summing 2 measurements gives +6db not +3db ??

[mamba76]

Hi all. When adding 2 measurements I noticed where amplitude is equal this gives a +6db sum at that point and everything Iv read or heard say summing 2 equal sources should be +3db. Would this be because the measurements Im using are averaged data so both sources have equal phase which is zero.

 

[mamba76]

http://www.sengpielaudio.com/calculator-coherentsources.htm

Looks like the difference is this is a 'coherent' signal and not incoherent. Who knew !! D

ps. Could someone kindly tell me how I would put this equation in Excel?
thanks

if adding 2 coherent signals of 80db I should gt 86db.

This is my excel equation =20*LOG10((10*80/20)+(10*80/20)) = 38.0618 ?

 

[John Mulcahy]

10^(80/20), not 10*80/20.

As of 5.19 beta 7 REW treats signals without phase data as incoherent.

 

[mamba76]

Ah ok, that explains all. Iv not used the '^' symbol before but will work it out. Thanks John.

 

[John Mulcahy]

Just means raised to the power of, so the Excel equivalent is Power(10, 80/20)

 

[ddude003]

Some math parsers will accept ** for raised to the power of... And some will accept both ^ and **...

For instance R will accept both: (10^80/20)+(10**80/20)

Or try http://www.wolframalpha.com

I do not normally use Excel nor do I recommend my friends use it... 8^) And something like: =20*log10((10^(80/20))+(10^(80/20))) may work...

 

[mamba76]

Thanks, I'm a bit out of touch with the complicated stuff.

I have another problem with an excel formula. Its a bit of algebra..
And a bit of a headache!

I know that the Excel formula 10*LOG10(10^(S1/10)+10^(S2/10)) = 46.049

I also know that S1(source1) is 93% of S2(source2), but how would I change the formula to find this?

Basically if both sources were 100% and sum to 46.049 then each source would be -3.01db or 43.039db. I need to reverse the formula some how but I can only remember basic algebra and the LOG function is new to me. Having trouble seeing it.

I feel I should apologize in advance for any headaches!

ps I am working on something where Iv expressed the reflected sound as a percentage of the direct sound.

 

[mamba76]

No worries I managed to do it. Tried to upload the xls file but not allowed. I will post anything interesting. In this case 93% would be S1=42.724 and S2=43.333.