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Quick dB question

Discussion in 'Electronic Basics' started by Grey, Sep 5, 2004.

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  1. Grey

    Grey Guest

    How do I calculatre if I have a 30v signal and I reduce it by -6dB?

    Graham
     
  2. John Miller

    John Miller Guest

    To reduce it by -6 dB, multiply times four. (Although you probably intended
    to reduce it by 6 dB, which would mean dividing by four.)

    --
    John Miller
    Email address: domain, n4vu.com; username, jsm

    Punishment becomes ineffective after a certain point. Men become
    insensitive.
    -Eneg, "Patterns of Force", stardate 2534.7
     
  3. Dbowey

    Dbowey Guest

    Grey posted:

    << How do I calculatre if I have a 30v signal and I reduce it by -6dB? >>

    You will have a 15V signal after you do that.

    dB= 20*log E1/E2.

    Don
     
  4. Graham Knott

    Graham Knott Guest

  5. Gareth

    Gareth Guest

    That is, of course, you divide the POWER by four. The voltage will be
    reduced by a factor of two since power is proportional to voltage squared.

    Gareth.

    --
     
  6. John Miller

    John Miller Guest

    Yes, exactly right. I shouldn't post before waking up.

    --
    John Miller
    Email address: domain, n4vu.com; username, jsm

    I was the best I ever had.
    -Woody Allen
     
  7. Jamie

    Jamie Guest

    lets see if my math will pop to my head.
    (20 * log(30))-6 = 15 DB
    of course you can put that back into the V signal after.
    V = InvertLog(15/20) = 5.6; etc///////////////

    which should give you something to work with..
    to the best of my knowledge that should be close
    enough.
     
  8. john jardine

    john jardine Guest

    You need to find what "-6dB" means in a real world so ...

    Divide "-6" by the magic number of "20"
    Get "-0.3"

    Find the antilog of "-0.3" (ie press the 10^x button)
    Get "0.501187"

    This is how much the 30V needs reducing.
    So ... 30V X .501187 = 15.035V

    This works for volts and amps. People normally just regard the -6dB as "a
    half" or "+6dB" as "2 times".

    regards
    john
     
  9. JeffM

    JeffM Guest

    To reduce it by -6 dB, multiply times four.
    Professor: Two negatives give a positive result,
    but two positives don't result in a negative.

    Voice from back of classroom: Yeah, right.
     
  10. Tim Auton

    Tim Auton Guest

    That's a (mis) quote of the late Sidney Morgenbesser. OK, perhaps he
    wasn't the only person to say it, but he's arguably the most famous.

    "The most celebrated Morgenbesser anecdote involved visiting Oxford
    philosopher J. L. Austin, who noted that it was peculiar that although
    there are many languages in which a double negative makes a positive,
    no example existed where two positives expressed a negative. In a
    dismissive voice, Morgenbesser replied from the audience, "Yeah,
    yeah..."

    (from the New York Sun website, but it's elsewhere too)


    Tim
     
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