide, 19' Long, 14' tall. Following your advice, my best way to fill the space with your ratios of would be H=11'3", W=14'1", L=18.0" for my situation. 2847 Cubic feet.

[avare]

Originally Posted by Chris Kress

1.) At what point does cubic volume create diminishing returns against cost? I have seen 1500 cubic feet as a minimum.
...for my situation. 2847 Cubic feet.

There is no hard and fast diminishing point. The low limit is based on tolerable acoustics for one listener. That is the 1500 ft^3 that you mentioned. The EBU/ITU specs for monitoring spaces use a nominal 3,500 ft^3 in their reverb time equation. that is 100 m^3. Obviously chosen as a round figure. Usually control room size is dictated by size of equipment number of people desired to listen at the same time.

You didn't ask, but it fits in here. The NARAS Production and Engineers wing recommends a minimum height of 11' for mixing spaces for surround mixing.

2.) I have designed 3 rooms to date and I very much enjoy it. I am an acoustician hobbyist. This is the first time I have had this much flexibility with my room dimensions. As soon as I consider the proper ratios (thanks to help from nice people like you), I'll be ready to run the calculations I learned in this book

"How to build a small budget recording studio from scratch", Everest and Shea.

Great! I strongly getting three more books:

Everest: "The Master Book of Acoustics" fourth edition;
Gervais: "Home Studio: Build it Like the Pros"; and,
Newell: "Recording Studio Design"

The BBC R&D has some of the most technically advanced reports relating to studio acoustics. Ones from 1970 to 1996 are available for free here.

I have a spreadsheet to calculate reverb time, while adding materials such as 703 and bass traps, also considering NRC of all surface materials.

Great! So you are not afraid of math.

NRC is not appropriate for music. It does not consider low frequencies.

Andre

[Chris Kress]

Originally Posted by avare

NRC is not appropriate for music. It does not consider low frequencies.
Andre

Sorry, I used the wrong description. I am using STC values ranging from 125hz to 4000hz. I'll post the spreadsheet for my new room next. I am set on 1:1.25:1.6 though. This was great advice that I was able to substantiate from several different sources.

[Chris Kress]

Avare, attached is a reverb time spreadsheet for the room you proposed. I have a few questions.

1.) I am concerned about a few of my sound absorption coefficients values:
- I am looking for good SAC data on a laminate wood floor which rests on the foam padding. Or I could consider the glue down kind.
- I am not sure if my SAC data for Drywall considers insulation behind it. I would be happy to eliminate the cost though if it is not necessarry. Do I need insulation for these inner walls? (sound containment is not required for my structure)

2.) I found a figure for a ceiling bass trap. For my design, I would implement it by cutting a 2' x 18' hole in my ceiling along the back wall. The inside of the ceiling area would be filled with randomly placed layers of 2" 703 that drop down vertically. I may price the option to simply fill it with "pink stuff"? Either way, I am pretty sure that I do not need to consider this as a part of my spreadsheet since it will be most effective below 125 hZ and will have no other impact. Ahh yes it will. It will end up being 36' of broadband absorber if the hole is covered.

Any help is appreciated.. Chris

EDIT - Attachments were lost somehow......? Others are below

[avare]

Originally Posted by Chris Kress

Avare, attached is a reverb time spreadsheet for the room you proposed.

I can't tell because it is a jpg. Does the spreadsheet use the Sabine or Eyring equation? Considering how you are spreadsheet literate and enjoy the design process, I would suggest using the Fitzroy-Eyring formula for best calculation results.

The relationship between theory and practice is not clear. BBC RD 1991-07 shows in fig 7 how high frequency reverb time INCREASED with added absorption. It didn't take long to get the BBC RD Reports specifically into your thread

- I am looking for good SAC data on a laminate wood floor which rests on the foam padding. Or I could consider the glue down kind.

