It's All About the Decibels, Baby!

So…this week's topic is something that I think most of us take for granted, we just kind of learn our software and levels and get on with it. But I was staring at my waveform in Adobe Audition the other day and lamenting the fact that I always ALWAYS get clipping when I laugh loudly on mic. And it occurred to me that I didn't really know why the scale tops out at 0dB. In fact, I didn't actually know what a decibel IS - which is kind of embarrassing because I went to college for engineering and I'm certain I knew it off by heart at one point. Anyway, today I'm going to talk about decibels and what the heck is up with that scale?

So let's start with a decibel. According to Merriam-Webster's dictionary, there are a couple different definitions…one of the definitions is "a unit for expressing the relative intensity of sounds on a scale from zero for the average least perceptible sound to about 130 for the average pain level." It's also used in the vernacular as a degree of loudness, which I think is what most of us associate with the word "decibel", at least the way we've learned it growing up. But that doesn't explain the negative scale in Audition.

The other definition of decibel is "a unit for expressing the ratio of two amounts of electric or acoustic signal power equal to 10 times the common logarithm of this ratio." That last one is a mouthful.

So here's the thing: a decibel, unlike an inch, or liter, or kilogram, is not an absolute unit of measure. It only ever measures the relative whatever of two things. In the first definition, what we're talking about is actually the decibel value of the Sound Pressure Level, sometimes denoted dB(SPL). It's comparing the sound pressure of whatever you're listening to, to the sound pressure of a barely audible noise. For reference, a noisy restaurant would be about 70 decibels, but a jet engine would be 140 decibels.

By contrast, the decibels showing in Adobe Audition are comparing the output amplitude of a signal to the system's full signal output. Think of it this way: if you were to output a sound as loud as you could without clipping or distortion, that would be the same as the full signal output. By the power of math, that works out to be 0dB. You can't go higher than full output (unless you're in Spinal Tap), you can only go quieter. So in a way, you can think of the negative values as how much you're attenuating the full scale output. To give you some reference values, the loudness of a commercially produced pop song is usually around -10dB, an audiobook produced for Audible.com should be around -20dB. A noise floor of -60dB and below is pretty good for a home recording.

Of course, when I'm talking about loudness, I'm not talking about peak amplitude…that's a topic for another day!