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Thread: Negative dB numbers

  1. #1
    Jack Hammer is offline Dedicated Member
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    Negative dB numbers

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    I do not understand negative dB numbers. What does this really mean. Obviously, there is no negative sound, there is some level, sme amplitude, some volume, so that must correlate to positive something. So how come negative measurements are used or should I say negative designtions

    I know this may seem like a dumb question but I am new to engineering. Sure, I have played an instrument for years and, yes, I have been in many, many studios but...it seems that it is taken for granted that you know certain things and possibly smoe people are afraid to ask obvious questions becuase they do not want ot sound dumb or inexperienced. God forbid your not the coolest engineer in town! But I want to know what the story is with these negative designations on the threshold contorl of a compressor for instance.

    So, what is the story with negative dB threshold amonts.

  2. #2
    Blue Bear Sound's Avatar
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    Well... it's all a relative scale for different references....

    If you're talking about digital Full-Scale, then 0db is where digital clipping occurs (which is to be avoided).... of course if 0dbFS is the top, then anything level less than that has to be a negative db value....

    It all depends on what meter scale you're looking at, at any one time. For analog meters it can be different again....

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    TexRoadkill's Avatar
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    The simple explanation is that 0db is the best quality signal. Anything above that is subject to distortion and anything below that is subject to lower signal to noise ratio.

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    If that didn't make sense figure that with digital, like BB said, 0db is the strongest possible signal. So if 0 is the loudest possible signal than you can only go down from there. I believe an increase of 6db is actually a 2x increase in strength. (I'm probably wrong on that number so please correct me anybody).

    It would be too hard to measure from quiet on up to loud because how do you determine how quiet something is unless you compare it to something? It is easier to state a known maximum level, 0db, and go down from there.

    Analog audio does have positive db values and the quality of equipment is often determined by how far it can go above 0db and still sound great.

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    Blue Bear Sound's Avatar
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    what I still don't quite get is how come 24 bit audio or 8bit audio, or 64bit audio etc...all have the same maximum volume.

    but, i am *much* closer to understanding the whole thing then i was before joining this board a year ago and having BB and Tex explain this same question to me...don't lose sleep over it anymore at least

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    hmm, apparently i have only been here 6 months.

    scary.

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    What may be causing the confusion is that the term dB is often misused to describe a number of different measurements.

    By itself, dB is only an expression of a ratio, not an absolute measurement. As such, it is a way of comparing two sounds, but does not represent any absolute value by itself.

    When dB is attached to a reference, then it actually has a specific value. The standard references differ, depending on application, and are represented by an additional letter added to dB, like dBV, dBW, dBm, dBu, etc.

    If you can find it, there is an excellent discussion of dB in all it's forms in the December 2000 issue of Pro Audio Review in an article by Edward J. Foster. Not being very technical myself, I keep it handy for reference.

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    dB is relative. +6db is a higher level, and -6db is a lower level. 0db is the "reference" level, against which all other levels are measured.

    Now which level is 0db is completely up to whoever makes the scale. As mentioned above, 0db on mixers is usually the "optimum" level.

    0db on other stuff could be another level. For example, gear is often said to be -10db or +4db. This is information on what the optimum level of the inputs and outputs of the gear is. They use other reference levels, obviously.

    Since levels are all relative like this, 8bit and 24bit audio does not really have the same maximum levels. Neither do they have different maximum levels. :-) It's just numbers, representing the availiable levels from 0 to maximum, with different amount of steps inbetween.

    The "maximum level" doesn't mean anything more than "hey, go any higher and you get distortion". What this level is depends on other variables.

    If you aren't more confused yet I can make you.
    Random Pavarotti Disease Victim.

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    Also, don't forget that dB is a logarithmic scale. I hate to link to another BBS but ProRec has a pretty good article on in it. You can check it out here. It helped me understand it better.

    http://www.prorec.com/prorec/article...25675400514576

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