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