Room Treatment Calculation Guide

Discussion in 'Listening Spaces' started by RevMen, Aug 17, 2011.

  1. RevMen

    RevMen The Reverend Menacer

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    Here are a set of steps you can follow to estimate your room's reverberation decay time and the amount of acoustical absorption you should add to dial in your room response. This is meant to be a first draft, so any suggestions for improvements are very welcome.

    I created a spreadsheet to do RT60 and room mode calculations for a simple, 6-sided room. You can download the file HERE and change it however you like. I will use it for these instructions. It's in Excel format. If you don't have a copy of Excel, you should be able to use the spreadsheet in Libre Office.

    The primary purpose of the spreadsheet is to perform a RT60 analysis of a room, based on the dimensions and surface types you enter into it. The RT in RT60 means Reverberation Time, which means the amount of time, in seconds, it takes for a sound to die down in the room. 60 means 60 dB, which is the total change in sound pressure between the sound's peak (when the decay starts) and the end of the reverb time. In other words, RT60 is the time it takes a sound to decrease by 60 dB. RT60 is considered for each octave band individually, which means there are 8 simultaneous calculations.

    RT60 is used very frequently by acousticians when designing rooms with acoustical sensitivity. Conference rooms, auditoriums, atria, home theaters, classrooms, even offices have preferred RT60 values for the best compromise between sound reinforcement (high values) and speech clarity (low values).

    Generally it's preferable to perform RT measurements in a room to assess it's response. Sometimes the room doesn't yet exist, or sometimes the equipment just isn't available. In those cases calculation or computer modeling must suffice.

    For listening rooms, a good RT60 to shoot for is 0.5 seconds in each of the octave bands.

    The spreadsheet has 3 tabs:

    RT60 - This is where all of the input information goes, including the room dimensions and the sizes and nature of every wall surface and significant object in the room. At the bottom you will find 2 versions of the RT60 calculation, one for more live rooms (Sabine) and one for more dead rooms (Eyring). Use whichever results match your average absorption the best.

    Reference Absorption Values - A list of absorption coefficients for some common coefficients. You use these values by copying them over to the RT60 worksheet. This is certainly not a comprehensive list. There are absorption coefficients for many more materials all over the internet, particularly from manufacturers of acoustical products.

    Room Modes - A quick calculation of the primary axial and tangential room modes, based on the room's dimensions. This will give you a hint as to what frequencies might cause problems for you between flat parallel walls or between corners.

    Let's get started.

    Step 1 - Room Dimensions. Download Simple-Room-Calcs.xls from the above link and open it up. Open it to the RT60 tab and put in the length, width, and height of your room. The spreadsheet refers to the walls as North, South, East, and West, to help you keep track of your surfaces and so it knows which direction is which. Call any room you want North and name the rest accordingly.

    Step 2 - Base Surfaces. There are several default materials and values filled in to give you an idea of how the spreadsheet should be used. The first surface to consider is the "Base" surface on the top line of each section. This is going to be the main surface of your walls, most likely a single layer of gypsum wallboard. If your wall is the party wall of a condo or apartment building, it may have 2 layers of wallboard. If you want to use something other than the default, you can copy the values from anywhere else on the spreadsheet or from the Reference Absorption Values tab (paste special... to save formatting). You do not need to set the area of the base material, it's calculated based on your room dimensions and the amount of other materials on the walls.

    Step 3 - Existing Surface Coverings. Copy in materials for each surface that is separate from your base surface. This will include doors, windows, wall hangings, and bookshelves. Use the left-most column for notes so you know what it is, set the surface area in the Area column, and fill in the Material and absorption coefficient columns from the Reference worksheet, or with other data that you find elsewhere. Every area that you enter in will subtract from the base surface area, so that the total area for each surface will add up correctly.

    If you have an opening to another room, you can model it as a surface with absorption coefficients of 1.0 in each band.

    The default state of the spreadsheet shows 0 area 703 panels on each wall. As long as their area remains 0, they will not affect your calculation. You can leave them there, or you can overwrite them with other materials if you need the space.

    Step 4 - Room Contents. Below the 6 surface sections is a Room Contents area. This is where you enter things like people and furniture that have area but are not attached to any of the room's surfaces.

    For items that say something like (per person, not per square foot), enter the number of items in the Area column, not the actual square footage. For instance, if you expect 2 people to be using your listening room, put a 2 in the Area column.

