1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Bel is unit for what physical quantity?

  1. May 9, 2014 #1
    If newton is unit for measure force, meter is unit for measure length and radian is unit for measure angle, so, bel is unit for measure which physical(/mathematical) quantity?
  2. jcsd
  3. May 9, 2014 #2
    The belt is a unit of sound intensity. It is a large unit so we more often use the decibel.
  4. May 9, 2014 #3


    User Avatar
    Science Advisor
    Gold Member

    It is not a unit as such. The deci-bel s just used to express relative logarithmic relationships (in base 10, hence deci).
    The most common example is sound, (where it tells you the sound pressure relative to a standardized pressure, which if I remember correctly is something like 1 micropascal) but is also used for things like RF/microwave engineering or indeed any other field where we work with relative quantities that differ by orders of magnitude. One can for example talk about 10 dB attentuation of sound (or microwaves).

    There are also some commonly used "proper" units which are "related" the the decibel: a good example is dBm which stands for decibel relative to 1 mW. This is used to express power in RF/microwave engineering.
  5. May 9, 2014 #4


    User Avatar
    Science Advisor
    Homework Helper

    The "standard" reference levels in acoustics are different for sounds in air and in other media, and also different for sound pressure level, sound intensity level, and sound power level :cry:

    See http://en.wikipedia.org/wiki/Decibel#Suffixes_and_reference_values for even more standard levels for other applications
  6. May 9, 2014 #5
    The bel (B) and the smaller decibel (dB) are units of measurement of sound intensity invented by Bell Labs and named after Alexander Graham Bell.
    You can find more references in the Wikipedia article on him. This sounds like a homework question to me.
  7. May 9, 2014 #6
    I never need to use dB or B for anything, but now, in Control Theory, the argument of the transfer function is the phase shift and the absolute is the gain and the gain is measured in dB. Gain and sound intensity are different concepts, I think, but they have the same unit, dB. Which is the difference and iguality concepctual between gain and sound intensity?
  8. May 9, 2014 #7


    User Avatar

    Staff: Mentor

    "Gain" is the ratio between output and input (or between final and initlal) values of some quantity. The decibel is a way of describing that gain in logarithmic terms: # of decibels = 10 * log (final/initial). As f95stoli wrote, it's commonly used for sound intensity (with the "initial" value being some standard value chosen by convention), but it's also used for things like the ratio between output and input power in an electrical circuit, or the sensitivity of an antenna, etc.

  9. May 9, 2014 #8


    User Avatar
    Science Advisor
    Gold Member

    For intensity the value of the field is squared -
    so you get I = 10*Log10[ A^2/B^2] = 20*Log10[A/B]. This is in deciBels.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook