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vu meter, is it linear ?

Discussion in 'Electronic Basics' started by fred, Jan 30, 2004.

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  1. fred

    fred Guest

    Is the "needle device" (I think it is called a vu meter)
    of a sound level meter is it linear, if so, how do we
    get a non linear scale, namely -10 at extreme left,
    zero in the middle, and +6 at extreme right?
  2. The meter movement is linear but you are measuring a logarithmic
    signal, so the meter scale has to reflect this.
  3. It is the units of measurement, rather than the signal, that is

    The units indicated on a VU meter are decibels, which are
    20 log(Vin/Vref).

    If you were to add a voltage scale to the VU meter, that scale would
    be linear.
  4. fred

    fred Guest

    Can you explain this then:

    why is -10db equal to +6db in terms of needle displacement ?

    If 20 db is times 10 then 10 db is times root 10 i.e. 3.16
    Hence, the voltage must increase times 3.16 to cause the needle
    to move from -10 to 0 and from 0 to +6 the voltage doubles.
    Since the needle is linear this does not make sense, what am
    I doing wrong ?

  5. Are you sure the -10dB point is *exactly* at the beginning of the scale?
    Look carefully. If your meter has the +6dB at extreme right and the 0dB
    exactly at the centre, then the extreme left *has* to be -oo (-infinity). If
    there was an offset in the scale to bring the -10dB at extreme left, then
    either the 0dB wouldn't be at the centre, or the +6dB wouldn't be at extreme
    right... Plus you'd need to subtract a voltage from the measured signal to
    have a correct measurement in this case.

    In dB meters, because of the logarithmic scaling, the dB values near the
    beginning of the scale become so dense that it isn't easy to read them. I
    suspect the discrepancy in your calculations is because of this.

    Costas Vlachos Email:
    SPAM-TRAPPED: Please remove "-X-" before replying
  6. fred

    fred Guest

    In dB meters, because of the logarithmic scaling, the dB values near the

    yes, you are right.
  7. fred

    fred Guest

    I thought I understood this but now I am confused again:
    When the needle moves from center to the +6 at the
    extreme right, the displacement doubles, and so does
    the voltage, so this makes perfect sense. However, when
    the voltage doubles, sound pressure level quadruples,
    so the reading of the sound level will be wrong, what
    am I doing wrong here?
  8. Bob Masta

    Bob Masta Guest

    An increase of 6 dB is a doubling of voltage or pressure,
    but a quadrupling of *power*. The formula for dB is
    20 * log (V / Vref)
    20 * log (p / pref)
    10 * log (P / Pref)
    where p = pressure and P = power.
    SPL = 20 * log (p / 20^10-6)
    where p is pressure in Pascals.
    (A Pascal = 1 Newton / meter^2)

    Hope this helps!

    Bob Masta

    D A Q A R T A
    Data AcQuisition And Real-Time Analysis

  9. OK, let's see what we have here. The meter scale is in dB, but the meter's
    needle really measures voltage. We also have the following relation:

    dB = 20 * log ( P / Po )

    where P and Po are the two sound pressure levels we're comparing. So, if you
    feed your meter with a voltage that is proportional to sound pressure level
    (SPL), then the readings will be correct.

    Costas Vlachos Email:
    SPAM-TRAPPED: Please remove "-X-" before replying

  10. Hmmm, I shouldn't have used SPL in my reply above, as SPL has a fixed point
    of reference. Please read the above as simply "sound pressure", *not* SPL.

    So, if you feed your meter with a signal whose voltage is proportional to
    sound pressure, then the readings will be correct. This is because of the
    20*log relationship shown above. In addition to that, if your reference Po
    is equal to a sound pressure of 20 microPascal (the threshold of hearing),
    then the readings will be in dB [SPL].

    Sorry for any confusion caused.

    Costas Vlachos Email:
    SPAM-TRAPPED: Please remove "-X-" before replying
  11. fred

    fred Guest

    I think it simply works out that a 6db voltage increase and
    a 6db sound level increase are the same think, which can be
    concluded from the above mentioned formulas.
  12. BobGardner

    BobGardner Guest

    The meter reads avg voltage, the scale is marked in dB
  13. Guest

    It's easier than you think, if you double the voltage then twice the
    current will be forced to flow (through the same load) quadrupling the

    | power
    |________ I

    | 4 * power
    |________________ 2I

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