    Step 5 - Existing RT60. After all of your room dimensions, surfaces, and contents are set, the resulting reverberation times will be at the bottom of the spreadsheet. There are 2 calculations, but only 1 will be appropriate, based on the average absorption of your room.

    Sabine - This calculation should be used if your room's average absorption coefficients are below 0.2. You can see the Average Absorption Coefficient calculated on the line above "Sabine." Sabine is the classic RT60 calculation from the early 1900's and still holds up.

    Eyring - Use these values if your Average Absorption Coefficients are above 0.2.

    For small rooms, the accuracy of both of these calculations suffers for the lowest frequencies. In fact, the 63 Hz band may not be dependable at all. Even so, you can at least get a good sense of what your room is doing.

    Step 6 - Experiment with Absorption. If your reverb values are well above 0.5 seconds, you will want to add absorptive panels to bring your RT down. In its default state, the spreadsheet has 0 area 2" and 3" OC703 absorbers for each wall. Owens Corning 703 is, by far, the most common room treatment material and the bulk of off-the-shelf absorptive panels are made with this product or an equivalent.

    You can "add absorbers" by increasing the areas of these panels. Typical panel sizes are 4, 8, and 16 square feet, so you should add area in 4 s.f. increments. The specific wall that you add these panels to doesn't affect the calculation results, but you should keep track of how much absorption you want on each wall so that you can try to keep absorption symmetrical left to right and front to back.

    Do not forget that you can add absorption to your ceiling. In fact, this is often the best place to add absorption if your room has a lot of windows, doors, and coverings that don't leave room for panels.

    If you have other acoustical treatments besides generic OC703 panels that you want to try, just enter the values like you would for any other wall covering. Enter bass traps that will sit in a corner in the Room Contents section, so that their area doesn't count against the total surface area of the room.

    In addition to getting your RT values at or below 0.5 seconds, you will want to aim for as smooth an absorption spectrum as possible. In other words, you want all of your values to be as close to each other as they can be, especially for 500 Hz and above. If all of your values are within 0.05 seconds of each other, and they're near or below 0.5 s, you are likely to have a very nice sounding room.

    Step 7 - Acquire and Install Room Treatment. When you know the thickness and area of panels you want to install, it becomes much easier to purchase them with confidence.

    The calculation doesn't care where the panels are located, but your system will. Key places to install are:

    -On the back wall behind your speakers. Place them at first reflection points* first. If you're putting a lot of absorption on this wall, just generally line the areas behind your speakers. Some people like to put absorption halfway between their speakers to reduce inter-speaker interaction.

    -Side walls at first reflection points. Center them at about ear level (assuming your speakers are also at ear level).

    -Back wall first reflection points, or just generally behind the listening position. You may want to mount them a little higher than ear level if you have a sofa.

    -Ceiling first reflection points. The ceiling is a great place to put thicker absorption if you can't put it on the walls due to wife resistance. Sometimes people don't notice things that are on the ceiling, like spiders.

    (I know you just looked up).

    -Bass traps generally go in corners. The manufacturer of your bass trap will probably provide you with installation suggestions. Although they do the same thing, implementation is a little different from trap to trap. Between speakers on the back wall is another common location.

    * First reflection points are spots where there is a direct reflection between a speaker and your noggin. The classic trick is to sit in your listening position while a friend or spouse moves a mirror along the wall. When you can see one of your speakers in the mirror, mark the location of the mirror as a first reflection point.

    Step 8 - Enjoy. And report back to us with your impressions and pictures.
     
    Last edited: Apr 14, 2015
    bronto64 and sKiZo like this.

     

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  2. djnagle

    djnagle Lunatic Member

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    Very cool. Thanks for all your hard work.
     
  3. addaz

    addaz Well-Known Member

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    I can't wait to play with this! Thanks!
     
  4. djnagle

    djnagle Lunatic Member

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    Me too Add. I just moved into a new house and starting to set up my system. My last listening space was designed by the old owner who was the Cheif accostics engineer at Ford and Paul Klipsch......god I miss that space.
     
  5. Arkay

    Arkay Lunatic Member

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    Cool and useful tool - thanks for sharing it! :thmbsp:

    You mention furniture that isn't attached to a wall's surface. Do you mean physically attached, as in bolted to the wall, or just up against it in space (pushed against the wall)? What if, for example, a large cabinet is nearly against a wall, but separated by the thickness of a baseboard?
     
