Dynamics 101: Compressors and Limiters. Introduction.

Introduction: Rick Naqvi is one of the best people I know to teach on the care and feeding of compressors and limiters, which makes sense, as he works for Presonus, which makes excellent compressors and limiters. He has given us permission to include it here on the ChurchSoundGuy blog. So as to avoid being overwhelming, it will be posted in several installments.

Common Questions Regarding Dynamics Processing…

What is dynamic range?

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 +24dBu and the output noise floor is -92dBu, then the processor has a total dynamic range of 24 + 92 = 116dB.

The average dynamic range of an orchestral performance can range from - 50dBu to +10dBu on average. This equates to a 60dB dynamic range. 60dB may not appear to be a large dynamic range but do the math and you’ll discover that +10dBu is 1000 times louder than -50dBu!

Rock music on the other hand has a much smaller dynamic range, typically - 10dBu to +10dBu, or 20dB. 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 a average dynamic range of 20dB. You wish to add an un-compressed vocal to the mix.

The average dynamic range of an un-compressed vocal is around 40dB. In other words a vocal performance can go from -30dBu to +10dBu. The passages that are +10dBu and higher will be heard over the mix, no problem. However, the passages that are at - 30dBu 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 10dB. The vocal can now be placed at around +5dBu. At this level, the dynamic range of the vocal is from 0dBu to +10dBu. 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…

Does every instrument need compression?

This question may lead many folks to say ‘absolutely not, overcompression is horrible’. That statement can be qualified by defining ‘overcompression’. The term itself, ‘overcompression’ must have been derived from the fact the you can hear the compressor working. A well designed and properly adjusted compressor should not be audible! (Of course this can make a well designed compressor difficult to demonstrate!) Therefore, the overcompressed sound is likely to be an improper adjustment on a particular instrument. Why do the best consoles in the world put compressors on every channel? The answer is simply that most instruments need some form of compression, oftentimes very subtle, to be properly heard in a mix.

Why do you need noise gates?

Consider the compressed vocal example above and you now have a 20dB dynamic range for the vocal channel. Problems arise when there is noise or instruments in the background of the vocal mic that became more audible after the lower end of the dynamic range was raised. (air conditioner, loud drummer, etc.) You might attempt to mute the vocal between phrases in an attempt to remove the unwanted signals, however this would probably end disastrous. A better method is to use a noise gate. The noise gate threshold could be set at the bottom of the dynamic range of the vocal, say -10dBu, such that the gate would ‘close’ out the unwanted signals between the phrases.

If you have ever mixed live you know well the problem cymbals can add to your job by bleeding through your tom mics. As soon as you add some highs to get some snap out of the tom the cymbals come crashing through, placing the horn drivers into a small orbit. Gating those toms so that the cymbals no longer ring through the tom mics will give you an enormous boost in cleaning up the overall mix.

Courtesy Presonus. Used by permission.