A Bit About Audio Compression – Tuesday Voiceover Tech Tip

As you begin to record in your home voiceover studio, you’ll start hearing about “Compression.” Compression is one of the least well understood audio tools available to us.

During one of the first VO classes I took, another student was explaining their rule for recording. They said, “…you have to use compression”  Emphatic. No room for discussion.

This struck me as kind of strange. I’d used Compressors in the recording studio – real ones with dials and knobs and stuff – and had a pretty good idea how they worked and where they normally got used. However, I was struggling to understand their use in VO. If voiceover was supposed to be “conversational”, “real” and “non-announcery”, then how did Compression help that? Compression tends to make things punchy and processed sounding – like a radio DJ voice.

Using Compression? Be careful!

The first thing to realize is that Compression, like any other complex audio effect, cannot be “undone” once applied to the audio. As with any effect applied to your audio, if you aren’t sure how to use it, it’s probably best not to.

A simple gain adjustment – something altered using “Amplify” for example – can likely be fixed or adjusted by a client without impacting the audio quality. But, if we use a complex process like Compression on our audio it can definitely cause problems further on in the production process.

If we hand off a file which has been Compressed (or Limited, or even had Noise Reduction applied), that has become part of the audio in a way that cannot be removed. (Even EQ can cause this). When a client asks for “Raw” audio, they are most likely requesting that you do not use any type of “dynamic Effect.” Compression is a dynamic Effect by definition.

How does an audio Compression Effect work?

Compression is triggered by the recorded audio reaching a certain volume level. This is the “Threshold” and can be adjusted directly on the Effect. Usually the settings on Compressors are for the incoming Loudness from the recorded track. They will attenuate audio as the loudness goes above that Threshold. This means that if the volume is below the threshold set on the Compressor, it will not have any effect upon the audio. You can think of the Threshold as a thermostat on your home HVAC system. Nothing occurs until the device kicks on. 

Once the Compression Threshold is reached, the Effect begins reducing any additional increase of volume by a specific amount. This is the Ratio of Compression.

A low ratio of 2:1 lets one decibel through for every two that enter the Compressor. A higher ratio of 10:1 lets one decibel through for every 10 that arrive. With a high Compression Ratio, once the volume of your audio reaches the threshold, it hardly gets any louder at all which can sound quite artificial. A Compressor with a very high ratio starts to behave a lot like a Limiter.

Compressors have “Knees”?

This weird phrase which is used on many Compressor controls refers to the transition area of the Threshold. The “softer” the knee, the more gently it transitions through the Threshold point. A “sharp” knee is makes the action of the Compressor very abrupt and noticeable. On the other hand, if the angle of the knee is too soft it might actually start working before we really want it to do so. 

Compressors do not make things louder – so, we need “Make Up Gain”

This is where a Compressor differs from a true Limiter. With a Limiter, we are increasing the volume “into” the Effect. That lets us Limit the Peaks with a high degree of accuracy. 

By contrast, Compression will only make things quieter. Using the combination of Threshold and Ratio, a Compression effect only reduces volume in our audio. That is how it gets its name – it compresses the dynamic range and loudness. If we then want our resulting audio to be louder, we need to use a “Make Up Gain” adjustment after the audio is compressed. This amplifies the compressed audio we just created. 

Because we’ve limited the dynamic range of the recording, and then made everything louder, the resulting audio will tend to sound “punchier”. This can easily be overdone and is often not desirable if anyone else will be doing any type of processing to our audio. It’s generally a bad practice to use multiple passes of dynamics tools on audio.

Attack and Release – Don’t be lagging!

Attack and Release values control the speed with which a Compressor reacts to the incoming volume (Attack) and how quickly it stops working (Release). For voiceover recording, we generally want a quick attack and release. If a Compressor reacts too slowly to the audio we’ll tend to notice the effect taking place – it can sound like our vocal track is suddenly getting squished. Not all Compressors allow you to control these variables.

Coloration, Saturation, Distortion – oh my!

As with preamps (see the section “Can Distortion Sound Good?” in my resource on audio interfaces), certain types of Compressors were historically used because the distortion or sonic coloration they added to the signal was pleasing within a musical context. Often described as a “Classic” effect, these are generally less helpful in a voiceover recording context, where transparent sound and clarity are key.

Next: When to use a Compressor, and a few of my favorite ones

Compression Resource Update: I’ve created a companion video demonstrating some of the settings in typical compressors – check it out @JustAskJimVO on YouTube.

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