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I need help with a db question.
 
 
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Author I need help with a db question.
tompty
Wiggling with Experience


Joined: 04 Aug 2008
Last Visit: 19 Aug 2014

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PostPosted: Tue Jan 18, 2011 10:31 am    Post subject: I need help with a db question. Reply with quote Add User to Ignore List

If i have a 70hz sine wave and a 5khz sine wave both running into my converters at -18dbfs will the final outcome be -18dbfs?

I have always wondered whether the overall db goes up with two non-clashing frequencies are summed or whether its just when frequencies overlap.


thanks!
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tompty
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PostPosted: Wed Jan 19, 2011 12:09 am    Post subject: Reply with quote Add User to Ignore List

:(
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megaohm
Super Deluxe Wiggler


Joined: 15 Jun 2009
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PostPosted: Wed Jan 19, 2011 1:24 am    Post subject: Re: I need help with a db question. Reply with quote Add User to Ignore List

tompty wrote:
If i have a 70hz sine wave and a 5khz sine wave both running into my converters at -18dbfs will the final outcome be -18dbfs?


It depends on when you 'look' at it.
I think your general grasp of what is going on is correct.
But two waves at different frequencies will add together at certain times depending on how their frequencies are related.

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megaohm
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PostPosted: Wed Jan 19, 2011 1:26 am    Post subject: Reply with quote Add User to Ignore List

Also, I'm a dumb ass when using dB and the like. I always refer to this page of calculators/info:

http://www.sengpielaudio.com/calculator-db-volt.htm

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joshnapkins
Learning to Wiggle


Joined: 30 Nov 2010
Last Visit: 08 Feb 2014

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PostPosted: Wed Jan 19, 2011 9:12 am    Post subject: Reply with quote Add User to Ignore List

it doesn't matter if they're not the same frequency. think of adding two sine waves of different frequency, at some points their maxima will line up perfectly and you'll get constructive interference at twice the amplitude.

and, as always, when doing any sort of math with dB make sure to convert from log scale to linear scale, do the addition/subtraction/mult/etc. and then convert back to the dB scale. i.e....

10log10(x) = -18 dB, so x = 10^(-18/10) = .016

now we add the max amplitude of each signal 0.016+0.016=0.032

now convert back to dB, 10log10(0.032) = -15dBfs
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Ranxerox
Dirk Diggler Wiggler


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PostPosted: Wed Jan 19, 2011 9:37 am    Post subject: Reply with quote Add User to Ignore List

Yes, frequency has nowt to do with it. As a general rule of thumb adding any two signals of the same amplitude together will result in an increase of ~3dB at the output.
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tompty
Wiggling with Experience


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PostPosted: Wed Jan 19, 2011 10:59 am    Post subject: Reply with quote Add User to Ignore List

brilliant!

thank you all for the helping me out with this.
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