Dynamic range can be defined as the distance between the loudest possible level to the lowest possible level. For example, if a processor states that the maximum input level before distortion is +24 dBu and the output noise floor is -92 dBu, then the processor has a total dynamic range of 24 + 92 = 116 dB.
However, the average dynamic range of an orchestral performance can range from - 50 dBu to +10 dBu on average. This equates to a 60 dB dynamic range, which may not appear to be a large dynamic range, but do the math and you’ll discover that +10 dBu is 1000 times louder than -50 dBu!
Rock music on the other hand has a much smaller dynamic range, typically - 10 dBu to +10 dBu, or 20 dB. This makes mixing the various signals of a rock performance together a much more tedious task.
Why do we need compression?
Consider the previous discussion: You are mixing a rock performance with an average dynamic range of 20 dB. You wish to add an un-compressed vocal to the mix. The average dynamic range of an un-compressed vocal is around 40 dB. In other words a vocal performance can go from -30 dBu to +10 dBu. The passages that are +10 dBu and higher will be heard over the mix, no problem.
However, the passages that are at - 30 dBu and below will never be heard over the roar of the rest of the mix. A compressor can be used in this situation to reduce (compress) the dynamic range of the vocal to around 10 dB. The vocal can now be placed at around +5 dBu. At this level, the dynamic range of the vocal is from 0 dBu to +10 dBu. The lower level phrases will now be well above the lower level of the mix and louder phrases will not overpower the mix, allowing the vocal to ‘sit in the track’.
The same discussion can be made about any instrument in the mix. Each instrument has it’s place and a good compressor can assist the engineer in the overall blend of each instrument. This brings our discussion to a another good question…
An in-depth yet easy-to-understand discussion of wireless systems, how they operate, issues that can plague performance, and solutions that do the trick in the vast majority of situations.
Continuing, when comparing audio levels that are already referenced to a common scale (dBu is a reference to .775 V), it's important to discard this reference and make level comparisons in simple dB. This might seem like nit-picking until we consider that the 60 dB range described in this article may very well have originated at microphone levels (maybe -75 dBu to -15 dBu), continued through line-level processing (maybe -48 dBu to +12 dBu) and they will almost certainly end up coming out of a power amplifier at much higher levels (maybe -22 dBu to +38 dBu).
Rather than continue criticizing, I'll make my point here:
The good points in this article are overshadowed by its faults and errors. I'm tired of working from the deficit that this kind of bad reference material generates.
Raise your game folks, or find somewhere else to play.
-Andy Sickle
Associate Professor
Audio Arts and Acoustics
Columbia College Chicago