# Decibels

Discussion in 'General Electronics Chat' started by Jimit Kavathia, Oct 25, 2015.

1. ### Jimit Kavathia Thread Starter New Member

Nov 30, 2014
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Why exactly do we use 600 ohms or 60 ohms as expalined in the artcile? I mean how do we decide the load and the power across it?
Practically speaking, how the audio that we listen to is calibrated?

2. ### kubeek AAC Fanatic!

Sep 20, 2005
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Which article?

3. ### JamesBond007 New Member

Oct 25, 2015
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Generally, audio has 2 ohms, 4 ohms and 8 ohms for speakers.
With PA - Public Announcement - transformers are used on a high voltage line, typically 70V, to run each speaker. Think Show Day.
These are usually 600 ohm.
That is, impedance matching 600 ohm line to 8 ohm speaker.
If you are doing a UNI course, you are expected to figure this out.
Luck.

4. ### dl324 Distinguished Member

Mar 30, 2015
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Audio typically uses dBm to describe power, which is usually referenced to 600Ω.

Your article reference didn't make it...

5. ### Jimit Kavathia Thread Starter New Member

Nov 30, 2014
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By the term 'Article' , I meant this website's page explaining decibels.

Apr 5, 2008
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7. ### Papabravo Expert

Feb 24, 2006
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Decibels is a general purpose technique for measuring practically anything. For example if you want to know the relationship of the US national debt to the median family income, you proceed as follows:
1. Form the ratio of the US national debt (≈ \$18,418,630,000,000) to the median family income for 2014 (≈ \$53,657)
2. Take the logarithm to the base 10
3. Multiply by 20
I get approximately 171 dB
Your mileage may differ and it is growing by the second.

8. ### JamesBond007 New Member

Oct 25, 2015
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dB is loudness. But it the ratio of input to output. So 1.0 dB can be soft, or very #\$%^ loud. It does not have units.

9. ### joeyd999 AAC Fanatic!

Jun 6, 2011
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Explain your rational for using a factor of 20 instead of 10 in this example. In other words, where is the square?

10. ### blocco a spirale AAC Fanatic!

Jun 18, 2008
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The 600Ω standard has nothing to do with 70V loudspeaker lines.

11. ### Jimit Kavathia Thread Starter New Member

Nov 30, 2014
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Thanks Bertus and Papabravo!

12. ### JamesBond007 New Member

Oct 25, 2015
24
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It has with PA

13. ### blocco a spirale AAC Fanatic!

Jun 18, 2008
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No, it has not.

The 600Ω standard applies to low level signal impedance matching where 0dBm = 1mW or 0.7746V across 600Ω.

14. ### Papabravo Expert

Feb 24, 2006
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Voltage and current ratios use a factor of 20, whilst power ratios use a factor of 10 because power is proportional to voltage squared or current squared. If you want to use a factor of 10 for dollar ratios I don't think I could present a cogent argument against it, except, by analogy to power, why are dollars proportional to something squared.

15. ### AnalogKid Distinguished Member

Aug 1, 2013
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While applying dB to something other than electrical concepts as a way to express ratios and quantitative relationships is not unheard of, it gets into trouble because of the lack of a defined reference plane. Still, I think that for scalar quantities like volts, amps, dollars, marbles, length, etc., I'd go with the 20log calc. For things that are based on two or more other things, like area (as opposed to length), the 10 log would be the better analog. Still, the lack of any universally agreed upon reference means that the calc would have to be stated explicitly. Even within the EE community, the reference can not be assumed. A conversation among a telecom guy, a radio guy, and a TV guy will get into trouble because they have three different definitions of what 0 dB on a VU meter means.

ak

16. ### Veracohr Well-Known Member

Jan 3, 2011
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It's used because the dBm reference, based on a 600 ohm load, was defined when that was a common load. dBu, the most common audio voltage reference, is based on dBm so also based on a 600 ohm load. For voltage transmission between equipment 600 ohms may no longer be a normal load impedance, but since the reference was already there people decided to keep using it.