Some of the most difficult to understand effects processors are dynamics processors such as compressors, expanders, noise gates and limiters. While we all know effects like distortion, equalisers and reverbs and what they sound like, dynamics processors are not usually detectable unless they are used heavily.
What are “dynamics”?
Dynamics are the differences in volume in a signal. Most sounds have parts which are, and stand out louder than others. A piece of music is “dynamic” if there are large differences between the quieter and louder parts. The difference between the quietest and the loudest parts of a sound is called the dynamic range.
Take a look at the wave files below of: a distorted electric guitar on the left and an acoustic guitar track on the right:
While they both have the same peak level, the electric guitar track is denser, with less dynamics, while the acoustic is more dynamic.
Sometimes you want to make a track more dynamic because a dynamically squashed track doesn’t always stand out, while one with good dynamics will. Other times you may want a track less dynamic – if the overall track is too quiet and you cannot raise the level without clipping.
What Compressors Do
The basic function of a compressor is to alter the dynamics of a track. It does this by boosting the volume of the quiet parts and reducing the volume of the loud parts. The difference between the loud and quiet parts is lessened, so the track’s dynamics are reduced helping it to “sit” in the mix so it is always audible, but never too loud. By lowering the dynamics, the overall track level can be boosted, and the track can be louder without distorting.
Compressor Controls
Threshold
The level above which the compressor starts working the volume. When the threshold set at its highest point, the compressor will not change the sound’s volume. As the threshold is lowered, peaks will trigger the volume reduction effect.
Ratio
Ratio is how much the volume will be reduced. A ratio of 2:1 will reduce the volume of peaks crossing the threshold by a factor of two – this means that a peak 5 dB above the threshold level will be reduced to only 2.5 dB above the threshold level. A ratio of 5:1 and the peaks will have their volume reduced by a factor of five – the same peak will now be only 1dB above the threshold level.
Attack
Attack is the time it takes before the compressor reduces the volume. A longer attack time lets more of the peak through before compression.
Release
The time it takes before the compressor allows the volume to return to normal after the peak has fallen below the threshold.
Basic compressor use
Evening levels
If you have a track which has an occasional peak which is louder than the rest of the track, sometimes when it is mixed loud enough to hear the quieter parts, the peaks will be too loud and mixing it lower makes most of the track disappear. In short, it is too dynamic. This might look like this:
Compressing this track results in the following, which has a more even volume:
Note: I used more compression than I normally would to make the effect obvious: threshold -12 db, ratio 10:1, attack 10 ms, release 40 ms.
Controlling peaks
Sometimes the track will be fine aside from a few peaks which jump out. Fixing this is easy – use the compressor as you did above, but set the threshold carefully so it clamps down on nothing but the peaks.
Accentuating Attack
You can also use compression to make a track jump out of a mix. If a track doesn’t bite, it’s because it doesn’t have enough attack. As mentioned, the attack control on a compressor lets a certain amount of signal pass through uncompressed before compression kicks in.
Here’s a track that had almost no attack before compression:
Note that there is no difference in volume between the attack portion and the rest of the note. Using a compressor with a 5:1 ratio, threshold of -16 db, attack of 100 ms, and release of 10 ms produces this:
The attack of each note now stands out more from the sustain.
We just used our compressor to increase the dynamics of the track instead of decreasing it.
Written by : Alan Ratcliffe