I am not aware of any either. If was consulting on a project, I would not recommend it without getting test data, whether from other sources or paying for tests. Glue down would be the way

- I am not sure if my SAC data for Drywall considers insulation behind it. I would be happy to eliminate the cost though if it is not necessarry. Do I need insulation for these inner walls? (sound containment is not required for my structure)

I would use insulation simply because it reduces the MAM frequency by about 1/3 of an octave. IOW the walls will are bass traps.

2.) I found a figure for a ceiling bass trap. For my design, I would implement it by cutting a 2' x 18' hole in my ceiling along the back wall. The inside of the ceiling area would be filled with randomly placed layers of 2" 703 that drop down vertically. I may price the option to simply fill it with "pink stuff"?

Fantastic! If you are going to fill it, do the pink stuff route. There is no advantage (actually disadvantaqe) to using 703.

Andre

[Chris Kress]

I'll be reading this again. It seems to discuss the consideration of the empty room as a starting point I think???? instead of calculation to get that starting point.

After reading the conclusion, I get the idea that reverberation room measurements (which is what I think I am currently using) are reasonable if the acoustic materials are spread evenly over the entire room and ceiling. They also mention that effective diffusion should be used as well. I believe that I can do all of these things. But it seems that you recommend Fitzroy-Eyring as being better so I would be silly not to find out more about it.

[avare]

For the benefit of people reading the thread for the first time and not knowing what you may be writing about, in future could you include references to document(s) you are referring to? We are discussing BBC RD 1995-08 linked in my previous post.

Originally Posted by Chris Kress

I'll be reading this again. It seems to discuss the consideration of the empty room as a starting point I think???? instead of calculation to get that starting point.

Uhm, well yes. I think you started your calculations from an empty room.

After reading the conclusion, I get the idea that reverberation room measurements (which is what I think I am currently using) are reasonable if the acoustic materials are spread evenly over the entire room and ceiling. They also mention that effective diffusion should be used as well.

For Sabine to be effective, yes. Reality keeps getting in the way though. Notice how they put no 8" thick absorbers on the floor?

I believe that I can do all of these things. But it seems that you recommend Fitzroy-Eyring as being better so I would be silly not to find out more about it.

I have not written the following sentence in over 5 days now. Good studio building is 90% design and 10% construction.

Andre

[Chris Kress]

I'll be reading BBC RD 1991-07 again. I hope to find the specifics about Fitzroy-Eyring. I'll do this before asking any more questions. This is uncharted territory and I expect that it may take a while. Let me know if I am going in the wrong direction.. Thanks, Chris

[Chris Kress]

I have been focused on the room construction. Problems I am having:

- I'll have to face the longest 18' wall (for listening) in order for the room to function.
- My room ratios ended up being closer to 1:1.2:1.5 (12'H-14'5"D-18"W) and then I decided to be creative. I widened the room and then angled the left and right walls inward, hopefully providing compensation for the slight loss in cubic volume. I also angled the ceiling in a way that should keep the cubic volume the same.

Questions:
- Above what frequency will non-parallel walls be effective at diffusing sound?

[Chris Kress]

The .pdf shows a top view of the room and also has room mode calculations based on these dimensions 18.15'W - 14.4'D - 12'H.

[avare]

Above what frequency will non-parallel walls be effective at diffusing sound?

Sorry, they do not diffuse sound. What they do do is control early reflections and flutter echo. For more about splayed surfaces this link will help.

For small rooms, as in home studios the limit is either the frequency where the dimensions become equivalent to a wavelength; or where the room dimensions become significant and sound starts to act in a modal behaviour. The easy equation to use for the latter is the Davis frequency equation. It is three times the frequency where twice the smallest dimension equals the wavelength.

The next step in difficulty is the Schroeder frequency which is:

f=1200((v/rt60^0.5))

where:

f= Schroeder frequency
v=room volume in m^3
rt60=reverb time

Congratulations on having a ceiling height that exceeds NARAS recommendations of 11 ft minimum. You have a nice space and design. Keep us posted with pictures!

Calculatingly:
Andre

[Chris Kress]

Andre, Thanks for the link in the previous thread. That was good and practical information. Otherwise, I am glad you feel the room has potential. I'll keep adding pictures this month as we progress.

