Tutorial: The Compression Connection

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How to Give Your Audio That Pro Sound

When you listen to any professional broadcast, you hear audio that sounds clear and natural and has, mostly, the same loudness. This type of audio has narration that cuts through music, sound effects and noise. It just sounds impressive. But while tweaking the audio levels on your latest video project, you just don’t seem to be able to get that same great sound. What’s the secret?

All professional audio producers use compressors to even out the peaks and valleys of their wavefiles to make their audio sing. And getting you up to speed on just how to do this is the subject of this article.

The Order of Sweetening

You can get different results depending on the order in which you apply filters and effects. While personal taste and program material can change this suggested order, we prefer to first reduce the noise, then apply compression/expansion/limiting, do any equalization that’s needed and then normalize to -1dB. This order will ensure that your audio maintains the highest signal-to-noise ratio possible and will even provide a little headroom for later sweetening.

1. The Problem

To get the best audio, you always want your input signal to be as close to 0dB as possible. But, unlike in the old days of tape, digital recordings will begin to distort when the signal goes beyond 0dB. Since it is nearly impossible to salvage a distorted signal, many of us run our audio levels too low. This forces us to boost the overall signal, along with the noise, during editing. To avoid this, you have three ways to optimize the levels of your audio.

If you are recording a voiceover, use proper mic technique and avoid many of the normal spikes in your signal by not popping your “p’s & t’s.” Simply turn your head away slightly from the mic each time you pronounce these and other plosive and fricative sounds. It takes practice, but it really works. You can further smooth out those audio peaks and valleys by using a hardware audio compressor/limiter when you first record. For the greatest flexibility in post, you can minimize the compressor effect and just use the limiting function to ensure you don’t distort your original recording. These units most often come in rack-mount configurations, so they are most convenient to use in a studio setting. The last and perhaps most common solution to optimizing your signal is using the built-in capabilities of your audio editing software or using specialized third-party plug-ins.

2. Noise and Compression

If you’re not careful, compression can make the quieter parts of your audio, like noise, louder. So it’s a good idea to do some noise reduction first. Depending on your audio software, noise reduction may come in several flavors. Hiss-reduction is little more than a bandpass filter that allows or inhibits certain frequencies in the final mix. Others, like click and pop eliminators, automatically find the sharp transients characteristic of these types of noises and reduce or eliminate their presence. But the type that gives you the greatest control will be generically named “noise reduction.”

We’ll use the noise reduction effect in Adobe Audition for a quick example, but most other programs will follow a similar process. First highlight a section of your file that contains the specific noise you want to remove. Open the noise reduction filter and click Capture Profile. Hit the Preview button and begin to move the Noise Reduction Level slider as you listen to the playback. Once you find the balance between noise reduction and adverse impact on your main signal, choose Select Entire File and click OK. Now you’re ready to add compression.

Noise and Compression
Noise and Compression

3. Compression Controls

A compressor is essentially a really fast volume control. It looks ahead and, when it sees loud sections coming, turns down the volume, then brings it back up during quieter sections. You control the amount and quality of compression that gets applied to your signal by adjusting attack, release, threshold, ratio and gain. These controls may go by slightly different names in your software, but knowing what each type does is important.

Attack controls how quickly the effect will kick in. For standard narration, a good place to start is around 25 milliseconds (ms). Release sets the time it takes for the compressor to get back to the original volume. Try starting with 100ms for narration.

The threshold and ratio controls are very important, and they also interact with each other. When your signal level rises above the threshold setting you’ve chosen, the volume will be reduced by the amount of your ratio setting. Set your threshold too low and your audio will begin to sound muddy and overly compressed. Start by setting your ratio to 3:1, then lower the threshold just until you begin to hear a reduction in the louder sections. If this sounds good, you’re set. But most likely you’ll need to do a bit of tweaking of both threshold and ratio before you find the sweet spot.

For those of us who hated math, a little more about what the ratio setting does is in order. The ratio is simply the gain reduction relation of the input to output. A 3:1 setting means that, for every 3dB of input (above your threshold setting), the output will be 1dB. Higher ratios and lower threshold settings bring up the relative volume of softer elements in your signal, like breaths, sibilance and even noise, which is why it’s important to experiment. At its highest setting, infinity:1, the compression ratio is now a limiter, preventing any increase in volume above the threshold level.

Finally, the gain setting controls the overall output of your signal. Depending on your threshold and ratio settings, your average volume will be lower than your original signal. So you will usually need to add back some overall volume. Targeting a final output level of -1dB will give your wavefile some headroom if you later need to make some adjustments.

Compression Controls
Compression Controls

4. Multiband Compression

Unlike a single compressor that impacts all frequencies, some software allows you to control the compression of separate frequency ranges or “bands.” This is an incredibly powerful tool that can really mess up your signal unless you are careful. If you understand the basics of how to apply single-band compression and cannot get the result you need, then this may be your solution. We strongly suggest you experiment first with some of the presets to see if they get you in the ballpark. The VST multiband compressor plug-in that comes with Adobe Audition features real-time color graphics that represent four separate compressor settings, which control as many frequency bands. The Broadcast preset is a good starting point if you want to quickly maximize your narration. Be sure you select the Brickwall Limiter option to prevent signal distortion.

Nearly all of your audio can be improved by using a compressor wisely. The suggested settings we’ve made will get you started in the right direction. But nothing will replace careful listening, tweaking and listening again. So practice, play and repeat!

Multiband Compression
Multiband Compression

Contributing editor Brian Peterson is a video production consultant, trainer and lecturer.

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