Dynamic Range Compression: What Is The Threshold Control?

Dynamic range compression is easily one of the most important audio effects/processes. The threshold control of a compressor is a key parameter in how the compressor will process audio signals.

In this article, we'll discuss the threshold control of compressors in greater detail to help you to better understand and use compression in your audio endeavours.

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A Brief Discussion On Audio Dynamic Range Compression

Before we get into the specifics of compressor threshold control, let's quickly go over what dynamic range compression is in terms of audio.

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 as the name suggests, a compressor will effectively compress/reduce the dynamic range of a signal.

Compression can be visualized as a volume fader that automatically ducks the loudest parts of a signal, bringing the loudest parts down while keeping all the quieter parts the same.

Compression is one of the most common processes to apply to audio signals. Its general 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)

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What Is The Threshold Control Of A Compressor?

The threshold control/parameter of a compressor sets the input signal amplitude threshold that will cause the compressor to engage and begin attenuating the signal.

Previously, we discussed how a compressor will attenuate the “loudest parts” of a signal. The threshold effectively tells the compressor when the signal is “loud enough”, which engages the compressor to begin working.

If the audio signal never surpasses the threshold, the compressor will never engage. Conversely, the compressor will never disengage if the signal amplitude is constantly greater than the set threshold.

So as the input signal level surpasses the set threshold, the compressor will engage. The amount of attenuation is defined by the compressor's ratio, and the time it takes for the compressor to fully realize this ratio is defined by the attack time parameter.

As the input signal drops back down below the set threshold, the compressor will disengage. Note that it will take some amount of time (defined by the release time parameter) for the compressor to fully disengage/stop attenuating the signal.

To visualize the threshold control, let's look at an illustration of a simple audio signal and a compressed version of that signal.

In the following image, the amplitude is along the y-axis, and time is along the x-axis. The threshold is defined as the red dotted line. Notice how the compressed signal is only attenuated in section where the original signal amplitude is greater than the threshold amplitude.

Also, note that the ratio is undefined in the image above, and the attack/release times are not illustrated.

To deepen our understanding of compressor threshold, let's look at another image. This graph has the input level of the compressor (in decibels) along the x-axis and the output level of the compressor (in decibels) along the y-axis:

We see in the graph above that the threshold's dotted line only intercepts the x-axis.

Before the threshold, the output level is the same as the input level. However, as the input level surpasses the threshold, we can see clearly that the output level becomes less than the input level.

The amount of compression that takes place above the threshold is determined by the ratio.

Once again, the compressor will be engaged as long as the input signal amplitude is above the threshold. As the input signal drops below the threshold, the output becomes directly proportional to the input once again.

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How To Set The Threshold Control Of A Compressor

Setting up a compressor is highly subjective. Depending on the results you want, you'll have to set the compression parameters differently.

That being said, if you want a more transparent kind of compression, I'd suggest the following:

Set the ratio between 2:1 and 6:1 and set attack time and release time to “medium” settings before getting to the threshold.

Next, try setting the threshold so that an average of about 3 to 6 dB of gain reduction is applied to the signal.

From there, adjust to your liking.

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

To maintain a strong output level, set the compressor's makeup gain to match the average amount of gain reduction/attenuation.

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Other Uses Of Threshold Parameters In Audio

Compressors are not the only processors that utilize threshold parameters. Threshold controls can be found in the following audio processes:

• Limiters
• Expanders
• Noise Gates
• De-essers
• Dynamic EQ

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

Limiters

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.

Expanders

The expander is essentially the opposite of 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, limiting and de-essing).

Noise Gates

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

De-essers

The de-esser is a narrow-band compressor that targets the sibilance of voice/vocal signals. It can act upon narrow bands, upper-frequency-only bands, or the entirety of the signal.

Dynamic EQ

Dynamic EQs are similar to multiband compressors in the way that they hone in on a specific band of frequencies. As the input amplitude within this range exceeds the threshold, the EQ (typically a bell-type cut will engage and affect the output amplitude within this band.

To learn more about the differences between dynamic EQ and multiband compression, check out my YouTube video on the matter:

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Related Questions

What are the main controls of a compressor? The main controls/parameters of a dynamic range compressor are as follows (with links to in-depth articles):

• Threshold
• Ratio
• Attack Time
• Release Time
• Knee
• Makeup Gain

What is audio data compression? Audio data compression is the process of encoding digital audio information into fewer bits than the original signal/file, thereby compressing/reducing the file size. Data compression can be either lossless (eliminating redundant info) or lossy (eliminating unnecessary or “less-important” info).

Popular lossless audio compression formats include:

• FLAC (Free Lossless Audio Codec)
• ALAC (Apple Lossless Audio Codec)
• APE (Monkey’s Audio)
• OFR (OptimFROG)
• WV (WavPak)
• TTA (True Audio)
• WMAL (Windows Media Audio Lossless)
• Dolby TrueHD
• MLP (Meridian Lossless Packing)
• MPEG-4 ALS (Audio Lossless Coding)
• MPEG-4 SLS (Scalable Lossless Coding)
• RealAudio Lossless

Popular lossy audio compression formats include:

• Dolby Digital
• Dolby Digital Plus
• DTS Coherent
• MPEG-1
• MPEG-2
• MPEG-4
• MPEG-H
• Vorbis
• WMA (Windows Media Audio)