  6. RevMen

    RevMen The Reverend Menacer

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    The room contents are is for objects that are out in the room, providing absorptive surface area without affecting the room's physical dimensions. Furniture will cover up parts of walls or flooring, and if it's a large piece of furniture you might want to subtract the area of its footprint from the base area of a wall or floor. Generally, though, the amount of area they take up isn't enough to make an important difference in the calculation (which isn't extremely accurate to begin with).

    In the case of a book shelf with a back that's up against a wall, I would count that as part of a wall (I think I did that in the example calculation in the spreadsheet), since its sides aren't very big and since there's so little room between its back and the wall.

    So, basically, if the amount of floor or wall that the object covers is small compared to its own area (like a couch or bed), put it in the room contents area. And if an object covers a large big of floor or wall compared to its own area, like a bookshelf, count it as part of the wall or floor. "Covers" would mean within inches. A dining table doesn't cover any floor because it's separated by 30 to 40 inches.
     

     

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  7. pcb121055

    pcb121055 Member

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    Hey, this was a fun experiment. I finally downloaded the spreadsheet and worked thru the details to get it to reflect (no pun intended) my room. I generally liked the sound of my room but felt that the lower midrange and upper bass were a bit cavernous and RevMen's calculator confirmed it. I have a typical Eyring value of about 2.1 from 1K and up but at the 125, 250, 500 entries, I have about 2.8. Now I have to come up with a way to bring that first range down to the 2.1 level. Thanks for the calculator, RevMen. Suggestions welcome.
     
  8. RevMen

    RevMen The Reverend Menacer

    Messages:
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    Location:
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    I've been working on a graphical application to do the same calculation. It's going to be part of a larger suite of acoustical calculation programs, but the RT calculator is the only thing working right now.

    no longer available

    If you want to use it, just sign up there and you can launch the application from the Reverberation Time page.

    It's rough. Like, really rough. But it's pretty easy to use, everything works that you need to work, and the calculation works just fine. You can even download your results as a .csv file.

    When you launch it you may get a nag screen about isomorphic software. I actually own the license, I just haven't uploaded the new license files yet because I've been lazy. The software should still work the same after you click through it.

    There are instructions on how to use it at the website, but if you have any questions feel free to ask. It should be pretty intuitive. The only trick is making sure you select the project and room when you start (that's the part that really needs work).
     
    Last edited: Apr 14, 2015
  9. Ken Boyd

    Ken Boyd AK Subscriber Subscriber

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    I think this is over my head but I need to fix a large room for a customer and friends of mine. She called me yesterday wanting to know what I could do to get rid of all the echoing in the room when its full of people, not to mention the sound system that I installed is a nightmare to get sounding good. I knew it would be a problem but they wanted a sound system and then a large screen Flat monitor and then disco balls and nightclub lighting, fog machines, all of the lighting, fog machines, monitors I have mounted to the ceilings with the Flat screen hanging down from the center of the ceiling down to about 8 feet above the floor, the job just never ended they kept asking me what else I could do. Now they are not complaining about the music (I am) just about having a hard time hearing each other because when it gets crowded their is just to much reflective sounds from people talking. The problem lies also that it has quite a few windows all the way around, and high vaulted ceilings and some of the walls were built with old bricks that they might not want to cover up. Any idea's? The room measures about 30 feet wide by about 120 feet long, 18 foot ceiling, tile floors. I really don't know where to start. I thought about suggesting building some bifoldable panels with sound deading materials that they could stand around the edges of the room that way if they want to remove them they can, and they won't have to fasten them to the walls, and they could put them in closer when crowds were smaller. It is mainly used for family get togethers, but sometimes they let others use it for special occasions. Any ideas?
     
  10. RevMen

    RevMen The Reverend Menacer

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    Carpet on the floor is an obvious step, but that won't be enough by itself.

    If they don't want to change the walls at all, then the ceiling is about all you have left. What does the ceiling look like? Is it drywall? Fiberglass panels on the ceiling would work the best, and they could get some nice colors and patterns for the fabric, but it still might not be the look they want. If they're willing to spend some money, look into BASWAphon, which is an absorptive covering that looks like drywall. It looks awesome and it works well for situations like this.
     
