Dynamic range compression is easily one of the most important audio effects/processes and the threshold control of a compressor is a key parameter in how the compressor will process audio signals.
What is the threshold of a compressor? The threshold of a compressor is a set amplitude limit that dictates when the compressor will engage and disengage. As the input exceeds the threshold, the compressor begins compressing/attenuating the signal and as the input drops back down below the threshold, the compressor disengages.
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.
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 processed 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
- “Gluing” a mix together (making it more cohesive)
To learn more about audio dynamic range compression as a whole, check out my in-depth article The Complete Guide To Audio Compression & Compressors.
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 attenuated 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 surpassed the threshold, the compressor will never engage. Conversely, the signal amplitude is constantly greater than the set threshold, the compressor will never disengage.
So as the input signal level surpasses the set threshold, the compressor will engage. The amount of attenuation is defined by the ratio of the compressor and the amount of time it takes for the compressor to fully realize the ratio is defined by the compressor’s attack time parameter.
To learn more about compressor ratio, check out my article Dynamic Range Compression: What Is The Ratio Control?
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 have a look at an illustration of a simple audio signal and a compressed version of that signal.
In the following image, 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, in the image above, the ratio is undefined 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, so long as the input signal amplitude is above the threshold, the compressor will be engaged. As the input signal drops below the threshold, the output becomes directly proportional to the input once again.
To learn more about the somewhat confusing topic of decibels, check out my article What Are Decibels? The Ultimate dB Guide For Audio & Sound.
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 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.
To learn more about compressor attack and release times, check out my article Dynamic Range Compression: Attack & Release Controls.
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 not 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.
For more information on compressor makeup gain, check out my article Dynamic Range Compression: What Is The Makeup Gain Control?
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:
- Noise Gates
- Dynamic EQ
Note that these effects all act on a signal’s dynamic range.
- The limiter can be thought of as a compressor with a ratio of ∞:1.
- The expander is essentially the opposite of a compressor.
- The noise gate is to an expander what a limiter is to a compressor.
- The de-esser is a narrow-band compressor that targets the sibilance of voice/vocal signals.
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).
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.
For more information on these audio processes and many other audio effects, check out my article Full List: Audio Effects & Processes For Mixing/Production.
What are the main controls of a compressor? The main controls/parameters of a dynamic range compressor are as follows:
- Attack Time
- Release Time
- Makeup Gain
What is audio data compression? Audio data compression is the process of encoding digital audio information into few 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
- WMA (Windows Media Audio)