Audio Compression 101: A Practical Introduction to Dynamic Processors.
This opening episode focuses on dynamic processors, and more specifically, on compressors—one of the most widely used tools in audio recording studios throughout the world.
Compressors are fundamental tools for shaping the dynamics of an audio recording. Because they directly control changes in volume over time, they are categorized as dynamic processors or dynamic effects. Their role is not decorative but functional: they manage how loud and how soft parts of a signal behave relative to each other.
What Is a Compressor, and How Is It Used?
A compressor is normally used as an insert effect, meaning it affects only the sound on the channel it is inserted into.
For example:
- A compressor might be inserted on a vocal track
- When the vocalist gets too loud, the compressor will automatically reduce the volume of that vocal track
This happens in real time, without manual intervention.
A compressor reduces the volume of an audio track, just like you would do with the fader on a mixing console—but much faster than you could ever do manually.
This speed and precision is what makes compression such a powerful tool.
The Basic Idea Behind Compression
At its core, a compressor is designed to make loud sounds softer.
That is the traditional and most straightforward way to describe what a compressor does.
The process works by reacting to the level of the incoming audio signal. Once certain conditions are met, the compressor begins to reduce volume automatically.
The Threshold: When Compression Begins
The threshold is the level at which the compressor starts working.
- When the audio signal reaches the threshold
- And then goes above it
- The compressor engages and begins compressing
If the signal never crosses the threshold, the compressor will not do anything.
Important practical note:
- If the threshold is set too high, the compressor won’t work
- You must lower the threshold so the signal peaks over it
The Ratio: How Much Compression Is Applied
Once the signal exceeds the threshold, the ratio determines how much compression is applied.
The ratio defines the relationship between:
- How much the signal goes over the threshold
- And how much level is actually allowed to come out
Common Ratio Examples
- 2:1 Ratio (Soft Compression)
- For every 2 dB above the threshold
- Only 1 dB is output
- 20:1 Ratio (Hard Compression)
- For every 20 dB above the threshold
- Only 1 dB is output
- 1:1 Ratio (No Compression)
- For every 1 dB above the threshold
- 1 dB is output
- This is a neutral setting with no compression at all
The higher the ratio, the harder the compressor works.
Hearing Compression in Action
When listening to a dynamic vocal or drum track without compression, volume differences are very noticeable.
As compression is applied:
- Ratio 2:1 gently controls dynamics
- Ratio 4:1 tightens the signal further
- A ratio of 10:1, especially with a lower threshold, makes the sound significantly softer
As you could hear, once a 10:1 ratio is applied and the threshold is lowered, the sound becomes much more controlled—and noticeably reduced in level.
At this point, only the threshold and ratio controls are being used. No other settings have been adjusted yet.
Makeup Gain: Compensating for Gain Reduction
When compression reduces volume, it also reduces overall level.
This is where makeup gain (also called compensational gain) comes in.
Makeup gain allows you to:
- Bring the compressed signal back up
- Match the original input level
What Actually Happens
- Loud sounds are reduced significantly
- Soft sounds remain closer to their original level
- The overall signal is then boosted
We reduced the dynamic range and then brought the overall volume back up.
The result:
- Loud and soft sounds are closer together
- The signal sounds more aggressive
- Details like high-hats become more prominent
Reducing Dynamic Range
By using compression and makeup gain together, you are effectively reducing the dynamic range of the signal.
This means:
- Loud sounds are made softer
- Soft sounds are brought closer to the loud ones
- The entire signal becomes more consistent
This technique is especially noticeable on drums, where punch and presence become more controlled and impactful.
The Core Compressor Controls So Far
At this stage, the most basic compressor controls covered are
- Threshold
The level at which compression starts - Ratio
How hard the compressor works above the threshold - Makeup Gain
Compensates for volume lost due to compression
These form the foundation of compression.
Attack and Release: Timing Is Everything
Two additional controls are extremely important:
- Attack Time
- Release Time
Both are measured in milliseconds and define how quickly the compressor reacts.
Some compressors simplify these controls with options like
- Fast attack
- Slow attack
- Auto release
But the underlying concept remains the same.
Attack Time: How Fast Compression Starts
The attack time determines:
- How fast the compressor applies compression
- After the signal exceeds the threshold
If the attack is too slow:
- The compressor may barely react
- Especially on fast transients like a snare drum
If we set a really slow attack on a snare drum, the compressor will probably not work at all.
Attack time directly affects punch, especially on kicks and snares.
Release Time: How Fast Compression Stops
The release time tells the compressor:
- How long it takes to stop compressing
- After the signal drops below the threshold
Together, attack and release define how fast the compressor actually works.
Adjusting these settings changes:
- The feel of the groove
- The punch of drums
- The responsiveness of the compression
Listening while adjusting these values allows you to hear and see their impact simultaneously.
What Comes Next
This concludes part one of the first tutorial on compression techniques.
Upcoming episodes will cover:
- How to properly set up a compressor
- Practical usage in real mixes
- Techniques such as
- New York drum compression
- Side-chain compression
- A comparison of different compressor brands
- Hearing the distinct sound of each compressor type