Level, level, level. It’s one of the core concerns of post production sound. How loud is my show? Why are those dumb commercials so much louder? How loud should my dialog be compared to my music and effects? Will my mix be within specification of the broadcast network? Will my movie sound right in the theater? Will my webisode distort on laptop speakers?

The answer to these questions lies in good audio metering.

Simply put, audio meters are tools that show how loud or quiet your program material is, measured in decibels.  Below is an image of the classic VU meter.

Of course, the reality of audio metering for post is complex.

First, there’s the issue of decibel scale. When we’re talking about loudness in the real world, in the air around us, we’re measuring dB sound pressure level (dB SPL). In the SPL scale, 0dB is the quietest possible thing we can hear, up to 140dB which is so loud is causes physical pain. Professional post production sound mixing is done with the speakers set to 82dB SPL. This is plenty loud enough to hear everything clearly and have as flat a response as possible across the frequency spectrum, but low enough to not cause hearing damage!

When we’re talking about the decibel scale in Pro Tools we’re talking about dB Full Scale (dBFS). In digital audio 0dB is “full scale” where things are as loud as they can possibly go. All decibel values that we work with are below this point, measured in negative dB. “But my fader in Pro Tools can go above 0dB” you say! Remember, decibels measure change, and the fader does allow you to increase level. Therefore, it does have positive territory (up to +12dB). But the overall level for the final digital audio coming out your master outputs still hits the limit at 0dBFS. If you push up on the fader and the audio on the track is already at 0dBFS, you’ll just clip and distort.

Now, how does the real world of dB SPL relate to the digital world of dBFS? What we do is calibrate our speakers to play pink noise at 82dB SPL when Pro Tools is set to -20dBFS RMS. Don’t worry too much about the math behind the whole RMS thing. Suffice to say, it means that we’re measuring decibels in a way that most closely approximates how our ears actually perceive the sound as an average over time (as opposed to the absolute peak value that we could measure with a test microphone).

So we’ve calibrated our speakers to allow for 20dB of headroom in the dBFS scale. We see how decibels in Pro Tools equate to decibels in the real world. Next time we’ll take a look at how we measure those decibels in Pro Tools and what those numbers mean to us as we mix.