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 greater voltage 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…