Dynamic Range Compression: What Is The Ratio Control?

Dynamic range compression is one of the most utilized and important audio effects/processes in existence. The ratio control of a compressor is a key parameter in determining how the compressor will process audio signals.

What is the ratio of a compressor? The ratio of a compressor defines the ratio of input signal amplitude above the set threshold to the output signal amplitude above the threshold. It controls the relative amount of attenuation the compressor will apply, when engaged, to the signal.

In this article, we'll deepen our understanding of the ratio control of compressors to improve our knowledge of compression and audio in general.

A Brief Discussion On Audio Dynamic Range Compression

Let's begin this article will a quick discussion on dynamic range compression as a whole. By understanding the function of a compressor, we can better understand how the ratio control ties into these processors.

What is dynamic range compression? Dynamic range compression is the process of reducing the dynamic range of an audio signal (the difference in amplitude between the highest and lowest points). Compression does so by attenuating the signal amplitude above a set threshold point.

So then, a compressor will effectively compress/reduce the dynamic range of a signal.

When thinking of the actual effect a compressor has on a signal, we can visualize compression as a volume fader that automatically drops the signal volume during the loudest parts of the signal. The loudest parts get ducked while the quieter parts are outputted at full volume.

A compressor can be thought of as an automatic volume control in this manner.

As was mentioned at the beginning of this article, compression is one of the most commonly used processes in audio. Its uses include, but are not limited to, the following:

Maintaining a more consistent level across the entirety of the audio signal/track
Preventing overloading/clipping
Sidechaining elements together
Enhancing sustain
Enhancing transients
Adding “movement” to a signal
Adding depth to a mix
Uncovering nuanced information in an audio signal
De-essing
“Gluing” a mix together (making it more cohesive)

What Is The Ratio Control Of A Compressor?

The ratio control/parameter of a compressor sets the ratio of the input signal amplitude above the threshold to the output signal amplitude above the compressor's threshold.

The ratio, then, can be summed up in the following line:

[input signal dB above the threshold] : [output signal dB above the threshold]

The higher the ratio, the more the compressor will attenuate the signal as it surpasses the set threshold.

The threshold, by the way, is the compressor parameter that is set at a defined input signal amplitude. As the input signal surpasses the threshold level, the compressor will be engaged, and as the input signal amplitude drops back down below the threshold, the compressor disengages.

Let's have a look at some common compressor ratios on the following graph. This graph has the input level/amplitude on the x-axis and the output level/amplitude on the y-axis:

Let's make sense of the various ratios on the graph:

1:1 ratio: no compression at all.
2:1 ratio: for every 2 dB the input signal is above the threshold, the compressor will output 1 dB.
4:1 ratio: for every 4 dB the input signal is above the threshold, the compressor will output 1 dB.
10:1 ratio: for every 10 dB the input signal is above the threshold, the compressor will output 1 dB.
∞:1 ratio: limiting (sometimes called hard compression) where the compressor will not output any level above the threshold.

With these examples, we can better understand how the ratio control works.

To explain things a bit further, it's worth noting that compressors act continuously rather than discretely, in principle. So then, a 10:1 ratio would output 0.5 decibels above the threshold if the input was 5 dB above the threshold and 0.25 dB if the input was 2.5 dB above, so on and so forth.

A ratio of ∞:1 means that the output signal amplitude will never exceed the threshold level (assuming, of course, that the compressor would have zero attack time). However, in understanding ratio, this would be the case with a ∞:1 ratio.

Conversely, the first spot in the ratio must have a number greater than 1 for the compressor to actually work as a compressor.

A 1:1 ratio would mean that the compressor would not attenuate the signal above the threshold at all. It would simply output a signal with the same level as the input signal.

Theoretically speaking, if the first number in the ratio was less than one, the signal would be amplified into overload/feedback as the threshold was surpassed.

To solidify our understanding of compression ratios, let's have a look at a few waveforms with different ratios:

In the graph above, the threshold is defined by the red dotted line. In the majority of cases, the compressor will act to compress the entirety of the waveform (rather than only affect the positive and negative peaks of the signal). This is largely due to the frequency of audio signals and the attack/release times of the typical compressor.

That being said, we can see how, all else being equal, a higher ratio will attenuate the signal more.

If the input signal surpasses the threshold by 8 dB (as is the case in the graph above), then a 2:1 ratio would cause the output to be 4 dB above the threshold, and a 4:1 ratio would cause the output to be only 2 dB above the threshold.

How To Set The Ratio Of A Compressor

The ratio setting of a compressor will vary greatly depending on the purpose of the specific compressor.

For example, a master bus compressor may only require a ratio of 1.5:1 in order to help “glue” the mix together without having an overly noticeable effect on the dynamic range.

Conversely, a parallel “New York” style compression may require a 20:1 or even ∞:1 ratio to absolutely crush the dynamic range of the incoming signal.

For most cases, a 4:1 (or anything between 2:1 and 6:1) ratio is a good starting point. Combine this with a medium/fast attack and a medium release time and set the threshold so that 3 to 6 dB of gain reduction happens on average. Set the makeup gain to bring the signal up by 3 to 6 dB.

Playing around with the ratio here will affect the sound. Remember that higher ratios lead to greater attenuation.

Of course, it's also important to adjust the other parameters and to listen intently to get the results you're after.

Again, there's no perfect one-setting-fits-all for compression. The suggestions above will get you to a good starting point.

Other Uses Of Ratio Parameters In Audio

Compressors are not the only processors that utilize ratio controls. Ratio controls can be found in the following audio processes:

Limiters
Expanders
Noise Gates

Note that these effects all act on a signal's dynamic range.

Limiter Ratios

The limiter can be thought of as a compressor with a ratio greater than 10:1, and a brickwall limiter can be thought of as a compressor with a ratio of ∞:1.

Expander Ratios

The expander is essentially the opposite of a compressor.

Noise Gate Ratios

The noise gate is to an expander what a limiter is to a compressor.

With expanders and noise gates, the threshold is set up in such a way that the input signal must drop below the threshold for the effects to engage (rather than rising above the set threshold, as is the case with compression and limiting).

Call To Action!

Experiment with the ratio control of a compressor you're familiar with. Start with a low ratio (1.5:1 or 2:1) and dial in the other parameters to achieve an appropriate amount of compression (3 to 6 dB of gain reduction on transient peaks is a good starting point).

Proceed to increase the ratio and listen critically to how the sound of the compression changes. Make a note of how the sound changes.

Repeat the process on different tracks and even with different compressors to develop a better understanding of how the ratio control affects the sound of your compressors.

Dynamic Range Compression: What Is The Ratio Control?

A Brief Discussion On Audio Dynamic Range Compression

What Is The Ratio Control Of A Compressor?

How To Set The Ratio Of A Compressor