Mastering 101: Compression

Compression can play many roles, from making a track punchier or fuller-sounding to increasing overall loudness.

This post is the fourth article in our introductory guide to mastering. If you’ve missed an entry, click on any topic below to catch up:

  1. What is mastering?
  2. Signal flow & metering
  3. EQ
  4. Compression
  5. Limiting
  6. Preparation

This week, let’s take a look at compression.

There are plenty of tutorials and articles out there about compression that discuss what it is and what it isn’t. In this article, we’re going to specifically focus on compression within the context of mastering and how we can use it for your D.I.Y. masters.


What does a compressor do?

A compressor reduces dynamic range. More specifically, it reduces the difference between the loudest and softest parts of your mix, resulting in a track with less dynamics and movement. The tradeoff here is that, with a lower dynamic range, you can push the mix louder than if it were uncompressed.

This matters in mastering. Compression can do many other things like add punchiness, increase detail, or make things sound fuller, but in mastering a compressor is primarily used to increase loudness.

Although mastering engineers use limiters to make mixes louder, solely relying on a limiter to do all the legwork will produce unwanted effects such as pumping and distortion. Splitting the work between a compressor and limiter yields a more natural result.

Does the mix need compression?

About 80% of today’s mixes come in so hot and slammed that a compressor isn’t required. Use your ears when you receive a mix – if you find that the mix itself is already pretty compressed, adding compression for the master probably won’t help it much.

Getting hold of a meter such as Brainworx’s bx_meter will help you identify the dynamic range of your mix / master. The lower the number, the less dynamic your mix:


How much compression is enough?

In truth, mastering engineers hardly use any compression. Even if they do, it’s at low ratios and high thresholds. Here are some general guidelines if you want to use compression while mastering:

  • Start your ratio at 1.25:1 or 1.5:1. Going past a ratio of more than 2:1 is not recommended.
  • Set your threshold pretty high so that you’re getting 2 dB of gain reduction at most.
  • Use your ears – If you apply compression and don’t like how it changes your master, don’t hesitate to take it out.
  • Keep asking yourself after every move: “Am I making the music sound better?”

Multi-band vs single-band compression

Pros of a multi-band compressor:

  1. Isolated individual bands across frequencies
    • As you know, compressors react differently to different frequencies. For example, bass frequencies are longer in wavelength, and having a separate band to process the low end without affecting the rest of the frequencies can be ideal.
  2. The ability to use different settings across different bands
    • With a multi-band compressor, you can set different attack, release, and threshold settings for each band. You can divide the frequency bands into 3 or 4 bands (low, low mids, mids, and highs).
  3. Tailored compression to your program mix
    • Because multi-band gives you the ability to set different settings for each band, you’re essentially tailoring your compression for your particular mix.

Cons of a multi-band compressor:

  1. Many bands can be your enemy, too
    • For a multi-band compressor to work, it needs to use crossover filters to separate your audio into different bands. Whenever you run audio through a filter, you lose a little bit of fidelity. It adds a little bit of ringing, noise, and distortion.
  2. Over-processing can mess things up
    • With mastering, the number one rule is to not over-process. Over-processing with a multi-band compressor can easily skew the frequencies and phase of your master and cause it to lose fidelity. I would recommend only using a multi-band compressor if you really know what you’re doing.

Summing it up

  • Compression can be a useful tool if your ears tell you that the mix needs it.
  • Work in moderation – try not to over-process, and keep the gain reduction to no more than 2 dB.

That’s it for this entry – in the next post, let’s talk about limiting.

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February 22, 2016

Reuben Raman Product Marketing Manager at Splice