  11. Rex Aeterna

    Rex Aeterna Gigolo of Gigolos

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    i'm curious if you got any mineral wool calculations? recently i bought some r15 3/12'' thick roxul mineral wool at home dept mainly for a small project but they only had 50lb bags(48 something more exact) for 35 bucks but since i was planning on buying fiberglass anyways i said ''what the heck'' and gave it a try and bought the whole bag.

    i threw some in my one corner while playing a 50hz tone and noticed how efficient it became absorbing the low-bass and that was just 1 panel, so i decided to put 3 panels in the corner(front,back,side. like forming little chamber/box) and put the left over 9'' thick fiberglass combination with it and seems nothing can get through it.

    might only be 12 panels for 35 bucks but seems overall you get more for your money compared to fiberglass and safer to handle. i read somewhere that mineral wool R value is much higher compared to fiberglass. i might buy another big bag of it and finish my ceiling next time around.

    only thing sucks bout mineral wool is you can't shape it like fiberglass since it's not nearly as flexible
     

     

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  12. djnagle

    djnagle Lunatic Member

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    OK, I've used this program twice. Once in my rood and once at a friends house. In both cases we then took measurements with the PE OmniMic and the measurement results showed the same as the program.

    VERY WELL DONE Rev.
     
  13. RevMen

    RevMen The Reverend Menacer

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    I'm glad it's working well for you. Credit for the math goes to Dr. Sabine

    I actually just changed jobs so I don't have access to all of that great alpha data that I did before. Mineral wool should be very similar to batt fiberglass in terms of absorption, maybe a little better. Data for batt fiberglass should be close enough for the purposes of using the spreadsheet.
     
  14. djnagle

    djnagle Lunatic Member

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    I just did a search online and found the Mineral Wool much more expensive then the 703 or Safe-n-Sound.
     
  15. Ess El Emm

    Ess El Emm Active Member

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    That's odd. I used this sheet to try to calulate based off of my new basement, and it says my times are right around 0.4 seconds (anywhere from 0.36-0.44).

    However, I can tell you for a fact that that room does not sound good at all with no treatment. It is somewhere around 12'x18' with drywall all around. It is a drop ceiling and has thin office style carpet on top of concrete.

    Without treatment it... well... sounds like you are playing a stereo in a basement. :thumbsdn:

    Even just a few batts of safe n sound stood up on the walls in about the right places makes it sound much better. Maybe I will just have to do this by ear.
     
  16. RevMen

    RevMen The Reverend Menacer

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    When I put in a 12 x 18 x 8 room with gypboard walls and ceilings and a carpet on concrete floor I get values that are much higher than 0.4. Check your numbers.
     

     

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  17. Ess El Emm

    Ess El Emm Active Member

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    That's odd, I'm not getting that. Well, can you tell me how many sq. ft. of 703 absorption panels it says I need to bring my numbers down to a good level and/or post a screenshot of the sheet so I can see what I'm doing wrong?
     
  18. RevMen

    RevMen The Reverend Menacer

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    Since the Base wall and ceiling surface is 1 layer of GWB, all you have to do is set all the other areas to 0 for walls and ceilings. Then substitute carpet on concrete for the floor surface data, adjust your furniture, and you're done. Remember to only change values in red.

    Remember that this calculation only concerns itself with reverb time. It doesn't predict other audible effects like slap-back echo and loss of imaging due to strong primary reflections, things that make it sound like you're in a basement.

    Adjust the spreadsheet to your desired RT in each band by changing the area of 703 panels on each wall. Then put those panels on the first reflection points to start.
     

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  19. vanquishfist

    vanquishfist Loudness Off

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    What's a good RT60 to shoot for in a recording studio? I'm working on a recording room in a friend's basement and would like to get it sounding as good as possible for our band recordings.

    Thanks!:)
     
  20. RevMen

    RevMen The Reverend Menacer

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    It depends on which type of room you're talking about. If it's a vocal booth you want it to be very dry (maybe 0.3 s). If it's a drum room you'll probably want it to be wetter, like 1 s, but not into auditorium range (2 s). For the control room it's a matter of taste, but somewhere between 0.4 and 0.8 s is a good goal.

    If you're not sure how you want it to sound, or you have a single recording room that will be used for different types of instruments, consider designing your room so you can use variable acoustics. A series of panels that are hard on one side and absorptive on the other can be flipped to tune the room to taste. Or permanently installed absorption that can be covered or revealed with removable hard panels.
     

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