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(Updated 2/4/12) Despite a formal education and real world training at some serious recording studios, I’ve always been amazed at how far along I had to come as an engineer just to learn some very basic information and points of confusion (a big reason for starting this blog by the way). When you do something for the better part of your life, you sometimes forget how much you really know. That’s part of why the best audio engineers are not always the best teachers.

When I started out, the decibel confused me.

I had read that a jet engine was 140 dB, a library was 30 dB, a rock concert was 115 dB, etc. Why then, I wondered, do digital and analog meters go from negative infinity (silence) to 0 dB (absolute loudest)? And why is it that track faders can go up to +6 or +12 dB?

I’m not going to get into all the gruesome details here, I’m just going to set forth one basic, but extremely important concept: there is more than one kind of decibel.

Decibels in the digital realm are called dBFS or “Decibels Full Scale.” Decibels in the real world (the kind used to measure a jet engine) are called dB SPL or “Sound Pressure Level.”

Now let’s apply this knowledge to a WAV file you might receive back from a mastering engineer. In the end, how loudly people hear your master is going to be determined by the playback system (iPod, car stereo, studio monitors, etc.) and whoever controls the volume knob.* This means that in the real world, even a recording that peaks at 0 dBFS and is -5 dBFS on average (very loud) can be as quiet as a library or as loud as a jet engine upon playback.

So the dBFS of your master really only indicates how loud your song will be relative to other songs when played back under identical circumstances. So if someone plays a Notorious BIG track that’s -8 dBFS on average, and your track is -6 dBFS, you’ll be louder than Biggie every time (unless some asshole decides to turn it down when your song comes on).

dB Adjustments Within Pro Tools and on Actual Recording Consoles

It’s also important to note that when you set your snare drum’s volume fader to +6 dB in Pro Tools or on a console, you’re not making the snare 6 dB SPL, your snare has not become one twenty-third of a jet engine or one fifth of a library. You’re also not making it 6 dBFS (which doesn’t exist) either. You’re just adding 6 dBFS to whatever the sound file’s initial volume was. So if you record that snare into Pro Tools hitting -5 dB on the meters, and set the fader to +4 dB, your snare is now -1 dBFS. And like I said before, the loudest anything can ever be in the digital realm is 0 dBFS. This also called “digital zero.” (Going over digital zero is what causes clipping.)

There are other types of decibels, but these two are the most important to grasp for now.

Any questions?

*Further still, final perceived loudness must also take into account the distance between the listener and the speakers, as well as dB SPL.


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4 Responses to “What The Hell Is 0 dB? (A Quick Word About Decibels)”

  1. Andrew Kay says:

    Actually, dB SPL and dBFS are both absolute scales, with 0dB SPL defined to be the threshold of human hearing, and 0dBFS defined to be the greatest amplitude representable digitally.

    dB itself is a relative scale, a number of dB is not the volume of any particular sound, but the difference in volumes between two sounds. So if you move your fader to +6dB, the amplitude post-fader is (very nearly) twice the amplitude pre-fader. This is true even on analog mixers which obviously do not have a 0dBFS to measure against.

  2. Keith says:

    Thanks Andrew- I’ve updated the wording to be more awesome.

  3. ZisforZebra says:

    So just to make sure I have this straight, “unity” on a mixer’s volume fader means no change, whereas moving it up or down from that 0dB (or is it 4dB?) mark would be adding to or subtracting from that initial dBFS amount?

  4. […] will listen for is a great drum track. A minor flub in the bass line can be worked out in the mixing of the tracks. It has to do with the fact that a drum set is actually many instruments with […]

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