Questions:
- Is there any problem with placing the speakers on the 18'2" wall? This gives me a 14'5" room depth.
- The wall studs are so very tall that I am sure they'll resonate. In previous installations I have filled the entire wall cavity with cellulose fiber. It seemd to nicely deaden the wall. Any thoughts?

BTW. I still plan to stuff the huge ceiling cavity full with "pink stuff" and leave the edge of the ceiling open to allow bass trapping. I may leave a 1'4" opening so that bass may enter the ceiling cavity.

P.S. I added the modal distribution chart to page 2 of the PDF from 2 posts back (as I initially intended to)

Thanks again very much... Chris

[avare]

Is there any problem with placing the speakers on the 18'2" wall? This gives me a 14'5" room depth

Ouch, I missed that. Assuming 38% back leaves 8', not enough for commercial diffusors. You would have to build them yourself. It also leaves only 6' for console and balance of chair space.

The wall studs are so very tall that I am sure they'll resonate

They will be attached to the drywall/plywood you face the walls with. Not a problem.

Nice modal analysis. Eric's (as in Eric Desart in the splay thread I linked) analysis program is most comprehensive I now of.

Can you repost the reverb time calcs or email the spreadsheet to me? My email is in my profile.

Using the Davis equation, the limit for modal action is around 140 Hz. You will have absorption below that frequency in the walls and ceiling.

Andre

[Ethan Winer]

Originally Posted by avare

Assuming 38% back leaves 8', not enough for commercial diffusors.

Just to clarify, RealTraps sells "near style" diffusors that can be used within about 3 feet of the listening position. The diffusing depth is three inches, plus there's another three inches of bass trapping. In fairness, Auralex also sells 3-inch deep diffusors, though they reflect bass frequencies rather than absorb.

--Ethan

[avare]

Originally Posted by Ethan Winer

Just to clarify, RealTraps sells "near style" diffusors that can be used within about 3 feet of the listening position. The diffusing depth is three inches, plus there's another three inches of bass trapping. In fairness, Auralex also sells 3-inch deep diffusors, though they reflect bass frequencies rather than absorb.

Thanks Ethan;

What is lowest frequency your diffusors diffuse at?

Andre

[Chris Kress]

Originally Posted by avare

Ouch, I missed that. Assuming 38% back leaves 8', not enough for commercial diffusors. You would have to build them yourself. It also leaves only 6' for console and balance of chair space.

Andre

I knew this would be a problem...... : ( I had no other choice. In desperation I made a splayed wall directly behind the mix position that is 8' wide and comes out 1' to a point in the middle. I do understand now that this will not diffuse sound. I am also heartbroken because I would like a little distance between myself and the speakers for once...

Otherwise, I'll probably be using a mastering type desk with my console 90 degrees out of the normal position to my right. I'll have a compensating desk to my left. I would really like to be at least 6' from the surface of my speakers..

[Ethan Winer]

Originally Posted by avare

What is lowest frequency your diffusors diffuse at?

The 6-inch deep model should be good to below 800 Hz, and the 3-inch should be good to an octave higher. We haven't measured them, but that's the calculated specs for our particular QRD pattern.

--Ethan

[avare]

Good news. From Ethan's data his bigger diffusor is just about ideal.

Andre

[Chris Kress]

Originally Posted by avare

Can you repost the reverb time calcs or email the spreadsheet to me? My email is in my profile.
Andre

Sorry for the delay, here is a pdf of the spreadsheet. I'll send you my e-mail address in case you would like to communicate directly for the .xls.

BTW, A huge thanks to both of you.

[avare]

The RT60 is high compared to EBU/ITU specs. Around 240 ms is the target using their formula.

Andre

[Chris Kress]

Originally Posted by avare

The RT60 is high compared to EBU/ITU specs. Around 240 ms is the target using their formula.

