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Decibel Scale and Meters

Discussion in 'Recording' started by sgpatel, Jan 11, 2006.

  1. sgpatel

    sgpatel Guest

    I am confused about the decibel scale that is used on meters. I have been reading and trying to understand how meters work. I came up on a article where it was mentioned that 0db is the 'threshold of hearing' and then later in the same article it was mentioned that digital audio has a limit of 0db. This is what confuses me as to if 0db is the 'threshold of hearing' then how 0db be the maximum limit in digital audio. I apologize in advance for such a stupid question. Also the meter in wavelab and most meters display number like -48,-40,-32,-24,-16,-8, 0 db. Why are these negative if the threshold of hearing starts from 0db and upwards. . The article had the following chart:
    0dB Threshold of hearing
    20dB Quiet living room
    50dB Average office
    70dB Busy street
    80dB Average factory
    110dB Thunder
    120dB Airport runway
    133dB Threshold of pain

    so then shouldnt the meters start from 0db,8db,16db etc....
    Once again i do apologize since this might be very basic when it comes most board members.....any explanation would be helpful.....
     
  2. McCheese

    McCheese Well-Known Member

    Even though they're both in dB, they're referring to two different things. The scale you mentioned is, as you said, the scale of SPL emitted by certain objects.

    Your meters are different. They're (supposedly) calibrated so that the max you tape machine or whatever recording medium can handle corresponds to 0dB. In reality things are calibrated to +4 or some other number depending on the equipment being used, and some circuits are designed so that at 0dB+ they start distorting, but don't really exceed the output limits. So when you see -12 dB, you've got 12 more DB before you hit the limit.

    There is no way to say that what's on your tape at XdB will be broadcast at said dB IRL, since that relies on what sort of amplification and speaker system you are using. When you turn down the knob on your car CD player, you're not actually quieting the recorded data, you're just not amplifying it as much.

    Make sense?
     
  3. sgpatel

    sgpatel Guest

    Mccheese thanks for your answer...
    ut then is there any relation between SPL scale and the VU meter....whatsoever cos the audio signal would also have some corresponding SPL reading if measured that way. So for example if a audio waveform is reading -12 db on the meter it would have some corresponding reading lets say 80 SPL. Is this a true statement or am is still way off in my understanding.....
     
  4. David French

    David French Well-Known Member

    Decibels are a ratio of one thing to another, that is, a way of comparing the sizes of things. When I say that something is '100 dB', that means it's 100 dB louder than the quietest thing we can hear, which is 0 dB, or 20 micro Pascals (the Pascal is a unit of air pressure). The human ear has a very large range of air pressures that it can withstand, and this translates to a very large set of numbers to deal with, so we use the log scale to compress these extreme values into something more manageable. This is only one type of dB, and it is called the dB SPL for Sound Pressure Level. There are other types of dB for other quantities (not just air pressure, but the magnitude of one voltage level to another for example), for other media (not just air, but voltage in a wire for example), and for other reference levels besides the 20 mirco Pascals used to calculate dB SPL. The dB in your computer's meters is known as dB DFS or just dB FS for Digital Full Scale. As Cheese said, 0 dB here is very different that 0 dB SPL, since we are measuring different things. In the computer, we are measuring numbers from 0 to 65,535 in 16 bit audio, and 0 to 16,777,215 is 24 bit. 0 dBFS represents the biggest possible number before you run out of numbers and get digital clipping.
     
  5. sgpatel

    sgpatel Guest

    Ok that makes sense....i went back and checked wavelab meters and voila....the lowest reading is -96db on the meter....however on the upper end it reads +5 db above the 0 level.....i also checked ableton live meter and it allows me to go 6db over 0db.......does this mean that they have calibrated the software so that the +5 db and + 6db are actually 0dbFS...and that they are giving some more headroom....
     
  6. SME

    SME Guest

    http://www.prorec.com/prorec/articles.nsf/files/EA68A9018C905AFB8625675400514576
     
  7. David French

    David French Well-Known Member

    those questions are funny. 1, 2, 3 (the first one), and 6 don't have definitive answers due to lack of source correlation and the perceptually-based definition of 'loudness'. The rest at first glance seems perfectly fine though.
     
  8. sgpatel

    sgpatel Guest

    David,
    thanks for all your help...i apologize for not sounding clear. it clearly shows i need to read and learn alot more.

    thanks,
    sgp
     
  9. David French

    David French Well-Known Member

    No apologies necessary. This is a place of learning, and you are most welcome! :D
     
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