We talked about the main components of a sound when it comes to Time and Amplitude: Attack (or Transients) and Sustain. Then we examined how the sound is stored in digital land and how we cannot go over 0 dB.
So, a compressor will be essential to store (and reproduce) more significant sounds without distorting. Making sure nothing goes over it, and making sure every sound that we want to hear is pushed forward enough within the absolute limit.
A compressor’s main purpose is to reduce the Amplitude (level) of a sound during its lifetime.
You can think of it as a fader or volume control, but one that is automatic and can act extremely quickly, reducing the level of the sound at various phases, depending on a few parameters…
Evening out levels
This can be very useful for sounds that vary a lot, like a vocal for example… It’s not unusual for a vocal to have a lot of variation in amplitude, even during of one single vocal line. For example, look at this vocal take:
Compression here is going to help evening out the performance by lowering the highest parts (the peaks) … Once everything is at a similar level we can then make the whole thing louder and upfront as it should be in a song.
If we tried to raise the level of this sequence as a whole, to bring out the lowest parts, the highest parts would go over the limit of 0 dB, so they would be clipped and distort (the nasty sort of distortion). By first lowering the highest part (evening out the whole sequence) with a compressor, we can then raise the level of the sequence without going over the limit and without distortion!
This is the typical and simplest way to use a compressor. And it’s used A LOT during the mixing phase.
Fattening a sound
Now, another way to use a compressor will be to even out the difference between the attack and the sustain inside a single sound/a single note (not a whole performance like above), making it appear “fatter”. How so?
Remember that a compressor can act very very fast (some modern compressors can see the peaks before they even appear and play – it’s called look-ahead -, and they can react in mere nanoseconds), so it can act during the lifetime of a single note at a time, and this is where it will be used to alter the sound and make it fatter.
Let’s see how this goes. First, you need to understand one of the main parameters of a compressor which is its threshold. The threshold is the volume level over which a compressor will start acting. The picture below should tell you what a threshold is:
Everything that is over the threshold will be processed by the compressor. Everything under it will stay untouched. So, with the threshold parameter, we can tell the compressor which parts it should work (reduce) on and which ones it should leave alone.
Let’s have look at a typical snare hit before compression:
If we were trying to make this snare hit louder as it is, it could go over 0 dB which is not desirable.
But if we apply a fast compression, we can reduce the attack peaks, like this:
You can see that the threshold was set so that the Attack of the snare was reduced relative to its sustain (which was left untouched). Now, because the attack has been reduced, we can actually make the whole sound louder and it will not distort. If we do so now (using another parameter of the compressor, called gain which is applied AFTER the reduction and will raise the overall output level), it will look like this:
The initial attack is back roughly to where it was, but notice that the sustain has been made louder, thus making the snare sound “fatter”!
Next time we’ll look at some other usage of a compressor and a few other parameters that are used to alter a sound, mainly attack time, release time and ratio. Then we’ll talk about limiting. And finally, we’ll talk about loudness, the loudness war and why it’s important to know about it. See you then!