Nick's Tech Tip of the Month: DYNAMICS & COMPRESSORS

Sounds are made up of vibrations or ripples of air. Our ear is designed to detect these ripples and our brain takes this information and compiles an idea in our minds. When the ripples are close together, the pitch of the sound will be 'high' in pitch and the opposite when the ripples are far apart. For example, the ripples from the rumbling of a lightning strike may be separated by 20 about metres and the open 'A' string of a guitar tuned to 440Hz will be 781mm. In nearly all sounds (apart from an artificially generated sine wave), there will be sub-ripples of a factor of Hertz from the largest ripples and other sub-ripples bearing no mathematical relationship to the 'carrier' ripple. This is all thrilling and wondrous, but it's not on topic for the discussion of 'dynamics'. I simply wanted to make a clear distinction before we progress.
The above is a discussion of frequency, dynamics is a discussion of amplitude, or sonic energy over time. To discuss dynamics, we use the concept of an envelope, which is really just a graph of the volume of the signal over time. Different instruments have different shaped graphs, and this is the first thing to learn. A piano note will have a graph that rises up to its highest level very soon after it's been played, before slowly dropping to zero as the key is held down. A flute does not necessarily need to share this profile, as the player may delicately control the volume of the instrument to nearly any 'profile' they chose.
So what's a compressor? A compressor is a tool designed to give you the power over your recordings that flautists have enjoyed for years! It's a device/program/VST for controlling the dynamic profile of a recorded track or a live signal feed. Most compressors have four main controls and I'll detail each in the following points.
THRESHOLD: This is the signal level above which the compressor starts to do its business. Any signal below the level set by the threshold control will not be altered by the compression process.
RATIO: Signals rising above the threshold level will be diminished in level by a factor dependant on this setting. For example, a ratio setting of 4:1 will diminish the threshold exceeding signal by a factor of four. If the above threshold level was four decibels, the post compressor level will be one decibel.
ATTACK!: The compressor need not reduce all over-threshold signals instantly. This control sets a delay between detection of an over-threshold signal and the onset of the compressors level reduction. This control is where the dynamics shaping really starts to come into the picture. If this setting allows the compressor to act too quickly, all the sounds passing through this compressor will have soft edges to them. The piano mentioned above will lose its percussive initial strike and may end up sounding like a flute in some cases! The basic rule here is to set the attack to be longer than the natural dynamic profile of an instrument, IF that profile is a defining factor of the instrument. If you don't, you'll make the instrument sound strange, which of course, may be a desired effect.
RELEASE: The compressor will hold the level down for the amount of time specified here.
MAKEUP GAIN: All of this over-threshold level lowering has lowered the level! Duh... Makeup gain is simply a preamp after the main compressor circuitry to allow for the lost volume to be 'made' up.

If you're like me, you may be wondering about that last control. You may be wondering why you'd want to turn stuff back up again after you went through the tedium of fine tuning your fandangal compressor. The answer is simple. The Makeup Gain control doesn't have a threshold. It increases the level of EVERYTHING passing through the compressor. In effect, you're lowering the loudest levels and increasing the lowest levels. The difference between the quietest and the loudest sounds is now less. Interestingly, a heavily compressed signal will sound louder than a non-compressed version because there is more average signal energy present. Human ears are better at perceiving differences in volume than they are at perceiving differences in pitch. The TV commercials do not have any part of their signals louder than the show you're watching. The just have more of their signal near the top of the scale!
I hope this was enlightening for at least one other than myself, and please feel free to ask questions!

 

Nice guide!

However I will make one addition

Should be HOLD. Release is the time taken for the signal to go to 0db attenuation once the HOLD time has expired (obviously can be overridden by continual above threshold signal)]

Should we expect a tip per month? Could be useful given the increased home studio talk around here.

 

No conventional compressors have a 'hold' setting. I think you're possibly thinking more along the ADSR settings on synths, but I get what you mean. I'll try to do one a month, but there doesn't seem much interest. Maybe something more advanced is needed?

 

I think more so you're re-inventing the wheel, there's a wealth of information on this stuff everywhere, I'd be more interested in people passing on sites, links, names of books that have really helped them with sound engineering, as to filter out the rubbish from the good stuff, I know myself I've bought many a dud book.

 

For my taste, that was probably too advanced
Your introduction was extremely dry/not exactly relevant to the topic, and I'm sure would've put a few readers off continuing on past it.
A simple introduction to the concept/applicability of compressors would be nice imo - there are predominantly guitarists on this forum so referencing guitar pedals explicitly would also be of value.

You achieved your goal anyway - it was enlightening to me , but as mentioned before, I think you may have lost a few readers with your introduction (as you did me the first time).

 

Fair point, I've been staring at digital consoles too much it seems (honestly I cant remember when I last mixed on analogue, maybe 2-3 years?)

 

Good post, technically accurate but a little too wall-o-text for my liking.

+1 for relevance since pretty much all pro guitarists have a compression pedal (or multi's which include a compressor) in their arsenal, and they are obviously one of the principal tools for engineers. And there's a lot of mystery/misinformation about how to use them correctly.

It's a difficult topic to teach people about without getting bogged down in backstory about gain structure and SNR, some understanding of which is essential to use compressors properly (see make-up gain etc.).

 

Thing is,did Jimi Hendrix use compressors?..Still Good stuff here,I am learning all the time..

 

I believe Jimi used a special kind of compression called "Marshall-turned-up-to-11".

It provides overdrive, sustain and compression with the simple turn of a knob.

 

I found this post very useful. I was trying to explain to someone the other day how to use a compressor and realised I didn't fully understand it myself, now I do. I might point them in the direction of this post. Thanks

 

Eddie Kramer did. ;-)

 

I'm going to try this on my Peavey (called 'Squeeze').

So for lead work, I imagine this will shine? Would you use compression for any rhythm playing when its short and sharp (master of puppets) or would it be suited to slightly longer sustained notes (harvester of sorrow/my friend of misery).