# Average SPL measurements - first time user



## BDE10 (Nov 2, 2011)

Hi there. 

:help:
I am trying to use the SPL measurements, and while I do get some fine results, the problem is that I want an average. I want to know what the average is in dB, Is there any way to do this in REW V5?

for example.. I run 3 measurements and generate an average of these 3, now that this has been done, I want an average of the measured dB, how do I do this? 

Thanks in advance.


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## aackthpt (Jan 24, 2011)

Welcome to the forum, BDE10.

Try checking out these links:
http://www.hometheatershack.com/for...thmetic-missing-latest-jan-11-v5-release.html
http://www.hometheatershack.com/forums/rew-forum/43411-newbie-where-arithmetic-tab.html

Regards,
John


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## BDE10 (Nov 2, 2011)

aackthpt said:


> Welcome to the forum, BDE10.
> 
> Try checking out these links:
> http://www.hometheatershack.com/for...thmetic-missing-latest-jan-11-v5-release.html
> ...


Hi John.

Thanks for the quick response.

I am using ALL SPL, and there is indeed a tab that indicates that it will find the average, however it doesn't work for me (or I don't understand how to use it correctly, which is more likely), heres an example:

I take 3 tests, and want to make an average on these (lets say they are all 40dB), then somestimes the average comes out as 38dB, and that just doesn't make any sense to me. 

Is there by any chance a video tutorial on how to use specific parts of the program?


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## aackthpt (Jan 24, 2011)

BDE10 said:


> Hi John.
> 
> Thanks for the quick response.
> 
> ...


I don't understand the method used to do the arithmetic calculations so I can't really answer the question. I don't know if it is using the phase data to take vector sums such that the arithmetic doesn't come out the way one would expect or what. I think JohnM is probably the only person who can competently answer that question.

I don't even know how one would average three tests in REW. If the test results are A, B, and C then I don't think doing A and B averaged, then averaging that result with C is correct. Take, for example, the data 3, 5, 8. The triple average of then (3+5+8)/2=5.333. Using the other method, the average of 3 and 5 is 4, the average of 4 and 8 is 6. Since clearly 5.333!=6, the methods are not equivalent in normal real mathematics (again, all bets are off if it is using vector math).

The main advocate of response averaging seems to be Earl Geddes, who uses HolmImpulse for his acoustical measurements so I think that is what most followers of his ideas use to make their acoustical measurements. I did also find a post he placed onto diyAudio in which he said he does his own data manipulation because then he knows exactly what has been done rather than having to trust someone else's software - which doesn't sound like a rousing endorsement to me. Nonetheless, I did also find this post in which someone discusses their methods for averaging spatially-separated measurements. Perhaps you can figure out how to do an analog of that method with REW, or else just download Holm and use it. I, myself, am going to want to figure out how to do this pretty soon so I'm interested to know what method seems to work and give reasonable results.


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## BDE10 (Nov 2, 2011)

That's odd, it doesn't seem that my last response is posted... oh well. 


We have found a way to do it manually, we extract the data in text form and put it into excel, from here we can get the dB for whatever Hz we want, and also get an average and whatnot. 

This however has unleashed a new question, there seems to be negative dB readings. At 500 Hz, the measured dB is -15dB, however most other places the measured is ofc. positive. Is this just a mistake and means that it would be (+)15dB, or is it supposed to be 0dB?


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## aackthpt (Jan 24, 2011)

BDE10 said:


> We have found a way to do it manually, we extract the data in text form and put it into excel, from here we can get the dB for whatever Hz we want, and also get an average and whatnot.
> 
> This however has unleashed a new question, there seems to be negative dB readings. At 500 Hz, the measured dB is -15dB, however most other places the measured is ofc. positive. Is this just a mistake and means that it would be (+)15dB, or is it supposed to be 0dB?


Please post one of the graphs you are using so we can see the scale. Either way, it shouldn't matter if the data is negative as long as it is all to the same reference (same gain on the input chain). Averages work fine with negative numbers.


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## BDE10 (Nov 2, 2011)

Heres a link to the graph (don't mind the pictures of the setup, they are old photos). 
imageshack.us/g/9/kasse1.jpg/ 

If it's the negative measurements you wanted to see,then I would have to find it in the mountains of data, so I hope it isn't so.  

But we found out that the difference between the negative measurement and when we adjust it to 0, is on 0,4dB


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