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Thread: RTA and Spatial Averaging Usage

  1. #1
    Tester Extraordinaire ErinH's Avatar
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    RTA and Spatial Averaging Usage

    Really quickly, spatial averaging is nothing more than taking a measurement at various positions in the desired seating location and averaging them together. Geddes wrote a paper on this back when he worked for Ford, called the "Localized Power Sound Method". A link is here:
    http://rapidshare.com/files/41377257...und_Method.pdf

    What I talk about below is discussed in more detail in Geddes' paper linked above, so I suggest you read it if you really want to know more.

    The reason you do this is because basing a tune of a single mic measurement is quite possibly the worst thing you can do (especially in a car). Comb filtering/reflections cause erroneous data, that if used, can often make your car sound even worse than if you hadn't used it.
    In most cars I've tested, the data is non-repeatable above 500-1000hz. In other words, this is where reflections start to take over. If you were to move your mic a couple inches in any direction and measure again, you would see different results.

    So, it's imperative that those of you who use the RTA to help you tune really make an effort to move the mic around. In the car, all you have to do is tilt the mic to the left, right, up, down in various angles and run a measurement with each location. Afterward, simply average those results into one. While it may take more time for you to measure your car, it's time well spent.

    If you have TrueRTA (like most of us do) then it's a total breeze.

    Here's how you get a spatial average:
    1. Measure Center. Save results by going to the 'view' menu and saving or simply hit 'Alt+1'.
    2. Move the mic about 45 degrees to the right or left of center. Mesure. Save results in #2 (Alt+2).
    3. Move the mic to the opposite side of center (left or right). Measure. Alt+3 to save.
    4. If you can, move the mic up about 2-3". Measure in the center. Save as #4. Alt+4.
    5. With the mic in the same Y position (height position), move the mic left/right of center. Measure. Save. Alt+5.
    6. Move the mic to the other side of center. Measure. Save. Alt+6.
    7. Finally, average all the results together by doing this: Go to the 'Utilities' menu. Then 'Average'. 'Select Input': this will be #1-#6. Now 'Select Memory to use for Result': this will be #7 or whichever number you want to use. Just don't override one of your measurements.
    8. That's it. Now, look at the difference between each singular measurement and the 'averaged' measurement. Notice the difference? ;)


    You have now measured and completed a spatial average of your system. Use this averaged measurement to tune for best results.

    Here's an illustration of the first singular measurement of my system, with the mic located at the headrest, pointed straight towards the system in green vs. the final spatial average in purple.
    You'll see that, for the most part, the two measurements are the same until you get to about 400hz. Then the results take a sharp detour from each other.



    Secondly, here are the 6 measurements before the average. You can see just how different each measurement is.


    Imagine trying to base your tune off a single one of these results.
    If I were to have done this, I would have likely been trying to correct problems that weren't as big a deal as appeared.
    Also, BE SURE TO LISTEN as you make changes. The RTA cannot replace your ear. I've 'corrected' many things via the RTA method, only to get in the car and find that it sounded like junk.
    My best results have followed after I picked up on spatial averaging. My one golden rule for using an RTA is to focus on the big problems and sweat the small stuff later. For example, in the picture above, my first mode of attack would be the peak at 250hz.
    The dip at 5khz is something I like, as well as the boosted upper end response. There's no right or wrong. Let your ears decide.

    Hope that helps you guys.

  2. #2
    Devil's Advocate Adam_MSS's Avatar
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    I thought I'd piggy back on Erin's post and show an example of how I've used spatial averaging.

    My setup was inspired by Geddes multiple subwoofers technique, and numerous email conversations with him while I was building the Nathan 10s. I've got three different sources, two IB manifolds in the ceiling (each with 2 IB15s) and then a Maelstrom in the front corner of the room.

    The measurement locations:


    And the measurements in location order:








    There are obviously some areas of uneven response at the edges of the room, but in the main seating area, the average response is very good:
    You don't use science to show that you're right, you use science to become right. - R.Munroe

    The important thing in science is not so much to obtain new facts as to discover new ways of thinking about them. - W.L.Bragg



  3. #3
    Founding Member 86mr2's Avatar
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    I'm guessing it's safe to say you are happy with the multiple sub approach. Did you move the Maelstrom or the IB manifolds based on testing to get that response, or did it just work out that well to begin with?

  4. #4
    Devil's Advocate Adam_MSS's Avatar
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    I changed the direction the maelstrom was facing. Everything else was done with the dcx and deq.
    You don't use science to show that you're right, you use science to become right. - R.Munroe

    The important thing in science is not so much to obtain new facts as to discover new ways of thinking about them. - W.L.Bragg



  5. #5
    Founding Member n_olympios's Avatar
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    This is very interesting! When I played with an RTA I found out on my own that it works best this way. It would've been much easier if I knew the theory behind it though. Erin, could you please upload the pdf file again?
    Nick
    Virtus probata florescit: reversio

  6. #6
    Founding Member wtfBBQ's Avatar
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    It would be great if someone could write a macro or some code to do this, so when we have 4 hours to do some tuning, we can get a spatial average and do some tuning on the fly.

  7. #7
    Tester Extraordinaire ErinH's Avatar
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    why would you need code or a macro? you just move the mic in 6 different positions and then average the response together. I'm all for writing code when it makes life easier, but the 'code' in this case is the 'average' feature built into the software. ;)
    Your ears: The best tools you have... and they're free, too!

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