# Audio question, +4/-10 ?



## derekleffew (Jan 23, 2012)

(Since it's been quite a while: The intent of this forum is to be educational, directed at students. Unless specifically stated otherwise, professionals should not answer (but may kibbitz) until at least one week from the time of the original post.)

"What is the difference between -10dBV and +4dBu ?" 
Hint - it is not 14dB.


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## chausman (Jan 23, 2012)

dBV refers to voltage, whereas dBu refers to unloaded, or with no impedance.


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## rochem (Jan 23, 2012)

derekleffew said:


> "What is the difference between ...


 
Like, mathematically? In that case I think it'd be 11.8dB? Or do you want a discussion of the differences in the dB references?


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## Mutton (Jan 24, 2012)

derekleffew said:


> "What is the difference between -10dBV and +4dBu ?"
> Hint - it is not 14dB.



A dB isn't actually anything, it requires a reference.

dBV is referenced against 1 volt RMS
dbU is referenced against .775 volts RMS

dBs are also logarithmic, which is always fun.

To work in two separate scales you either need to convert one to the other or both to a linear scale.

Because we most likely care about the voltage difference I'll take them both to volts.

-10dBV = 20 log (V / Ref)
-10dBV = 20 log (V/1)
V=10^(-10/20) x 1
-10dBV = .316V

V = 10^(db/20) x Ref
V = 10^(4/20) x .775
4dBU = 1.288V

1.288V - .316V = .912V

The difference between -10dBV and +4dBU is .912 volts.


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## Chris15 (Jan 24, 2012)

Mutton said:


> A dB isn't actually anything, it requires a reference.


 
Care to elucidate?


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## Mutton (Jan 24, 2012)

Chris15 said:


> Care to elucidate?


 
A decibel is a referenced logarithmic scale OR a ratio of two numbers. Saying "97 dB" doesn't actually mean anything until you put it against a reference.

A dB is a ratio of power where dB = 10 log (P1/P2)

Through some math you can algebra Ohm's law into a formula for dB where for voltage (or force eg dB SPL) dB = 20 log (V1/V2)

Note that power and voltage/force use different multipliers.

If you lock P2 or V2 as a refernce value that's a dB scale.

The reference of dBV is 1 volt RMS
So dBV = 20 log (V/1 volt RMS)
20 log because voltage uses a multiplier of 20. That allows you to turn any voltage into dBV.

As a point of interest, a dB is a decibel. Deci as in 10. 
In the 20s Bell Telephone worked with the Bel to measure power loss over a mile of telephone cable. A Bel was simply log (P1/P2) but their numbers were always fractions so they put the 10 multiplier in and called it a decibel.

I'm pretty sure that all makes sense.


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## sk8rsdad (Jan 24, 2012)

Mutton said:


> As a point of interest, a dB is a decibel. Deci as in 10.



That would be deci as in _1/10th_. dec*a* is 10.


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## museav (Jan 26, 2012)

Mutton said:


> A decibel is a referenced logarithmic scale OR a ratio of two numbers. Saying "97 dB" doesn't actually mean anything until you put it against a reference.


Well, it does mean something, it's just something relative rather than absolute unless an absolute reference is provided.

A good example of this is mixing consoles. A level on an input or output level meter typically reflects some absolute XdB(u/V/FS) level, with the u/V/FS defining a standard reference level. Conversely, the numbers on a fader represent a relative difference rather than an absolute level and those are indicating XdB more or less rather than an absolute XdB(u/V/FS) value. Both mean something specific and both are valid uses of the Decibel.


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