TL;DR: Audio compression is key in balancing audio levels, and it’s governed by parameters such as threshold, ratio, gain, attack, release, and knee. Understanding and effectively utilizing these elements can help create a balanced, professional-sounding audio mix.
In the realm of audio production, one of the most crucial elements to grasp is audio compression. Its purpose? Balancing the disparity between audio levels to enable listeners to appreciate the nuances of the performance. A compressor comes in handy for this job. It can sound daunting initially, but once you master the concept, you’ll find it a powerful tool in shaping your sound.
Threshold: The Gatekeeper
At the heart of any dynamics processor is the threshold. It’s the point at which the compressor starts its job of processing the signal. Imagine it as a gatekeeper. It allows quiet sounds to pass through while regulating louder ones that exceed the threshold. In simpler terms, the compressor will attenuate signals above the threshold, while signals below it remain unaffected. Hence, setting the threshold appropriately is crucial to any audio compression.
Ratio: The Amount of Compression
The ratio determines the amount of compression applied to a signal once it crosses the threshold. If the ratio is high, the signal will undergo significant attenuation once it crosses the threshold. For instance, a ratio of 2:1 applies light compression, while a ratio of 10:1 is heavy and often referred to as a limiter.
Gain Reduction and Output Gain
When a signal gets compressed, the resulting reduction can be expressed in dB – this is known as gain reduction. As we lower the threshold or increase the ratio, the gain reduction increases. But don’t worry, any volume lost from the overall signal can be recovered using output or makeup gain. This is applied after the compressor has lowered the signals above the threshold, raising the uncompressed values and blending them into the mix. A good starting point might be choosing a threshold that results in 4 – 6 dB of gain reduction, then use the output gain to bring the signal level back to its pre-compression state.
Attack and Release: Timing is Everything
The reaction time of the compressor to a signal crossing the threshold is governed by attack and release controls. The attack is how long it takes the compressor to kick in and attenuate the signal, while the release is how long it takes to revert to the uncompressed signal.
These times, usually measured in milliseconds, need to be set just right. If your attack is too fast, you may shave off too much signal, causing it to sound dull. Conversely, if it’s too slow, the compressor might completely miss the signal as it crosses the threshold. Fast release times can inflate and bring out the sustain, while too fast release times may lead to a pumping effect and distortion on low-frequency signals.
Knee: The Smooth Operator
In a compression graph, the knee represents the point where the compressor kicks in, attenuating any signals above the threshold. Some compressors allow you to use a ‘soft knee’, which means a more gradual attenuation of the signal as it crosses the threshold. This results in a smoother, more natural sounding compression which works well on vocals, bass, guitar and on the master mix. In contrast, a ‘hard knee’ is often preferable for fast, percussive sounds.
Audio compression, when done correctly, can make a significant difference in the quality of your mix. By understanding and effectively utilizing elements like threshold, ratio, gain, attack, release, and knee, you can create a well-balanced, professional-sounding audio mix.