How to Use Compressors, Limiters and Noise Gates

At last, you've finished recording all the various components of your audio track and have them carefully assembled in your time line. Now for that final, professional touch,we'll show you how to ensure that your audio is well balanced and free of both low end noise and high end distortion by using compressors, limiters and noise gates to sound its very best.

Wouldn't it be wonderful if we could simply record our various audio assets, drop them onto our time line, export to our preferred distribution medium and call it a day? Of course, we could do exactly that, but then our work wouldn't have that high degree of professionalism we constantly strive for. In reality, a lot of work goes into achieving a finely tuned audio mix worthy of an audience's time. Let's take a look at some of the tools commonly used to do this.

A typical audio waveform is filled with peaks and valleys. Sometimes these encompass both extremes from the highest highs to the lowest lows. This uneven audio mix can cause your audience to ride the volume controls in order to hear the softer portions without being blown out of the room by the louder ones - an annoying requirement at best. A compressor is used to even out the audio waveform by compressing it so it conforms to values specified by you. It softens the loud parts and boosts the soft ones to create a more even sound. In this tutorial we'll be using Adobe's Premiere Pro CS5 to demonstrate these tools. Most editing software will have similar capabilities and dedicated audio editors will have an even more extensive tool set available. With your project open in Premiere go to the search textbox in the Effects window and type Dynamics. The Audio Effects folder opens, as well as three sub-folders, each displaying the Dynamics effect. Choose Dynamics from the folder that matches the type of audio track your mix is located on: 5.1 surround sound, stereo or mono. Drag Dynamics onto your audio clip, then find it in the Effect Controls window. Twirl the effect open and you'll see a checkbox labeled Bypass, which allows you to toggle your adjustments on and off so you can hear the effect they are having. Below Bypass are Custom Setup and Individual Parameters. Twirl open Individual Parameters for an array of fine tuning options that an audio engineer's dreams are made of. The rest of us, however, will do just fine with Custom Setup. Twirl it open and you'll see a graphic interface reminiscent of a hardware based compressor/ limiter. The Dynamics effect consists of several different, yet similar, audio processing effects having similarly labeled controls. Threshold sets the level at which processing begins and Ratio controls the degree of processing that will take place. For example, if Ratio is set to 3, or a ratio of 3 to 1, then as the input level increases by 3 decibels the output increases by only 1 decibel. Attack determines how quickly the effect will kick in after the Threshold has been breached and Release refers to how quickly the effect will stop working. Click the Toggle looping audio playback followed by the Play only the audio for this clip buttons located in the lower right portion of the Effect Controls window to loop your audio clip. Listen carefully through a set of quality headphones while making adjustments so you can hear exactly the effect your adjustments are making. The Compressor controls make up the middle section of the graphic interface. Make sure Compressor is checked at the top. Start by setting Ratio to around 2-1/2 to 3 and Threshold between -8 and -14. This should put you in the ballpark and you can fine tune from there. Avoid over compression as this can result in a dulling effect due to the reduction of the higher frequencies. After making adjustments you may find that the volume has been reduced. Use the MakeUp control to restore gain lost due to compression. When you think you've got it, stop looping and play your clip through on the time line as you watch the audio meters. You'll want the volume at about -6 decibels with peaks approaching, but not hitting, zero.

Oftentimes, a portion of our audio will extend far enough north that it crosses into the red. This is known as clipping and is extremely undesirable. It is hard on the ears, is often distorted and ugly sounding and can cause damage to audio equipment. To prevent clipping, a limiter is used. A limiter employs a much higher compression ratio than a compressor and prevents clipping by acting as a brick wall through which signal peaks may not pass. The overall strategy is to use compression to bring the highs down out of the stratosphere, then apply a limiter to stop that occasional rogue signal from going "over the wall." Click the Limiter checkbox in the lower right portion of the Dynamics interface. Setting the Threshold to -0.20 dB for example, will prevent anything higher from getting through; it will be automatically reduced down to -0.20 dB and will prevent the signal from ever clipping. Play through the clip while watching the meters to check your results.

A noise gate is typically used to remove low level sounds and is oftentimes employed to remove background noise such as during pauses in a voiceover. The threshold acts as a gate that closes, cutting off a signal when it falls below a preset value and opening again to allow it to pass through when it rises above that value. On the left side of the Dynamics interface click the AutoGate checkbox. While looping, listen carefully as you adjust the controls. A color coded display indicates the gate's status. When open it displays as green, yellow during attack and release, and red when closed. Adjust the controls until you have achieved the desired result.

For cleaner, evenly balanced and finely tuned audio, try using compressors, limiters and noise gates on your next project. Using the tips and techniques we've outlined here, your audience will be captivated by both your story and its polished, professional sound.