What are Threshold and Ratio on a Compressor? Audio Compression for Beginners Ep. 2


Thales Matos


Do you want to know what a compressor's Threshold and Ratio do? Keep reading this easy compression tutorial for beginners.

This post is the second episode of our series on Audio Compression for Beginners. Here's Episode 1, in case you missed it:

The Threshold and Ratio work together. To summarize, they tell the Compressor:

"When the audio goes above this level (threshold), trim it by this amount (ratio)."

How sound is measured

The intensity of sound is measured in decibels (dB).

Some compressors call it Volume, Gain, and some call it Amplitude. I've used those terms as synonyms here.

What does the Threshold do on a Compressor?

So, the Threshold is like a volume ceiling.

Any audio that goes higher than the Threshold line gets reduced. Any audio below the ceiling you chose will remain the same.

Say you want to compress your vocals or guitar whenever it exceeds -10dB.

Simply set up your Threshold to -10dB.

I've run a simple sine wave through Neutron Compressor in the following examples.

Neutron compressor - threshold and ratio

It has a good visual representation of what happens with the audio.

How much Gain reduction do you want?

Compressor Threshold and Ratio Example 1

If you want to reduce half of the signal above the -10dB threshold, use a Ratio of 2:1.

For every 2dB entering the Compressor, only 1dB gets out on the other side.

For example, this Audio is peaking at 8dB above the Threshold.

Neutron compressor - example 1

Look at the grey line at -2dB.

With a ratio of 2:1, the following happens to those 8dB that were above the threshold line:

Neutron compressor - threshold and ratio example 1 - after compression
  • The Compressor removes 4dB and;
  • The Compressor lets the other 4dB pass unaffected;
  • Therefore, the output signal peaks at -6dB instead of -2dB.

This happens because the Compressor removed half of the signal above the Threshold.

If you wanted to trim, even more, you could increase the Ratio.

Compressor Threshold and Ratio Example 2

A ratio of 8:1 means that for every 8dB above the Threshold, only 1 will pass the Compressor, and the rest will be cut.

Say the sound initially peaks at 8dB above the Threshold.

A compressor with a Ratio of 8:1 does the following:

Neutron compressor - example 2
  • It divides the 8dB by eight, which is the Ratio, and only preserves 1dB.
  • The other 7dB will never make it through the Compressor.
  • Therefore, the example sine wave leaves the Compressor at -9dB. Only 1dB above the threshold line.

What does Ratio do on a compressor?

A ratio of 4:1 means the Compressor will keep 1/4 of the decibels that crossed the Threshold line.

A ratio of 5:1 means the Compressor will keep 1/5 of the decibels above that Threshold ceiling.

1:1 Ratio

When the Ratio is 1:1, nothing gets compressed. For every 1dB above the Threshold, the Compressor will, in turn, let 1dB pass.

Neutron compressor - ratio 1 to 1

So no compression. Input and Output levels will be the same.

Compressors with High Ratios

On the other hand, some compressors allow for very high ratios, like 30:1, sometimes even ∞:1 ratios.

Neutron compressor - ratio 30 to 1

A compressor set up like that will barely let any signal pass above the Threshold level, much like a Limiter does.

But even with a ratio of 100:1, if the audio signal doesn't hit the Threshold, nothing will happen. It won't be enough to trigger the Compressor.

How to set up the Threshold and Ratio on a Compressor

There isn't a specific Ratio for certain applications. Nor is there a recommended Threshold level.

There are only guidelines because it all depends on your audio source.

  • Use lower ratios to preserve more of the original dynamics (gentle compression).
  • Use higher ratios for more extreme compression. Careful, though: it can suck the life out of vocal performances if overdone.

But sometimes, you'll need extreme compression to tame specific performances.

Especially when the performance is all over the place — quiet, then loud, then quiet again.

Sometimes you'll need no compression at all. It depends on the recorded performance.

The best thing you can do is experience compression. Try different settings, and play with your Compressor.

Now it's your turn

Now that you know what Threshold and Ratio do, start compressing stuff.

Listen to the results and the different sounds you'll get with each setting.

It may take you some time to actually start hearing compression. It's a bit of a challenge at first. But nothing beats experience.

Do it over and over again until it clicks. And when it sounds right, it's right.

In the next episode, we'll factor in the time element.

You'll learn how to use the Attack and Release controls to get more precise with your compression.

If you have any questions, let me know in the comment section below.

For more music production and mixing resources, visit my website.


Thales Matos

September 2, 2022


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