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Decibels usage?

Discussion in 'Electronic Basics' started by terryS, Feb 3, 2008.

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  1. terryS

    terryS Guest

    Based on one tenth of the Bel named in memory of Alexander Graham of
    telephone fame. Have always understood that Decibels are the
    logarithmic ratio (base 10) of 'two' or more, power levels.

    So am familar that doubling (or halving) a power ratio is approx +3
    (minus 3) decibels. etc.

    But in day to day parlance one sees (and often hears) that something
    is loud, at say 90 dBs, very loud at say 105 dBs. or liable to damage
    ones hearing at say 125 dBs.

    Or that 'normal office noise is 76 dBs or something.

    But in relation to what?

    On other hand have seen auto magazine articles reviewing vehicles
    that, very carefully, will say, for example, "Interior noise at 'x'
    mph. is so many dBA".

    What is that 'A' ? Ambient or something?????

    Also recall doing 'Noise measurements' on communication circuits using
    dBa (small 'a'). Meaning IIRC 'adjusted'? Those were using a 'Typical
    weighted circuit response' with zero dBa appearing to be somewhere
    around minus 90 dBm?

    Various 'trades' also seem use dBm, i.e relative to dB Zero being
    being one milliwatt; dBw, i.e. relative to one watt and others.

    And then there are loud speaker measurements; something to do with
    Sound Pressure Levels at a certain distance from the speaker?

    But is there any one single power level or standard that is generally
    accepted as being a 'Normal' reference. Hence people tending to drop
    the reference in everyday usage?
  2. Tom Biasi

    Tom Biasi Guest

    There is a power point presentation here; II page twelve.htm

  3. JeffM

    JeffM Guest

  4. Eeyore

    Eeyore Guest

    In relation to the threshold of hearing.

    Also they are sound PRESSURE levels - not Power.

    A weighting. The LF and HF is filtered.

    I'm sure Wikipedia explains it very well.

  5. terryS

    terryS Guest

    Thanks guys. TerryS
  6. Tony Burch

    Tony Burch Guest

    The other guys have given some good info and links, but I thought I'd just
    also give my short summary...

    As you said Terry, decibels always relate to ratios of power levels. Power
    is measured in Watts.

    dB = 10log(P1 / P2).

    When we are talking about "How many Decibels" we always need to know what
    the "reference power" is first. This is the number P2, in the equation

    For sound, the proper notation to use is "dB (SPL)" or "dBSPL". People often
    leave off the "SPL" part of the notation.

    The equation is:

    dB (SPL) = 20log(SPL1 / SPL2)

    Note that it is 20log, instead of 10log because SPL1 and SPL2 are measured
    in micropascals instead of Watts.

    For sound, it was decided that the reference SPL2 is 20 micropascals. 20
    micropascals is a very quiet sound. A 20 micropascal sound is 0dB (SPL).

    So if we have a sound meter that measures SPL1 in micropascals, the meter
    gives us the reading dBsound = 20log(SPL1 / 20 micropascals)

    Also, here's a Wikipedia reference

    Anthony Burch
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