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Decibels

Discussion in 'Sound, Music, and Intercom' started by Schniapereli, Jan 20, 2007.

  1. Schniapereli

    Schniapereli Active Member

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    This thing has always confused me. I always seen negative dB and dBU, and I have no Idea what it means? I'm pretty sure dBU is referring to how much gain there is... So, here are all my questions.

    1. What is dBV? (and dBU, and anything else)

    2. Is dBU (if it is measured by gain) defined by line level from the mixer?

    3. Can someone just re-explain the whole thing to me as if I were a 6th grader? (or does anybody know a good site)

    4. My friend's home TV speakers just say "-32 dB" and go from -82 to 0 db. Is this a different scale? It just says dB.

    5. I have heard jokes of teenagers wanting sound to go up to 11 (10 being the highest) What scale is this on? Is this on dBU?

    Any other good information on decibel measurements. (not the one like '100dB is like a plane engine' and '3 dB is like a whisper'. (not actual the actual #'s)) How does this scale relate to the dBU?

    Also, randomly:
    6. What is bench focusing (I have looked all over for what it is)
    Is this focusing the bulb inside of the reflector housing? (with the little knob on the back?)
     
  2. TomyN

    TomyN Member

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    Hi,

    all dB values have in common that they are relations between two numbers.

    A simple relation is

    x/refernce

    On dB, wie took a logarithem with a base of 10 from this relation

    log10(x/reference)

    This gives real small numbers, so wie multiply that by 10 (thus the 'dezi'), so wie finally have

    10*log10(x/reference)

    By definition, a Bell is a relation of power. Because power goes with the square of voltage, we got

    10*log10(sqr(x/reference)), which yields 2*10*log10(x/reference) (This is pure math), when we look at voltages.

    This is the one part. The other part is the value used for reference. dBu has a reference of 1/sqrt(2) volts (approx 0.707), dBV has just 1Volt, dBSpl has a defined sound pressure as reference.

    For a 6th grader: Dezibell means the relation of two values, treated with a math function called log10, and then multiplied by 10. The letter after dB defines which value was used as reference.

    Tomy (who hopes that he translates the math terms correctly)
     
    Last edited: Jan 20, 2007
  3. the_dude

    the_dude Member

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    Buy the sound engineers handbook. It has pages better, more complete information then anyone is willing to type.
     
  4. Dillon

    Dillon Active Member

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    Re: 11

    11 is a joke in reference to the movie "This is Spinal Tap". See it.
     
  5. BillESC

    BillESC Well-Known Member Premium Member

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    Re: 11

    In sixth grade terms.

    0 dB means no addition to signal strenth

    + dB means you have boosted the signal

    - dB means you have reduced the signal

    0 dB is also referred to as unity and is where most adjustments would like to reside.
     
  6. mbenonis

    mbenonis Wireless Guy Administrator Premium Member

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    I second the suggestion of getting The Yamaha Sound Reinforcement Handbook, by Gary Davis. It's available from a number of places, and is around $30. It's definitely the best book you'll ever buy. :)
     
  7. Schniapereli

    Schniapereli Active Member

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    I'll see if they have that book at the library, or the college library.

    I've never been able to find any good sound books. They have eleventy billion on lights, but only a few on sound. Thanks for the referal.
     
  8. BNBSound

    BNBSound Active Member

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    Check out decibels on Wikipedia. They're quite well referenced.
     
  9. Schniapereli

    Schniapereli Active Member

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    How loud is the 0 on the mixer dBU scale on the meters in decibels?

    Is 0 just as loud as the singer is, or just as loud as the singer plus preamp?

    How can I tell from looking at the mixer meter, and tell how many decibels it is.
     
  10. mbenonis

    mbenonis Wireless Guy Administrator Premium Member

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    When you say Decibels, I'm assuming you mean dB SPL (Sound Pressure Level - i.e., the volume you hear). The short answer is: You can't.

    I should qualify that statement. If you set the levels on your amplifier, equalizer, and the like to a specific setting and never change it, you can have a correlation between the meters on the board and the volume you hear out of your speakers. However, if you change these, you'll have a new gain structure and everything will be "off."

    One thing I don't see mentioned here is the fact that the decibel is always with reference to something. You can have dBV (with respect to one Volt), dBm (with respect to one milliwatt), dB SPL (with respect to some pressure level which I don't know off the top of my head), etc. You can't have just a decibel by itself, because that would be illogical. When many non-technical people say dB when referring to sound volume, they almost always mean dB SPL.
     
  11. Andy_Leviss

    Andy_Leviss Active Member Premium Member

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    Well, first, I'm going to assume you really are asking, "How loud is the 0 on the mixer dBu scale in dB SPL?, since the answer to the question as you wrote it is 0 dBu (note the lowercase u; there's no such thing as dBU, dBu is used to clarify the confusion that resulted from the difference between dBv and dBV, which ARE referenced differently).
    That said, the answer is that it isn't any particular level in dB SPL, the two measurements don't have a fixed correlation. While being dB ratings, dBu and dB SPL will track relative to each other, they have no fixed relation to each other. I'll explain more below.

