Compression In Music Explained and Defined!

Writer Kevin Ott At Rocking Gods HouseCompression is possibly the most common effect for audio, yet it is often misunderstood. Over the years, as I have searched for a clear explanation of compression, all the guides I’ve found could not break down its abstract functions into concrete, practical language. There was always too much technical insider lingo. When I was a beginner trying to grasp the concept in down-to-earth common sense terms, it was a frustrating experience.

Although there are some techie terms that are unavoidable, this article will cut through the industry talk and zero in on what it all really means.

Let’s start with a quick and easy explanation of why you want to use compression: It makes loud sounds quieter and quiet sounds louder.

It evens things out and avoids extremes on either end of the volume spectrum. Whoever invented compression was probably working on a track and thinking, “Wow, at the beginning of this song the guitar is blasting at full volume and causing everything to distort and making my coffee cup rattle on my desk, and then two seconds later it’s so quiet I can’t even hear it. This is impossible to mix with the rest of the song. It’s driving me crazy! I think I will invent something that gets rid of these extreme spikes and sudden dips in the volume.”

Let’s say you’ve just begun a sound check for a rock band. The lead singer has the vocal range of Johnny Cash and Mariah Carey combined. One moment they’re singing these impossibly low, quiet notes that rumble around in the room, and the next moment they’re hitting notes so high and loud that the International Space Station can hear them.

That’s when compression comes in handy. You flip it on, adjust the dials, and smile during the low Johnny Cash notes that have been boosted and are easier to hear thanks to compression. You nod your head in approval during the glass-shattering Mariah Carey notes because their ferocious loudness has been tamed and brought down to a reasonable volume without changing the other notes.

So how do you use compression and what on earth do the words in the compression’s parameters mean? Although more advanced compressors might have extra bells and whistles added to them, there are always four primary features in every compressor. If you understand these four basic things, you’ll be fine whether or not you use any extra features:

Threshold: This first setting called “threshold” decides when your compressor kicks into gear and starts squashing audio.

For example, if you set the threshold of a compressor to -5 decibels (see the decibel scale alongside a fader on the mixer to find this number—i.e., the knob that slides up and down to adjust volume), the compressor will kick in whenever the sound rises above -5 decibels in volume. It’s like a bouncer or a border patrol agent. It guards the perimeter of a specified territory, and anything that tries to go beyond that border without permission gets squashed.

Ratio: This parameter decides how hard the compressor squashes the audio once it goes above the threshold boundary. If you set the ratio to 3:1, for example, then for every one decibel that spikes above the threshold limit, the compressor squashes it down three decibels. So, say the singer hits a really loud note and her signal rises four decibels above the set threshold boundary. This means the compressor will squash her signal down by twelve decibels. Why? Because the ratio is set to 3:1. For every one decibel of audio that rises above the limit, the audio is reduced by three decibels. It went four decibels above the limit set in the threshold, and 4×3=12, so it was reduced by 12 decibels.

Attack: Once the compressor begins to squash the audio signal, the “attack” parameter decides how quickly the compressor latches onto the audio and squishes it. If you’re dealing with a fast striking instrument—like a snare drum or some other instrument that is hitting very rapid, short notes—turn the attack down to zero. This will make it work as fast as possible. If you’re dealing with a slower instrument like a mellow guitar or a vocal, you don’t need the compressor to react with lightning speed. You can leave the attack around two or higher, depending on the instrument. It’s easiest to simply play the track with the instrument and adjust the attack by ear until it sounds right. If the attack is too fast for the instrument, you’ll notice some odd sounds—a pumping sound or something similar.

Gain: Once the compressor has squashed the loud parts of the audio signal, this final parameter allows you to boost the overall signal to compensate for the squashed loudness. Because of all the squishing and squashing involved, compression tends to make a track quieter. The gain function allows you to bring it back into the mix with the proper volume after you’ve gotten rid of all of the extreme peaks of loudness. Using this gain function after you’ve compressed the signal also boosts the quiet moments of the track and makes them more audible in the mix. It is now safe to do this because you’ve gotten rid of the annoying spikes of loudness that were making it impossible to properly mix the audio track with the rest of the song.

When you use your compressor, always adjust the settings in the order above: Threshold, Ratio, Attack, and Gain. Doing everything in that order will ensure you’re getting the most out of the compressor and using it in a way that makes sense. Compression is a fantastic tool, and it is meant to eliminate some headaches for engineers when sudden spikes in loudness make it impossible to place a track into the mix properly. Be careful not to over-compress, however, as this can change the tone of the track in dramatic ways.

For live situations, the compressor is especially useful for keeping unpredictable vocalists in line. You never know what singers might do. Compressors give you the ability to control their loudness even if they go wild. Compressors can also help you enhance the snare and kick drum in live mixes, tighten up the tone of the guitarist, keys, and bass, and give you more control in fitting all of the instruments into a pleasing sound.

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