The basics on compressing and limiting audio for live music
When I was younger I remember placing an old penny on a railroad track and waiting for a train to come along and flatten and smooth it out into an oval shaped copper badge. Smooth on both sides and devoid of texture, it didn’t resemble the object it started as only a few moments before. Mixing audio uses a wide variety of types of compression and limiting with the goal being a beautiful, stable mix, but all too often we end up flattening the penny. The goal of this post is to help understand how compressors work and a good approach for how to apply them in a positive manner.
“A device whose output level decreases as its input level increases”
- used to reduce the dynamic range of the signal passing through it
- “Compressors make the loud sounds quieter and the quiet sounds louder.”
So it makes it the same volume and sometimes louder? Yes. Explaining how a compressor works is one of the more difficult things to understand, especially in the digital age where electricity doesn’t pass along in an analog format.
Sound enters a microphone and is turned into electricity or comes directly as electricity from a direct source like an acoustic guitar or keyboard. That sound enters the sound console and is amplified via the preamp or input gain of the mixer. What this preamp is doing is increasing the amount of electricity of the sound signal making it larger and able to be manipulated by the soundboards EQ, compressors, and gates as well as a multitude of other devices.
What a compressor does to this electricity is actually a very basic concept but is achieved via a large range of specific changes. At its core a compressor takes in all of the electricity of the source and restricts the amount that is allowed to flow out. The electricity being restricted has to go somewhere- it cannot simply disappear, so the compressor absorbs the excess electricity lowering the level, which is where the tone-changing aspect of a compressor comes into play. As the compressor reduces volume it takes the excess electricity (volume) and compresses it through a resistor and/or capacitor, which is where the squishy compressor sound comes from.
There are many aspects of a compressor that allow for varying and tailoring specific aspects of how the compressor works and the amount of compression that happens. We will go into that in a later post.
The important terms and controls of any compressor are the threshold, ratio, and output gain. The other two most common terms are attack and release. For our quick use we will use these definitions.
- Threshold: The amount or moment in time at which a change will start to happen.
- Ration: How large of a change will occur when the threshold is crossed. This is displayed as a ratio number. It determines the amount of compression…steepness of the slope. A ratio of 1:1 means no change. A ratio of 2:1 means: every 2 dB increase in the input level above the threshold gives 1 dB increase in the output level.
- Attack: How fast the compressor will begin to restrict audio. Useful to allow some uncompressed audio to pass through before a change happens.
- Release: How fast a compressor will stop changing the audio after the signal drops below the threshold.
- Output gain(make up gain): This is used to add extra volume to the overall signal after the compressor to make up for any lost volume.
Now that we have covered the super fun stuff let’s look at what I believe are the two main ways a compressor is used in modern music. Those two are to control and to level out volumes of musical parts and to shape the overall tone of the sound.
There are so many ways to use and apply compression that it’s almost impossible to give a good “rule of thumb”. However, I do believe there are a few simple concepts that can help us make wise use of a compressor. Imagine you are pushing a rolling cart down a brick paved hallway. On this cart you have sixteen or twenty-four cups of water with open tops. The water is three fourths of the way full-enough that with a large bump in the hallway some may spill out. Not only is spilling water a potential problem, but the water of each cup is sloshing about in its own direction and speed. The water is the amount of volume being produced by your source and the cup is the channel on the soundboard. You can tilt the cups or reposition the cups, but the water is still going to slosh about and may even spill. To better control the water we need to compress the top level of the water just enough to hold it in the desired place and keep it from spilling. We will need to drink some of that water so we have a straw hole in the lid allowing only what we want out as we have need. Now we have nice compressed cups of water that won’t spill but still have enough room to allow the water to move independently.
I’m going to walk you though how I apply a compressor (and why I do it the way I do), then move on to tone shaping.
Whether I do a sound check with a large amount of time or a quick festival style check I always use this same process.
- Tap it
- Tighten it
- Bring it
1. By tap it I mean I will bring my threshold down till I see the compressor is just barely tapping some small amount on the compression meter. I have just put a loose-fitting lid on my cup of water, keeping it from spilling, but not forcing the water into a pressurized chamber.
2. After I have tapped everything and I know I’m in a safe, spill-free zone I will begin to tighten up my compressors. Some inputs don’t need much more than to be lightly tapped with compression and can be skipped over. Depending on the song and style of music the amount of compression will very greatly. As a good rule I will aim for 3-6 dB of gain reduction on the compressor meter when I feel the source needs to be compressed. That is an amount that allows for volume change, but not a great deal of tone shaping. I call this my standard level of compression. On specific instruments I will aim for a larger amount of reduction like with a snare drum or bass guitar. I believe these two normally have a better quality sound when we compress them harder and we don’t lose the natural sound with more compression.
3. Compressing a sound source will lower the peaks in volume and as a result usually lower the overall volume level. This is when I apply makeup gain to bring the level up to an appropriate level for the mix, making up for the lost volume.
This is where we need to remember our old friend the railroad penny. If we lower the threshold too much or use too high a ratio we can create a situation where the penny gets flattened and loses all its penny qualities. The term “nuke” or “nuking an input” is a reference to this over-compressed sound quality. However if applied wisely, a heavily compressed signal can have a pleasing sound. This is when we would be using the compressor to shape the tone.
The tone that is created by a compressor absorbing the extra signal can be either a positive or negative. Different manufactures us different components and ways of going about changing the sound. Because of this some compressors have tone altering qualities that are warming and pleasant while others sound like that penny under the train: flattened out and shapeless. A good example of a compressor being used to shape the tone of a sound is Katy Perry’s song “Dark Horse”. Her voice is compressed a great deal but it was to give it a very distinct tone quality. This was most likely a very high ratio and was probably applied at the very end of the vocal mixing process. Most likely they would have used standard levels of compression earlier to level out volumes and mix the parts together so their cups of water don’t spill and then apply another heavy compression setting at the end to shape the overall tone. In live sound when analogue gear is limited and/or digital processing is not flexible it may be best to apply standard levels of compression to your channels and then use a “tone shaping” compression setting on the group you’re wishing to shape. Most large format digital consoles allow for multiple types of compression on a single channel as well as “stacking” analog gear on a single input to achieve the same result. This is how you can achieve a controlled signal that has a nice warm and solid sound.
By applying compressors this way I have now set my inputs so they won’t spill all over the cart and floor and have tone-shaped a few inputs leaving me with a very controlled and warm sounding mix.