# Using the RTA for tuning: The importance of spatial averaging



## ErinH (Feb 14, 2007)

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". I've attached it for you guys to read. Thanks to Thadman for hooking me up with it. The only way to get it is if you're an AES member, so make sure you download it here while it's still up.
What I talk about below is discussed in more detail in Geddes' paper, 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:

Measure Center. Save results by going to the 'view' menu and saving or simply hit 'Alt+1'.
Move the mic about 45 degrees to the right or left of center. Mesure. Save results in #2 (Alt+2).
Move the mic to the opposite side of center (left or right). Measure. Alt+3 to save.
If you can, move the mic up about 2-3". Measure in the center. Save as #4. Alt+4.
With the mic in the same Y position (height position), move the mic left/right of center. Measure. Save. Alt+5.
Move the mic to the other side of center. Measure. Save. Alt+6.
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.
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.

- Erin


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## basshead (Sep 12, 2008)

*Re: Using the RTA for tuning: The importance of spacial averaging*

Thanks for doing all the hard work for us!

what are you using to hold the mic when you test?


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## ErinH (Feb 14, 2007)

*Re: Using the RTA for tuning: The importance of spacial averaging*

It's really not hard work. The hardest part is opening the car door to move the mic. 

I just squeeze it between the headrest and seat. Nothing special. All of this RTA work is just approximation. I don't worry about a stand. Stick it under the head rest, measure. 
When I move it to a different height, I put it on top of the headrest. 
Then I move it left/right across the headrest. Nothing special.


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## jsun_g (Jun 12, 2008)

*Re: Using the RTA for tuning: The importance of spacial averaging*

Thanks for all of this information. IMHO it is important to consider that your ears are on the sides of your head and reflections off the side door windows can be troublesome as well as the shape of your head itself affecting the response you hear. Did you try any mic measurements where the mic is pointed to either side (~90 degrees) or by using earbud mics? I am curious to see how it measures against the other response curves.


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## titansfan (Jan 27, 2009)

*Re: Using the RTA for tuning: The importance of spacial averaging*

Thanks for the article Erin...nice read. I'm going to try this myself and see if I get better results than my current settings.


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## ErinH (Feb 14, 2007)

*Re: Using the RTA for tuning: The importance of spacial averaging*



jsun_g said:


> Thanks for all of this information. IMHO it is important to consider that your ears are on the sides of your head and reflections off the side door windows can be troublesome as well as the shape of your head itself affecting the response you hear. Did you try any mic measurements where the mic is pointed to either side (~90 degrees) or by using earbud mics? I am curious to see how it measures against the other response curves.


I've measured where your ears are, yes. 
IMHO, measuing a few points around your head area is just another approximation, but it's a closer approximation than measuring from a single point.
If you'd like, you can measure 90*, 45*, 0* and average all of those.

I do own binaural mics and have used those, but the hardware configuration makes it a PITA so I've pretty much stopped doing that.

The downside to using only a mic is it doesn't account for HRTF (head related transfer function) nor does it account for the space your body consumes. 

Try whatever you like. If you find something works well, let us know. The spatial averaging is a more logical way of approaching measuring. You're free to choose how ever many points and locations you want to measure and average. I stuck with 6 because Geddes' work showed that anything over that point is unnecessary.


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## Knobby Digital (Aug 17, 2008)

*Re: Using the RTA for tuning: The importance of spacial averaging*

Interesting; moving the mic around the head area.

Usually, I average 8 measurements on a horizontal plane. While sitting in the seat, 2 straight-forward w/ my head on either side of the mic, and then 45, 90, 135 deg in both directions. Also, I use a gooseneck drum mic clip on the 'legs' (or whatever you wanna call them) of the headrest.

Like this guy:










Next time I whip out the RTA I'll give that way a go and see if what I come up w/.


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