Compression occurs when a signal hits a specified threshold (in dB) and is reduced in loudness by a set ratio. More can be learned about basic compression here.
The term Parallel Compression comes from the idea of two or more copies of the same signal playing at the exact same time, with one signal processed by a compressor. With ample mixing, this technique can be used to add more presence to a certain instrument or sound.
Parallel compression is commonly used on drum kits, especially in rock and metal sub-genres, to highlight the driving force of the performance. There are several different techniques you can use in parallel compression for different purposes in order to achieve different results.
Since I mainly use FL Studio, here’s a video by EDMProd clearly explains the basic routing (signal flow) process for parallel compression in FL Studio 11:
Specifically with drums, you can choose to reinforce the whole drum kit by using a brick-wall limiter (pretty much anything higher than a 55:1 compression ratio) and dialling in as much of that compressed signal as you need. However, it does not stop there – you can adjust the attack and release parameters to compress certain transients that may be quieter than the rest of the kit.
For example, the snare may not feel ‘strong’ or ‘punchy’ enough compared to the rest of the kit, but with the use of the API-2500 compressor by Waves, you can set the ‘tone’ of the compressor to either “Loud”, “Medium”, or “Normal”. Both “Loud” and “Medium” compress the low and high frequencies, with the former favouring the low frequencies, and the latter favouring the mid frequencies. This, combined with slow attack and release times can result in the snare being given more presence in the mix.
In electronic music productions, parallel compression is not just used for ‘reinforcing the sound’, but also for creativity. Last year I worked with a classmate on a pop track entitled “Move On“, which used parallel compression in a way that was experimental (by my standards!). I took the output of my drum sub-mix, side-chained it to a new track, put some effects on the track such as distortion, delay, mono-summing, reverb, and a high pass filter, then essentially squashed the audio. Then I put a volume envelope on the compressed signal to give it a sweeping sound. Blended with the original audio, it gave it a pretty cool backing effect that I was fairly proud of. Here’s an example of the drum beat before and after I did this:
It’s very important to note that you may come across phasing issues in your productions if you aren’t careful with your routing, but there are ways around it. One workaround I use in particular is putting a compressor on the dry track that you don’t actually want to compress (sacrilege!) but not adjusting any of the parameters since the default attack time, if remained untouched on the compressed signal, would match up with the wet channel’s attack time ‘latency’, thus the signals would be “in phase” again. This won’t work though if you’re feeding the dry channel into the wet channel, as it will just give the wet signal even more latency and will still be out of phase.
Parallel compression can be used for corrective and creative purposes, which proves that it is a must-know technique for anyone who is keen for work with audio productions.
Kärkkäinen, I. (2011, October, 22) Smashed up – a parallel compression tutorial [Web Article] Retrived from http://www.resoundsound.com/smashed-up-a-parallel-compression-tutorial/
Robjohns, H. (2013, February) Parallel Compression – The Real Benefits [Web Article] Retrieved from http://www.soundonsound.com/techniques/parallel-compression
Weiss, M. (2011, October, 17) 2 Effective Ways to Use Parallel Compression [Web Article] Retrieved from http://theproaudiofiles.com/two-ways-to-use-parallel-compression/