These Ads will no longer appear once you have logged into ControlBooth.

  1. This site uses cookies. By continuing to use this site, you are agreeing to our use of cookies. Learn More.

Audio question, +4/-10 ?

Discussion in 'Question of the Day' started by derekleffew, Jan 23, 2012.

  1. derekleffew

    derekleffew Senior Team Senior Team Premium Member

    Messages:
    4,853
    Likes Received:
    1,988
    Location:
    Las Vegas, NV, USA
    (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.
    Last edited: Jan 24, 2012
  2. chausman

    chausman Chase

    Messages:
    2,834
    Likes Received:
    199
    Location:
    Spokane, WA
    dBV refers to voltage, whereas dBu refers to unloaded, or with no impedance.
  3. rochem

    rochem Well-Known Member

    Messages:
    1,228
    Likes Received:
    187
    Location:
    New York, NY
    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?
    Last edited by a moderator: Jan 24, 2012
  4. Mutton

    Mutton New Member

    Messages:
    21
    Likes Received:
    2
    Location:
    Buffalo NY/Ithaca NY
    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.
  5. Chris15

    Chris15 CBMod CB Mods Premium Member

    Messages:
    3,653
    Likes Received:
    270
    Location:
    Sydney, Australia
    Care to elucidate?
  6. Mutton

    Mutton New Member

    Messages:
    21
    Likes Received:
    2
    Location:
    Buffalo NY/Ithaca NY
    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. :grin:
    xander and (deleted member) like this.
  7. sk8rsdad

    sk8rsdad Well-Known Member Premium Member

    Messages:
    1,430
    Likes Received:
    301
    Location:
    Ottawa
    That would be deci as in 1/10th. deca is 10.
  8. museav

    museav CBMod CB Mods Premium Member

    Messages:
    4,021
    Likes Received:
    553
    Occupation:
    Acoustical, audio and audiovisual consultant
    Location:
    Marietta, GA
    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.

Share This Page