    I think you're getting confused about what dB is, which is understandable, as it's a very confusing subject. dB is ONLY a ratio, it is NOT, I repeat NOT a unit of measurement in and of itself. When you say that something is so many dB louder than something else, you're just saying that it is so many times louder than that something else. It's like saying that your car is twice as fast as my car; we're not talking fixed values, we don't know what speed either car goes, or what scale that speed is measured on. All we know is that, on any scale we do measure it on, yours is twice as fast as mine.

    Without a unit after the dB, as in dBu, dBV, or dB SPL, all you've got is a ration, not a measurement. Just because all three are measured in decibels doesn't mean they're the same, or that they relate directly. Because dBu and dBV are measurements of the same thing, just referenced to a different 0 value, you can convert directly between them. dB SPL, however, is not a voltage measurement, so it doesn't have any fixed relationship to the other two. More on this below...

    0 what? 0 dBu, I presume, but you need to be specific (yes, I'm being very anal-retentive here, but you NEED to be when referencing dB, because there are soooooooo many different scales measured in dB).
    That said, it's neither. 0 dB in any scale is a particular reference voltage. In the case of 0 dBu, that means that the signal has an RMS average of approximately 0.77 volts. It's effectively a measurement of how loud that input/output's signal is--every single part of the signal chain contributes to the signal's level, the singer, the microphone, the preamp, the EQ, any inserts, the gain stage, any summing amps that may be involved if it's a bus output you're metering, etc.

    Again, decibels what? Decibels SPL? You can't, directly. The mixer has no idea what system EQ, amps, etc. you have running at what gain settings into how many of what speakers.

    Within a given fixed system, as long as none of those factors change, you can measure the system at 0 dBu with an SPL meter, and then use that as a reference. If your system meters at 85 dB SPL A-weighted, for example, than if you're running at -10 dBu, you've got 75 dB SPL. That said, this is only a rough estimate, if you really want to know what SPL your system is putting out, use the proper tool for the job, an SPL meter.
    Hope this helps,
    Andy
     
  12. Andy_Leviss

    Andy_Leviss Active Member Premium Member

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    Seems Mike's slightly faster on the keyboard than me, at least when I'm being this verbose!
     
  13. Schniapereli

    Schniapereli Active Member

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    Sorry. Whenever I said decibel plain, I meant SPL.
    It's just that the only things I have ever learned about decibels was in my science textbook in 6th grade when it said "decibels were a unit for measuring sound" and then it had a table of a common noise matched with a decibel number. I am just barely learning that they meant SPL, but didn't want to get too technical. (or, didn't know themselves..?)

    I'll try to ask my question better.
    I guess I'm trying to find a good way to EQ, and set the gain structure.

    I was wondering how 0 dBu on the meters on the mixer compares to the actual singer's voice in dB SPL. I wanted to know if 0 dBu on the meters is the same SPL for singer #1, as it is to a much quieter singer #2 with the gain turned up. So, is it good to look at the meters when doing a mic check?

    What is a good SPL to have your mix at? I heard that a good mix is at -10 dBu and +7 dBu, but I do not know how much that is in SPL. I want to know how loud the general public like to have it. I like to get it to the point where I just feel it. (you know? can't explain it. Just to where the sound envelopes you, and you get a few goosebumps.) I have also heard of Broadway "unplugged" where they do not use any mics, and some people like it more because they have to sort-of strain to hear it, and they have to concentrate a little more to understand, and so they get more out of it from just having the music shoved into their brain. Just one night the directot told me the mics were a little too hot, when I thought the singers and the pit needed to be louder to get to the sound enveloping feeling. It's a shame because I like sound so much just because of the few good songs that you can turn up like that. (not blasting though. Songs like Madame Guillotine, Masquerade, etc. that seem that they should be at that level.)

    I understood dBu on the mixer as how much gain is added. Does 0 dBu mean that whatever SPL the singer is at, that is the level it sends to the amps.

    How much does an amp increase a sound in SPL if the mixer is at 0 dBu? (and the attenuation is set to about 5)

    Sorry if my questions don't make any sense. I still don't think I understand the principle of dBu's to dB SPL, so I my questions may be totally irrelevant. I am just a peon trying to learn enough to be an almost-peon.
     
  14. saxman0317

    saxman0317 Active Member

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    You know, if Im working with someone who hasnt taken his math classes and such...this is what i tell them
    Those are the numbers that you use for placement referancees when keeping notes. The closer to zero you can get everything the better, but other than that...its just there so that you can keep accurate notes.

    Works good for the "non-sound" guys doing it in a pinch, or the fresh guy that you got a few days before a show and cant properly train.
     
  15. Peter

    Peter Well-Known Member

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    About what a good SPL is to be running at is... I'm REALLY not sure if a blanket statement can answer that. I have run shows over a very wide dynamic range, from very soft talking events to a "Wall Of Sound" which was producing SPLs higher then our meter could read (You NEEDED hearing protection to be anywhere near it). It all depends on the event, venue, artist, audience, background noise, and about a million other things. The best way to figure this out is to just keep doing events and listening and learning. Trying to approach sound from an overly mathematical basis is a recipe for disaster if you ask me.

    That being said, it's good to know the numbers, but REALLY dont rely on them.
     

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