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Calculating Dimensions in an odd shaped room

Discussion in 'Studio Acoustics' started by DonnyThompson, Sep 14, 2015.

  1. DonnyThompson

    DonnyThompson Moderator (Distinguished Member) Resource Member

    Nov 25, 2012
    Akron/Cleveland, OH
    Home Page:
    @Brien Holcombe

    Brien - using Bob Gold's calculator, how do I calculate a room of varying widths?

    I'll be more specific ... I'm looking at moving into another area of my house - it's a Living Room area - where in half of the section, a staircase cuts into the width by 3 ft.

    So, Length is 23' uninterrupted ( with the exception of the doors, doorways to other areas, and windows), and in half of the area, the width is 14', 6"; but in the other half, the width is only 11',6" because of the staircase. The staircase is not fully walled-off on the side facing into the room though, the drywall follows the steps as they go up, so from a side view it looks like this:


    Ceiling Height in the main area is 7', 6" - except for above the steps, where it's canted to follow the staircase, going up to an eventual height at 14' 7" (measured from the floor in the main area to the ceiling at the very top of the staircase).

    I understand Bob Gold's calculation software is for "basic" L/W/H room dimensions; but the room I'm considering isn't really all that common, because of the way that the staircase intrudes into the space.
    I'm baffled ( no pun intended LOL) on how to accurately measure this room... not only from the approach of plugging in the measurements into Gold's calculator, but also acoustically as well.

    Here's a quick and dirty bird's eye layout of the area, with measurements:


    Other details: Walls and ceiling(s) were put up in the early 1940's, so whatever type of sheetrock was being used at that time is probably what it is. It's not "typical" modern drywall...
    Floors are hardwood, uncarpeted; right now I don't even have any area rugs... (although I do have two sofas which are cloth covered and which have some mass because of how they are filled...)

    I guess I should just come out and ask if in considering using this area as a mixing space, am I begging for problems here?

    Any help you could give would be greatly appreciated.
  2. Brien Holcombe

    Brien Holcombe Active Member

    Mar 31, 2015
    Exit 4
    Home Page:
    So exactly where do you think you will be positioned in this space? The staircase presents a coupled area, kinda like a reverb chamber. Depending on if you have it to the right of your head or to your back which would be the better of the two, it is going to influence what you hear. A heavy drape might subdue the chamber. But that room is square (11'-6"X11'6") with a shallow ceiling so it is out of the question.

    But we know that the more cubic footage we can get the better so that puts you with the front door to your back and the reverb chamber in your left ear.

    But let us for get about that for a minute. If you want to figure room modes by hand it is a simple equation:

    c= The Speed of Sound at sea level is considered to move at a static speed of 1,130 feet per second.
    W= The measurement in question.
    f = The frequency that correlates to the results of the equation.
    Then divide that by 2. Reason being is that you have to make a round trip, from one wall to the other.

    The room at the staircase, the square one is a good place to start. 11'-6" looks like 11.5 to a calculator.

    1130/11.5=98.26 /2=49.13 or 49Hz

    If you do the other wall you get the same results of 49Hz. So these frequencies will support each other meaning that you will get a 49Hz Axial mode from one set of parallel walls and the same 49Hz Axial mode from the other set of parallel walls. Two strong and disruptive room modes that support each other and we barely started.

    Some will take the "measurement in question" and multiple by two before dividing into "f", but the results are the same. For instance, 1130/(11.5X2)=49.13 or 49Hz.

    Since we are in this room let's take a look at the ceiling height 7'-6" 1130/7.5=150.66 /2=75.33 or 75Hz.

    These are the first order Axial Modes, 49Hz,49Hz,75Hz. In order to determine other possibilities you have to consider multiples of these frequencies to see if there is continued support or exactly how smooth the room response will be. Will the frequencies be spread out(good) or will they be clumped up(not so good).

    If the first order Axial of 49Hz is brought to the third order, 49HzX3, it becomes an third order 147Hz room mode.

