Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Are there any formulae that involve decibels?

  1. Jan 26, 2012 #1
  2. jcsd
  3. Jan 26, 2012 #2


    User Avatar
    Staff Emeritus
    Science Advisor

    Any formula that deals with "loudness" can be put in terms of decibels. The simplest, of course, is that one decibel is one tenth of a bel!
  4. Jan 26, 2012 #3


    User Avatar
    Gold Member

    Ralphonsicus, Google and Wikipedia are your friends...learn to use them to find answers to your questions plus references for where to look for more detailed research. I found this in about one minute! Here are two paragraphs from the Wiki page on “decibel”:

    “The decibel (dB) is a logarithmic unit that indicates the ratio of a physical quantity (usually power or intensity) relative to a specified or implied reference level. A ratio in decibels is ten times the logarithm to base 10 of the ratio of two power quantities.[1] A decibel is one tenth of a bel, a seldom-used unit.

    The decibel is used for a wide variety of measurements in science and engineering, most prominently in acoustics, electronics, and control theory. In electronics, the gains of amplifiers, attenuation of signals, and signal-to-noise ratios are often expressed in decibels. The decibel confers a number of advantages, such as the ability to conveniently represent very large or small numbers, and the ability to carry out multiplication of ratios by simple addition and subtraction.”

  5. Jan 26, 2012 #4


    User Avatar

    Staff: Mentor

    Please try to give more context and detail in questions like this. You do not say why you are asking the question, you do not specify which areas of science you are asking about, etc.

    It makes it a lot easier for folks who want to help you if you post what you know and what research you have done on the question so far, and what *specifically* you are looking for.
  6. Jan 26, 2012 #5


    User Avatar
    Science Advisor
    Gold Member

    Beware the huge amount of rubbish that it talked about the dreaded dB.
    In my experience, many people tend to struggle with Logarithms until they get familiar with them from extended use - I mean lots of example problems.
    Likewise with dB. You will eventually get a feel for them and you find 30dB will automatically mean a Power ratio of 1000 to you, without thinking about it.

    You can reduce the risk of going wrong if you always remember that dB is defined as a ratio of POWERS. It so happens that, when you are operating at the same impedance, you can relate Voltage or Current ratios to power ratios but it really is a nonsense just to say that a multiplication of volts implies a 'gain in dB'. A 1:10 transformer will have a (power) gain of less than 0dB, despite producing ten times as many volts and a 'voltage follower' circuit may have a gain of 60dB, although the voltage gain is 1.

    Anyone who you hear using the term " Volts dB" is on shifty ground unless they specify a constant impedance. 60dBμV is OK as long as, somewhere, there is a 50Ω specified.
  7. Jan 27, 2012 #6
    Last edited: Jan 27, 2012
  8. Jan 27, 2012 #7


    User Avatar
    Science Advisor
    Gold Member

    Shame he breaks the first rule at the start. dB is a ratio of powers so he isn't helping by starting off with Voltage ratios. I am not nitpicking here and his approach can lead to serious misconceptions. Too glib and chatty to be safe, imo.
  9. Jan 27, 2012 #8
    Well power = voltage^2 / resistance , isnt it? So even if dB is actually a rule which applies for power, we can use it for voltage also given the load remains same (which is most of the case).
  10. Jan 27, 2012 #9


    User Avatar
    Science Advisor
    Gold Member

    Is it the same for a transformer or an amplifier? Like I said, it's dangerous.

    I seem to remember someone trying 'decilogs' as a possible way out.
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Similar Discussions: Are there any formulae that involve decibels?
  1. Decibel Question (Replies: 2)