Need another set of eyes: Confirming room modes

Discussion in 'Home Theater Projects' started by Jay Mitchosky, Feb 16, 2004.

  1. Jay Mitchosky

    Jay Mitchosky Producer

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    Hey All

    I'm in the process of finalizing seating locations in my theater-in-progress so I can start building my riser and such. Been plugging away at the Stereophile Guide to HT spreadsheet and would like someone to confirm my results before I lock this puppy down.

    I tried loading my complete sheet to my website but ran into technical difficulties, so if someone could plug the following numbers into theirs and offer thoughts that would be great.

    ROOM DIMENSIONS
    • Length @ 23'8" (284")
    • Width @ 16'4" (196")
    • Height @ 8' (96")

    SEATING DISTANCE
    • From back wall @ 152" (11' from front screen)
    • From side wall @ 83"
    • From floor @ 44"

    Based on my interpretation there seems to be only two problem frequencies at this location: 214.7Hz and 241.9Hz, both peaks and easily attenuated. What's more, when I plug in the locations of the remaining front seats (at 57", 104", and 128" from the side walls respectively) they share the same peaks at seats 1 and 2, only the 214.7 peak at seat 3, and the 214.9 flips to a null at seat 4. For the back seats (same distances from side walls, 69" back from the front seats, elevated 12" compared to the front seats) 167Hz replaces 214.7Hz as the problem peak in seats 1-4, and 241.9 repeats in 1 and 2 as peaks, 4 as a null just like the front row.

    So if I'm reading this correctly, for all eight seats I have three peaks to contend with (214, 241, and 167Hz) and one null (241). The biggest problem I see is for the "cheap seats" at the far right in both rows where they not only have the natural 241Hz null but it will be completely exacerbated by the correction of its peak in seats 1-3 in both rows. But those aren't my seats so I'm not concerned. [​IMG] There are other problem frequencies noted in the room but if I understand correctly the ones I'm concerned with are located where people actually sit.

    Another question: what other information can be gleaned from this spreadsheet? How can it be used to determine the best locations for speakers and subs? This is a rectangular room but there are jogged sections at the front and a bulkhead around the ceiling perimeter. For the sake of this analysis I have ignored them and just used the longest dimensions. Don't know how that will affect the final result. My original seating position was at 12' from the screen but that introduced a PILE of trouble areas. The movement up one foot made a big difference. This also determines my screen size which is now looking to be 87" wide 16:9 (previously planned for 96" wide but at 11' distance that's a 40 degree viewing angle and maybe a little too extreme).

    All thoughts would be greatly appreciated. Thanks.

    FYI, here are pictures of my construction in case they may be of help.
     
  2. Cees Alons

    Cees Alons Moderator
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    Jay,

    Great work! I noticed that you have chosen the same "theme" name for your project as I did: Metropolis. Not that it's very original, but I even bought some posters already that will accentuate the theme later. [​IMG]

    Now about those 'modes'. (Actually, I hate the word. During the last HT-Cruise, someone asked the panel who had talked a lot about room-modes all the time, "I wonder why you totally leave out the problems of reflections and standing waves in HT rooms!" [​IMG] )
    You have chosen a room without any windows, so treating the walls to make them as 'dead' (in-reflective) as you want will not be much of a problem, isn't it? I wonder if you will have any problem at all with "room-modes". Just don't forget the ceiling and the floor [​IMG].

    The spreadsheet shows potential problems, but treating the walls (which doesn't even have to be too expensive, IMO) should take care of that.

    Personally, I even like to leave some (sound-) reflective places on the walls, to not make the room too dead, just not opposite to each other and parallel.

    I think you interpreted that spreadsheet quite right, but I hate the idea of leaving standing waves to exist in the HT and just make sure I don't sit at the worst places. Better dampen them out (almost) totally if you can. I think you can do that.

    Just my $0.02.

    Cees
     
  3. Jay Mitchosky

    Jay Mitchosky Producer

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    Hey Cees

    Hmmm, so I guess I should rethink hardwood and stainless steel then?
     
  4. Tim Bargar

    Tim Bargar Agent

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    I agree with Cees. But also, understand that the spreadsheet assumes perfectly reflective boundaries, which isn't the case, meaning the predicted frequencies and locations may not be accurate. The best thing to do is address reverberation time first and then do an acoustic analysis in the room at completion to determine problematic frequencies that can be addressed with specific absorbers.

    See this website

    http://www.proufo.netfirms.com/fam/oc.htm

    Pablo has put together a very good spreadsheet for calculating room modes and, more importantly, the RT60 for a room. Email him and ask him to email the spreadsheet to you.
     
  5. Cees Alons

    Cees Alons Moderator
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    Jay,

    No, by all means, go ahead. The effect will be ... interesting and different... [​IMG]

    Cees
     
  6. Jay Mitchosky

    Jay Mitchosky Producer

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    E-mailed for that spreadsheet. But do I not need a starting point first? It's a raw room and the initial seating locations will determine the riser location that needs to be built.
     
  7. Tim Bargar

    Tim Bargar Agent

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    You've probably received his spreadsheet by now, so you'll have an idea of what I'm talking about. Your room dimensions "look" OK just from looking at your website. That is the only thing it would be too late to deal with. Put the dimensions into his spreadsheet and it will tell you for sure. The "ideal" dimesions only lessen the number of peaks and nulls that will need to be dealt with later on through room treatments. It will tell you some of the same things that the SGHT spreadsheet does about the resonances, but as I said earlier, it assumes perfect reflectivity that does not likely exist in the room.

    The best part of the spreadsheet is on the last worksheet where it predicts the RT60 based on the different materials used for carpeting, seating, wall treatments, wall construction, etc. The MRSS spreadsheet is linked to another spreadsheet he sent you, abscoeff. On that other spreadsheet is a database of various materials along with their absorption data at frequencies from 250Hz to 4kHz, those frequencies that will most efficiently be blocked by drywall. The RT60 for the room is calculated using that database. You can then determine what percentage of the walls and ceiling need to be 'covered' by that material to achieve the desired RT60. Getting that right will ensure your HT is not too reverberant or too dead. Then you can place any absorption in the correct places to ensure against first reflections.

    Then the only thing to deal with, and it is probably the most difficult, is low frequency (
     
  8. Jason D.

    Jason D. Agent

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    I just wanted to drop my $.02 in on this one as well.....I too have used that spreadsheet, and spent hours playing with different room and seating dimensions. I have been planning the most intricate parts of my HT, and have probably changed designs a dozen times. It seems like you are as anal (not meant as a ding btw) as I am, so I just warn you not to fall in love with that calculator/spreadsheet. First, you will drive yourself crazy doing it, and second it really was meant as a reference tool to help you make some general decisions....i.e. you don't want a cube. I think that is what Tim was saying above. Your room will surely NOT have the same peaks and nulls that the calculator predicts as there are things the spreadsheet can not account for. With that said though, I believe it is a VERY wise move to use it as a tool to help limit the problems you will have with sound, and go from there. Good luck with your project, and I can tell from your post you will be having a blast designing and building it.....if you are anything like me...the problem will be when you finish it and you are sitting in it trying to enjoy it....thinking to yourself....WHAT'S NEXT[​IMG]

    -Jason
     

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