Attached is a compensation:

Responses from earlier conversation:
- I am using Sabine equation
- I have not been able to implement the Fitzroy-Eyring equations yet

[avare]

Originally Posted by Chris Kress

Attached is a compensation:

Responses from earlier conversation:
- I am using Sabine equation
- I have not been able to implement the Fitzroy-Eyring equations yet

It looks good. Probably higher in life because Sabine is good for live rooms. You might want to try the Eyring equation. Replace, or add depending on your view, "a" in the Sabine equation with "-e(1-a)" where "e" is the natural logarithm.

Andre

[Chris Kress]

Slow going. Pre-wiring is done. Drywall is in. I think it will be finished 3 weeks or so. Also, I have almost figured out how to plug this new equation in...

[Chris Kress]

And a few more...

[andrebrito]

6 inches diffuser is target to I think 650 Hz. Look at the RPG QRD 734 datasheet to confirm this. I also sell similar ones but never tested it.

[Chris Kress]

I also build a 6" diffuser.... I think??. This is a picture of a unit built and installed in a different studio. Although this version is only a 2" difference in radius, I could build my own with a 6" total difference in radius. Here are pictures of the unit which is filled with regular "pink stuff" insulation. Results have been very good for the couch listening position adjacent to these....Admittedly, I am a hobbyist at acoustics.. Does anyone know if a simple radius surface will function properly as a diffuser?

EDIT: I found the answer that this is not a diffuser.. but a deflector..

[Chris Kress]

Since the ceiling cavity above my room averages 2' deep and is approximately 19' x 15', I have made it into a bass trap at the recommendation of Avare. The ceiling is splayed. So this cavity for the bass trap is 2.5 feet at the front of the room and 1.5 feet in the rear. I filled nearly all the space with "pink stuff" insulation, also at Avare's recommendation.

These pictures show an 8" hole which wraps around the rear of the room. And one with a 2' x 2' hole in the front-center of the room which opens to a very dense area of insulation. These are for the purpose of allowing bass to enter the cavity.

- Is 8" a big enough gap for the rear bass trap?

Otherwise, I am going to have to start with -inferior- calculations on setting up my room acoustics. Avare recommend a different equation for my spreadsheet and I have not had time (or proper knowledge of how) to set it up. However he mentions that I will likely have a higher reverb time, which I want anyway.

I just ordered 20 boxes (2' x 2 ' x 4' boxes) of 2" 703. I'll follow my calculations (Reverberation-Time-Chesterfield-REV1.pdf is attached above) as a starting point and then measure using a free program (which I downloaded from an acoustics forum) and my earthworks measurement mic (I have two of them). I have about 3-4 days established to have a construction guy manipulate the materials until the room balances. I'll expect to use finished birch and other materials listed on the attached spreadsheet to vary the acoustics.

- would anyone have recommendations for a first time measurer using software?
- Is it overkill to use a 6" depth of 703 for the approximately 560 square foot coverage area... or should I start with 4" and expect to get additional low frequency absorption from the bass trap instead?

I added a 3-d sketchup of the room. It is zipped and attached below.

Average dimensions
18.15'W - 14.4'D - 12'H

[avare]

It looks good.

The 8" is consistent with electrical Audio's bass traps that they are very proud of.

If you have the material, start with 6". If nothing else it lowers the effective frequency for high normal incidence sound insulation.

Did I mention that it looks great!

Andre

[andrebrito]

Recommendations:

- use the simplest software you can find (I suggest REQ Wizard)

- make the measurements long (at least 5 s)

- the sound has to be loud enough to excite the room but not extremely loud it can damage the speaker or the sound has too much distortion due to the "efforts" of the speaker's cone displacement

- use just one sound source and for reverberation time measurements placed it away from the walls

- use earplugs

[Chris Kress]

Originally Posted by avare

It looks good.

The 8" is consistent with electrical Audio's bass traps that they are very proud of.

If you have the material, start with 6". If nothing else it lowers the effective frequency for high normal incidence sound insulation.

Did I mention that it looks great!

Andre

Wow, what a nice boost as I am a little worn out by pressing the construction team for all of the little details.. and it has taken forever. I'll stay strong.....