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Audio Compressors 101

By Edited Aug 17, 2016 0 0

What Is a Compressor?

A Compressor, more specifically called a Dynamics Range Compressor, is a device, can be software or hardware, that is used to lower the dynamic range of an audio file. Simply this means that what ever audio signal is put into a compressor, the signal that will come out will be more balanced in volume and the differences between the higher and lower levels will be "compressed" so that they are closer together and the overall sound which comes out sounds more balanced.

- How a compressor does this differs from software to hardware as well as compressor to compressor as many are very different and can affect an audio signal differently depending on which compressor is used. The topic of exactly how they work is very complicated and involves algorithms beyond my knowledge and is left for a more advanced topic by an expert on compressors.

What I will focus on is how they are used in the recording and mixing domain in current studios and how you can use a compressor to both touch up you tracks and give your overall song that extra push it needs to sound commercial.

Dynamic Range Compressor Hardware

How to Use a Compressor

When to Use a Compressor (Possible Scenarios where one is necessary)

The first step you should do before beginning to use a compressor is determine if your current project requires use of a Compressor. Most of the time the answer to this question is YES. But there are occasions where using to much compression can destroy a track so becareful and considerate when determining to use a compressor.

The ways to determine if a track or piece of audio needs compression are simple:

  • There are big differences between the lower volumes and higher volumes. Example:                                                      
    High/Low End Examples
  • If you hear a sudden loud part in the audio. This can be in the form of a scream or anything of that nature or a louder drum hit, etc.
  • If your trying to balance out an entire session of instrumental notes on a piano or drum kit and you hits are louder than others. (You want to becareful with this because dynamics in this sense are great to keep because they give the recording a live feel to it.)

If any of the above are true of the audio you are examining then your best bet is to use a Compressor. Of course the use and recommendation of compressors differ from engineer to engineer, but in my successful career as a Sound Engineer, I have used the above criteria and never seemed to fail in my mixing as far as compressors go.

How to Pick and Choose the Correct Tool for the Job

Compressors come in all different shapes and sizes. You must first go through the hurdle of deciding on a hardware compressor or to go with the newer software version. Right of the bat I can tell you that a hardware compressor will most likely cost you a greater amount than that of a software compressor. This is due to all the time and work, as well as the complicated pieces that go into creating a compressor. There are costly software compressors out there as well but there are alot more freeware compressors which are just as great.

Remember price doesn't determine quality. I've came across a list of freeware Compressors that have more then been enough for the job I needed. But don't get me wrong the hardware compressors will give you a better sound a majority of time.

There are two basic typs of Compressors : Soft Knee and Hard Knee.

  1. Hard Knee Compressors: The most common type of compressor. This compressor applies the ratio (explained later) immediatly after the signal level suprasses the Threshold (also explained later) in full force to the setting it is set to.
  2. Soft Knee Compressors: A more advance style of compressor. This compressor begins to apply the ratio even before the signal surpasses the Threshold. The closer the signal gets to the Threshold the more ratio  will be applied to the audio signal until the audio signal suprasses the threshold, in which case the ratio would be applied in full force to the setting which it is set to.

I will not publicly endorse any specific brand of compressors but will say to take your time and look through the freeware compressors out on the web before you being to invest and see if they can accomplish the certain task you need to be completed. If you have a budget for a hardware Compressor or to invest in a more high scale software compressor then please, please ,please do your research into the different brands and types there are because the choice to choose from is large and the one you choose WILL make a difference. Notice the reviews and ratings on all the products before you purchase any. (This should be a strategy used in purchasing anything.)

Audio Compressor - Software

How to Implement a Compressor

Picking and choosing the Compressor is just the first step into using one. The next step involves actually placing the compressor into the certain audio piece you need it for, whether that be a certain word the artists recorded too loud or a drum sequence that needs to have less dynamics.

After you install/download/setup your Compressor correctly (shouldn't be difficult task if it came with a manual.) then you are ready to implement it into your recording software.

The process is fairly simple and involves you going to the certain track and inserting it into the effects area. Different software programs have different areas so it is difficult for me to explain exactly where this can be located but if you are familiar with the software you are using this shouldn't be an issue.

SN: Some engineers prefer to place the effect on a Bus or Auxillary track and then send the audio signal into this track. I personally just place it onto the specific track I need, or secion of audio, and apply it to that specific part. This allows me to place different compressors on different tracks and gives me more freedom of how each different track is compressed. One could also do this with the technique of the Bus or Aux Tracks but I believe it gets alot more complicated and gets more confusing than it needs to be. The choice, of course, is yours.

What do all the Knobs and Settings Mean?

Below are the basic settings provided in the majority of Compressors:

  • Input: Level in which any audio is imported into the Compressor.
  • Threshold: Level at which the signal must reach before the Compressor kicks in.
  • Ratio: The amount of compression applied is determined by the ratio of the Decibel rise in signal level to the level which surpasses the Threshold.
  • Attack: How quickly the Compressor is activated once the Threshold has been passed.
  • Release: How quickly the Compressor deactivates once the signal level has dropped below the Threshold.
  • Link: Unites Left signal and Right signal for stereo operation. 
  • Output: Level in which audio is exported from the Compressor

Some Compressors also offer optional settings such as the ones listed below:

  • Auto Attack & Release: Some compressors have an auto switch which will adjust the Attack and Release automatically to fit the dynamics of the incoming audio signal. This is used sometimes when you have yo compress an entire track and the dynamics change as the song progresses. This setting can emit unwanted results and must be carefully examined before being completely implemented to make sure the sound you get is the sound you want.
  • Limiting: This option is usually implemented manually by setting the Ratio to infinity and setting the Threshold whereever you want the audio to stop. By setting the Ratio to infinity the Compressor will know not to allow any audio past the threshold, hence the term limiting.

Conclusion

As I've stated above a Compressor is a very important tool in the arsenal of a Sound Engineer no matter the style of music or size of studio. Most audio signals will need compression of some sort and I can garauntee the majority of Sound Engineers needing to know how to use one throughout their entire career or they will not be up to par as the next engineer.

To recape we discussed exactly what a Compressor is, the different types and which to use for what scenario. We looked into the settings of a common compressor and also some optional settings that one may run into. Some compressors can get alot more complicated than we discussed while others are very straight forward and simple. It all depends on the job at hand for you to decide which one is perfect for you. Once you get some experience with Compressors you will be on your way to mastering the art of mixing as a sound engineer, though there is much more to learn.

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