    Stay with me....

    If the first order Axial of 75Hz is brought to the second order, 75HzX2, you get a second order room mode of 150Hz.

    You now have a third order 147Hz mode from wall to wall being supported by a second order from floor to ceiling at 150Hz. Only 3 points separate them from being a direct hit, so to speak.

    So the square area has proven to be an ineffective area mathematically for our want. Also, this will continue to rule it out in future decisions unless modification of some sort is considered.

    So that leaves you with the front door area...is there where you would like to be?
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  3. DonnyThompson

    DonnyThompson Moderator (Distinguished Member) Resource Member

    Nov 25, 2012
    Akron/Cleveland, OH
    Home Page:
    First of all, thanks for writing a great post that I was able to follow - something I don't think I could have done a year ago. ;)

    Truthfully, I hadn't really gone as far as to determine where I would be located in the space. The area is something I was considering, because it would give me more room to work than the space I currently occupy.

    I understand that the squared area ( 11.5 x 11.5) is not only not ideal, it's problematic; and when we add into the scenario the hollow area under the staircase, it presents even further issues...( reverb chamber.. and not even a good one LOL)

    This area I was considering is looking unrealistic at this point. While the space I'm currently in is smaller and not as easy to work in ( cramped when you factor in the gear), it is a better area to work in acoustically, and it has been treated with traps, BB absorbers and a 3" OC cloud, to the point where I am able to turn out mixes that translate fairly well. I've recently completed an album project in that smaller area, and I was pretty happy with the results - as was the mastering engineer - so I think I should probably just bite the bullet and stay there.

    I've learned a lot from you Brien - along with past posts from Andre ( @avare ), and I can't begin to thank you enough for taking the time to help me to further understand that which I should ( and need) to know in order to determine the acoustic practicality of possible location(s) in which to work.

    I feel this post is quite valuable - if we were to consider that someone might be doing a similar topic search on the subject ( not that their room dimensions and properties would be the same as mine) it certainly brings home the point that you can't choose just any random area to mix in, and expect it to be productive... to the contrary ... it may very well make that task even harder. It also makes it pretty clear that you can't fix these issues by simply throwing up 1" auralex tiles onto your walls, and expect them to fix - or to make any difference at all - in low frequencies, or frequencies below their attenuation rating.

    Home studios face some tough obstacles, because houses aren't built with audio recording /mixing in mind (and my house, which was built in the early 40's, makes this even tougher, I think).

    The transfer of sound - not only from room to room - but from outside to inside, too - makes recording difficult, and the rooms within the house are commonly not the best spaces in which to mix.

    There is one other area in my house ( a finished attic) that would also provide more room to work, but I haven't yet taken any measurements; I've not taken it seriously because of the shape of the space, ( the walls are canted about 1/3 of the way up to match the pitch of the roof) and I've made an assumption that because of this, it would also turn out to be very problematic as well:


    Anyway... Thanks Brien... for taking the time to respond and explain why that Living Room space would be unrealistic.

    Great post. ;)


    Attached Files:

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  4. Brien Holcombe

    Brien Holcombe Active Member

    Mar 31, 2015
    Exit 4
    Home Page:
    " It also makes it pretty clear that you can't fix these issues by simply throwing up 1" Auralex tiles onto your walls, and expect them to fix - or to make any difference at all - in low frequencies, or frequencies below their attenuation rating."

    It also reveals that "testing" is for the lazy.

    Anyone who wanted to can learn this equation, learn the difference between Axial, Tangential and Oblique modes, come to an understanding of 1,2,3,4, etc. order of modes...can understand a room as if they had actually been in it :)

    It's all in the Master Handbook of Acoustics...but ya have to read it 15 times to get it. And far, far too many want to be seen as knowledgeable after learning a few tips when in fact, this is an on going never ending thing. You do it because you want to learn and the more you learn the less you know so you go back and start all over again until you "get it".

    Anyway, thanks for allowing me to be involved in some small way.

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