Pro Audio Files

Introduction

In this article, we will delve into the often misunderstood and overlooked parameter of dynamic compression: knee. We will examine how a compressor’s ratio setting affects its behavior and the various ways we can utilize it in music production.

Compression Basics

Compressors come in a wide range of colors and flavors, but their primary function remains the same – reducing dynamic range. Although this description may oversimplify the capabilities of compressors, it highlights their versatility in shaping and coloring tracks. Despite their seemingly simple purpose, the practical implementation of compression is nuanced and adaptable.

Key features that contribute to this versatility are a compressor’s attack and release times, along with the ratio setting. The attack and release parameters determine the timing of gain reduction, potentially resulting in significant changes to the compressor’s sound and behavior. The ratio setting determines the intensity of the reduction.

At the core of a compressor’s functionality lies its threshold. This threshold establishes the point at which gain reduction takes effect. When the incoming signal falls below this threshold, the compressor remains inactive. Above the threshold, the compressor engages. These parameters, including attack, release, ratio, and threshold, may be user-adjustable or inherent to the compressor’s design.

Understanding Knee

To fully comprehend knee, it is essential to grasp the concept of ratio in compression. If you are new to compression or haven’t read our article on ratio, we recommend checking it out to gain a necessary foundation. Knee essentially refers to the interaction between a compressor’s threshold and ratio settings.

While knee is often quantified numerically in compressor controls, we typically describe it qualitatively as “soft,” “hard,” or somewhere in between. Hard knee compression operates as compression appears to do on paper – when the signal crosses the threshold, gain reduction occurs at the predefined ratio. No reduction occurs below the threshold.

Soft knee compression, on the other hand, follows a similar principle, with the ratio determining the intensity of gain reduction. However, instead of reduction occurring abruptly at the threshold, the compressor introduces reduction gradually. It eases in from a lower ratio, starting below the actual threshold.

The diagram below illustrates this concept:

The Importance of Knee Knowledge

If you are new to compression and find this information overwhelming, fear not! Many compressors feature a knee behavior that is preconfigured and known to engineers. In fact, if you have a favorite compressor, chances are you’re already familiar with its knee behavior, even if you weren’t aware of it.

Knee helps determine how “smoothly” a compressor behaves. Do you desire a subtle or noticeable effect? If you’ve ever found a compressor to be too harsh and aggressive or not aggressive enough, the knee setting likely played a significant role.

To further clarify the concept of knee, let’s explore a few scenarios.

Scenario #1: Moderate Knee Whack-a-Mole Compression

The “whack-a-mole” compression technique, introduced to me by my colleague Max Foreman, offers effective dynamic control for any sound source. This approach targets only the loudest moments of a track, resulting in increased consistency and facilitating better processing decisions down the line.

A soft knee compression curve isn’t ideal for this type of compression. We seek to capture the dynamic outliers, not the quieter parts. Conversely, hard knee compression may prove too aggressive. Therefore, a moderate knee setting strikes the right balance. It allows surgical compression with finesse, leaving minimal impact on the source material.

This compression technique works best for applying modest gain reduction, typically a couple of dB at most. It serves as the foundation for other dynamic processing, preparing the track for subsequent treatments.

Scenario #2: Transparent Vocal Leveling

Smoothness is the keyword for soft knee compression, as it enables a gradual and natural transition from uncompressed to fully compressed. While this technique is commonly associated with vocals, it can benefit other legato source materials such as strings, woodwinds, and synths.

The LA-2A compressor, renowned for its exceptional vocal compression, offers an inherently soft knee. Though a hardware version may be out of your budget, numerous plugin emulations of the LA-2A are available. The Waves CLA-2A, often on sale for $30-40, is highly recommended.

For those unable to access LA-2A emulations, achieving smooth and silky vocal compression is still achievable using your DAW’s stock compressor. Simply dial in a soft knee, moderate ratio (around 2:1 to 3:1), gentle attack and release times, and set the threshold for 2-4 dB of compression.

This soft knee vocal compression pairs well with targeted dynamic control, as described in our ratio article. With a combination of smoothness and surgical precision, you can confidently position the lead vocal at the forefront of your mix.

Scenario #3: Strictly Business Hard Knee Drum Compression

Drum compression encompasses various applications, each differing based on the intended outcome. In this scenario, we focus on basic single-channel drum compression, typically applied to kick or snare tracks.

The primary goal here is to enhance consistency, shape transients and sustain, and introduce pleasant harmonic coloration. In these instances, hard knee compression yields optimal results, as the transparency associated with soft knee compression is unnecessary.

The remaining compressor settings will depend on personal preference and the requirements of the song. For added snap, employ a slow attack time; for weight, go with a faster setting. Adjust the release time to complement the musicality of the part. Experiment with higher ratios for aggressive and modern productions, but keep the ratio lower for a vintage and subtle sound.

Conclusion

While knee may not be as critical as parameters like attack and release times, it significantly contributes to a compressor’s musical behavior. When selecting a knee setting, consider whether a “smooth” quality aligns with your desired outcome. By understanding the role of knee, you can confidently use compression to shape your sounds and elevate your music production.